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Journal of Petrology | Volume 44 | Number 1 | Pages 113-140 | 2003
© Oxford University Press 2003
Geochemistry of Lavas from the Emperor Seamounts, and the Geochemical Evolution of Hawaiian Magmatism from 85 to 42 Ma
MAX-PLANCK INSTITUT FÜR CHEMIE, ABTEILUNG GEOCHEMIE, POSTFACH 3060, 55020 MAINZ, GERMANY
RECEIVED July 10, 2001; ACCEPTED July 16, 2002
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe Hawaiian–Emperor Seamount Chain (ESC), in the northern Pacific Ocean, was produced during the passage of the Pacific Plate over the Hawaiian hotspot. Major and trace element concentrations and Sr–Nd–Pb isotopic compositions of shield and post-shield lavas from nine of the Emperor Seamounts provide a 43 Myr record of the chemistry of the oldest preserved Hawaiian magmatism during the Late Mesozoic and Early Cenozoic (from 85 to 42 Ma). These data demonstrate that there were large variations in the composition of Hawaiian magmatism over this period. Tholeiitic basalts from Meiji Seamount (85 Ma), at the northernmost end of the ESC, have low concentrations of incompatible trace elements, and unradiogenic Sr isotopic compositions, compared with younger lavas from the volcanoes of the Hawaiian Chain (<43 Ma). Lavas from Detroit Seamount (81 Ma) have highly depleted incompatible trace element and Sr–Nd isotopic compositions, which are similar to those of Pacific mid-ocean ridge basalts. Lavas from the younger Emperor Seamounts (62–42 Ma) have trace element compositions similar to those of lavas from the Hawaiian Islands, but initial 87Sr/86Sr ratios extend to lower values. From 81 to 42 Ma there was a systematic increase in 87Sr/86Sr of both tholeiitic and alkalic lavas. The age of the oceanic lithosphere at the time of seamount formation decreases northwards along the Emperor Seamount Chain, and the oldest Emperor Seamounts were built upon young, thin lithosphere close to a former spreading centre. However, the inferred distance of the Hawaiian plume from a former spreading centre, and the isotopic compositions of the oldest Emperor lavas appear to rule out plume–ridge interaction as an explanation for their depleted compositions. We suggest that the observed temporal chemical and isotopic variations may instead be due to variations in the degree of melting of a heterogeneous mantle, resulting from differences in the thickness of the oceanic lithosphere upon which the Emperor Seamounts were constructed. During the Cretaceous, when the Hawaiian plume was situated beneath young, thin lithosphere, the degree of melting within the plume was greater, and incompatible trace element depleted, refractory mantle components contributed more to melting.
KEY WORDS: Emperor Seamounts; Hawaiian plume; lava geochemistry; lithosphere thickness; mantle heterogeneity
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INTRODUCTION |
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Long-lived, intra-plate oceanic (‘hotspot’) magmatism is generally thought to be the surface expression of mantle plumes—columns of relatively hot material rising from deeper in the mantle (Morgan, 1971). Geochemical studies of intra-plate magmatism can therefore potentially give insights into the structure and composition of the Earth’s deep mantle. Previous studies have shown that the chemical and isotopic compositions of intra-plate oceanic lavas are more diverse than those of lavas from mid-ocean spreading centres, which sample only the uppermost mantle (e.g. Cohen & O’Nions, 1982; White & Hofmann, 1982; Hofmann, 1997). The origin of the chemical and isotopic heterogeneity in intra-plate oceanic lavas is unclear, although subduction of oceanic crust and sediment is probably responsible for generating heterogeneity in the deeper mantle (Hofmann & White, 1982; Hofmann, 1997).
Seamount chains, created when oceanic plates move over mantle plumes, record changes in the composition of lavas erupted above a single plume over time. The study of temporal changes in the chemistry of intra-plate magmatism can give insights into the thermal and chemical structure of mantle plumes (Class et al., 1993; White et al., 1993; Hauri et al., 1996; Hoernle et al., 2000), the influence of the oceanic lithosphere on the chemistry of intra-plate lavas (Dupuy et al., 1993; Basu & Faggart, 1996; Chauvel et al., 1997), and the dynamics of mantle plume–spreading ridge interaction (Fisk et al., 1989; Gautier et al., 1990; Class et al., 1993; Cheng et al., 1999). In order to use the geochemistry of intra-plate lavas to probe the composition of the deeper mantle, it is important to know the influence of shallow-level processes such as these on the chemistry of oceanic island lavas. In spite of this, there have been few very detailed studies of the temporal geochemical variations among lavas erupted above mantle plumes. The existing data show that along some seamount chains, there are significant, systematic variations in lava chemistry, whereas other hotspots appear to have erupted lavas with very similar composition over long periods of time. For example, the composition of the lavas erupted above the Louisville hotspot in the southern Pacific has changed very little over the past 70 Myr (Cheng et al., 1987). In contrast, lavas from the Kerguelen Plateau, Ninetyeast Ridge and Kerguelen Archipelago in the Indian Ocean, which were erupted above the Kerguelen hotspot between 120 Ma and the present, appear to show temporal changes in trace element and isotope chemistry. The isotopic variations have been attributed both to radioactive decay in the mantle source of the lavas (Class et al., 1993), and to variable proportions of continental lithosphere, depleted upper-mantle and plume source components, perhaps related to variations in the distance of the plume from a former spreading centre (Gautier et al., 1990; Frey & Weis, 1995; Frey et al., 2000). Geochemical variations among lavas from the Reunion hotspot track in the western Indian Ocean (Fisk et al., 1989), and the Easter Seamount Chain in the eastern Pacific (Cheng et al., 1999) have also been explained by variations in the distance from spreading centres. Lavas from several other seamount chains (e.g. the Line Islands and the Society and Austral chains) have variable compositions, but show no systematic chemical change with time (Nakamura & Tatsumoto, 1988; Garcia et al., 1993; Hémond et al., 1994a; White & Duncan, 1996).
In this paper, we present the results of a geochemical study of lavas from the Hawaiian–Emperor Seamount Chain (ESC) in the northern Pacific Ocean, which provide a 43 Myr record (from 
85 to 42 Ma) of the geochemistry of the Hawaiian mantle plume. Previous geochemical studies of lavas from the ESC have shown that the trace element and isotopic compositions of the lavas from the oldest, northernmost seamounts differ significantly from those of younger lavas erupted from volcanoes on the Hawaiian Islands (Lanphere et al., 1980; Regelous & Hofmann, 1999; Keller et al., 2000). The existing data also show that the chemical and isotopic variations along the ESC are large compared with most other seamount chains. We therefore carried out a detailed geochemical and isotopic study of lavas from the Emperor Seamounts, to document in detail the temporal variations in Hawaiian magmatism and determine their origin.
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GEOCHRONOLOGY AND PETROLOGY OF THE EMPEROR SEAMOUNTS |
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Active volcanoes on the Hawaiian Islands represent the current site of intra-plate volcanism which, over the past 85 Myr, has built the Hawaiian–Emperor Seamount Chain in the northern Pacific Ocean (Fig. 1). This seamount chain is almost 6000 km long, and is composed of >100 shield volcanoes with a combined volume of over 106 km3 (Bargar & Jackson, 1974).
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Although there have been numerous geochemical studies of lavas from the Hawaiian Islands, much less is known about the geochemistry of Hawaiian volcanoes older than 5 Ma, because most are now submerged and are therefore difficult to sample. The Emperor Seamounts are the oldest preserved products of Hawaiian magmatism. Geochronological studies of lavas from these seamounts have shown that the ages of the volcanoes increase progressively northwards, from Daikakuji (42 Ma) to Detroit (81 Ma). Dalrymple et al. (1980b) reported a minimum K–Ar age of 61·9 ± 5 Ma for Meiji Seamount, although microfossil assemblages in the overlying sediments indicate that the age of Meiji is at least 68–70 Ma (Worsley, 1973). The radiometric age data for Emperor Seamount lavas have been summarized by Clague & Dalrymple (1989).
