| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Journal of Petrology | Volume 43 | Number 3 | Pages 449-484 | 2002
© Oxford University Press 2002
Basement Geochemistry and Geochronology of Central Malaita, Solomon Islands, with Implications for the Origin and Evolution of the Ontong Java Plateau
1NATIONAL INSTITUTE OF GEOLOGICAL SCIENCES, UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY, 1101 PHILIPPINES
2SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII, HONOLULU, HI 96822, USA
3DEPARTMENT OF CIVIL ENGINEERING AND GEOLOGICAL SCIENCES, UNIVERSITY OF NOTRE DAME, NOTRE DAME, IN 46556, USA
4COLLEGE OF OCEANIC AND ATMOSPHERIC SCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR 97331, USA
5BRITISH GEOLOGICAL SURVEY, ONSHORE MINERALS AND ENERGY RESOURCES, KEYWORTH NG12 5GG, UK
Received October 10, 2000; Revised typescript accepted September 7, 2001
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONSTUDY AREA AND GENERAL...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Sections of Ontong Java Plateau basalt basement in central Malaita (Solomon Islands) are 0·5–3·5 km thick and resemble a much-expanded version of that recovered at Ocean Drilling Program Site 807. 40Ar–39Ar ages (121–125 Ma) are identical to those for Site 807, southern Malaita, Ramos Island, parts of the island of Santa Isabel, and Deep Sea Drilling Project Site 289; the
90 Ma eruptive episode seen in Santa Isabel, San Cristobal, and at drill sites 803 and 288 is not represented. The central Malaitan basalts provide further evidence of two distinct ocean-island-like mantle sources, and the combined data preclude a significant contribution from normal ocean-ridge-type mantle. As at Site 807, two geochemically distinct stratigraphic groups are present, the Singgalo Formation (750 m thick in central Malaita) and the Kwaimbaita Formation (>2700 m thick). Both a peridotite plume-head and eclogite-bearing plume-head may account for the geochemical characteristics, but the observed stratigraphic succession requires special conditions for the latter model. A number of first-order features of the Ontong Java Plateau do not obviously fit the predictions of any plume-head model: for example, at least two important, geochemically similar eruptive episodes 30 my apart, the lack of an obvious plume-tail trace, and lack of evidence for emergence or significant uplift. KEY WORDS: Ontong Java Plateau; geochemistry; mantle sources; petrogenesis; eruptive stratigraphy
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONSTUDY AREA AND GENERAL...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
The Solomon islands of Malaita, Ramos, Ulawa, the northeastern part of Santa Isabel, and probably part of San Cristobal (a.k.a. Makira) form the obducted and uplifted southwestern margin of the world’s largest volcanic oceanic plateau, the 5 x 107 km3 Ontong Java Plateau (OJP; Fig. 1) (e.g. Kroenke, 1972
; Hughes & Turner, 1977; Coleman et al., 1978; Coleman & Kroenke, 1981; Petterson et al., 1997; Phinney et al., 1999). Reconnaissance studies show that the Cretaceous igneous basement of these islands is almost indistinguishable in age from, and closely similar in composition to, that drilled at Ocean Drilling Program (ODP) Sites 807 and 803 and Deep Sea Drilling Project (DSDP) Site 289 on the northern and north–central plateau (Mahoney et al., 1993; Parkinson et al., 1996; Tejada et al., 1996; Neal et al., 1997). Recently, we sampled accessible portions of 0·5- and 3·5-km-thick sections of OJP crust in central Malaita. As the deepest exposures of the plateau’s crust yet sampled systematically, these sections present a unique source of information on age, eruptive stratigraphy, petrogenesis, and mantle sources. In this paper, we characterize the geochemical stratigraphy of the southern OJP crust recorded in these sections, and build upon previous work to examine what the isotopic and chemical variations reveal about the plateau’s origin and the implications for the mantle source regions that fed plateau magmatism.
|
|
STUDY AREA AND GENERAL FIELD CHARACTERISTICS OF OJP CRUST IN MALAITA |
|---|
|
TOPABSTRACTINTRODUCTIONSTUDY AREA AND GENERAL...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Cretaceous igneous OJP crust forms the cores of several large anticlines in Malaita (Fig. 2a) and is exposed in stream gullies and waterfalls on this heavily vegetated island. The basement rocks, formerly known as the Older Series or Older Basalts in the south (Hughes & Turner, 1976
, 1977), the Fiu Lavas in the north (Rickwood, 1957), and the Older Malaita Volcanics in the northernmost part of the island (Barron, 1993), recently have been termed the Malaita Volcanic Group (Petterson et al., 1997). The Malaita Volcanic Group is distinguished in the field from an overlying, much less extensive volcanic series, previously called the Younger Series or Younger Basalts (Hughes & Turner, 1976, 1977) but now termed the Maramasike Formation (Fm.; Petterson et al., 1997), by the vesicularity of the younger volcanic rocks and the intercalation of these rocks with Eocene pelagic limestones.
|
Samples for this study were collected in conjunction with geological mapping surveys by the Solomon Islands Ministry of Energy, Mineral and Water Resources in 1993 (see Petterson, 1995). The great bulk of the Malaita Volcanic Group exposed in the central part of the island consists of massive and pillowed lava flows very similar to those in southern Malaita, and to the Sigana Basalts (Hawkins & Barron, 1991) in northeastern Santa Isabel. The flows are aphanitic to phaneritic, aphyric to plagioclase + clinopyroxene (± olivine)-phyric basalts, and coarser-grained diabases and microgabbros [see Petterson (1995) and Babbs (1997) for detailed descriptions]. Important findings of the surveys are that all of the lavas were emplaced in a submarine environment, that interbeds of chert or mudstone are very rare, and that no limestone interbeds were found. Very few dikes were encountered, although gabbroic intrusions were found to the north, within the Fateleka Anticline (Fig. 2a). Except in disturbed sections, the basalts in central Malaita are everywhere overlain conformably by the early to middle Cretaceous Kwara’ae Mudstone Formation, consisting of siliceous, parallel-laminated radiolarian pelagic mudstones.
Our principal study area was the northern part of the Kwaio Anticline, where the thickest exposures of the Malaita Volcanic Group occur (Fig. 2a). The anticline is accessible via rivers that cut the basement and sedimentary sections in a SW–NE direction roughly perpendicular to the axis of the anticline, facilitating thickness determination of all the lithostratigraphic units. The majority of the samples we discuss here were collected from the Singgalo and Kwaimbaita rivers, where a lava section of estimated 3·5 km thickness (Petterson et al., 1997) forms the core of the anticline (Fig. 2b). The other sample area we discuss is the Kwara’ae Anticline (Fig. 2a), accessible via the Kwaiafa’a River and its tributary, the Bisula, where we sampled a basement section of 500 m thickness.
Samples were taken mainly from interior portions of pillows and massive flows, to minimize alteration. The modal mineralogy of the microgabbros and the groundmass in the aphyric and plagioclase-phyric basalts consists of almost equal proportions of unaltered plagioclase (up to 58%) and clinopyroxene (up to 45%), with minor altered olivine (5–7%) and oxides (Petterson, 1995). Plagioclase and clinopyroxene are usually in an ophitic to subophitic relationship, indicating contemporaneous crystallization, although in some sections plagioclase crystals are found included in clinopyroxene grains, possibly indicating that the plagioclase crystallized earlier. Modal abundance of mesostasis varies from 10 to 30% but usually is <20%. In the samples chosen for geochemical work, limited low-grade alteration is seen in rare amygdules containing zeolite, chlorite, and calcite, and is manifested in mesostasis and olivine, which are partially to wholly replaced by brown clay and iddingsite (Babbs, 1997). In some of the more altered samples (e.g. ML476) plagioclase is partially replaced by a mixture of clay and calcite, whereas in others (e.g. SGB24), secondary phases replace olivine completely and fill the interstices between feldspar laths. In the Kwaio Anticline, a minor but striking facies of the Malaita Volcanic Group, consisting of basalts with ‘spherulitic’ or ‘orbicular’ texture, was found as boulders in the river beds (Petterson, 1995). This facies consists of 1–6 cm fragments of ophitic gabbro and plagioclase or clinopyroxene megacrysts in basaltic to diabasic host rock.
|
ANALYTICAL METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONSTUDY AREA AND GENERAL...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
We collected 152 igneous rock samples, 64 of which we chose for geochemical study: 30 from the Singgalo River, 20 from the Kwaimbaita River, and 14 from the Kwaiafa’a River drainages. Samples were trimmed with a water-cooled rock saw to remove alteration rinds and split into three parts; the freshest interior portions were reserved for 40Ar–39Ar and isotopic analysis. Splits for bulk-rock major and trace element analysis were chipped into pieces 0·3–0·5 cm across, cleaned ultrasonically in deionized water, and dried; chips showing macroscopic alteration significantly greater than the average were removed, and the remainder powdered in an alumina mill. Major elements and a suite of trace elements (Table 1; note that samples are listed in stratigraphic order from top to bottom of each section) were analyzed by X-ray fluorescence (XRF) spectrometry at the University of Hawaii, using the methods of Norrish & Chappell (1977)
. Rare earth elements and several other trace elements (Table 2) were analyzed on 0·2 g splits of the powders by inductively coupled plasma–mass spectrometry (ICP-MS) at the University of Notre Dame using the modification of Jenner et al.’s (1990) method proposed by McGinnis et al. (1997); see Jain & Neal (1996) for details. Of the elements analyzed by both XRF and ICP-MS, the agreement is generally 
5% for Zr and 15% for Y; agreement is within 15% for Co and Nb and 20% for Cr (except for one, four, and six samples, respectively). The ICP-MS values for Rb are systematically higher than the XRF values by as much as 0·9 ppm; however, the analytical uncertainty of the XRF measurements is relatively large as Rb contents are generally near the XRF detection limit.
|
|
Splits of eight whole-rock samples were prepared at Oregon State University for 40Ar–39Ar incremental-heating age determinations. Small cores (0·7 g) cut from the freshest portions of samples were loaded in evacuated quartz vials and irradiated for 6 h at 1 MW power at the Oregon State University TRIGA reactor. Biotite standard FCT-3 (27·55 ± 0·12 Ma, equivalent to 513·9 Ma for hornblende Mmhb-1; Lanphere et al., 1990) was used to monitor the neutron fluence. The isotopic composition of Ar was determined for each of 4–6 temperature steps using an AEI MS-10S mass spectrometer [see Duncan et al. (1997) for experimental details]. Table 3 includes plateau ages (the weighted mean of concordant, adjacent step ages), isochron ages (determined from the linear regression of step compositions), and integrated or total fusion ages (calculated by adding all step compositions together).
|
Sample preparation and analysis for Sr, Nd, and Pb isotopes were performed at the University of Hawaii [for methods, see Peng & Mahoney (1995) and Tejada et al. (1996)]. Isotope-dilution abundance data for Sr, Rb, Nd, Sm, Pb, Th, and U, used for isotopic age corrections, are presented together with isotopic ratios in Table 4. As in our previous studies, elemental abundances determined by isotope dilution do not necessarily represent those of the bulk rocks because, for isotopic work, small chips were handpicked from the bulk rock and cleaned in acid before powdering; also, some measurements were made on strongly acid-leached splits of these powders. In addition to the basaltic lavas, a plagioclase megacryst and its host basalt (SGB21) from a sample of the ‘orbicular’ facies rocks were analyzed isotopically to evaluate the genetic relationship of the megacrysts and lavas.
