Journal of Petrology Advance Access originally published online on January 21, 2005
Journal of Petrology 2005 46(4):829-858; doi:10.1093/petrology/egi002
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Evidence for a Widespread Tethyan Upper Mantle with Indian-Ocean-Type Isotopic Characteristics
1 SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII, HONOLULU, HI 96822, USA
2 CHINA UNIVERSITY OF GEOSCIENCES, BEIJING 100083, P. R. CHINA
3 GEOLOGY DEPARTMENT, GEORGIA STATE UNIVERSITY, ATLANTA, GA 30303, USA
4 INSTITUTE OF MINERALOGY, UNIVERSITY OF FERRARA, 44100 FERRARA, ITALY
5 CENTRE FOR ORE DEPOSIT STUDIES, UNIVERSITY OF TASMANIA, HOBART, TAS. 7001, AUSTRALIA
RECEIVED APRIL 21, 2004; ACCEPTED NOVEMBER 23, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODS AND SAMPLESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The mantle sources of Tethyan basalts and gabbros from Iran, Tibet, the eastern Himalayas, the seafloor off Australia, and possibly Albania were isotopically similar to those of present-day Indian Ocean ridges and hotspots. Alteration-resistant incompatible element compositions of many samples resemble those of ocean-ridge basalts, although ocean-island-like compositions are also present. Indian-Ocean-type mantle was widespread beneath the Neotethys in the Jurassic and Early Cretaceous, and present beneath at least parts of the Paleotethys as long ago as the Early Carboniferous. The mantle beneath the Indian Ocean today thus may be largely ‘inherited’ Tethyan mantle. Although some of the Tethyan rocks may have formed in intra-oceanic back-arcs or fore-arcs, contamination of the asthenosphere by material subducted shortly before magmatism cannot be a general explanation for their Indian-Ocean-ridge-like low-206Pb/204Pb signatures. Supply of low-206Pb/204Pb material to the asthenosphere via plumes is not supported by either present-day Indian Ocean hotspots or the ocean-island-like Tethyan rocks. Old continental lower crust or lithospheric mantle, including accreted, little-dehydrated marine sedimentary material, provides a potential low-206Pb/204Pb reservoir only if sufficient amounts of such material can be introduced into the asthenosphere over time. Anciently subducted marine sediment is a possible low-206Pb/204Pb source only if the large increase of U/Pb that occurs during subduction-related dewatering is somehow avoided. Fluxing of low-U/Pb fluids directly into the asthenosphere during ancient dewatering and introduction of ancient pyroxenitic lower-crustal restite or basaltic lower-arc crust into the asthenosphere provide two other means of creating Tethyan–Indian Ocean mantle, but these mechanisms, too, have potentially significant problems.
KEY WORDS: Indian Ocean; mantle geochemical domains; ophiolites; Tethyan Ocean
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODS AND SAMPLESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The origin of the vast Indian Ocean mantle domain is unknown. This domain stretches from south of Australia along the Southeast Indian Ridge (Klein et al., 1988
; Pyle et al., 1992) to south of Africa on the Southwest Indian Ridge (Mahoney et al., 1992), northward along the Central Indian and Carlsberg ridges, and into the Red Sea (Volker et al., 1993). Basalts formed along these ridges are characterized by lower 206Pb/204Pb relative to 
Nd, 208Pb/204Pb, and 207Pb/204Pb than the great majority (>95%) of Pacific and North Atlantic mid-ocean ridge basalts (MORB), and tend to have comparatively high 87Sr/86Sr and low Nd (e.g. Subbarao & Hedge, 1973; Hedge et al., 1979; Dupre & Allègre, 1983; Hamelin et al., 1986; Michard et al., 1986; Price et al., 1986; Dosso et al., 1988; Mahoney et al., 1989, 2002; Rekhkämper & Hofmann, 1997). Ocean-island basalts (OIB) in the Indian Ocean exhibit qualitatively similar Pb–Nd–Sr isotopic characteristics (e.g. Hart, 1988) but have higher 206Pb/204Pb than most Indian MORB. Isotopically Indian-MORB-like mantle also feeds part of the Southern Mid-Atlantic Ridge (e.g. Hanan et al., 1986; Douglass et al., 1999; Kamenetsky et al., 2001) and broadly similar isotopic signatures are characteristic of several marginal basins and island arcs in the Western Pacific (e.g. Mukasa et al., 1987; Stern et al., 1993; Crawford et al., 1995; Hickey-Vargas et al., 1995; Castillo, 1996; Spadea et al., 1996; Hickey-Vargas, 1998).