A wide range of volcanic rock types have been recovered from the ESC. These include tholeiitic and alkalic basalts and their differentiates, and silica-undersaturated lavas such as basanite, nephelinite and nepheline melilitite (Clague & Dalrymple, 1989; Lonsdale et al., 1993). These rock types are similar to those occurring on the Hawaiian Islands. On Suiko and Ojin Seamounts, drilling has recovered alkali basalts overlying tholeiites (Kirkpatrick et al., 1980), and both tholeiitic (shield) and alkalic (post-shield) lavas are present on Detroit, Koko, Yuryaku and Daikakuji Seamounts (Clague & Dalrymple, 1989; Keller et al., 1995). The similarities in volcano morphology, lava petrology, and the stratigraphic distribution of rock types suggest that the volcanoes of the ESC passed through a series of evolutionary stages similar to those of young Hawaiian volcanoes (Clague & Dalrymple, 1989; Lonsdale et al., 1993).
Previous studies indicate that lavas from Daikakuji, Yuryaku, Kimmei, Koko, Jingu, Ojin, Nintoku and Suiko Seamounts have major and trace element compositions that are very similar to those of younger lavas from the Hawaiian Islands (Clague & Dalrymple, 1973; Clague et al., 1975; Dalrymple & Clague, 1976; Bence et al., 1980; Clague & Frey, 1980; Dalrymple & Garcia, 1980; Kirkpatrick et al., 1980), and from seamounts along the Hawaiian segment of the Hawaiian–Emperor Chain (Dalrymple et al., 1974, 1981; Garcia et al., 1987). On the other hand, Keller et al. (1995) and Regelous & Hofmann (1999) have shown that lavas from Meiji and Detroit Seamounts have lower concentrations of incompatible elements than most lavas from the Hawaiian Islands.
There have been few previous isotopic studies of ESC lavas. Lanphere et al. (1980) carried out Sr isotope measurements on lavas from seven of the younger Emperor Seamounts. Those workers showed that tholeiites from the Emperor Seamounts generally have lower age-corrected 87Sr/86Sr ratios than tholeiitic lavas from the Hawaiian Islands, and that 87Sr/86Sr values decrease northwards along the ESC. Lanphere et al. (1980) suggested that this was due to variations in the difference in age between each seamount and the oceanic crust on which it was built. Recent studies of lavas from Meiji and Detroit Seamounts (Keller et al., 2000) have shown that these lavas have more depleted Sr and Nd isotope compositions than all other Hawaiian–Emperor lavas. Keller et al. (2000) speculated that this was the result of plume–ridge interaction. Here we present combined major and trace element, and Sr, Nd and Pb isotope data for Emperor Seamount lavas, and show that lavas from the oldest Emperor Seamounts also have depleted incompatible trace element compositions compared with younger Hawaiian–Emperor lavas. We suggest that the chemical and isotopic variations in lavas from the ESC result from differences in the degree of melting of a heterogeneous mantle, as a result of variations in the thickness of the lithosphere upon which the seamounts were built.
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SAMPLE LOCATIONS |
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We analysed 44 lava samples from nine seamounts along the entire length of the Emperor Seamount Chain (Fig. 1). The samples provide an
43 Myr record of the geochemistry of Hawaiian magmatism, between 85 and 42 Ma. Samples from the northern seamounts were collected by drilling, and all dredge samples were collected south of latitude 37°N. None of these samples can therefore represent glacial dropstones, which occur only north of latitude 41°N (Lonsdale et al., 1993). Sample locations are given in Table 1.
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Deep Sea Drilling Project (DSDP) Leg 19, Hole 192A, recovered an 13 m thickness of pillow basalts from near the summit of Meiji Guyot (Stewart et al., 1973). The lavas were initially classified as alkali basalts on the basis of their mineralogy, but subsequent microprobe analyses of glass and pyroxene suggested that they are tholeiitic (Dalrymple et al., 1980a). At least five flow units were represented (Stewart et al., 1973). Samples from four flows were selected for analysis in this study.
Drilling at Site 883, Ocean Drilling Program (ODP) Leg 145, on Detroit Seamount penetrated basement and recovered 24 m and 38 m of plagioclase-phyric basalt in two holes located 20 m apart. The lavas were described as transitional in composition (Keller et al., 1995). Six samples from these two holes were analysed in this study. Hole 884E penetrated 87 m into basement, and recovered basaltic pillows and massive flows, which were divided into 13 lithological units. Major and trace element analyses (Keller et al., 1995) showed these to be of tholeiitic composition. Samples from five units were analysed in this study.
During DSDP Leg 55, three holes were drilled into the volcanic basement of Suiko Seamount. A thickness of 11 m of alkali basalt was recovered from Hole 433A, and 19 m (two flow units) of alkali basalt from Hole 433B. Hole 433C penetrated almost 400 m into at least 114 subaerially erupted lava flows consisting of tholeiitic basalt, picrite and overlying alkali basalt (Kirkpatrick et al., 1980). We analysed eight samples from these three holes; an alkali basalt from each of Holes 433A and 433B, and five tholeiitic basalts and an alkali basalt from Hole 433C.
Ojin and Nintoku Seamounts were drilled on DSDP Leg 55. Hole 430A (Ojin Seamount) penetrated 28 m (five flow units) of basalt. Subsequent major element analyses of these rocks showed the lowermost unit to be tholeiitic in composition, whereas the overlying units are hawaiites (Kirkpatrick et al., 1980). Four samples, including the tholeiite, were analysed in this study. Hole 432A (Nintoku Seamount) penetrated 32 m of basement, comprising three flow units of alkali basalt, and an overlying conglomerate containing volcanic cobbles. We analysed samples from each of the three flows, and a hawaiite cobble from the conglomerate previously analysed by Kirkpatrick et al. (1980).
We also studied samples that were collected from the southern Emperor Seamounts by dredging during the Scripps Institution of Oceanography cruise AIRES VII, in 1971. This cruise recovered volcanic rocks from Koko, Kimmei, Yuryaku and Daikakuji Seamounts. Major element and some trace element analyses, as well as K–Ar and/or Ar–Ar data for samples from these dredge hauls have been previously reported by Clague and co-workers (Clague & Dalrymple, 1973, 1987; Clague et al., 1975; Dalrymple &d Clague, 1976). Two dredges on Koko Seamount recovered a range of rock types, including tholeiitic and alkalic basalt, hawaiite, mugearite, trachyte and phonolite. Both tholeiitic and alkalic basalts were recovered from Yuryaku, whereas only alkali basalts were dredged from Kimmei and Daikakuji Seamounts. Thirteen of the least altered samples from five dredges from these four seamounts were analysed in this study. The dredge samples are in the form of pebbles and blocks, varying from 5 to 40 cm in diameter, many of which had Fe–Mn coatings. The dredged samples are generally more altered than the drilled samples (Table 1).
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ANALYTICAL TECHNIQUES |
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Dredge samples were crushed to 0·5–2 cm fragments, and material was hand-picked from the interior of the pebbles, to avoid the Fe–Mn coatings on some of these samples. Drill core material was crushed to chips of 2–3 mm size, and the freshest material was hand-picked. The chips were washed twice in distilled water in an ultrasonic bath, dried, and crushed in an agate swing mill. Rock chips for trace element analyses were handpicked to avoid phenocrysts, but several of the samples analysed (e.g. 55-13, 145-10) included olivine or plagioclase phenocryst fragments.