|
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONSTUDY AREA AND GENERAL...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
40Ar–39Ar ages
Previous work on the OJP has revealed two distinct sets of basement ages (Fig. 3). Lavas in southern Malaita, Ramos Island, parts of Santa Isabel, at DSDP Site 289, and at ODP Site 807 all have ages that are indistinguishable, within errors, around a mean of 122 Ma (total range is 119–125 Ma). In contrast, basalts at ODP Site 803, in much of Santa Isabel and parts of San Cristobal, and ash layers at DSDP Site 288 yield ages around 90 Ma (total range is 86–94 Ma; Mahoney et al., 1993
; Birkhold-VanDyke et al., 1996; Parkinson et al., 1996; Tejada et al., 1996). Compositionally similar tholeiites with 40Ar–39Ar ages of 62 Ma and 34 Ma are also present on San Cristobal (Birkhold-VanDyke et al., 1996), but their relationship to the older OJP basalts is as yet unclear. Our 40Ar–39Ar ages for rocks from the Singgalo, Kwaimbaita, and Kwaiafa’a river sections all fall within the 122 Ma group (Table 3). Five of the samples produced acceptable plateau ages; that is, concordant step-ages comprising >50% of the total 39Ar released. An additional two samples (SGB10, KF14) gave similar plateau ages, based on slightly less than half of the total gas; one sample (ML423) produced a recoil age-spectrum pattern, from which only a minimum (integrated) age estimate could be determined. Isochron ages are concordant with plateau ages for all samples, but generally have larger uncertainties because of the small dispersion of the few step compositions. For this reason, we take the acceptable plateau ages as the best estimate of the timing of volcanism. These five ages are indistinguishable at the 95% confidence level, and provide a mean age for the entire section of 123·1 ± 1·4 Ma. There is no evidence of any significant time difference between the lowermost and uppermost lavas, from either the 40Ar–39Ar ages or the volcanic stratigraphy (e.g. sedimentary interbeds are almost nonexistent). Combined with previously reported ages, these results show the 122 Ma volcanic event was widespread across the OJP, occurring at locations separated by as much as 1600 km (see Tejada et al., 1996). Furthermore, the combined data now available suggest that the 90 Ma event is not represented on Malaita.
|
, mean ages reported by Parkinson et al. (1996)
Volcanic stratigraphy
Effects of alteration on chemical composition
Relative to many of the samples from southern Malaita and Santa Isabel studied by Tejada et al. (1996), the majority of the central Malaitan lavas are only weakly altered, as indicated by, for example, their relatively low (<1·5 wt %) weight loss on ignition (LOI) values (Table 1); also, only a few samples with MgO values between 7·0 and 8·5 wt % have comparatively high K2O and Na2O values (>0·25 and >3·00 wt %, respectively). This is consistent with Babbs’ (1997) conclusion that alteration is limited mainly to clays replacing olivine and mesostasis. As a result, data for elements and oxides known to be affected by higher levels of submarine alteration, such as Na2O, CaO, P2O5, Cr, Ni, Zn, Cu, and V (e.g. Bienvenu et al., 1990), still show some coherence with data for more immobile elements such as TiO2, Fe2O3, Al2O3, Nb, Y, Zr, and the lanthanide rare earths. Nevertheless, most of our petrogenetic interpretations are based on the alteration-resistant elements.
Isotopic and chemical variations
Kwaio Anticline. Previous studies have revealed two distinct, ocean-island-like isotopic groups of OJP basement lavas; at Site 807, where both groups were sampled, they are stratigraphically separated (Mahoney & Spencer, 1991; Mahoney et al., 1993; Tejada et al., 1996). Two stratigraphically separated, isotopically distinct groups of lavas also are present in the Kwaio Anticline (Fig. 4; Table 4). The upper stratigraphic group has initial 
Nd(t) = +3·8 to +3·9, (87Sr/86Sr)t = 0·7040–0·7042, (206Pb/204Pb)t = 17·71–17·85, (207Pb/204Pb)t = 15·47–15·49, and (208Pb/204Pb)t = 37·87–37·98. The stratigraphically lower group has Nd(t) = +5·4 to +6·0, (87Sr/86Sr)t = 0·7037–0·7039, (206Pb/204Pb)t = 18·12–18·40, (207Pb/204Pb)t = 15·51–15·53, and (208Pb/204Pb)t = 38·15–38·23 (Figs 4–6
). Basalts of the Kwaelungga River section south of the Singgalo River (Babbs, 1997) show similar characteristics. The plagioclase megacryst of sample SGB21 has an identical isotopic composition, within error, to that of its host rock, which belongs with the lower stratigraphic group. Malaita Volcanic Group lavas in southern Malaita (Mahoney & Spencer, 1991; Tejada et al., 1996) are isotopically equivalent to the upper group. Moreover, the two isotopic groups in the Kwaio Anticline are virtually indistinguishable from those of northeastern Santa Isabel (Tejada et al., 1996) and similar to the upper (Unit A) and lower (Units C–G) groups, respectively, of lavas at ODP Site 807 (Mahoney et al., 1993). In detail, the upper group in the Kwaio Anticline has slightly lower Nd(t) and Pb isotope ratios than the flows of Unit A at Site 807 (e.g. with Nd(t) = +4·5 to +5·1), whereas the lower group possesses slightly higher (87Sr/86Sr)t than the Units C–G basalts (with values of 0·7034–0·7035), much like the lavas of northeastern Santa Isabel (0·7036–0·7037).
|
|
|
Except for compositionally extreme samples ML475 (MgO 10·0 wt %) and ML476 (MgO 5·8 wt %), the major element compositions of the Kwaio Anticline basalts have a relatively small range in MgO (6·3–8·9 wt %), molar mg-number [100 x Mg/(Mg + Fe2+) = 51–61, assuming Fe2+ is 0·85 of total Fe], SiO2 (48·6–50·5 wt %), TiO2 (0·9–1·8 wt %), Al2O3 (13·5–15·9 wt %), CaO (10·4–13·2 wt %), and total Fe as Fe2O3 (Fe2O3* 11·9–14·6 wt %) (Fig. 7; Table 1). These results are consistent with those of previous studies, which have documented a much smaller range of major element variation in OJP lavas than seen in, for example, the much smaller Manihiki or Caribbean plateaux (e.g. Mahoney et al., 1993; Tejada et al., 1996; Kerr et al., 1997). In general, lavas of the stratigraphically upper isotopic group have lower CaO and MgO/Fe2O3* but higher TiO2, and Fe2O3* for a given MgO content than the underlying lavas (Fig. 7).
|
As with the two groups of lavas at Site 807, incompatible elements in the upper group of the Kwaio Anticline are slightly more abundant than in the underlying group (Tables 1 and 2): for example, TiO2 1·5 ± 0·3 (total range) vs 1·1 ± 0·2 wt %, P2O5 0·14 ± 0·02 wt % vs 0·10 ± 0·02 wt %, La 5·4 ± 1·0 ppm vs 3·4 ± 0·5 ppm, Ce 14·4 ± 2·4 ppm vs 9·8 ± 0·8 ppm, Nb 5·9 ± 0·5 ppm vs 4·0 ± 1·0 ppm, and Th 0·53 ± 0·14 ppm vs 0·30 ± 0·06 ppm. Except for samples ML407, ML475, and ML476, Cr and Ni contents are generally higher in the lower group of lavas (128 ± 28 vs 112 ± 56 ppm and 110 ± 15 vs 68 ± 28 ppm, respectively), probably indicating slightly lesser amounts of crystal fractionation than in the stratigraphically upper group. Sample ML475, with 446 ppm Cr, appears to contain abundant excess (cumulus) clinopyroxene
Kwara’ae Anticline. The basalt flows exposed in the Kwara’ae Anticline are chemically very similar to the stratigraphically upper group of lavas in the Kwaio Anticline; for example, in TiO2 (1·5 ± 0·2 wt %), P2O5 (0·11 ± 0·03 wt %), La (5·2 ± 1·2 ppm), Ce (15·3 ± 4·3 ppm), Nb (4·8 ± 1·3 ppm), and Th (0·48 ± 0·20 ppm; Tables 1 and 2). The two samples analyzed isotopically, KF1 and KF4, from the top and bottom, respectively, of the Kwaiafa’a River section (Figs 4–6) also have similar isotopic compositions to the upper group of Kwaio Anticline lavas, except that KF1 has higher (87Sr/86Sr)t (0·70441) even after multi-step acid leaching. This sample was taken from the topmost lava flow, in contact with the Kwara’ae Mudstone and separated from the rest of the lava sequence by a thin bed of chert. Its high (87Sr/86Sr)t value is probably a result of seawater alteration.
Proposed formation names, estimated thickness, and regional correlations
We propose that the geochemically distinct upper sequence of flows in central Malaita, characterized by lower Nd(t) and (206Pb/204Pb)t, higher (87Sr/86Sr)t, and greater abundances of the more highly incompatible elements, be designated as the Singgalo Formation (Fm.), and that the lower sequence, with higher Nd(t) and (206Pb/204Pb)t, and lower (87Sr/86Sr)t and incompatible-element contents, be termed the Kwaimbaita Fm. These designations are appropriate because they are taken from the names of the two major rivers where the thickest exposures of each magma type have been found (Figs 2 and 4). On the basis of the close isotopic and elemental affinity noted above, 122 Ma lavas sampled in southern Malaita, northernmost Malaita, Ramos Island, and many of those in northeastern Santa Isabel also can be classified as Singgalo type, whereas a few lavas sampled in northeastern Santa Isabel are Kwaimbaita type (Figs 5–8). Isotopic ratios of the single flow sampled at Site 289 indicate that it is also of the Kwaimbaita type. At Site 807, the upper (Unit A) and lower (Units C–G) groups of flows likewise correspond, respectively, to the Singgalo and Kwaimbaita types, although physical continuity with the Malaitan volcano-stratigraphic formations remains to be demonstrated. In contrast, the 90 Ma lavas of Site 803, Santa Isabel, and San Cristobal, although isotopically similar to the 122 Ma basalts, cannot be related directly to either formation because they are some 30 my younger; however, when discussing petrogenesis, we refer to them as Kwaimbaita-like or Singgalo-like.
|
Petterson (1995) estimated the maximum stratigraphic thickness of the OJP basement section exposed in central Malaita to be 4 km. The precise determination of the thickness of the Singgalo Fm. is complicated by the presence of faults in lavas of this formation in the relevant portion of the Kwaimbaita River section and the upstream portion of the Singgalo River section (Fig. 2b). However, in the downstream portion of the Singgalo River traverse, an apparently continuous and undisturbed section exists, in which the top of the Singgalo Fm. is overlain conformably by the Kwara’ae Mudstone Fm. and the base is marked by a sharp change to Kwaimbaita Fm. isotopic and chemical compositions (Fig. 4, Table 1). The Singgalo Fm.’s thickness here is 750 m. The Singgalo Fm. appears to be much thicker (1290 m) in the upstream portion of the Singgalo River section, but the presence of several low-angle faults in this area (Fig. 2b) suggests a repetition of strata, which would explain why no evidence was found for a significant increase with depth in the grade of alteration (Babbs, 1997). The Singgalo Fm. section along the Kwaiafa’a River is undisturbed and 500 m thick; however, the base of the formation is not exposed. In contrast, the thickness of the Singgalo-type flows in the basement section at Site 807 (i.e. Unit A) is only 46 m (Mahoney et al., 1993; Fig. 4); furthermore, the Singgalo- and Kwaimbaita-type lava groups at this site are divided by a pelagic limestone unit of 0·5 m thickness not seen in Malaita. At Site 289, Singgalo-type lavas are missing entirely. Thus, the Singgalo magma type appears likely to be much less voluminous in the northern part of the plateau than in the south.
For the Kwaimbaita Fm. only a minimum thickness can be estimated, because its base is not exposed anywhere in the sections we sampled. From the structure of the Kwaio Anticline and the location of its fold axis (Petterson et al., 1997), we estimate that the exposed thickness of the Kwaimbaita Fm. is 2700 m in the eastern limb, where no major faults were found within this formation, and 2600 m in the western limb (the latter value includes the 310 m exposed in the lower portion of the Singgalo River; see Fig. 2b). At Site 807, only 102 m of Kwaimbaita-type basalts (i.e. Units C–G) were penetrated before drilling was terminated.