Information on the history of this mantle domain is provided by basalts from drill and dredge sites on old Indian Ocean seafloor (Lanyon, 1995; Pyle et al., 1995; Weis & Frey, 1996; Mahoney et al., 1998). Isotopic data for such sites show that mantle similar to that beneath the modern Indian Ocean was present, at least in places, as long ago as 140 Ma, the age of the oldest true Indian Ocean crust yet sampled. Additional information on the history of this domain may be provided by remnants of Tethyan seafloor, because the now-vanished Tethyan Ocean covered much of the same geographical region currently occupied by the Indian Ocean, which began opening at around 160 Ma (e.g. Lawver & Gahagan, 1993; Lawver et al., 2000; Stampfli & Borel, 2002). Although Tethyan seafloor has been largely subducted, a small portion remains intact off northwestern Australia, and fragments of both Neotethyan (primarily Jurassic and Cretaceous) and Paleotethyan (primarily pre-Triassic) lithosphere are preserved in ophiolites and mélanges along the Tethyan suture zones of Eurasia (Fig. 1).
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Reconnaissance study of Early Cretaceous (
120 Ma) Neotethyan basalts in the Xigaze ophiolite of Tibet has revealed strongly Indian-MORB-type Nd–Pb isotopic characteristics (Mahoney et al., 1998). The Nd–Pb isotopic signatures of Neotethyan rocks of the middle Cretaceous Samail ophiolite of Oman (Chen & Pallister, 1981; McCulloch et al., 1981; Benoit, 1997) also point to an isotopically Indian-Ocean-type mantle source. However, Late Jurassic MORB-type basalts and gabbros of the Masirah ophiolite of Oman, which represents 150 Ma oceanic lithosphere formed near the boundary between the southern Neotethys and the nascent Indian Ocean, lack clear Indian-MORB-type characteristics (Mahoney et al., 1998). Data for Neotethyan MORB of similar age at Deep Sea Drilling Project (DSDP) Site 261 off northwestern Australia indicated that those rocks also lack an Indian-MORB-type signature (Weis & Frey, 1996). On the basis of these results, Mahoney et al. (1998) suggested that the Indian Ocean mantle domain may pre-date the 160 Ma opening of the Indian Ocean by only a few tens of millions of years, and speculated that asthenosphere from beneath the young Indian Ocean may have flowed into the Tethyan region in the Early Cretaceous. An even younger origin for the Indian Ocean mantle domain was proposed by Weis & Frey (1996), who linked it to the appearance of the Kerguelen hotspot, now estimated at 120 Ma (Duncan, 2002). In contrast, the possibility of a much older origin for this mantle domain is suggested by Nd and Pb isotopic data for the Paleotethyan Mian–Lue northern ophiolite of central China and the Jinsha River and Shuanggou ophiolites of southwestern China, which display age-corrected values in or rather close to the range expected for Indian MORB mantle at 350 Ma (Xu et al., 2002; Xu & Castillo, 2004). However, indications of disturbance to the Pb isotope system, such as high (207Pb/204Pb)t values (where the subscript t indicates age-corrected) and anomalously high Pb concentrations, were present in some of the samples.
Here, we report age-corrected Nd–Pb–Sr isotope data for 100–350 Ma basalt, diabase, and gabbro from widely separated Tethyan locations in Tibet, Iran, Albania, the eastern Himalayan syntaxis, and the seafloor off NW Australia (Fig. 1). Our results indicate that isotopically Indian-Ocean-type mantle underlay much of the Neotethys in the Jurassic and Early Cretaceous, and support its presence below at least part of the Paleotethys as long ago as the Early Carboniferous.