Major element analyses (Table 2) were carried out by X-ray fluorescence (XRF) at the Universität Mainz, Germany. Trace element concentrations (Table 2) were determined by inductively coupled plasma mass spectrometry (ICP-MS) using a Fisons Plasmaquad II instrument at the University of Queensland, Australia. Full details of the procedure have been given by Niu & Batiza (1997). External precision on the concentrations of most of the trace elements measured is between 1 and 3% (Table 2).
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Sr, Nd and Pb isotope analyses were carried out at the Max-Planck-Institut für Chemie, Mainz. Before dissolution, sample powders for Sr and Pb isotope analysis were subjected to an acid leaching procedure to remove alteration products. Approximately 4–5 g of rock powder was weighed into a Teflon beaker, and 5–10 ml once-distilled 6M HCl added. The samples were placed in an ultrasonic bath for 2 h; the acid was changed every 20 min. The samples were then leached in hot (80°C) 6M HCl for a further 5 h; the acid was changed every 60 min. The residue was soaked in deionized water at 80°C for 30 min, rinsed twice, and dried. The weight loss as a result of leaching was between 55 and 71%. Microscopic examination of the residue showed that the leaching procedure leaves a residue of plagioclase, clinopyroxene, ± olivine and opaque minerals. Rb and Sr concentrations in the leached powders were determined by isotope dilution, and U, Th and Pb concentrations were measured by ICP-MS.
The leached sample powder (100–200 mg) was digested in HF–HNO3 for Pb isotope analysis, and Pb was separated from the sample matrix by anion exchange in HNO3–HBr mixtures, as described by Lugmair & Galer (1992). Rb and Sr were separated using conventional cation exchange techniques from a separate dissolution of leached sample powder. Nd isotope analyses were carried out on unleached powders. The rare earth elements (REE) were separated from the sample matrix using standard cation exchange procedures, and Nd was then separated from the other REE by cation exchange using 
-hydroxyisobutyric acid as eluant. Total procedural blanks were below 500, 80 and 60 pg for Sr, Nd and Pb, respectively.
Isotope analyses were carried out in static multicollection mode using a Finnigan MAT-261 mass spectrometer. Sr and Nd isotope ratios were corrected for fractionation using 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219. The NBS-987 Sr and La Jolla Nd standards gave 87Sr/86Sr = 0·710223 ± 18 and 143Nd/144Nd = 0·511872 ± 10 (2
), respectively, during the period of analysis. Sr and Nd isotope data in Table 3 have been normalized to values of 0·710245 and 0·511855 for these standards. Pb isotope analyses were carried out using a triple-spike technique to correct for instrumental mass fractionation (Galer, 1999). After elution of the Pb fraction from the ion exchange columns, 5–10% was transferred to a second beaker, and spiked with an optimal amount of 204Pb–206Pb–207Pb triple spike. The spiked and unspiked Pb fractions were measured separately on the mass spectrometer, and the data were then combined to obtain the fractionation-corrected Pb isotope composition of the sample (Galer, 1999). During this study, the NBS-981 Pb standard gave 206Pb/204Pb 16·9403 ± 22, 207Pb/204Pb 15·4974 ± 20, 208Pb/204Pb 36·7246 ± 58 (2, n = 19).
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EFFECTS OF LEACHING AND AGE CORRECTION ON THE ISOTOPE DATA |
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Few of the samples analysed have both Ce/Pb and Nb/U ratios within the range of fresh oceanic lavas (Hofmann et al., 1986), indicating that alteration has modified the U and Pb concentrations of many of the lavas since crystallization. Pb and Sr isotope analyses were therefore carried out on leached sample powders, in an attempt to obtain meaningful Sr and Pb isotope data from samples that gained or lost Rb, Sr, U and Pb during subsolidus alteration.
The leaching procedure we used is similar to that of previous studies, and has been shown to be effective at removing alteration products from altered basaltic rocks (Cheng et al., 1987; Mahoney, 1987). Experiments carried out in this study showed that more intense leaching did not lead to a significant further decrease in initial 87Sr/86Sr ratios (Table 3). Compared with the unleached rock powder, the leached residues generally have significantly lower Rb concentrations, and similar or higher Sr contents (Tables 2 and 3). These changes are probably the result of removal of clay minerals and concentration of plagioclase in the residue during leaching.
Seawater has an extremely low Pb concentration, and so if alteration changes U/Pb soon after eruption, reliable initial Pb isotope ratios could be obtained from unleached powders (Staudigel et al., 1995). However, recent studies have shown that this is often not the case (Mahoney et al., 1998). We analysed unleached powders of three samples. Two of these (19-1 and 145-10) yielded very different initial Pb isotope compositions from those of the corresponding leached powders, whereas initial Pb isotope ratios of leached and unleached aliquots of sample 145-11 were very similar (Table 4). Leached and unleached samples do not differ in any systematic way (Table 4). As a result of the low U/Pb, Th/Pb ratios in plagioclase, measured Pb isotope ratios in this mineral should be close to the whole-rock initial value. A plagioclase separate from sample 145-11 yielded similar Pb isotope ratios to the age-corrected leached whole-rock powder (Table 4). These results suggest that accurate Pb isotope data for old, altered basaltic rocks may be obtained from leached rock powders. We therefore carried out Pb isotopic measurements on leached sample powders. U, Th and Pb concentrations are all significantly lower in the leached powders than in the unleached samples (Table 4).
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The measured Ce and Nb concentrations of the samples could be used to estimate their Pb and U concentrations before alteration, assuming that the rocks had Ce/Pb and Nb/U ratios typical of other oceanic basalts before alteration. However, U/Pb ratios calculated in this way can be used for age correction only if the alteration occurred recently, but many samples contain petrographic evidence for multiple alteration events (e.g. zoning of secondary minerals infilling vesicles). In addition, Ce/Pb ratios of many of the samples are lower than those typical of most oceanic basalts, indicating that they have gained Pb, probably from seawater, during the alteration process.
Although moderate degrees of sea-floor alteration have no significant effect upon Nd isotope compositions of basalts (e.g. Staudigel et al., 1995), the Fe–Mn coatings on some of the samples are potentially a source of Nd (and Pb) contamination. Samples for isotope measurements were therefore carefully handpicked to avoid these coatings. We carried out Nd isotope analyses on three samples that had been subjected to the leaching procedure. Of these, two samples yielded initial 143Nd/144Nd ratios within error of those of the unleached powders (despite a large decrease in Sm and Nd concentrations; see Table 3), whereas the other leached sample had a somewhat higher initial 143Nd/144Nd ratio than that of the unleached powder. As the leaching procedure would have removed any Fe–Mn coating present (Cheng et al. 1987), we conclude that alteration has not significantly modified the Nd isotope ratios of the unleached samples.
Measured Sr, Nd and Pb isotope ratios were corrected for radioactive decay since eruption using the known age of each seamount (Table 1). A potential problem with age-correcting isotope data for leached samples is that the leaching procedure may have altered the parent/daughter ratios of the primary minerals left in the residue or preferentially removed radiogenic nuclides from damaged sites in the crystal structure. However, the fact that aliquots of the sample 55-10 that were leached for 10, 12 and 24 h (69–75% weight loss) yielded initial 87Sr/86Sr ratios similar to those of the same sample subjected to the usual 7 h procedure (61% weight loss) suggests that reliable initial Sr isotope ratios can be obtained from leached powders (see Table 3). A plagioclase separate from sample 145-11 yielded measured Pb isotope ratios very similar to the initial ratios calculated for the leached whole-rock sample (Table 4). As the measured Pb isotope compositions of plagioclase should be close to the initial ratio of the whole rock, we believe that the initial Pb isotope ratios calculated for the leached rock powders are also reliable.