Chemical variations within each magma type
The Malaitan lavas display a broader range of chemical composition than the basalts at Site 807, probably largely reflecting the much greater thickness of basement sampled in central Malaita. Collectively, however, data for the few, but widespread, OJP localities sampled thus far indicate that the lavas of each magma type are compositionally very uniform overall, except for rare high-MgO examples such as ML475 and ML68, and rare evolved ones such as ML476 and the High-Ti Sigana Basalts of Santa Isabel (see Fig. 7d; Tejada et al., 1996). Singgalo-type basalts can conveniently be divided into three general chemical types (Fig. 7d): (1) a high-Ti group represented by the High-Ti Sigana Basalts of Santa Isabel; (2) lavas with TiO2 between 1·5 and 2·0 wt %, represented by Unit A at Site 807, most of the 122 Ma Sigana Basalts, and the majority of central, northern, and southern Malaitan Singgalo-type lavas; (3) basalts with TiO2 <1·5 wt %, overlapping with Site 803 and some Kwaimbaita Fm. compositions (upper dashed outline in Fig. 7d). Similarly, Kwaimbaita-type lavas can be grouped into high- and low-Ti types: (1) those with TiO2 between 1·0 and 1·5 wt %, representing the majority of central Malaitan Kwaimbaita Fm. lavas, the Site 807 Units C–G flows, and the Site 289 basalt; (2) those with TiO2 1 wt % in central Malaita (lower dashed outline in Fig. 7d). The 90 Ma Site 803 and Santa Isabel tholeiites, which are isotopically Kwaimbaita-like, are also chemically similar to the high-Ti Kwaimbaita Fm. lavas (see Tejada et al., 1996).
As with Ti, relatively small variations are evident in incompatible trace element abundances within each magma type, both among flows in different locations and at a given location (Fig. 8). For example, the low-Ti Singgalo Fm. lavas of the Kwara’ae Anticline, KF1 and KF32, have lower incompatible element abundances than most other lavas in the same area (Fig. 8b). Regionally, the average incompatible element contents of the Singgalo-type basalts in central Malaita are slightly higher than the averages for southern Malaita and Santa Isabel. In addition, the Kwaio Anticline lavas have similar average abundances of highly incompatible elements (Zr to Th in Fig. 8) to those of their Site 807 counterparts, but slightly higher average abundances of moderately incompatible elements (Sm to Lu). The Kwara’ae Anticline lavas have slightly greater average abundances of most incompatible trace elements than similar units at Site 807, southern Malaita, and Santa Isabel.
Among Kwaimbaita-type basalts, a few central Malaitan low-Ti lavas with the lowest incompatible element abundances (i.e. with patterns at the lower boundary of the shaded band in Fig. 8c) display larger troughs at P, Ti, and Y relative to others from the Kwaio Anticline. However, the incompatible element patterns of the Units C–G flows at Site 807, the Site 289 basalt, and most Santa Isabel flows are similar to those of most central Malaitan Kwaimbaita Fm. lavas. With the exception of evolved sample ML476, Kwaimbaita-type lavas exhibit either no peak or a variably positive peak at Sr, in contrast to the trough at Sr seen for all Malaitan Singgalo Fm. lavas.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONSTUDY AREA AND GENERAL...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Bimodal ages and emplacement rates
The combined age data now available for the plateau provide no evidence of a lateral age progression and thus an Icelandic-type origin by progressive lateral accretion. Instead, data for the widely separated drill-hole and island basement lavas reveal a strongly bimodal distribution of ages, at
122 Ma and 90 Ma (Fig. 3). For the older event, the total range of 40Ar–39Ar ages (excluding those for samples SGB10 and SGB25, which have large uncertainties; Table 3) is 119–125 Ma, yielding a maximum possible duration of 6 my for this event. Table 5 lists estimates of the average emplacement rates, or magmatic fluxes, corresponding to maximum and minimum OJP volumes estimated by Coffin & Eldholm (1994) and Neal et al. (1997), the smaller value assuming that the plateau was emplaced entirely in an off-ridge setting and the larger assuming, unrealistically, that all emplacement took place at a mid-ocean ridge. Assuming that the 90 ± 4 Ma event recorded in lavas at Site 803, Santa Isabel, and San Cristobal and in ash layers at Site 288 was volumetrically negligible and that essentially the entire plateau was emplaced in 6 my between 119 and 125 Ma gives minimum average emplacement rates of 4·5–10·2 km3/yr. These values are at the low end of previous estimates of 16–20 km3/yr (which assumed a duration of only 3 my; Coffin & Eldholm, 1994) and 8–22 km3/yr (assuming a duration of 3 my and an OJP volume of 24 x 106 to 65 x 106 km3; Tarduno et al., 1991).
|
However, besides the 90 ± 4 Ma event, the recognition of OJP-related lavas with ages of 111–115 Ma covering extensive areas in the Nauru and East Mariana basins (e.g. Castillo et al., 1994) may suggest a lower overall average emplacement rate (see also Ito & Clift, 1998; Ito & Taira, 2000). Assuming that the 90 ± 4 Ma event was volumetrically important, was characterized by similar eruption rates to those of the 122 Ma event, and that magmatism in the period between 94 and 119 Ma was minor, the maximum total duration of plateau emplacement in the two eruptive events would be 14 my and the minimum average emplacement rates would have been 1·9 km3/yr and 4·4 km3/yr for off- and on-ridge settings, respectively. These values are similar to maximum estimates for the Kerguelen Plateau (3·4–5·4 km3/yr; Coffin & Eldholm, 1994) but still much higher than estimates for other large igneous provinces. For average emplacement rates to fall as low as values estimated for the Deccan Traps (0·3–2·4 km3/yr; Coffin & Eldholm, 1994), North Atlantic Tertiary Province (0·6–2·4 km3/yr; Coffin & Eldholm, 1994), or Shatsky Rise’s South High (1·7 km3/yr; Sager & Han, 1993), for example, the observed bimodal age distribution on the OJP would have to be an artifact of the few basement locations currently sampled, such that additional pulses or relatively continuous volcanism occurred in the 86–126 Ma period, possibly for up to 39 my. In the latter case, the estimated minimum average emplacement rates would fall to 0·7 and 1·6 km3/yr. If the geochemically OJP-like volcanism in the East Mariana Basin (0·25 x 106 km3) and Nauru Basin (0·85 x 106 km3) is included, the estimated total crustal volume associated with the OJP for off-ridge and on-ridge emplacement increases to 28·1 x 106 (Neal et al., 1997) and 62·4 x 106 km3 (Coffin & Eldholm, 1994), respectively. In this case, the average emplacement rates would vary from 1·6 to 3·5 km3/yr if magmatism were episodic (total of 6, 4, and 8 my duration in the 122, 111–115, and 90 Ma events). The 62 and 34 Ma events documented in San Cristobal (Birkhold-VanDyke et al., 1996; Birkhold et al., in preparation) provide evidence of an unknown amount of compositionally OJP-like tholeiitic basalt magmatism younger than 90 Ma. We also note that an unknown volume of crust was added to the plateau at 44 Ma, as recorded in the alkalic Maramasike Fm. of Malaita (Tejada et al., 1996). If the extent of any of these events turns out to have been significant, then estimates of overall average emplacement rates would be reduced further.
Estimates of average emplacement rates will improve as more samples, 40Ar–39Ar dates, and crustal thickness data become available but, so far, all estimates are much higher than those calculated for modern hotspots such as Iceland and Hawaii (0·12–0·24 km3/yr and 0·03–0·16 km3/yr, respectively; R. S. White, 1993). Present indications are that the contribution of post-90 Ma magmatism to the plateau’s volume was relatively minor (Mahoney et al., 2002). If further sampling confirms that the bulk of the OJP was formed at 122 Ma or in two events at 122 and 90 Ma, it will be difficult to avoid the conclusion that the OJP represents melting of mantle on a scale not seen in either modern hotspots or other large igneous provinces.
Petrogenetic implications of chemical variations
Phase equilibria and possible fractionation paths from multiple parental melts
Major element compositions of the majority of OJP lavas coincide approximately with the low-pressure cotectic, liquid + olivine + plagioclase + clinopyroxene, consistent with the mineral phases observed and the ophitic to subophitic texture of clinopyroxene and plagioclase in the phyric lavas. The differentiation trajectories (arrows a–c) in Fig. 9a illustrate olivine-controlled fractionation trends. The spread of data between the 1 x 10-4 GPa (1 atm) and 0·8 GPa (8 kbar) cotectics (Fig. 9a and b) demonstrates the importance of crustal-level fractionation processes.
|
, invariant points at 1 x 10-4, 1, and 2 GPa from Longhi (1987)
OJP lavas also may be inferred to be derivative liquids that have undergone significant olivine fractionation at greater pressures; yet we do not see direct evidence for such fractionation because the effects of low-pressure fractionation effectively dominate the major element signature of the lavas (see Neal et al., 1997). Although some Kwaimbaita lavas may have differentiated from 1 GPa melts (arrow a in Fig. 9a), the majority of the data appear to fall at the ends of olivine-control trajectories originating from melts formed between 1 and 4 GPa (arrow b). Interestingly, trace element modeling also requires 3·5–4·0 GPa initial melting pressures to fit the OJP data (Tejada, 1998; see also Mahoney et al., 1993; Neal et al., 1997). In any case, the diagram indicates that, if they formed from peridotite source mantle, the erupted OJP lava compositions are significantly different from primary magma compositions (e.g. Herzberg & O’Hara, 1998).
Some data for Singgalo-type lavas fall at the end of the 4–6 GPa olivine-control trend (dashed arrow c is the 6 GPa olivine control), similar to most Hawaiian lavas (Herzberg & O’Hara, 1998). Although this type of diagram, derived from isobaric experimental data, provides only a rough idea of equilibration pressures of polybaric natural melts, the similarity with Hawaiian tholeiites is perhaps not too surprising considering that the Singgalo Fm. was erupted at the end of the 122 Ma episode, when the lithosphere of the OJP had probably been thickened significantly by the emplacement of the earlier Kwaimbaita-type magmas, cumulates, and melt-depleted residues. A deeper origin for Singgalo-type magmas is also consistent with trace element modeling suggesting higher initial pressures of melting for Singgalo-type magmas (Tejada, 1998).
Another important indication of polybaric fractionation is given by the Al:Ti ratios measured in clinopyroxene, present both as a phenocryst and groundmass phase, in lavas from central and northern Malaita. Values range between 4 and 10, implying pressures of formation from 1 x 10-4 GPa (1 atm) to a maximum of 1 GPa (Babbs, 1997). This finding is consistent with the position in Fig. 9 of the bulk of the whole-rock major element data, which fall between the 1 x 10-4 GPa and 0·8 GPa cotectics.
Source variation vs melting processes
Among the major elements (Fig. 7), TiO2, P2O5 (not shown) and, to a lesser extent, Fe2O3* show a clear distinction between Kwaimbaita-type and Singgalo-type lavas at similar mg-number. This difference could potentially reflect the composition of the mantle sources or be a result of differences in the amounts and/or pressures of partial melting. TiO2, for example, is sensitive to the amount of partial melting (e.g. Hirose & Kushiro, 1993), behaving as an incompatible element during mantle melting and attaining the highest concentrations in small-degree partial melts. The lower concentrations of TiO2 in the Kwaimbaita-type lavas at a given mg-number suggest larger fractions of partial melting than for the Singgalo-type basalts. Previous major and trace element modeling indicates that both groups of rocks were formed by large total amounts of partial melting (most estimates are in the 18–30% range, assuming peridotite sources), but suggest that the Kwaimbaita basalts represent several percent more melting (Tejada et al., 1996; Neal et al., 1997; Tejada, 1998).