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METHODS AND SAMPLES |
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TOPABSTRACTINTRODUCTIONMETHODS AND SAMPLESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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All of the rocks have been affected by seawater-mediated alteration, although to variable extents and under a range of temperature and pressure, from clay-forming conditions to upper greenschist facies (Girardeau et al., 1985
; Beccaluva et al., 1994; Crawford & von Rad, 1994; Mo et al., 1994, 1998; Hassanipak et al., 1996; Bortolotti et al., 2002; Shen et al., 2002a, 2002b; Zhou et al., 2002; Ghosh & Ray, 2003; Ghazi et al., 2004). In this respect the samples are very similar to the Tethyan rocks studied by Mahoney et al. (1998), and we employed identical sample preparation and analytical procedures. A critical first step is careful sample selection, followed by hand-picking of small chips under a stereomicroscope (rejecting samples or parts of sample with visible phosphate, ferromanganese oxide, sulfide, or carbonate, areas with veins, vesicle fillings, etc.). For age-correcting isotope ratios, concentrations of Sm, Nd, U, Th, Pb, Rb, and Sr were measured by isotope dilution on the same dissolution of hand-picked, acid-cleaned (5 min in 0·1 M HF + HNO3 in an ultrasonic bath) rock chips and, in some cases, strongly acid-leached powder, as that analyzed for Nd, Sr, and Pb isotopes (Tables 1 and 2). Because the preparation procedure tends to modify a sample's mineralogical composition relative to that of the bulk rock, the isotope-dilution concentrations generally cannot be taken to represent whole-rock values, particularly for coarser-grained or more altered samples. Whole-rock incompatible trace-element concentrations were determined for a subset of previously unanalyzed samples by inductively coupled plasma–mass spectrometry (Table 3). For major element and additional trace element data, we refer the reader to the references cited in the relevant sections below. A set of published major and trace element data is available for downloading from the Journal of Petrology website at http://www.petrology.oupjournals.org (Electronic Appendices 1 and 2).
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Because of alteration, age-corrected Sr isotope ratios generally cannot be assumed to represent magmatic values. Similarly, concentrations of Rb, U, and Ba have been modified substantially in many cases, in contrast to alteration-resistant elements such as Nb, Ta, Zr, Hf, Th, and the lanthanide rare earths. Also, although age-corrected Nd isotope ratios are not affected appreciably by the types and amounts of alteration present, the same cannot necessarily be assumed of age-corrected Pb isotope values. Moderate amounts of seawater-mediated alteration tend not to directly affect Pb isotope ratios in basalt significantly, because of the very low concentration of Pb in seawater; however, uptake of U from seawater-derived fluids and Pb (and rarely, Th) mobility within a rock during alteration can cause large changes in parent–daughter ratios [see Mahoney et al. (1998)
and references therein]. Such alteration typically occurs within the first few million years after rock formation (e.g. Staudigel et al., 1995), before Pb isotope ratios have changed significantly by radiogenic ingrowth; thus, if an altered rock behaves thereafter as a closed system to U, Th, and Pb, its age-corrected Pb isotope ratios will be close to its original, magmatic values. However, if subsequent modification of parent–daughter ratios occurs long afterward—for example, during a later alteration event(s) or even during leaching—then the age-adjusted Pb isotope ratios of old rocks will be erroneously high or low. The principal effect is on age-corrected (206Pb/204Pb)t, followed by (208Pb/204Pb)t; although the half-life of 235U (the parent of 207Pb) is much shorter than that of 238U (the parent of 206Pb), the effect on (207Pb/204Pb)t of a change in parent–daughter ratio is relatively small for Phanerozoic rocks because of the very low abundance of 235U. Of course, alteration at any stage that involves fluids containing Pb derived from marine sediment or continental crust can cause significant changes to all three Pb isotope ratios.