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RESULTS |
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Major and trace element composition
New major and trace element data for Emperor Seamount lavas are given in Table 2. Alteration has modified the compositions of most of the samples analysed, particularly the dredged samples. As a result, concentrations of mobile trace elements in many of the samples cannot be considered as primary. We have reported Rb, Sr, U and Pb concentrations in Table 2, to show the effects of leaching. Post-eruptive growth of secondary minerals (particularly calcite and clays) in vesicles, together with variable replacement of glass, olivine and plagioclase by alteration products such as smectite, calcite and Fe-oxyhydroxides, will have affected the concentrations of some major elements such as Ca, K, Na and P, given the observed abundance of these secondary minerals in the most altered samples. A rough estimate of the degree of alteration of the samples based on their petrography is included in Table 1.
Meiji Seamount (85 Ma)
The four samples analysed from DSDP Site 192 are highly altered plagioclase ± clinopyroxene-phyric basalts, with MgO contents of between 5·8 and 6·9%. On an alkali–silica diagram (Fig. 2a), three of the samples plot within the field of alkalic basalts as a result of their high K2O contents (Fig. 2f). However, Na2O concentrations of the Meiji lavas are similar to those of other Emperor tholeiites, and the relatively high K2O concentrations are probably the result of post-eruption alteration. In the TiO2–P2O5 classification diagram (Fig. 2b), the samples plot within the field of ocean island tholeiites. The Meiji Seamount lavas have lower concentrations of TiO2 and P2O5 than tholeiites from Suiko and younger seamounts (Fig. 2b).
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The Meiji tholeiites have lower concentrations of the highly incompatible trace elements than many young tholeiitic basalts from the Hawaiian Islands with similar MgO. Concentrations of Th and Nb lie at the depleted end of the range of Hawaiian lavas (Fig. 3), and are similar to those of shield-building lavas from Mauna Loa (Hofmann & Jochum, 1996) and Koolau (Frey et al., 1994). However, concentrations of the heavy rare earth elements (HREE) in the Meiji lavas are higher than those of young Hawaiian lavas (Fig. 4a), and La/Yb ratios are intermediate between those of tholeiites from the Hawaiian Islands and mid-ocean ridge basalt (MORB) from the East Pacific Rise (EPR) (Fig. 3).
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Detroit Seamount (81·0 Ma)
The five flows sampled from Hole 884E are tholeiitic basalts containing 6·2–8·5% MgO. Samples 145-9, 145-10 and 145-11 contain up to 30% plagioclase phenocrysts, which may account for their higher CaO and Al2O3 contents compared with those of the other Emperor lavas. The Detroit tholeiites have lower TiO2, P2O5 and K2O, compared with other Hawaiian–Emperor tholeiites, which cannot be explained simply by the diluting effect of plagioclase, as both TiO2 and P2O5 are a factor of 2–3 lower than in most Hawaiian–Emperor lavas (Fig. 2b), and are also low in the aphyric samples 145-7 and 145-8.
Compared with tholeiites from the Hawaiian Islands, the Detroit tholeiites have lower concentrations of the highly incompatible trace elements (Figs 3 and 4b). Concentrations of the most incompatible elements such as Th are about seven times lower than in average Kilauea tholeiites for a given MgO value (Fig. 4), and are similar to the Th concentrations in Pacific N-type MORB (Fig. 3). The concentrations of the HREE (Er to Lu) are slightly higher in the Detroit tholeiites than in young Hawaiian tholeiites. Incompatible trace element patterns are similar to those of Pacific N-MORB (Fig. 4). The trace element compositions of the Detroit tholeiites are unlike those of most other intra-plate tholeiites, except the highly depleted lavas from the Galapagos Islands (White et al., 1993).
The six samples from Site 883 are highly altered, plagioclase- and olivine-phyric lavas containing 1·8–7·2% MgO. Samples 145-4 and 145-6 have 1·8–2·0% MgO and are from different flows in Hole 883F. Keller et al. (1995) classified the Site 883 lavas as transitional between tholeiitic and alkalic. On a total alkali–silica diagram (Fig. 2), the lavas plot above the alkali–tholeiite dividing line. However, compared with alkalic lavas from other Emperor Seamounts, the Site 883 lavas have lower P2O5, TiO2 and Na2O, and fall within the tholeiitic field in Fig. 2b.
The Site 883 lavas have higher concentrations of incompatible elements, and higher ratios of more- to less-incompatible elements compared with the tholeiites from Site 884 (Fig. 3). Concentrations of all elements more incompatible than Zr are lower than in young tholeiitic lavas from Kilauea (Fig. 4), and far lower than in Hawaiian alkalic or transitional basalts with similar MgO. Incompatible trace element compositions of the Site 883 lavas are similar to those of the Meiji tholeiites. However, compared with the latter, the Detroit lavas have lower SiO2 for a given MgO, and in this respect are similar to young alkalic lavas from the Hawaiian Islands.
Younger seamounts (62–42 Ma)
Tholeiitic basalts were recovered from Suiko, Ojin, Koko, Yuryaku and Daikakuji Seamounts. One tholeiitic lava from Suiko Seamount (sample 55-13) has a picritic composition and contains cumulate olivine. Sample A55D-e, from Daikakuji, is a basaltic andesite. Alkali basalts occur on Suiko, Nintoku, Kimmei, Yuraku and Daikakuji Seamounts. Two phonolites (samples A43D-d and A43D-g) were dredged from the SE flank of Koko Seamount. Other samples from these seamounts include basalts of intermediate composition, hawaiites and a mugearite, all of which have similar major element compositions to lavas erupted during the post-shield volcanic stage on the Hawaiian Islands. Immobile trace element abundances of both tholeiitic and alkalic lavas from these seven seamounts are similar to those of equivalent rock types from the Hawaiian Islands (Figs 3 and 4, Table 2).
Sr–Nd–Pb isotope data
Sr and Nd isotopic data are given in Table 3, and Pb isotopic data in Table 4. All isotope data for Emperor Seamount lavas shown in the following figures have been corrected for radioactive decay since eruption; 
Sr and Nd values are reported relative to CHUR. Fields for young lavas from the EPR and the Hawaiian Islands at 80 Ma were calculated assuming 87Rb/86Sr, 147Sm/144Nd, 238U/204Pb and 232Th/204Pb ratios of 0·02, 0·24, 5 and 11 for the source of EPR lavas, and 0·06, 0·21, 11 and 35 for the source of Hawaiian lavas (White, 1993; Cohen & O’Nions, 1994; Hémond et al., 1994b; Sims et al., 1995; Mahoney et al., 1998).
Meiji Seamount
Initial Sr values of the Meiji tholeiitic basalts are lower than those of any tholeiites yet reported from the Hawaiian Islands. The Meiji tholeiites plot at the depleted end of the field for tholeiitic lavas from the Hawaiian Islands in Fig. 5. Sr and Nd isotopic compositions of the single Meiji sample analysed by Keller et al. (2000) fall within the range of our samples. Age-corrected Pb isotope compositions lie at the unradiogenic end of the array defined by young lavas from the Hawaiian Islands in Fig. 6, but compared with Hawaiian lavas, have higher 208Pb/204Pb for a given 206Pb/204Pb. The Meiji sample analysed by Keller et al. (2000) has similar 207Pb/204Pb and 208Pb/204Pb ratios to our samples, but significantly higher 206Pb/204Pb (Fig. 6). The age correction for the Meiji lavas is relatively large (85 Ma), and some uncertainty in the age correction for these highly altered and strongly leached samples may account for the apparent scatter in Fig. 6. Data for Meiji lavas overlap with the field defined by Hawaiian alkalic lavas in terms of their Pb–Sr–Nd isotopic compositions, but extend to lower 206Pb/204Pb (Fig. 7).