Alternatively, can the difference in chemical compositions between the Kwaimbaita and Singgalo magma types be a function of source differences? The values for many incompatible element ratios among rocks from different locations on the OJP overlap, particularly when interlaboratory variation and variable analytical precision are considered. In diagrams involving ratios of elements with significantly different bulk partition coefficients and having higher analytical precision, such as Ce/Sm, Zr/Nb, Gd/Yb, and Zr/Y (Fig. 10b, c, e and f), a weak correlation with La abundance is observed. On the other hand, ratios of elements with similar incompatibilities, such as La/Nb and Nb/Th (Fig. 10a and d), show no clear covariation with La (or with isotopic ratios). This observation is consistent with both the generally similar shapes of primitive-mantle-normalized incompatible element patterns for Singgalo- and Kwaimbaita-type lavas (Fig. 8) and the generally slightly higher abundances of incompatible elements in the Singgalo-type lavas, and probably reflects slightly lower degrees of melting for the latter. Because ratios of incompatible trace elements with similar incompatibilities, such as Nb/Th, and La/Nb, are not changed significantly by moderate amounts of magmatic differentiation or by variations in melt fraction at large total amounts of partial melting, they must mainly reflect the extent of variation in mantle source regions. Figure 10a and d suggests that although the sources of OJP lavas had distinct isotopic compositions, they had very similar ratios of highly incompatible trace elements (see also Neal et al., 1997).
|
Bimodal isotopic compositions: a vertically zoned mantle source for the OJP?
The combined isotopic data now available indicate that a mantle source containing material of two distinct compositions, the Singgalo and Kwaimbaita types, dominated magma production, at least for the later stages of volcanism represented by basalts in the upper few kilometers of OJP crust. Importantly, there is no evidence of either magma or source mixing between the two types during the 122 Ma event. It should be noted that the bimodal distribution of data on the Pb–Pb isotope diagrams (Fig. 6) was less sharply defined in previous studies because most Pb isotope ratios were not age-corrected (the age-corrected Pb isotopic data in Table 4 are the first for which parent and daughter nuclides were both measured on the same sample splits by isotope dilution).
A two-component mantle source also has been inferred, for example, for the Hawaiian hotspot, currently the world’s most active, with end-members represented by shield lavas of Kilauea and Koolau volcanoes (e.g. Rhodes & Hart, 1995; Chen et al., 1996). Intriguingly, the Kwaimbaita and Kilauea isotopic ranges overlap significantly (Figs 5 and 6). Koolau values range to lower Nd and 206Pb/204Pb than yet found for the OJP but Singgalo-type isotopic values are close to, or overlap with, those for historical lavas of Mauna Loa volcano, except that the Mauna Loa lavas have lower 207Pb/204Pb and, particularly, 208Pb/204Pb.
The geochemical stratigraphy in central Malaita and at Site 807 indicates a temporal change in the composition of the OJP mantle source. At Mauna Loa volcano, a somewhat similar shift from earlier (i.e. in the prehistoric lavas) Kilauea-like to more ‘typical’, lower-206Pb/204Pb and lower-Nd Mauna Loa compositions in the historical lavas (Kurz et al., 1995) coincided with declining magma production (Moore et al., 1990). The analogy for the OJP is limited, however, because the different Hawaiian shield volcanoes and/or lava sequences within a single shield appear to represent different amounts of mixing between the two end-member components, whereas no mixing between Kwaimbaita- and Singgalo-type sources or magmas is observed in the 122 Ma lavas. Rather, the shift from Kwaimbaita-type to Singgalo-type compositions is abrupt in the two locations where it has been found (i.e. Site 807 and Malaita). The lack of intermixtures between the two OJP isotopic groups, their consistent stratigraphic position in lavas emplaced at essentially the same time in two locations 1600 km apart, and the probably small amount of drift of the plateau during the 125–100 Ma period (Neal et al., 1997) suggest that the two mantle reservoirs in the OJP source were much larger in size than the scale of melting and, in the melting region, may have been vertically separated from one another.
Modeling the OJP’s mantle source
In recent years, most attempts to explain large igneous provinces have invoked mantle plumes. An exception is the plate reorganization model (e.g. Anderson et al., 1992; Smith & Lewis, 1999), in which flood basalt volcanism is initiated by plate reorganization above anomalously hot, shallow, non-plume mantle. Initially, melting is assumed to tap a hypothesized widespread, shallow mantle layer with ocean-island-like isotopic and trace element characteristics [termed the ‘perisphere’ by Anderson (1996)]; this layer is later replaced by upwelling mid-ocean ridge basalt (MORB)-type mantle that lies beneath it. The model thus predicts that MORB-like isotopic compositions should dominate the signature of late-stage lavas on oceanic plateaux. Although the OJP’s 122 Ma event, in particular, occurred relatively close in time to a major reorganization of the Pacific plate (Neal et al., 1997), the complete absence, thus far, of isotopically MORB-like lavas in the 122 Ma (and 90 Ma) OJP crustal section does not support the plate reorganization model. Rather, the Singgalo Fm. lavas are even less MORB-like than the earlier Kwaimbaita Fm. basalts.
Another class of model posits that oceanic plateaux are formed gradually over tens of millions of years by more or less steady-state, ridge-centered or near-ridge plumes, rather like the comparatively small Icelandic Plateau or Galápagos Platform and its associated aseismic ridges (e.g. Mahoney & Spencer, 1991; Ito & Clift, 1998). Here, involvement of MORB-type mantle also might be expected, particularly in locations distant from the plume center. Perhaps more importantly, this model finds no support in the currently available age data for OJP basement lavas, which indicate that eruption occurred in short-lived pulses 30 my apart.
By far the most frequently applied plume-based model for large igneous provinces in the last decade has been the starting-plume or plume-head model (e.g. Richards et al., 1989; Griffiths & Campbell, 1990). Some laboratory and numerical experiments suggest that, for all but the largest plume heads, significant amounts of ambient, mostly lower mantle may be entrained during the ascent of a plume head from a thermal boundary layer within the mantle, possibly from the core–mantle boundary, and therefore plume heads are predicted to be compositionally zoned or layered (e.g. Griffiths & Campbell, 1990). Thus, in open-ocean areas such as that in which the OJP formed (e.g. Neal et al., 1997), where complications arising from a plume’s interaction with continental lithosphere are absent, the isotopic compositions of the volcanic products are predicted to reflect the plume-source and some amount of entrained lower mantle. The different ocean-island-like isotopic compositions of the OJP lavas are not inconsistent with these predictions, as the Singgalo- and Kwaimbaita-type isotopic groups would represent internally homogeneous (relative to the scale of melting) mantle regions composed of slightly different proportions of plume-source mantle and entrained, presumably lower, mantle.
Although interpretation is very model-dependent, the Kwaimbaita-type basalts in the framework of this class of models would more probably reflect the greater amount of plume-source-derived mantle. First, both in central Malaita and at Site 807, the thickness of the earlier, Kwaimbaita-type lavas is much greater than that of the later, Singgalo-type lavas, which appears consistent with laboratory and numerical models that suggest large plume heads will contain much more plume-source material than entrained mantle (e.g. Griffiths & Campbell, 1990). Second, the chemical compositions of Kwaimbaita-type basalts indicate derivation by somewhat larger amounts of partial melting of peridotite, consistent with hotter, plume-source-dominated mantle (see above). Third, the 90 Ma eruptive event, which may have been centered on the plateau’s eastern lobe or salient (Tejada et al., 1996; Neal et al., 1997), produced lavas with Kwaimbaita-like compositions at Site 803, Santa Isabel, and San Cristobal, whereas 90 Ma Singgalo-type lavas have been found only in San Cristobal. Fourth, the 111–115 Ma Nauru, East Mariana, and Central Pacific Basin basalts, which appear to be closely related to emplacement of the OJP, have isotopic ratios that overlap with those of the Kwaimbaita Fm. (e.g. Figs 5 and 6). Thus, the Kwaimbaita-type signature appears to be expressed much more commonly than the Singgalo type, suggesting that the former was volumetrically more abundant in the OJP mantle source.
However, several critical aspects of the OJP do not obviously conform to the starting-plume model (see also Mahoney et al., 1993; Tejada et al., 1996; Neal et al., 1997; Sheth, 1999). The model predicts rapid emplacement in a single event; for the OJP this would require that the 90 Ma event was volumetrically negligible and caused by magmatism related to the plume tail that followed the plume head. At present, the magnitude of the 90 Ma episode remains unknown, but it was clearly widespread, showing up in locations as far apart as Site 803, Site 288, San Cristobal, and Santa Isabel. Also, plume-tail-related volcanism following the plume-head phase is predicted to form a post-plateau chain of seamounts adjacent to a plateau as plate motion moves the plateau away from the hotspot, but the seafloor adjacent to the OJP lacks a post-90-Ma chain; if one ever existed, it must have been obliterated by subduction along the old Solomon trench. Recent numerical modeling suggests that a different type of large plume head may form at mid-mantle depths beneath the 660 km discontinuity, rather than above it or the core–mantle boundary (Cserepes & Yuen, 2000). Such plume heads may not be accompanied by very long-lived tails (D. Yuen, personal communication, 1999); if so, this model may account for the lack of a post-OJP seamount chain. However, this and other plume-head models also predict that substantial uplift accompanies plateau emplacement, with large areas at shallow depths or even emergent; yet paleodepths appear never to have been shallow at any of the locations sampled thus far in Malaita, Santa Isabel, or at any of the drill sites (e.g. Neal et al., 1997; Ito & Clift, 1998; Michael, 1999; Ito & Taira, 2000; Mahoney et al., 2002). Furthermore, if the 90 Ma event was relatively minor, a truly enormous plume head is required to fuel melting of peridotite at a rate and volume capable of building the huge OJP in only a few million years or less at 122 Ma (e.g. Coffin & Eldholm, 1994).
More recently, several studies have modified the starting-plume model by proposing that plume heads, although consisting principally of peridotite, may contain significant amounts of embedded eclogitic material derived from subducted oceanic crust (e.g. Cordery et al., 1997; Yasuda et al., 1997). The idea behind this ‘composite diapir’ model is that the eclogite melts extensively before the peridotite begins to melt, but cannot escape the source until peridotite melting allows large-scale melt connectivity, at which point large volumes of melt may be released cataclysmically. Yasuda et al. (1997) showed that the major element compositions of hybrid magmas derived by melting of such mixed sources could be comparable with those of continental flood basalts, implying that some flood basalts could be near-primary magmas.