Fortunately, several criteria can be used to evaluate the degree to which Pb isotope systematics have been disturbed (see Mahoney et al., 1998): (1) for MORB- and OIB-type rocks, age-corrected Pb isotope data are suspect if they fall significantly outside the age-adjusted fields for modern MORB- and OIB-type mantle (i.e. adjusted for changes resulting from radiogenic ingrowth in the mantle); (2) age-corrected values for a group of related samples should usually have a smaller range than the present-day range; (3) acid-leached and unleached splits should yield similar age-corrected Pb isotope ratios; (4) substantial variation in (206Pb/204Pb)t at near-constant Nd(t) in a group of related samples (e.g. from a single ophiolite or drill site) is a sign of disturbance of the U–Pb system; (5) data for related samples that exhibit a large dispersion of present-day 208Pb/204Pb vs 232Th/204Pb and 206Pb/204Pb vs 238U/204Pb should roughly parallel reference isochrons of the correct age.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODS AND SAMPLESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Indus–Yarlung suture zone, Tibet
We collected Early Jurassic to Early Cretaceous Neotethyan magmatic rocks in 1998 from outcrops along
1300 km of the Indus–Yarlung suture zone (Fig. 2). Radiolarians in cherts in depositional contact with pillow lavas indicate an age of 120–125 Ma for crust of the Xigaze massif around Xialu and Dazhuqu–Dazhuka (Ziabrev et al., 2003). For both sections, we sampled both the upper (pillow lavas) and lower (sills or gabbros) crustal levels. Basalts near Langceling may be an eastward extension of the same terrane, termed the Dazhuqu terrane by Aitchison et al. (2000). For gabbroic bodies at Luobusha, a 177 Ma Sm–Nd age has been determined (Zhou et al., 2002). The Dangqiong samples are from sheeted dikes in the northern Zhongba ophiolite. Chert in this ophiolite contains latest Triassic to earliest Jurassic radiolarians and calcareous microfossils (Y.-J. Wang, unpublished data, 2001); accordingly, we use an age of 200 Ma. Basalts and gabbros in the Bar area are in an ophiolitic mélange and lack good age control, but to the west of Bar a Late Jurassic to Early Cretaceous age has been determined for radiolarian chert atop pillow basalt at Dajiweng (X.-X. Mo et al., unpublished data, 2002); an age of 130 Ma is used here for samples from both areas. For most of the Tibetan samples, age-corrected Nd(t) values and Pb and Sr isotope ratios are not very sensitive to the exact age used because Sm/Nd ratios are not too far removed from the chondritic value and U/Pb, Th/Pb, and Rb/Sr ratios are generally low.
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Incompatible elements
Alteration effects are most evident in highly variable concentrations of Rb, Ba, and U relative to elements such as Nb, La, and Th (Fig. 3). Small peaks or troughs at Pb are also likely to be a result of Pb mobility during alteration (see Mahoney et al., 1998
). Three Langceling patterns stand out in having particularly large peaks at U, and two of these also have prominent Pb peaks (Fig. 3d); it is not clear whether these peaks are solely an alteration feature or not, as these basalts are not visibly more altered than some from other locations.
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Four basalts from Dajiweng and one from Bar display OIB- or enriched- (E-) MORB-like patterns, with pronounced light rare earth element enrichment and (Nb/La)n and (Ta/La)n >1 (where the subscript n indicates normalization to primitive mantle). Dajiweng gabbro DJB98-19 exhibits a flatter pattern similar to those of transitional (T-) MORB. However, most of our samples resemble normal (N-) MORB in their alteration-resistant elements. For example, (La/Sm)n = 0·3–0·8, (Nb/La)n = 0·4–1·0, and (Th/Nb)n = 0·2–0·8 in all of the Dangqiong dikes, the Xialu and Dazhuqu–Dazhuka basalts and gabbros, two of the Langceling basalts, and the Luobusha and Bar gabbros. In many of these rocks, particularly the gabbros, the more incompatible alteration-resistant elements, although within the range for N-MORB globally, tend to be lower than the N-MORB average (see average N-MORB and ‘D-MORB’ patterns in Fig. 3a).