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Detroit Seamount
The Detroit tholeiites have lower initial Sr and higher Nd than all other Hawaiian–Emperor lavas (Fig. 5). Sr and Nd isotopic compositions of these lavas overlap with those of Pacific N-MORB, but the Detroit tholeiites are displaced to the high Sr side of the field for Pacific MORB, and lie on an extension of the Hawaiian Islands array in Fig. 5. Mahoney et al. (1998) observed a similar result for leached, old, highly altered basalts, and attributed this to incorporation of non-magmatic Sr into the crystal structure during replacement of plagioclase by secondary feldspar. However, plagioclase phenocrysts in the Detroit tholeiites show no evidence of replacement, and plagioclase-phyric and aphyric samples have similar Sr isotope compositions, indicating that Sr ratios of the leached samples are not affected by alteration. Unlike young Hawaiian lavas, the Detroit tholeiites have lower Sr and higher Nd than the transitional lavas from the same seamount. The low Sr and high Nd ratios of the Detroit lavas are unusual for intra-plate volcanics, although lavas with similar compositions have been reported from Iceland and the Galapagos Islands (Hémond et al., 1993; White et al., 1993).
The Detroit Seamount lavas have initial 206Pb/204Pb ratios within the range of lavas from the Hawaiian Islands (Fig. 6). Compared with young Hawaiian lavas, the Detroit lavas have lower 207Pb/204Pb and 208Pb/204Pb for a given 206Pb/204Pb. The transitional lavas from Site 883 have higher 208Pb/204Pb than the tholeiitic lavas from Site 884. New high-precision triple-spike isotope measurements (Abouchami et al., 2000; S. J. G. Galer et al., unpublished data, 2002) show that lavas from the Hawaiian Islands and the EPR show little overlap in Pb isotope diagrams (Fig. 6). Compared with EPR MORB, the Detroit lavas have lower 207Pb/204Pb for a given 206Pb/204Pb, but overlap with the unradiogenic end of the MORB field in the 206Pb/204Pb–208Pb/204Pb diagram (Fig. 6).
Younger seamounts (62–42 Ma)
Tholeiitic basalts from Suiko Seamount have lower initial Sr values than tholeiites from the Hawaiian Islands, and tholeiites from younger seamounts (Ojin, Koko and Yuryaku) have Sr ratios that lie at the depleted end of the field for Hawaiian lavas (Fig. 5). None of the ESC tholeiites have the high Sr and low Nd ratios that characterize young shield lavas from Koolau and Lanai. Alkalic lavas from Suiko, Nintoku, Ojin, Koko, Kimmei, Yuryaku and Daikakuji Seamounts have Sr and Nd isotopic compositions within the range found in young Hawaiian post-shield lavas (Fig. 5). Transitional basalts from Daikakuji Seamount at the southern end of the ESC have the highest Sr and lowest Nd values of all the samples analysed. Tholeiitic lavas from Suiko, Koko and Yuryaku Seamounts all have higher initial 87Sr/86Sr than the associated alkalic lavas; however, the single tholeiite recovered from Ojin Seamount has lower initial 87Sr/86Sr ratio than hawaiites from the same seamount.
Our Sr isotope data for these seamounts are similar to those measured by Lanphere et al. (1980); however, in our dataset the intra-seamount variation is smaller, and values for dredged lavas are generally shifted to less radiogenic values. This is probably due to the fact that our measurements were carried out on leached sample powders, and therefore more closely reflect the original magmatic values. The single Suiko sample analysed by Keller et al. (2000) has age-corrected Sr, Nd and Pb isotope ratios within the range shown by our data.
Initial Pb isotope compositions of tholeiitic and alkalic lavas from Suiko, Nintoku, Ojin, Koko, Kimmei, Yuryaku and Daikakuji Seamounts lie within the field for young lavas from the Hawaiian Islands (Fig. 6). Alkalic lavas generally have less radiogenic Pb isotope compositions than tholeiitic lavas from the same seamount. Compared with the other southern Emperor Seamount lavas, the two transitional lavas from Daikakuji have high 208Pb/204Pb for a given 206Pb/204Pb.
In the 206Pb/204Pb–Sr diagram, tholeiitic lavas from Suiko plot at the high 206Pb/204Pb end of the field for Hawaiian Islands tholeiites, with lower 87Sr/86Sr (Fig. 7). An alkalic basalt from Suiko Seamount, as well as tholeiitic and alkalic lavas from seamounts younger than 60 Ma (Ojin, Koko, Yuryaku and Daikakuji) lie within the field for Hawaiian lavas.
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CHEMICAL AND ISOTOPIC EVOLUTION OF HAWAIIAN MAGMATISM |
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Figure 8 compares the compositions of lavas from the Emperor Seamounts (85–43 Ma) with those of lavas from the Hawaiian Chain (<43 Ma), to illustrate the temporal chemical and isotopic changes in Hawaiian magmatism. Tholeiites from Meiji Seamount (85 Ma) have low concentrations of incompatible elements, and depleted trace element and Sr isotope ratios, compared with young Hawaiian Islands tholeiites. At 81 Ma, when Detroit Seamount was built, Hawaiian magmatism was even more depleted. Incompatible trace element compositions and Sr and Nd isotope compositions of Detroit tholeiites are unlike those of all other Hawaiian lavas, and are similar to those of Pacific N-MORB. Tholeiitic and alkalic lavas from Emperor Seamounts younger than 65 Ma (Suiko, Nintoku, Ojin, Koko, Kimmei, Yuryaku and Daikakuji) have trace element compositions that lie within the range of young lavas from the Hawaiian Islands. However, between 81 and 42 Ma, there appears to have been a systematic increase in initial
Sr of both tholeiitic and alkalic lavas (Fig. 8). Between 42 Ma and the present, there has not been any systematic temporal variation in trace element or isotopic composition of Hawaiian magmatism.
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The isotope variation within lavas from the Hawaiian Islands has often been explained in terms of mixing of least three end-member components (Staudigel et al., 1984; Stille et al., 1986; Roden et al., 1994; Eiler et al., 1996). However, recent high-precision Pb isotope data indicate that, in detail, many more than three end-members would be required (Abouchami et al., 2000; Eisele et al., in preparation). Our data show that lavas erupted between 56 and 43 Ma had similar Sr–Nd–Pb isotope compositions to lavas from the Hawaiian Islands (although none of the Emperor Seamount lavas analysed in this study has the high Sr and low Nd ratios that characterize young tholeiites from Lanai and Koolau). The same sources may therefore have contributed to Hawaiian magmatism during this period. Tholeiitic lavas from Suiko Seamount have some of the highest 206Pb/204Pb ratios of all Hawaiian lavas (Fig. 7). It has been suggested that the high 206Pb/204Pb (‘Kea’) component is derived from the underlying Pacific lithosphere (Tatsumoto, 1978; Stille et al., 1986). However, Abouchami et al. (2000) and Eisele et al. (in preparation) have shown that young Kea-type lavas have different Pb isotopic compositions from young Pacific MORB. Our Pb isotope data suggest further that the Kea component is unlikely to be aged Pacific lithosphere, as suggested by Tatsumoto (1978), because Suiko Seamount was built on oceanic lithosphere that was only 40 Myr old (compared with 90–100 Myr old beneath Mauna Kea). Thus, the ‘Kea’ component is present in both Suiko and Mauna Kea lavas, and appears to be unrelated to the age of the underlying lithosphere. Meiji lavas lie on an extension of the Hawaiian Islands post-shield lava array, but the Detroit lavas are displaced to lower Sr and higher Nd for a given 206Pb/204Pb ratio, compared with young Hawaiian lavas (Fig. 7). This suggests that between 85 and 81 Ma at least one additional source, having low 206Pb/204Pb and 87Sr/86Sr but high 143Nd/144Nd ratios, contributed to Hawaiian tholeiitic magmatism.