Intra-oceanic plateaux such as the OJP should be the best places to test the composite diapir model using isotopes, trace elements, and major elements, because direct continental lithospheric influence on magma compositions is lacking. Because subducted, recycled oceanic upper crust is expected to develop high 206Pb/204Pb ratios on time scales of 108 yr (e.g. Kogiso et al., 1997), the plume peridotite assumed to surround pockets of recycled crust must have Pb isotopic values as low as or lower than observed in the OJP basalts. Figure 11a–d illustrates that it is indeed possible to approximate the OJP’s isotopic characteristics by mixing 0·5–20% partial melts of peridotite, assumed here for simplicity to have model primitive-mantle isotopic and elemental compositions, with a 100% melt of recycled upper oceanic crust (eclogite), the isotopic characteristics of which have been modified by seafloor aging and subduction-related dehydration processes (see Table 6 for the model compositions used in constructing Fig. 11). The calculations show that an 80–98% proportion of eclogite melt is required to broadly reproduce the 206Pb/204Pb, Nd, and Sr isotopic values of OJP lavas if the plume-source peridotite melts only to a very small extent (0·5%), consistent with Cordery et al.’s (1997) ‘best’ melt volume scenario. However, the amount of peridotite partial melting that accompanies complete melting of the eclogite generally might be expected to be somewhat higher, considering that the eclogite itself ultimately represents MORB-type crust formed by at least several percent of partial melting of (non-plume) peridotite. If the plume-source peridotite melts to a greater extent (e.g. 20% in the example illustrated in Fig. 11), a smaller proportion (only 40–60% in the example) of recycled oceanic upper crust can produce the OJP isotopic compositions. In all cases, with the primitive-mantle and eclogite Pb isotope values used for the figure, the 207Pb/204Pb ratios of the postulated hybrid magmas are much lower than observed for the OJP lavas for any given 206Pb/204Pb value. Of course, the plume peridotite need not have primitive-mantle isotopic (or elemental) values; a more EM-1-like end-member, for example, provides higher relative 207Pb/204Pb and still yields reasonable fits in Fig. 11a–d. Even if the plume peridodite is primitive mantle, this may not present much of a problem because of the large uncertainties in, and the large range of, estimates of primitive-mantle Pb isotopic ratios (dashed field in Fig. 11e; e.g. Galer & Goldstein, 1996).
|
|
Model incompatible element patterns generated by mixing batch melts of primitive mantle and recycled oceanic crust are shown in Fig. 12. Patterns for unaltered and altered MORB modified by dehydration in a subduction zone were estimated using the trace element mobilities derived by Kogiso et al. (1997). The calculated mixtures do not reproduce the patterns of OJP lavas too successfully. Models assuming a purely peridotitic source give a better fit to the data (see Tejada et al., 1996; Neal et al., 1997; Tejada, 1998). In particular, the patterns of OJP lavas lack the U and Rb enrichment expected for altered, recycled MORB and have different overall shapes than the model patterns. Slightly better fits are achieved if the recycled crust is assumed to be unaltered when subducted and dehydrated. Although none are very good, the best fits are obtained with 100% melting of recycled oceanic crust and high proportions of crustal melt in the hybrid magma (40% or 90–100%, respectively, assuming the plume peridotite partially melts to 20% or 0·5%), as in the isotopic modeling. These proportions are similar to those hypothesized for continental flood basalts by Yasuda et al. (1997) from major element considerations.
|
To estimate the major element compositions of hybrid primary OJP magmas, we used Yasuda et al.’s (1997) data on melted MORB (column 1 in Table 7) and results from melting experiments on fertile peridotite KLB-1 (columns 2, 5, and 8 in Table 7; data of Hirose & Kushiro, 1993). Table 7 lists the compositions of hybrid magmas comprising large-degree partial melts (20%) of fertile peridotite and total melts of recycled upper oceanic crust [taken to be a 100% melt of crust plus 1% peridotite melt, following Yasuda et al. (1997); their other hypothetical mixes have higher SiO2 and Al2O3 contents than most OJP lavas]. Proportions of crustal melt in the mixtures shown are 0%, 40%, and 60%, and results for melts in the depth range 2·5, 2·0, and 1·5 GPa are presented. The mixtures show a range of SiO2, Al2O3, TiO2, and CaO contents, which overlap with those of OJP lavas, except that hybrid primary magmas containing large proportions of peridotite melt have lower Fe2O3* and much higher MgO contents (11–13 wt %) than seen for any OJP basalt. The difference in MgO contents can be removed by fractionating the model primary magmas to levels seen in OJP basalts. Fractionation modeling using the ‘MELTS’ program (Ghiorso & Sack, 1995) is generally consistent with up to 10% fractionation of olivine and spinel, followed by 10–40% removal of clinopyroxene and plagioclase, at pressures varying between 0·05 and 0·2 GPa (Fig. 13). [An exception is the path calculated for the pure crustal melt of Yasuda et al. (1997), which entails up to 15% olivine + feldspar fractionation followed by up to 15% fractionation of the olivine + clinopyroxene + feldspar.] The model fractionation paths, although somewhat lower in Fe2O3* and CaO and higher in TiO2 than the majority of OJP lavas, intersect the OJP range. If such fractionation is considered in the trace element modeling in Fig. 12, the principal effect is to increase the abundances of the incompatible elements. For example, 30% fractionation of an assemblage consisting of 15% clinopyroxene, 75% plagioclase, and 10% olivine leads to a factor of 1·4 and 1·3 increase, respectively, in the abundance of elements with bulk distribution coefficients of 0·04 (e.g. Th, Dy) and 0·20 (e.g. La). This increase slightly improves the concordance of the best-fit model patterns with those of the OJP averages, particularly from Th to Dy. In view of the uncertainties in source composition, fractionation history, modification of some elemental abundances by post-eruptive alteration, and the ability of the MELTS program to reproduce real-world processes, it appears that melting in a composite diapir may give rise to broadly OJP-type magmas under some conditions.
|
|
In summary, we cannot rule out the composite diapir model for the OJP on the basis of isotopic or chemical data alone, given the uncertainties in end-members and in effects of alteration and subduction on oceanic upper crust. A major advantage of this model, relative to the ‘simple’ plume-head model, is that it does not require an anomalously hot and huge plume head to produce the rate and volume of melting required for continental and oceanic flood basalts. However, in the composite diapir model temperature variations and variations in the distribution of eclogite within the plume head strongly affect the rate and volume of melting, and thus the geochemistry of magmas (Cordery et al., 1997). Yet one of the most outstanding characteristics of the OJP basalts, thus far, is their small overall range of geochemical variation. Furthermore, the stratigraphic succession of Singgalo-type lavas above Kwaimbaita-type lavas is the same in both locations (1600 km apart) where both lava types have been sampled in a single section. The calculations of Cordery et al. (1997) suggest that only the material in the uppermost portion of the plume head will melt significantly. Following Griffiths & Campbell (1990), they assumed the plume head to have a hot cylindrical core (axis) with a thin cap of hot plume-source-dominated material along its upper boundary; the remainder is composed of a cooler mixture of plume-source and entrained lower-mantle material. If so, the Kwaimbaita–Singgalo succession would appear to require an abundance of eclogite within the upper boundary layer, which would melt first to produce the Kwaimbaita-type lavas. Subsequent melting of plume mantle containing a lesser amount of eclogite would produce the Singgalo-type lavas.
One important feature that is difficult to explain with a non-uniform distribution of eclogite within the plume head is the observed abruptness of the change from one isotopically distinct, relatively uniform magma type to another. Also, most of the 90 Ma OJP basalts studied thus far are isotopically equivalent to the 122 Ma Kwaimbaita, not Singgalo, lavas, which would require that the plume tail, 30 my after the initial plateau-forming event, must again contain a high proportion of eclogite, a proportion very similar to that in the upper part of the starting-plume head at 122 Ma. Yasuda et al. (1997) argued that the eclogite is entrained as the rising plume head crosses an eclogite-rich zone near 660 km depth (where eclogite should be neutrally buoyant), which might explain how the upper part of a plume head could contain more recycled oceanic crustal material than the interior. However, the mantle near 660 km crossed by the plume would be expected to contain much less eclogite at 90 Ma than at 122 Ma, because much of it would have already been dragged upward by the plume head shortly before 122 Ma. Finally, if the proportion of eclogite involved in producing OJP magmas was high, as the model suggests, then the plateau’s huge size requires a very large amount of recycled oceanic crust to have been concentrated in a relatively small mantle region, so as to become incorporated in a rising plume head. If we assume, for example, that 50 x 106 km3 of eclogite melted, this value translates to an eclogite layer of 100 km thickness and 399 km in radius; assuming further that such a layer was derived ultimately from the basaltic upper 2·5 km of the oceanic crust, an immense original area of subducted seafloor (4472 km x 4472 km) is required. In short, it appears that, as with the ‘simple’ plume-head model, a number of contortions are required for the composite-diapir model to work for the OJP.
OJP’s crustal growth and mantle source structure: a synthesis
Crustal growth and structure
Schematic diagrams of possible OJP crustal and mantle source structure during the 122 Ma event, based on the results of this study and previous geochemical modeling (Mahoney et al., 1993; Tejada et al., 1996; Neal et al., 1997; Tejada, 1998), are shown in Fig. 14. An initial emplacement somewhat off-axis (Mahoney & Spencer, 1991; Coffin & Gahagan, 1995; Gladczenko et al., 1997), and a peridotitic (Fig. 14a) or composite (Fig. 14b) plume head are assumed, despite the first-order discrepancies noted in ‘Modeling the OJP’s mantle source’ between observed features of the OJP and predictions of all current plume-head models. Because of these limitations, the figure should be taken as only a rough working model for future studies. It emphasizes the importance of intrusion, underplating, and cumulate formation (e.g. Neal et al., 1997; Ito & Clift, 1998; Ito & Taira, 2000). Modeling assuming purely peridotite melting suggests that melt generation was polybaric and probably at high potential temperature and initial pressures of at least 4 GPa (Tejada, 1998; and ‘Petrogenic implications of chemical variations’); under these conditions, picritic, high-MgO primary melts were probably formed (e.g. Herzberg & Zhang, 1996; Herzberg & O’Hara, 1998). Exposed sections of the much smaller Caribbean Plateau indeed document the existence of picritic lavas at deep crustal levels (e.g. Kerr et al., 1997). The fact that most OJP lavas are relatively Fe-rich basalts, differentiated to roughly similar extents, suggests that primary high-MgO magmas were ponded below the crust or at the Moho (e.g. Cox, 1980; Neal et al., 1997; see Fig. 9). Fractionation of olivine and spinel would decrease the density of the melts and enable them to ascend and fractionate further in crustal magma chambers; some magmas undoubtedly froze en route in intrusions, whereas some reached the surface, leaving behind more gabbroic cumulates (e.g. Cox, 1980, 1993). The ‘hidden cumulates’ in the deeper levels of OJP crust probably represent a significant portion of the high-seismic-velocity lower-crustal layer of the plateau (Farnetani et al., 1996; Neal et al., 1997). In a composite diapir scenario, gabbroic cumulates would be less abundant because the hybrid primary magmas would be less Mg rich. The megacrysts and xenoliths in the ‘orbicular’ basalts in central Malaita described in the section on field characteristics exemplify relatively shallow-level gabbroic cumulates. Also, the high Cl/K ratios of basaltic glasses at Site 807 are suggestive of relatively shallow magma residence because they indicate some shallow-level assimilation of hydrothermally altered crust (Michael, 1999).
|
Because the portion of the Pacific plate containing the OJP appears to have been near the 125–100 Ma Pacific stage poles and thus drifted little relative to the putative plume (e.g. Neal et al., 1997), Fig. 14 shows vertical to slightly offlapping stacking of 122 Ma volcanic products. By the time of the Singgalo Fm. eruptions, the earlier volcanic pile and associated cumulates and intrusions had thickened the OJP’s crust enormously. This change is reflected in the chemical compositions of Singgalo-type lavas, which appear to represent lower mean fractions of peridotite melting and higher pressures of melt segregation than the Kwaimbaita basalts (Tejada et al., 1996; Neal et al., 1997; Tejada, 1998). In general, lesser total amounts of melting would mean decreased magma supply to magma chambers, possibly leading to slightly higher amounts of fractionation, on average, as inferred for the Singgalo Fm. (relative to the Kwaimbaita). Overall, although the crust of the OJP is much thicker (30–43 km; e.g. Miura et al., 1996; Gladczenko et al., 1997; Richardson et al., 2000) than that of the Caribbean Plateau (<20 km; Mauffret & Leroy, 1999), the gross crustal structure of the Caribbean and the Ontong Java Plateaux may be rather similar (e.g. Hussong et al., 1979; Kerr et al., 1997; Phinney et al., 1999).
Compositional structure of mantle source region
The time difference between emplacement of the Kwaimbaita and the Singgalo basalts is beyond the resolution of current 40Ar–39Ar dating capabilities, given the low K2O contents of most OJP basement rocks [but may just be distinguishable at Site 807 when biostratigraphic data are taken into account; see Larson & Erba (1999)]. Stratigraphically, the change in isotopic compositions is abrupt, and implies a source with two distinct compositional regions and probably a component of vertical stratification.