The Dazhuqu terrane, which includes the Xigaze massif, is structurally and petrologically the best studied area we sampled. It has been suggested to represent lithosphere formed at a mid-ocean ridge (e.g. Pozzi et al., 1984; Girardeau et al., 1985; Pearce & Deng, 1988), an intra-oceanic fore-arc (Aitchison et al., 2000; Zhang & Zhou, 2001), or an intra-oceanic back-arc spreading center (e.g. Zhang & Zhou, 2001; Griselin, 2001). The characteristics of alteration-resistant incompatible elements in our crustal samples from Xialu and Dazhuqu–Dazhuka are consistent with an ocean-ridge origin; however, they are also compatible with a back-arc origin and do not rule out a fore-arc setting for the terrane as a whole.
The next best studied ophiolite in Tibet is the Luobusha ophiolite, which consists largely of peridotite interpreted to be residual MORB-type mantle, through portions of which a boninitic melt percolated after a plate-boundary reorganization placed it above a subduction zone (Zhou et al., 1996). Our Luobusha samples are from gabbro bodies within the peridotite. Their essentially N-MORB-like element patterns in Fig. 3 [and basaltic major element composition (Mo et al., 2005); see Electronic Appendix] suggest they represent magmas derived from the original MORB-source mantle rather than later boninitic melts.
Element patterns diagnostic of subduction-zone settings are rare in the sections we sampled. Among the alteration-resistant elements, a pronounced depletion of Nb and Ta relative to Th is typical of arc-related volcanic rocks and common in back-arc environments (e.g. Sinton et al., 2003), but rare in MORB. Only one sample, Dajiweng gabbro DJB98-18, shows a significant trough at Nb–Ta relative to Th, with (Th/Nb)n = 3·0 (Fig. 3e). Because of limited exposure, this rock's field relationship with the nearby OIB-like pillow basalts and T-MORB-like gabbro is unclear. Our only other sample with (Th/Nb)n >1 is Langceling basalt LC98-4, with a value of 1·1. In contrast, two Dazhuqu-terrane gabbros and two basalts with (Th/Nb)n significantly greater than unity (1·5–2·0) were reported by Xu & Castillo (2004); their samples also possess markedly higher Pb concentrations (0·9–2 ppm) than any of our broadly N-MORB-like Tibetan rocks (0·1–0·66 ppm). Argued to be most consistent with a back-arc affinity, their samples are from an outcrop to the west of Xialu and one SE of Dazhuka. Adjacent to the Dazhuqu terrane, true arc-type rocks are known to be prevalent in the Zedong terrane (Aitchison et al., 2000; McDermid et al., 2002), which, however, we did not sample.
Isotope ratios
Age-corrected Nd(t) values of the samples with broadly MORB-like incompatible element patterns cover only a small range between +8·2 and +8·9. (Unless stated otherwise we refer to values for leached splits when both leached and unleached splits were analyzed.) Despite variable alteration, their age-corrected Pb isotope ratios also fall within a small range; for samples from a single area the maximum variation in (206Pb/204Pb)t is only 0·15, and the total range for the MORB-like rocks as a group is only 0·45 (17·41–17·86). Minor differences are apparent from one location to another, but the small overall Nd and Pb isotopic range is remarkable, considering that the samples represent widely separated sections along 1190 km of the suture zone (i.e. from Bar to Luobusha; Fig. 2) and span 80 Myr of time. Previous data for six pillow lavas from Xialu (Mahoney et al., 1998) are in the same range as our results; so too are Göpel et al.'s (1984) (207Pb/204Pb)t–(206Pb/204Pb)t data for several Xigaze-massif basalts. Importantly, all samples of this group display a strongly Indian-MORB-like character in Fig. 4 (for clarity, the figure is divided into panels according to sample age).