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A DEPLETED UPPER-MANTLE SOURCE IN CRETACEOUS HAWAIIAN LAVAS? |
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If the depleted compositions of the oldest Emperor lavas are due to mixing of Hawaiian plume material with the upper-mantle source of MORB, either by entrainment of Pacific upper mantle into the Cretaceous Hawaiian plume, or by plume–spreading ridge interaction (see below), then depleted Pacific-type upper mantle will be one component in the source of these lavas.
The Detroit tholeiites have incompatible element compositions that are broadly similar to those of young Pacific MORB from the northern EPR (Figs 3 and 4). If the Detroit tholeiites are interpreted as melts of such a mixed source, then their Sr isotope compositions (and incompatible trace element ratios) imply that the source is dominated (90%) by the depleted upper-mantle component, even if a relatively low Sr value is assumed for the enriched end-member (Fig. 9a). However, the isotopic compositions of the Meiji and Detroit tholeiites are not entirely consistent with such mixing. Sr and Nd isotope ratios of the Detroit tholeiites overlap with values for Pacific MORB, but the former have higher Sr for a given Nd, and lie on an extension of the Hawaiian Islands array (Fig. 5). Although sample powders for Sr isotope analysis were strongly acid leached, it is possible that apparent differences in Sr values for Detroit tholeiites and Pacific MORB are the result of alteration. However, combined Hf–Nd isotope variations in Detroit and Meiji lavas also appear to be inconsistent with mixing between Hawaiian plume mantle and depleted Pacific upper mantle (Kempton & Barry, 2001), and these elements are much less susceptible to seawater alteration. Compared with East Pacific Rise MORB (excepting the highly depleted lavas from the Garrett Transform; Wendt et al., 1999), the Detroit lavas have lower 207Pb/204Pb and slightly higher 208Pb/204Pb for a given 206Pb/204Pb. It should be noted that the measured 207Pb/204Pb ratios of the Detroit lavas (15·43–15·46) are lower than most Pacific MORB measured using the triple-spike technique (15·47–15·58), which indicates that uncertainty in the age correction is not responsible for the difference in 207Pb/204Pb between Detroit lavas and young Pacific MORB. Two samples of Mesozoic Pacific MORB from DSDP Site 307 (Janney & Castillo, 1997), which, like the oceanic crust underlying the ESC, was emplaced at the Pacific–Farallon spreading axis, also have 207Pb/204Pb ratios that are higher than those of the Detroit lavas (Fig. 6). On the other hand, Mesozoic Pacific MORB from DSDP Sites 303 and 304 (Pacific–Izanagi spreading centre) have Pb isotope compositions that overlap with those of the Detroit lavas (Fig. 6).
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In summary, the differences in isotopic composition between the depleted Detroit tholeiites and lavas from both active and extinct Pacific spreading centres are not readily explained by mixing between Hawaiian plume mantle and depleted Pacific upper mantle. We suggest that the depleted mantle component that contributes to Detroit and Meiji lavas may be intrinsic to the Hawaiian plume. A depleted plume component has also been identified in intra-plate lavas from the Galapagos (Hoernle et al., 2000) and from Iceland (Thirlwall, 1995; Fitton et al., 1997; Kempton et al., 2000), where it has been argued to represent recycled oceanic lithosphere (Chauvel & Hémond, 2000; Skovgaard et al., 2001). The non-primitive (high) Nb/Th ratios of the Detroit tholeiites (Fig. 4), together with their unradiogenic Sr and Pb isotopic compositions and high Nd values, are at least consistent with ancient, subducted lower oceanic lithosphere being the depleted component in the Hawaiian plume (Hofmann & White, 1982).
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ORIGIN OF TEMPORAL CHEMICAL AND ISOTOPIC VARIATIONS |
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Explanations for the temporal changes in Hawaiian magmatism must be able to account for (1) the highly depleted trace element and isotopic compositions of both tholeiitic and alkalic lavas from the oldest Emperor Seamounts, (2) the fact that such depleted lavas occur on the ESC but not the Hawaiian Chain, (3) the apparent systematic increase in initial
Sr values of both tholeiitic and alkalic lavas from 81 to 43 Ma, and (4) the observation that the incompatible element depleted Detroit tholeiites have different isotope compositions from those of most Pacific MORB.
Changes in mantle plume composition
A possible explanation for the unusual compositions of the oldest Emperor Seamount lavas is that the composition of the mantle ascending in the Hawaiian plume has changed over time. The total volume of magma erupted between 85 and 42 Ma was 5 x 105 km3 (Bargar & Jackson, 1974). If Hawaiian lavas represent 7% melting (Watson & McKenzie, 1991), then the volume of mantle that has been processed through the plume during this period is of the order of 3 x 106 km3. Plume mantle that was heterogeneous on length-scales of 100–1000 km could therefore account for variations in the chemistry of erupted lavas over time scales of 10–100 Ma (Janney & Castillo, 1999). On the other hand, tholeiitic lavas as depleted as those from Detroit Seamount are rare on other intra-plate seamount chains, and have not been found on the Hawaiian Chain. Nevertheless, it is difficult to rule out a change in source composition as an explanation for the temporal variations in Hawaiian magmatism.
Class et al. (1993) have argued that changes in the isotope compositions of lavas along the Ninetyeast Ridge (Indian Ocean) reflect radioactive decay within the mantle source of the lavas. However, this process cannot account for the observed variations of 87Sr/86Sr along the Hawaiian–Emperor Chain, because there is not a simple progression to higher Sr with decreasing age (Fig. 8). Furthermore, an unrealistically high Rb/Sr ratio (0·213) would be required for the Sr value of the mantle source to evolve from -26 to -15 between 81 and 43 Ma (Fig. 8). In addition, radioactive decay in the source of the lavas cannot account for the higher (more radiogenic) Nd ratios of the Detroit lavas compared with those of younger Emperor lavas (Fig. 8), nor the relatively depleted incompatible trace element compositions of the Meiji and Detroit lavas.
Plume–spreading ridge interaction
The Hawaiian Islands and the Hawaiian Seamounts are situated on oceanic lithosphere that was 80–100 Myr old at the time of intra-plate magmatism (Caplan-Auerbach et al., 2000) (Fig. 10). The difference in age between the seamount and the underlying crust decreases northwards along the Emperor Seamount Chain, from 80 Ma at Daikakuji Seamount, to 40 Ma at Suiko Seamount (Fig. 10). The northern Emperor Seamounts were constructed on young oceanic crust close to a former spreading centre between the Pacific and Kula, or Farallon Plates (Rea & Dixon, 1983; Lonsdale, 1988a; Mammerickx & Sharman, 1988). The age of the ocean floor beneath the northernmost Emperor Seamounts is not well constrained, because it was formed during the Cretaceous Quiet Period and therefore lacks magnetic lineations. However, the absence of a flexural moat close to the Obruchev Rise, the relatively low gravity signal around Meiji (Sandwell & Smith, 1997) and the low heights of Detroit and Meiji guyots (Caplan-Auerbach et al., 2000), suggest that the oldest Emperor Seamounts were formed on thin lithosphere that was <20 Myr old at that time.
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Lanphere et al. (1980) have proposed that variations in the distance to a former spreading centre could explain the isotopic changes along the Emperor Seamount Chain, and Keller et al. (2000) argued that such changes resulted from plume–ridge interaction. Plumes are known to influence the composition of lavas erupted at nearby spreading ridges (Schilling et al., 1985; Hanan et al., 1986). Conversely, hotspots located close to active spreading ridges often erupt lavas with relatively depleted compositions (Hémond et al., 1993; White et al., 1993; Haase et al., 1996; Hékinian et al., 1999). However, the physical process of plume–ridge interaction is not well understood. Two possible mechanisms, which have been used to explain the depleted compositions of lavas from Easter Island and its vicinity, are formation within a sublithospheric channel between the plume and the nearby ridge axis (Kingsley & Schilling, 1998), and flow of mantle material from the spreading ridge towards the plume (Haase et al., 1996).