In Griffiths & Campbell’s (1990) model, hot plume-source-derived material (light gray shading in Fig. 14a) is carried to the stagnation point at the cap of the plume and spreads laterally into thin laminae. In a broad area around the center of the head, plume-source-dominated peridotite lies above a region enriched in entrained peridotite (white in Fig. 14a). In this context, the Kwaimbaita and Singgalo basalts can be interpreted as corresponding, respectively, to the upper, plume-source-dominated zone and the underlying zone containing more entrained material. Griffiths & Campbell (1990) also argued that for a very large plume head, of which the OJP case is one of the two most viable examples, little entrainment of non-plume mantle would occur because the plume head would ascend too rapidly. For this reason, the amount of plume-source material depicted in Fig. 14a is much greater than that of entrained mantle. Furthermore, Van Keken’s (1998) numerical simulation indicated that much of the plume-source material would be contained in the plume head and that the trailing conduit at this stage would be composed largely of entrained non-plume mantle. Thus, Fig. 14a shows a reduced amount of plume-source material at greater depth and tapping of mostly entrained mantle in the later stages of 122 Ma volcanism. In Yasuda et al.’s (1997) composite diapir model, the upper part of the plume head contains most of the eclogite pods, and Fig. 14b shows most of them melting completely and mixing with partial melts of the peridotite matrix during the eruption of the Kwaimbaita basalts (Stage 1). At the time of the Singgalo eruptions (Stage 2) fewer entrained eclogitic fragments remain, resulting in a decreased contribution of melt derived from subducted oceanic crust to the total melt.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONSTUDY AREA AND GENERAL...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
- Geochemical and geochronological study of 0·5 and 3·5 km sections of pillowed and massive basaltic flows composing crustal basement in central Malaita reveals a sequence of 123·1 ± 1·4 Ma OJP crust resembling a greatly expanded section of that at ODP Site 807. The results further confirm the importance of a major 122 Ma eruptive episode and a nearly plateau-wide distribution of two isotopically distinct types of basalt in the world’s largest oceanic plateau.
- As at Site 807, the two types of basalt form stratigraphically separate groups. With the Kwaio Anticline as the type locality, we term the stratigraphically lower group (maximum thickness >2·7 km) the Kwaimbaita Fm. and the overlying group (maximum exposed thickness of 750 m in central Malaita) the Singgalo Fm. On the basis of closely similar isotopic and elemental characteristics, this nomenclature is extended to other locations where 122 Ma OJP basement has been sampled, including northeastern Santa Isabel, Ramos Island, southern Malaita, ODP Site 807, and DSDP Site 289. The Singgalo magma type appears to be much more voluminous in the south than in the northern plateau (e.g. only 46 m of flows at Site 807 and none at Site 289).
- Isotopic variation within each group is very limited and indicates an isotopically ocean-island-type mantle source with two distinct compositional regions, each of which was fairly homogeneous relative to the scale of melting. No evidence of normal-MORB-type mantle is present. The sharp stratigraphic boundary between the Kwaimbaita and Singgalo formations in Malaita and, 1600 km away, between the two corresponding groups at Site 807, coupled with the apparent lack of translational motion of this part of the Pacific plate between 125 and 100 Ma, suggests a vertically stratified structure for the OJP source mantle.
- The ocean-island-like isotopic signatures of OJP lavas can be accommodated with either a peridotite plume head (whether originating above a thermal boundary layer or at mid-mantle depths) or a composite diapir model, in which a dominantly peridotitic plume head is assumed to contain pockets of eclogite. Both models may also accommodate the major and trace element data, although the compositions of model melts of a mixed eclogite–peridotite source provide only rough fits to the OJP data. Better fits are obtained assuming a two-component peridotitic source (Mahoney et al., 1993; Tejada et al., 1996; Neal et al., 1997; Tejada, 1998). The composite diapir model also requires an enormous amount of eclogite to have been concentrated in a relatively small part of the mantle if OJP magmas are assumed to comprise a significant proportion of eclogite melt. Furthermore, the abrupt transition from Kwaimbaita- to Singgalo-type basalts, and the reappearance some 30 my later of Kwaimbaita-like lavas are not readily explained by the composite diapir model.
- In several important respects, the OJP fails to fit the predictions of, and thus poses a challenge to, any plume-head model: the absence of evidence for significant uplift despite construction of a crust of 35 km thickness during what should have been the impact of the largest plume head in the last 200 my, the lack of an attached seamount chain (plume-tail trace), and the presence of at least two important episodes of eruptive activity separated in time by 30 my.
|
ACKNOWLEDGEMENTS |
|---|
We thank T. Babbs and A. Saunders for allowing us to use their unpublished data, R. Magu for providing his field and petrographic notes, L. Kroenke, M. Garcia, J. Sinton, A. Pietruszka, and G. Fitton for helpful discussions, K. Spencer, K. Rubin, T. Hulsebosch, and J. C. Jain for help with the analytical work, and N. Hulbirt for improving several figures. Ian Parkinson and an anonymous reviewer provided valuable critical reviews. The Solomon Islands Ministry of Energy, Water, and Mineral Resources (Geology Mapping) provided invaluable logistical support, enthusiastic guidance, and diplomacy in the bush. This study was supported by NSF-EAR grants to J.J.M., C.R.N., and R.A.D., and a Faculty Development Grant from the University of the Philippines to M.L.G.T.
|
FOOTNOTES |
|---|
*Corresponding author. Telephone: 632-435-4837. Fax: 632-929-6047. E-mail: mtejada{at}nigs.upd.edu.ph
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONSTUDY AREA AND GENERAL...ANALYTICAL METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Anderson, D. L. (1996). Enriched asthenosphere and depleted plumes. International Geology Reviews 38, 1–21.
Anderson, D. L., Zhang, Y.-S. & Tanimoto, T. (1992). Plume heads, continental lithosphere, flood basalts, and tomography. In: Storey, B., Alabaster, T. & Pankhurst, R. J. (eds) Magmatism and Causes of Continental Break-up. Geological Society, London, Special Publications 68, 99–124.
Arth, J. G. (1976). Behavior of trace elements during magmatic processes—a summary of theoretical models and their applications. Journal of Research and Geological Survey 4, 41–47.
Babbs, T. L. (1997). Geochemical and petrological investigations of the deeper portions of the Ontong Java Plateau: Malaita, Solomon Islands. Ph.D. dissertation, Leicester University, 254 pp.
Barron, A. J. M. (1993). The Geology of Northernmost Malaita: a Description of Sheets 8/160/7 and 8/150/8. Geological Division Report. Honiara: Solomon Islands Ministry of Natural Resources, 42 pp.
Bedard, J. H. (1994). A procedure for calculating the equilibrium distribution of trace elements among the minerals of cumulate rocks, and the concentration of trace elements among the minerals of cumulate rocks, and the concentration of trace elements in the coexisting liquids. Chemical Geology 118, 143–153.
Berger, W. H., Kroenke, L. W., Mayer, L. A., et al. (eds) (1993). Proceedings of the Ocean Drilling Program, Scientific Results, 130. College Station, TX: Ocean Drilling Program.
Bienvenu, P., Bougault, H., Joron, J. L., Treuil, M. & Dmitriev, L. (1990). MORB alteration: rare-earth element/non-rare-earth hygromagmaphile element fractionation. Chemical Geology 82, 1–14.
Birkhold-VanDyke, A. L., Neal, C. R., Jain, J. C., Mahoney, J. J. & Duncan, R. A. (1996). Multi-stage growth for the Ontong Java Plateau (OJP)? A progress report from San Cristobal (Makira), Solomon Islands. EOS Transactions, American Geophysical Union 77, F714.
Castillo, P., Pringle, M. S. & Carlson, R. W. (1994). East Mariana Basin tholeiites: Jurassic ocean crust or Cretaceous rift basalts related to the Ontong Java plume? Earth and Planetary Science Letters 123, 139–154.
Chauvel, C., Hoffman, A. W. & Vidal, P. (1992). HIMU-EM: the French Polynesian connection. Earth and Planetary Science Letters 110, 99–119.[Web of Science]
Chen, C.-Y., Frey, F. A., Rhodes, J. M. & Easton, R. M. (1996). Temporal geochemical evolution of Kilauea volcano: comparison of Hilina and Puna basalt. In: Basu, A. R. & Hart, S. (eds) Earth Processes: Reading the Isotopic Code. Geophysical Monograph, American Geophysical Union 95, 161–181.
Coffin, M. F. & Eldholm, O. (1994). Large igneous provinces: crustal structure, dimensions, and external consequences. Reviews of Geophysics 32, 1–36.[Web of Science]
Coffin, M. F. & Gahagan, L. M. (1995). Ontong Java and Kerguelen Plateaux: Cretaceous Icelands? Journal of the Geological Society, London 152, 1047–1052.
Coleman, P. J. & Kroenke, L. W. (1981). Subduction without volcanism in the Solomon Islands Arc. Geo-Marine Letters 1, 129–134.
Coleman, P. J., McGowran, B. & Ramsay, W. R. H. (1978). New, Early Tertiary ages for basal pelagites, Northeastern Santa Isabel, Solomon Islands (central southwest flank, Ontong Java Plateau). Bulletin of the Australian Society of Exploration Geophysicists 9(3), 110–114.
Cordery, M. J., Davies, G. F. & Campbell, I. H. (1997). Genesis of flood basalts from eclogite-bearing mantle plumes. Journal of Geophysical Research 102, 20179–20197.
Cox, K. G. (1980). A model for flood basalt vulcanism. Journal of Petrology 21, 629–650.
Cox, K. G. (1993). Continental magmatic underplating. Philosophical Transactions of the Royal Society of London, Series A 342, 155–166.
Cserepes, L. & Yuen, D. A. (2000). On the possibility of a second kind of mantle plume. Earth and Planetary Science Letters 183, 61–71.
Davis, G. L. (1978). Zircons from the mantle. US Geological Survey Open File Report 78-701, 86–88.
Duncan, R. A., Hooper, P. R., Rehacek, J., Marsh, J. S. & Duncan, A. R. (1997). The timing and duration of the Karoo igneous event, southern Gondwana. Journal of Geophysical Research 102, 18127–18138.
Farnetani, C. G., Richards, M. A. & Ghiorso, M. S. (1996). Petrological models of magma evolution and deep crustal structure beneath hotspots and flood basalt provinces. Earth and Planetary Science Letters 143, 81–96.
Galer, S. J. G. & Goldstein, S. L. (1996). Influence of accretion on lead in the earth. In: Basu, A. R. & Hart, S. (eds) Earth Processes: Reading the Isotopic Code. Geophysical Monograph, American Geophysical Union 95, 75–98.
Garcia, M. O., Foss, D. J., West, H. B. & Mahoney, J. J. (1995). Geochemical and isotopic evolution of Loihi volcano, Hawaii. Journal of Petrology 36(6), 1647–1674.
Garcia, M. O., Rhodes, J. M., Trusdell, F. A. & Pietruszka, A. J. (1996). Petrology of lavas from Puu Oo eruption of Kilauea volcano: III. The Kupaianaha episode (1986–1992). Bulletin of Volcanology 58, 359–379.[Web of Science]
Ghiorso, M. S. & Sack, R. O. (1995). Chemical mass transfer in magmatic processes, IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid–solid equilibria in magmatic systems at elevated temperatures and pressure. Contributions to Mineralogy and Petrology 119, 197–212.[Web of Science]
Gladczenko, T. P., Coffin, M. F. & Eldholm, O. (1997). Crustal structure of the Ontong Java Plateau: modeling of new gravity and existing seismic data. Journal of Geophysical Research 102, 22711–22729.
Govindaraju, K. (1994). 1994 Compilation of working values and sample descriptions for 383 geostandards. Geostandards Newsletter, Special Issue 18, 15–16.
Green, T. H. (1994). Experimental studies of trace-element partitioning applicable to igneous petrogenesis—Sedona 16 years later. Chemical Geology 117, 1–36.[Web of Science]
Green, T. H., Sie, S. H., Ryan, C. G. & Cousens, D. R. (1989). Proton microprobe-determined partitioning of Nb, Ta, Zr, Sr, and Y between garnet, clinopyroxene and basaltic magma at high pressure and temperature. Chemical Geology 74, 210–216.
Griffiths, R. W. & Campbell, I. H. (1990). Stirring and structure in mantle starting plumes. Earth and Planetary Science Letters 99, 66–78.
Grove, T. L., Kinzler, R. J. & Bryan, W. B. (1992). Fractionation of mid-ocean ridge basalt (MORB). In: Phipps Morgan, J., Blackman, D. K. & Sinton, J. (eds) Mantle Flow and Melt Generation at Mid-Ocean Ridges. Geophysical Monograph, American Geophysical Union 71, 281–310.
Gurenko, A. A. & Chaussidon, M. (1995). Enriched and depleted primitive melts included in olivine from Icelandic tholeiites: origin by continuous melting of a single mantle column. Geochimica et Cosmochimica Acta 58, 2905–2917.
Hart, S. R. & Dunn, T. (1993). Experimental cpx/melt partitioning of 24 trace elements. Contributions to Mineralogy and Petrology 113, 1–8.[Web of Science]
Hauri, E. H., Wagner, T. & Grove, T. L. (1994). Experimental partitioning of Th, U, Pb and other trace elements between garnet, clinopyroxene and basaltic melts. Chemical Geology 117, 149–166.[Web of Science]
Hawkins, M. P. & Barron, J. M. (1991). The Geology and Mineral Resources of Santa Isabel. Geological Division Report J25. Honiara: Solomon Islands Ministry of Natural Resources, 114 pp.
Herzberg, C. & O’Hara, M. J. (1998). Phase equilibrium constraints on the origin of basalts, picrites, and komatiites. Earth-Science Reviews 44, 39–79.
Herzberg, C. & Zhang, J. (1996). Melting experiments on anhydrous peridotite KLB-1: compositions of magmas in the upper mantle and transition zone. Journal of Geophysical Research 101, 8271–8295.
Hirose, H. & Kushiro, I. (1993). Partial melting of dry peridotites at high pressures: determination of compositions of melts segregated from peridotite using aggregates of diamond. Earth and Planetary Science Letters 114, 477–489.[Web of Science]
Hughes, G. W. & Turner, C. C. (1976). Geology of Southern Malaita. Geological Survey Division, Bulletin 2. Honiara: Solomon Islands Ministry of Natural Resources, 80 pp.
Hughes, G. W. & Turner, C. C. (1977). Upraised Pacific Ocean floor, southern Malaita, Solomon Islands. Geological Society of America Bulletin 88, 412–424.
Hussong, D. M., Wipperman, L. K. & Kroenke, L. W. (1979). The crustal structure of the Ontong Java and Manihiki oceanic plateaus. Journal of Geophysical Research 84, 6003–6010.
Ito, G. T. & Clift, P. D. (1998). Subsidence and growth of Pacific Cretaceous plateaus. Earth and Planetary Science Letters 161, 85–100.
Ito, G. T. & Taira, A. (2000). Compensation of the Ontong Java Plateau by surface and subsurface loading. Journal of Geophysical Research 105, 11171–11183.
Jacobsen, S. B. & Wasserburg, G. J. (1980). Sm–Nd isotopic evolution of chondrites. Earth and Planetary Science Letters 50, 139–155.[Web of Science]
Jain, J. C. & Neal, C. R. (1996). Report of the inductively coupled plasma-mass spectrometry (ICP-MS) facility, University of Notre Dame 1993–1996. Open File Report, University of Notre Dame, 30 pp.
Jenner, G. A., Longerich, H. P., Jackson, S. E. & Fryer, B. J. (1990). ICP-MS—a powerful tool for high precision trace element analysis in earth sciences: evidence from analysis of selected U.S.G.S. reference samples. Chemical Geology 83, 133–148.
Kennedy, A. K., Lofgren, G. E. & Wasserburg, G. J. (1993). An experimental study of trace element partitioning between olivine, orthopyroxene, and melt in chondrules: equilibrium values and kinetic effects. Earth and Planetary Science Letters 115, 177–195.[Web of Science]
Kerr, A. C., Tarney, J., Marriner, G. F., Nivia, A. & Saunders, A. D. (1997). The Caribbean–Colombian Cretaceous igneous province: the internal anatomy of an oceanic plateau. In: Mahoney, J. J. & Coffin, M. F. (eds) Large Igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism. Geophysical Monograph, American Geophysical Union 100, 123–144.
Kogiso, T., Tatsumi, Y. & Nakano, S. (1997). Trace element transport during dehydration processes in the subducted oceanic crust: 1. Experiments and implications for the origin of ocean island basalts. Earth and Planetary Science Letters 148, 193–205.[Web of Science]
Kroenke, L. W. (1972). Geology of the Ontong Java Plateau. Hawaii Institute of Geophysics, Report 72-75. Honolulu: University of Hawaii, 119 pp.
Kurz, M. D., Kenna, T., Kammer, D., Rhodes, J. M. & Garcia, M. O. (1995). Isotopic evolution of Mauna Loa volcano: a view from the submarine southwest rift zone. In: Rhodes, J. M. & Lockwood, J. P. (eds) Mauna Loa Revealed: Structure, Composition, History, and Hazards. Geophysical Monograph, American Geophysical Union 92, 45–80.
Lanphere, M. A., Dalrymple, G. B., Fleck, R. S. & Pringle, M. S. (1990). Intercalibration of mineral standards for K–Ar and 40Ar–39Ar age measurements. EOS Transactions, American Geophysical Union 71, 1658.
Larson, R. L. & Erba, E. (1999). Onset of mid-Cretaceous greenhouse in the Barremian–Aptian: igneous events and the biological, sedimentary, and geochemical consequences. Paleoceanography 14, 663–678.[Web of Science]
Libourel, G., Boivin, P. & Biggar, G. M. (1989). The univariant curve liquid = forsterite + anorthite + diopside in the system CMAS at 1 bar: solid solutions and melt structure. Contributions to Mineralogy and Petrology 102, 406–421.
Longhi, J. (1987). Liquidus equilibria and solid solution in the system CaAl2Si2O8–Mg2SiO4–CaSiO3–SiO2 at low pressure. American Journal of Science 287, 265–331.[Web of Science]
Mahoney, J. J. (1987). An isotopic survey of Pacific oceanic plateaus: implications for their nature and origin. In: Keating, B., Fryer, P., Batiza, R. & Boehlert, G. (eds) Seamounts, Islands, and Atolls. Geophysical Monograph, American Geophysical Union 43, 207–220.
Mahoney, J. J. & Spencer, K. J. (1991). Isotopic evidence for the origin of the Manihiki and Ontong Java oceanic plateaus. Earth and Planetary Science Letters 104, 196–210.
Mahoney, J. J., Storey, M., Duncan, R. A., Spencer, K. J. & Pringle, M. (1993). Geochemisty and geochronology of the Ontong Java Plateau. In: Pringle, M., Sager, W., Sliter, W. & Stein, S. (eds) The Mesozoic Pacific. Geology, Tectonics, and Volcanism. Geophysical Monograph, American Geophysical Union 77, 233–261.
Mahoney, J. J., Sinton, J. M., Kurz, M. D., MacDougall, J. D., Spencer, K. J. & Lugmair, G. W. (1994). Isotope and trace element characteristics of a super-fast spreading ridge: East Pacific Rise, 13–23°S. Earth and Planetary Science Letters 121, 173–193.
Mahoney, J. J., Frei, R., Tejada, M. L. G., Mo, X. X., Leat, P. T. & Nägler, T. F. (1998). Tracing the Indian Ocean mantle domain through time: isotopic results from old West Indian, East Tethyan, and South Pacific seafloor. Journal of Petrology 39, 1285–1306.
Mahoney, J. J., Fitton, J. G., Wallace, P. J., et al. (eds) (2002). Proceedings of the Ocean Drilling Program, Initial Reports, 192. College Station, TX: Ocean Drilling Program, in press.
Mauffret, A. & Leroy, S. (1999). Seismic stratigraphy and structure of the Caribbean igneous province. Tectonophysics 283, 61–104.[Web of Science]
Michael, P. J. (1999). Implications for magmatic processes at Ontong Java Plateau from volatile and major element contents of Cretaceous basalt glasses. Geochemistry, Geophysics, Geosystems 1, paper no. 1999GC000025 [10 170 words, 6 figures, 2 tables].
Miura, S., Shinohara, M., Takahashi, N., Araki, E., Taira, A., Suyehiro, K., Coffin, M. F., Shipley, T. & Mann, P. (1996). OBS crustal structure of the Ontong Java Plateau converging into Solomon island arc (abstract). EOS Transactions, American Geophysical Union, 77F, 713.
McCulloch, M. T. & Black, L. P. (1984). Sm–Nd isotopic systematics of Enderby Land granulites and evidence for the redistribution of Sm and Nd during metamorphism. Earth and Planetary Science Letters 71, 46–58.
McDonough, W. F. & Sun, S.-s. (1995). The composition of the Earth. Chemical Geology 120, 223–253.[Web of Science]
McGinnis, C. E., Jain, J. C. & Neal, C. R. (1997). Characterization of memory effects and development of an effective wash protocol for the measurement of petrogenetically critical trace elements in geological samples by ICP-MS. Geostandards Newsletter 21, 289–305.[Web of Science]
McKenzie, D. & O’Nions, R. K. (1991). Partial melt distributions from inversion of rare earth element concentrations. Journal of Petrology 32, 1021–1091.
Moore, J. G., Normark, W. R. & Szabo, B. J. (1990). Reef growth and volcanism on the submarine southwest rift zone of Mauna Loa, Hawaii. Bulletin of Volcanology 52, 375–380.
Nakamura, Y. & Tatsumoto, M. (1988). Pb, Nd, and Sr isotopic evidence for a multicomponent source for rocks of Cook–Austral Island and heterogeneities of mantle plumes. Geochimica et Cosmochimica Acta 52, 2909–2924.
Neal, C. R., Mahoney, J. J., Kroenke, L. W., Duncan, R. A. & Petterson, M. G. (1997). The Ontong Java Plateau. In: Mahoney, J. J. & Coffin, M. F. (eds) Large Igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism. Geophysical Monograph, American Geophysical Union 100, 183–216.
Norrish, K. & Chappell, B. W. (1977). X-ray fluorescence spectrometry. In: Zussman, J. (ed.) Physical Methods in Determinative Mineralogy. New York: Academic Press, pp. 201–272.
Palacz, Z. & Saunders, A. D. (1986). Coupled trace element isotope enrichment on the Cook–Austral–Samoa Islands, southwest Pacific. Earth and Planetary Science Letters 79, 270–280.
Parkinson, I. J., Arculus, R. J. & Duncan, R. A. (1996). Geochemistry of Ontong Java Plateau basalt and gabbro sequences, Santa Isabel, Solomon Islands. EOS Transactions, American Geophysical Union 77, F715.
Peng, Z. X. & Mahoney, J. J. (1995). Drillhole lavas from the northwestern Deccan Traps, and the evolution of Reunion hotspot mantle. Earth and Planetary Science Letters 134, 169–185.[Web of Science]
Petterson, M. G. (1995). The Geology of North and Central Malaita, Solomon Islands (Including Implications of Geological Research on Makira, Savo Island, Guadalcanal, and Choiseul between 1992 and 1995). Water and Mineral Resources Division Geological Memoir 1/95. Honiara: Solomon Islands Ministry of Energy, Water, and Mineral Resources.