The ages of individual seamounts of the Emperor Chain increase progressively from south to north, which indicates that the oldest seamounts were not formed in a plume channel. Bi-directional flow and mixing of material between the plume and the ridge axis is an unlikely explanation for the compositions of the Meiji and Detroit tholeiites, because the highly depleted trace element and isotopic compositions of the latter (Fig. 9) would require the flow of material from ridge to plume to dominate the flow from plume to ridge.
Keller et al. (2000) suggested that beneath young, thin oceanic lithosphere, the melting column extends to shallower depths, and the ratio of depleted asthenosphere to enriched plume mantle that is melted may be larger. Alternatively, increased entrainment of depleted upper-mantle material into a plume may occur when the plume is close to a spreading centre (Keller et al., 2000). However, neither of these mechanisms can explain satisfactorily the isotopic compositions of the oldest Emperor lavas. As discussed previously, assuming that the North Pacific mantle at 80 Ma was similar in composition to that sampled along the length of the EPR (and taking into account the effects of radioactive decay), the combined Sr–Nd and Nd–Hf isotopic compositions of Detroit and Meiji lavas appear to be inconsistent with mixing between Pacific depleted upper mantle and Hawaiian plume mantle. Detroit lavas also have different Pb isotope compositions from those of both young lavas from the EPR and Mesozoic MORB from the Pacific–Farallon spreading centre. Moreover, it is unlikely that plume–ridge interaction of any form could explain the systematic increase in Sr that occurred from 81 to 43 Ma (Fig. 8; Lanphere et al., 1980), because the age difference between the youngest Emperor Seamounts and the underlying oceanic lithosphere is about 80 Ma (Fig. 10). This age difference would correspond to a plume–ridge distance of over 3000 km at the time the southern Emperor Seamounts were constructed. The Suiko tholeiites have lower Sr than young Hawaiian tholeiites (Fig. 5), yet Suiko Seamount was built on 40 Myr old lithosphere. Plume–ridge interaction has not previously been proposed for distances of more than 1700 km (Schilling et al., 1985; Schilling, 1991), and it is therefore unlikely that a ridge could influence the chemistry of hotspot lavas over such a distance.
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LITHOSPHERE THICKNESS AS A CONTROL ON MANTLE UPWELLING AND MELTING |
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Intra-plate lavas erupted onto younger, thinner lithosphere are produced by larger mean degrees of melting, at shallower average depth, than melts produced beneath thicker lithosphere. This is because the overlying lithosphere acts as a ‘lid’ that restricts the upper boundary of the melting column during decompression melting (Ellam, 1992; Haase, 1996). Both the degree and the depth of melting influence the chemistry of intra-plate magmas. For the same MgO, the Detroit and Meiji tholeiites have lower Fe contents than other Emperor Seamount tholeiites (Fig. 2), consistent with relatively high degrees of melting at low pressure beneath a thin lithosphere (Jaques & Green, 1980).
Variations in the depth of melting may also influence the trace element chemistry of intra-plate lavas, according to how much of the melting occurs within the stability field of garnet (Ellam, 1992; Haase, 1996). The Detroit tholeiites have low La/Yb and high Lu/Hf ratios compared with other Hawaiian–Emperor tholeiites and most EPR MORB (Figs 4 and 9). The trace element compositions of the Detroit tholeiites indicate that they are the product of relatively high degrees of mantle melting, and that much of the melt was generated at low pressure, within the stability field of spinel (Fig. 9). Larger mean degrees of melting beneath thin oceanic lithosphere may to some extent explain the low incompatible element concentrations of the Detroit and Meiji lavas. On the other hand, there is no indication from the volumes of Hawaiian–Emperor volcanoes that the average degree of mantle melting was greater before 60 Ma (Fig. 10).
Clearly, variable melting of a homogeneous source cannot explain the variations in highly incompatible trace element and isotope ratios in Emperor Seamount lavas. We suggest instead that the temporal variations in Emperor lava composition may be the result of variable degrees of disequilibium melting of heterogeneous plume mantle as a result of variations in the thickness of the overlying lithosphere (Fig. 11). Several workers have suggested that the mantle consists of low melting point, incompatible element enriched heterogeneities embedded in a more depleted, refractory matrix (Sun & Hanson, 1975; Sleep, 1984; Allègre & Turcotte, 1986; Phipps Morgan & Morgan, 1999; Niu et al., 1999, 2001; Hoernle et al., 2000). Phipps Morgan (1999) has argued that disequilibrium melting of a heterogeneous mantle may account for some of the isotopic heterogeneity observed in the lavas from individual oceanic islands, and the fact that these often define tube-like arrays in three-dimensional isotope space. With progressive melting of such a heterogeneous mantle, as a result of an increasingly thin lithosphere, the melts produced would have increasingly lower incompatible trace element contents, lower ratios of more- to less-incompatible elements, lower 87Sr/86Sr, and higher 143Nd/144Nd (Phipps Morgan, 1999). This hypothesis predicts variations in basalt chemistry with lithospheric thickness that are qualitatively similar to those observed along the ESC. In detail, the melt extraction trajectories created during this process are sensitive to the compositions and ease of melting of the various source components, although it is interesting that the Detroit lavas plot close to the end of the melting trajectory estimated for Hawaiian plume mantle by Phipps Morgan (1999). Phipps Morgan & Morgan (1999) suggested that MORB are the result of melting residual mantle created by melting beneath oceanic islands but, as discussed above, the Detroit tholeiites have different compositions from those of MORB. We suggest that the depleted compositions of the Detroit lavas are the result of melting a relatively refractory component contained within the ascending plume mantle (Fig. 11). This depleted component does not contribute to younger Hawaiian lavas, which were formed by lesser degrees of melting beneath thicker lithosphere. This could explain why the Pb isotopic compositions of the Detroit lavas are unlike those of Pacific MORB. Interestingly, a refractory plume component, which is chemically and isotopically depleted yet distinct from the upper-mantle source of MORB, has been identified in lavas from the Galapagos (Hoernle et al., 2000) and Iceland (Skovgaard et al., 2001), both of which are located on young, thin lithosphere.
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Geophysical measurements have shown that the thickness of the oceanic lithosphere increases as a function of age until the lithosphere is about 70 Myr old, after which time the thickness remains approximately constant (Parsons & Sclater, 1977). This could explain why systematic chemical and isotopic variations are observed along the Emperor Seamount Chain (situated on oceanic lithosphere that was <20–70 Myr old at the time of seamount magmatism), but not along the Hawaiian Seamount Chain (where the crust was 70–100 Myr old at the time of seamount magmatism). Variable melting of a heterogeneous mantle can also explain the increase in 87Sr/86Sr from 81 to 43 Ma, when the Hawaiian plume was situated up to 3000 km from the nearest spreading axis.
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EVIDENCE FROM OTHER SEAMOUNT CHAINS |
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If lithospheric thickness is an important control on the trace element and isotope composition of intra-plate lavas, then geochemical variations should also occur along other seamount chains that were built upon lithosphere of variable age. Unfortunately, there have not been many detailed studies of the long-term (
100 Myr) geochemical evolution of other seamount chains, and in most cases the variation in age of the underlying lithosphere is even more poorly known.
Volcanic rocks from the Ninetyeast Ridge (90 Ma to 38 Ma) and Kerguelen Archipelago (45 Ma to present) in the Indian Ocean record changes in the chemistry of magmatism above the Kerguelen plume. Much of the Ninetyeast Ridge was formed close to the Southeast Indian Ridge, but the ridge axis passed over the hotspot at 40 Ma, and since that time the hotspot has become progressively distant from the ridge (Royer & Sandwell, 1989). Lavas from the Ninetyeast Ridge have relatively depleted trace element and isotopic compositions and low La/Yb ratios (Frey & Weis, 1995), which may reflect their formation beneath relatively thin lithosphere close to the ridge axis. Younger lavas from the Kerguelen Archipelago, which were erupted onto older, thicker lithosphere, are generally more alkalic and have more enriched trace element and isotope compositions (Gautier et al., 1990; Frey et al., 2000).
The Easter Seamount Chain in the eastern Pacific is 3000 km long, and the age of magmatism varies from 0 Ma at Easter Island and Salas y Gomes, to 30 Ma at the eastern end of the chain (Cheng et al., 1999). The age of the underlying crust is not well known, but increases from 1–4 Ma at Easter Island to 8–10 Ma at Salas y Gomes. Sr, Nd and Pb isotope compositions of the lavas from seamounts at the eastern end of the chain, which were formed on older, thicker lithosphere, tend to be more enriched (Cheng et al., 1999). The age of the underlying lithosphere, and even the exact location of the plume, is not well known.
The Louisville Seamount Chain, in the SW Pacific, ranges in age from 66 Ma at the western end, to 0 Ma at the eastern end. Since 33 Ma, the Louisville plume has been situated on oceanic crust that was 45–52 Myr old at the time of magmatism, whereas estimates of the age of the crust underlying the older (33–66 Ma) seamounts at their time of formation range from 50 to 80 Ma (Lonsdale, 1988b; Watts et al., 1988; Lyons et al., 2000). The age-corrected isotopic compositions of lavas from the Louisville Seamounts show little variation (Cheng et al., 1987).
The isotope compositions of lavas erupted along the Pukapuka Ridge in the South Pacific vary with the age of the underlying sea floor, which was between 0 and 25 Myr old, at the time of intra-plate volcanism (Janney et al., 2000). Lavas erupted onto younger sea floor were formed by larger degrees of melting, at lower pressures, and have more depleted compositions than lavas erupted onto older lithosphere. The Pukapuka Ridge was not formed by hotspot activity, but by lithospheric extension (Sandwell et al., 1995), and thus plume–ridge interaction cannot be responsible for the isotopic variations. Instead, these might be the result of variations in distance from the enriched South Pacific Superswell (Janney et al., 2000). Alternatively, they could be the result of differences in the degree of melting of heterogeneous mantle beneath lithosphere of variable thickness.
Intra-plate lavas associated with the Canary hotspot in the NE Atlantic were erupted upon sea floor that was between 90 and 175 Myr old, and show little variation in isotopic composition (Geldmacher et al., 2001). In contrast, systematic temporal isotopic variations occur along the nearby Madeira seamount chain, which was built upon oceanic crust that was 60 to 130 Myr old, but these isotopic variations appear to be related to contamination by continental lithosphere (Geldmacher & Hoernle, 2000).
In summary, the geochemical variations along other seamount chains are consistent with lithospheric thickness being an important control on the extent of melting of plume mantle, and hence the compositions of the lavas produced. However, detailed geochemical and geochronological studies of other seamount chains, together with more precise constraints on the age of the underlying sea floor, are required, before the influence of lithosphere thickness on the compositions of intra-plate lavas can be fully quantified. Our hypothesis predicts that intra-plate lava chemistry will vary with the thickness of the lithosphere (rather than with plume–ridge distance, as required by plume–ridge interaction). This effect can occur only along those seamount chains where the age difference between the seamounts and the underlying oceanic lithosphere is less than 70 Myr. This implies that systematic geochemical and isotopic variations may be found along seamount chains that were constructed several thousand kilometres from the nearest spreading axis.
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CONCLUSIONS |
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- New major and trace element, and Sr, Nd and Pb isotopic analyses of 44 samples of volcanic rock from nine seamounts along the Hawaiian–Emperor Seamount Chain, provide a 43 Myr record (from 85 to 42 Ma) of the geochemistry of Hawaiian magmatism, and show that there were large temporal variations in trace element chemistry and isotopic composition of Hawaiian magmatism over this period.
- Lavas from the oldest seamounts sampled (Meiji and Detroit) have depleted incompatible trace element and Sr–Nd isotopic compositions, compared with those of young lavas from the Hawaiian Islands. Tholeiitic basalts from Detroit Seamount have incompatible trace element ratios, and Sr–Nd isotope compositions similar to those of modern Pacific mid-ocean ridge basalts, but higher 87Sr/86Sr for a given 143Nd/144Nd. Trace element compositions of Emperor Seamount lavas younger than 62 Ma are similar to those of young Hawaiian lavas, but 87Sr/86Sr ratios lie at the depleted end of the Hawaiian Islands array. From 81 to 42 Ma, there was a systematic increase in 87Sr/86Sr of both tholeiitic and alkalic lavas erupted above the Hawaiian mantle plume.
- Age-corrected Pb isotope compositions of most Emperor lavas lie within the field of young lavas from the Hawaiian Islands. The incompatible element depleted Detroit tholeiites have lower 207Pb/204Pb for a given 206Pb/204Pb, compared with MORB erupted along the East Pacific Rise and at the extinct Pacific–Farallon spreading centre.
- The trace element and isotope compositions of these lavas vary with the age of the underlying oceanic Pacific lithosphere at the time of seamount magmatism. The oldest Emperor Seamount lavas, which were erupted onto relatively young lithosphere close to a former spreading centre, have relatively depleted incompatible trace element and isotope compositions. In contrast, younger Hawaiian–Emperor lavas were erupted onto older lithosphere, and have more enriched compositions.
- The isotope compositions of the oldest Emperor lavas, together with the fact that the youngest Emperor Seamounts were formed >3000 km from the closest spreading axis, suggest that plume–ridge interaction was not responsible for the observed chemical and isotopic variations.
- Major and trace element compositions of Meiji and Detroit Seamount tholeiites indicate that they were formed by relatively large degrees of mantle melting, at lower pressures, compared with younger ESC tholeiitic lavas.
- We suggest that variable degrees of melting of a heterogeneous mantle may explain the temporal compositional changes in Hawaiian magmatism. When the Hawaiian plume was situated beneath young, thin lithosphere, melting was more extensive and extended to shallower depths. The melts produced had relatively depleted trace element and isotope compositions, because incompatible element depleted, more refractory source materials contributed more to the melting. In contrast, lavas from the younger seamounts, which were built on older, thicker crust, are more enriched because they were produced by smaller degrees of melting, and so the compositions of the melts were dominated by the contribution from incompatible-element-rich, easily melted mantle materials.
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ACKNOWLEDGEMENTS |
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This research used samples provided by the Ocean Drilling Program (ODP), which is sponsored by the US National Science Foundation and participating countries under management of Joint Oceanographic Institutions Inc. Warren Smith kindly provided samples from the Geological Collections of the Scripps Institution of Oceanography. We thank F. Frey, M. Garcia, an anonymous reviewer, and the editor M. Thirlwall for constructive comments, which improved the manuscript. M.R. thanks Y. Niu, J. Lassiter and I. Vlastélic for discussions and ideas, A. Greig for the trace element analyses, and S. Bederke-Raczek and H. Feldmann for technical assistance. This research was supported by the DFG and MPI.
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FOOTNOTES |
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*Corresponding author. Present address: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ. Tel: + 44 (0)117 954 5235. Fax: +44 (0)117 925 3385. E-mail: m.regelous{at}bris.ac.uk
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