Petterson, M. G., Neal, C. R., Mahoney, J. J., Kroenke, L. W., Saunders, A. D., Babbs, T. L., Duncan, R. A., Tolia, D., McGrail, B. & Barron, M. (1997). Structure and deformation of north and central Malaita, Solomon Islands: tectonic implications for the Ontong Java Plateau–Solomon Arc collision, and for the fate of oceanic plateaus. Tectonophysics 283, 1–33.[Web of Science]
Phinney, E. J., Mann, P., Coffin, M. F. & Shipley, T. H. (1999). Sequence stratigraphy, structure, and tectonic history of the southwestern Ontong Java Plateau adjacent to the North Solomon Trench and Solomon Islands arc. Journal of Geophysical Research 104, 20449–20466.
Rhodes, J. M. & Hart, S. R. (1995). Evolution of Mauna Loa volcano: episodic trace element and isotopic variations in historical Mauna Loa lavas: implications for magma and plume dynamics. In: Rhodes, J. M. & Lockwood, J. P. (eds) Mauna Loa Revealed: Structure, Composition, History, and Hazards. Geophysical Monograph, American Geophysical Union 92, 263–288.
Richards, M. A., Duncan, R. A. & Courtillot, V. (1989). Flood basalts and hot-spot tracks: plume heads and tails. Science 246, 103–107.
Richardson, W. P., Okal, E. A. & Van der Lee, S. (2000). Rayleigh-wave tomography of the Ontong Java Plateau. Physics of the Earth and Planetary Interiors 118, 29–51.
Rickwood, F. K. (1957). Geology of the island of Malaita. In: Geological Reconnaissance of Parts of the Central Islands of B.S.I.P. Colonial Geology and Mineral Resources 6 (3), 300–306.
Roden, M. F., Trull, T., Hart, S. R. & Frey, F. A. (1994). New He, Nd, Pb, and Sr isotopic constraints on the constitution of the Hawaiian plume: results from Koolau Volcano, Oahu, Hawaii, USA. Geochimica et Cosmochimica Acta 58(5), 1431–1440.
Sager, W. W. & Han, H.-C. (1993). Rapid formation of the Shatsky Rise oceanic plateau inferred from its magnetic anomaly. Nature 364, 610–613.
Sheth, H. C. (1999). Flood basalts and large igneous provinces from deep mantle plumes: fact, fiction, and fallacy. Tectonophysics 311, 1–29.
Smith, A. D. & Lewis, C. (1999). The planet beyond the plume hypothesis. Earth-Science Reviews 48, 135–182.
Smith, W. H. F. & Sandwell, D. T. (1996). New global seafloor topography from satellite altimetry. EOS Transactions, American Geophysical Union 77, F315.
Stille, P., Unruh, D. M. & Tatsumoto, M. (1983). Pb, Sr, Nd, and Hf isotopic evidence of multiple sources for Oahu, Hawaii basalts. Nature 304, 25–29.
Sun, S.-S. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313–345.
Tarduno, J. A., Sliter, W. V., Kroenke, L. W., Leckie, M., Mahoney, J. J., Musgrave, R. J., Storey, M. & Winterer, E. L. (1991). Rapid formation of the Ontong Java Plateau by Aptian mantle plume volcanism. Science 254, 399–403.
Tatsumoto, M. (1978). Isotopic composition of lead in oceanic basalt and its implication to mantle evolution. Earth and Planetary Science Letters 38, 63–87.
Tejada, M. L. G. (1998). Geochemical studies of Pacific oceanic plateaus: the Ontong Java Plateau and Shatsky Rise. Ph.D. dissertation, University of Hawaii, Honolulu, 328 pp.
Tejada, M. L. G., Mahoney, J. J., Duncan, R. A. & Hawkins, M. P. (1996). Age and geochemistry of basement and alkalic rocks of Malaita and Santa Isabel, Solomon Islands, southern margin of Ontong Java Plateau. Journal of Petrology 37(2), 361–394.
Todt, W., Cliff, R. A., Hanser, A. & Hoffman, A. W. (1996). Evaluation of a 202Pb–205Pb double spike for high-precision lead isotope analysis. In: Basu, A. & Hart, S. (eds) Earth Processes: Reading the Isotopic Code. Geophysical Monograph, American Geophysical Union 95, 429–437.
Van Keken, P. (1998). Evolution of starting mantle plumes: a comparison between numerical and laboratory models. Earth and Planetary Science Letters 148, 1–11.
Vidal, P., Chauvel, C. & Brousse, R. (1984). Large mantle heterogeneity beneath French Polynesia. Nature 307, 536–538.
Walter, M. J. & Presnall, D. C. (1994). Melting behavior of simplified lherzolite in the system CaO–MgO–Al2O3–SiO2–Na2O from 7 to 35 kbar. Journal of Petrology 35, 329–359.
White, R. S. (1993). Melt production rates in mantle plumes. Philosophical Transactions of the Royal Society of London 342, 137–153.[Web of Science][Medline]
White, W. M. (1993). 238U/204Pb in MORB and open system evolution of the depleted mantle. Earth and Planetary Science Letters 115, 211–226.
Xie, Q., McCuaig, T. C. & Kerrich, R. (1995). Secular trends in the melting depth of mantle plumes: evidence from HFSE/REE systematics of Archean high-Mg lavas and modern oceanic basalts. Chemical Geology 126, 29–42.
Yasuda, A., Fujii, T. & Kurita, K. (1997). A composite diapir model for extensive basaltic volcanism: magmas from subducted oceanic crust entrained within mantle plumes. Proceedings of the Japan Academy 73(10), 201–204.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  S. Schuth, C. Munker, S. Konig, C. Qopoto, S. Basi, D. Garbe-Schonberg, and C. Ballhaus Petrogenesis of Lavas along the Solomon Island Arc, SW Pacific: Coupling of Compositional Variations and Subduction Zone Geometry J. Petrology, May 1, 2009; 50(5): 781 - 811. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. Ingle, J. J. Mahoney, H. Sato, M. F. Coffin, J.-I. Kimura, N. Hirano, and M. Nakanishi Depleted mantle wedge and sediment fingerprint in unusual basalts from the Manihiki Plateau, central Pacific Ocean Geology, July 1, 2007; 35(7): 595 - 598. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Z. ZHANG, J. J. MAHONEY, J. MAO, and F. WANG Geochemistry of Picritic and Associated Basalt Flows of the Western Emeishan Flood Basalt Province, China J. Petrology, October 1, 2006; 47(10): 1997 - 2019. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 W. R. Dickinson Temper Sands in Prehistoric Oceanian Pottery: Geotectonics, Sedimentology, Petrography, Provenance Geological Society of America Special Papers, January 1, 2006; 406(0): 1 - 160. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Ishikawa, E. Nakamura, and J. J. Mahoney Jurassic oceanic lithosphere beneath the southern Ontong Java Plateau: Evidence from xenoliths in alnoite, Malaita, Solomon Islands Geology, May 1, 2005; 33(5): 393 - 396. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. ISHIKAWA, S. MARUYAMA, and T. KOMIYA Layered Lithospheric Mantle Beneath the Ontong Java Plateau: Implications from Xenoliths in Alnoite, Malaita, Solomon Islands J. Petrology, October 1, 2004; 45(10): 2011 - 2044. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. Weissert, H. Weissert, and E. Erba Volcanism, CO2 and palaeoclimate: a Late Jurassic-Early Cretaceous carbon and oxygen isotope record Journal of the Geological Society, July 1, 2004; 161(4): 695 - 702. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. G. Fitton, J. J. Mahoney, P. J. Wallace, and A. D. Saunders Origin and evolution of the Ontong Java Plateau: introduction Geological Society, London, Special Publications, January 1, 2004; 229(1): 1 - 8. [Abstract] [PDF]
|
|
|
|
|
|
L. W. Kroenke, P. Wessel, and A. Sterling Motion of the Ontong Java Plateau in the hot-spot frame of reference: 122 Ma-present Geological Society, London, Special Publications, January 1, 2004; 229(1): 9 - 20. [Abstract] [PDF]
|
|
|
|
|
|
P. Riisager, S. Hall, M. Antretter, and X. Zhao Early Cretaceous Pacific palaeomagnetic pole from Ontong Java Plateau basement rocks Geological Society, London, Special Publications, January 1, 2004; 229(1): 31 - 44. [Abstract] [PDF]
|
|
|
|
|
|
X. Zhao, M. Antretter, P. Riisager, and S. Hall Rock magnetic results from Ocean Drilling Program Leg 192: implications for Ontong Java Plateau emplacement and tectonics of the Pacific Geological Society, London, Special Publications, January 1, 2004; 229(1): 45 - 61. [Abstract] [PDF]
|
|
|
|
|
|
M. G. Petterson The geology of north and central Malaita, Solomon Islands: the thickest and most accessible part of the world's largest (Ontong Java) ocean plateau Geological Society, London, Special Publications, January 1, 2004; 229(1): 63 - 81. [Abstract] [PDF]
|
|
|
|
|
|
M. L. G. Tejada, J. J. Mahoney, P. R. Castillo, S. P. Ingle, H. C. Sheth, and D. Weis Pin-pricking the elephant: evidence on the origin of the Ontong Java Plateau from Pb-Sr-Hf-Nd isotopic characteristics of ODP Leg 192 basalts Geological Society, London, Special Publications, January 1, 2004; 229(1): 133 - 150. [Abstract] [PDF]
|
|
|
|
|
|
J. G. Fitton and M. Godard Origin and evolution of magmas on the Ontong Java Plateau Geological Society, London, Special Publications, January 1, 2004; 229(1): 151 - 178. [Abstract] [PDF]
|
|
|
|
|
|
T. Sano and S. Yamashita Experimental petrology of basement lavas from Ocean Drilling Program Leg 192: implications for differentiation processes in Ontong Java Plateau magmas Geological Society, London, Special Publications, January 1, 2004; 229(1): 185 - 218. [Abstract] [PDF]
|
|
|
|
|
|
W. J. Chazey III and C. R. Neal Large igneous province magma petrogenesis from source to surface: platinum-group element evidence from Ontong Java Plateau basalts recovered during ODP Legs 130 and 192 Geological Society, London, Special Publications, January 1, 2004; 229(1): 219 - 238. [Abstract] [PDF]
|
|
|
|
|
|
R. V. White, P. R. Castillo, C. R. Neal, J. G. Fitton, and M. Godard Phreatomagmatic eruptions on the Ontong Java Plateau: chemical and isotopic relationship to Ontong Java Plateau basalts Geological Society, London, Special Publications, January 1, 2004; 229(1): 307 - 323. [Abstract] [PDF]
|
|
|
|
|
|
L. M. Chambers, M. S. Pringle, and J. G. Fitton Phreatomagmatic eruptions on the Ontong Java Plateau: an Aptian 40Ar/39Ar age for volcaniclastic rocks at ODP Site 1184 Geological Society, London, Special Publications, January 1, 2004; 229(1): 325 - 331. [Abstract] [PDF]
|
|
|
|
|
|
J. T. Shafer, C. R. Neal, and P. R. Castillo Compositional variability in lavas from the Ontong Java Plateau: results from basalt clasts within the volcaniclastic succession at Ocean Drilling Program Site 1184 Geological Society, London, Special Publications, January 1, 2004; 229(1): 333 - 351. [Abstract] [PDF]
|
|
|
|
|
|
P. R. Castillo Geochemistry of Cretaceous volcaniclastic sediments in the Nauru and East Mariana basins provides insights into the mantle sources of giant oceanic plateaus Geological Society, London, Special Publications, January 1, 2004; 229(1): 353 - 368. [Abstract] [PDF]
|
|
|
|
|
|
R. A. DUNCAN A Time Frame for Construction of the Kerguelen Plateau and Broken Ridge J. Petrology, July 1, 2002; 43(7): 1109 - 1119. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||