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Indian-Ocean-type isotopic characteristics are also exhibited by the OIB-like Dajiweng and Bar rocks and the high-(Th/Nb)n Dajiweng gabbro from the two westernmost areas sampled. Data for the OIB-like rocks fall close to the age-adjusted field of source mantle for the modern Indian Ocean hotspot islands of Réunion, Mauritius, Crozet, and Amsterdam (Re–Cr–Am in Fig. 4). The high-(Th/Nb)n sample has slightly higher (206Pb/204Pb)t (17·92) and slightly lower
Nd(t) (+7·9) than the broadly MORB-like samples from more easterly sections. Xu & Castillo's (2004) high-(Th/Nb)n samples also have Nd–Pb and Pb–Pb isotope ratios close to or overlapping with those of our MORB-like rocks. In short, all of the Nd–Pb and Pb–Pb isotope data for samples from 1300 km of the Indus–Yarlung suture zone indicate a consistently Indian-Ocean-type character.
In Nd–Sr isotope space (Fig. 5), the data extend from within the MORB or Re–Cr–Am fields to higher (87Sr/86Sr)t. This variation is attributable to the variable but pervasive seawater-mediated alteration in these rocks, in which plagioclase (the main carrier of Sr) is invariably albitized, and olivine and once-glassy groundmass are replaced by secondary phases (e.g. Girardeau et al., 1985; Mo et al., 2005). We subjected three of the visibly most altered samples to multi-step leaching. Differences in Nd(t) (
0·2) and (206Pb/204Pb)t (0·02–0·06) between unleached and leached splits are negligible. Leaching reduced (87Sr/86Sr)t modestly for two samples, but by a large amount (0·70682 vs 0·70823) for OIB-like pillow lava DJB98-11. Such large decreases are commonly observed when alteration has not affected the main Sr-bearing phase(s) in a rock uniformly; in some cases, near-magmatic Sr isotope ratios have been recovered by leaching [e.g. Mahoney et al. (1998) and references therein]. However, in this instance the value of the leached split is still much greater than that of pillow lava DJB98-20 from the same section (0·70396). Moreover, the value of the unleached split of DJB98-11 is much higher than that of seawater itself around the time these lavas were erupted (0·7070–0·7073; e.g. Burke et al., 1982); thus, DJB98-11 must contain secondary Sr from another source besides seawater. We speculate that this rock was affected by later fluids carrying continentally derived Sr, perhaps during Tertiary uplift. In any case, the effect on Nd and Pb isotopes was rather small (compare values for DJB98-11 and -20 in Tables 1 and 2).
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Southern Iran
The Iranian rocks are from the Band-e-Zeyarat complex in the Makran prism, and the Neyriz ophiolite in the Zagros suture zone. Recent 40Ar–39Ar dating reveals an age around 143 Ma for the Band-e-Zeyarat complex (Ghazi et al., 2004
; see also Kananian et al., 2001), whereas major and trace element data indicate a generally T-MORB-like character (Hassanipak et al., 1996; Ghazi et al., 2004). For Neyriz, Babaie et al. (2003) reported 93 Ma 40Ar–39Ar ages. Mafic crustal rocks at Neyriz principally have MORB-like major and trace element compositions (Sarkarinejad, 1994), and are interpreted to represent lithosphere formed at two spreading segments offset by a transform fault (Nadimi, 2002). Few combined rare-earth and non-rare-earth trace element data are available, however, so we measured a suite of incompatible elements for three of our Neyriz basalts (Table 3). The alteration-resistant elements indeed illustrate a generally N-MORB-like character (Fig. 3f).
As with the Tibetan rocks, alteration is reflected in large (87Sr/86Sr)t variation accompanied by only limited variation in age-corrected Nd and Pb isotope ratios. Among the Band-e-Zeyarat samples, leached splits yielded systematically lower (87Sr/86Sr)t, by as much as 0·0006, indicating partial removal of the seawater-derived Sr overprint. The Nd(t) values range from +8·2 to +5·5; however, five of the eight Band-e-Zeyarat rocks have values between +7·2 and +6·2. The sample with the highest Nd(t) has the lowest (206Pb/204Pb)t (17·62), and its leached and unleached splits gave essentially identical age-corrected Pb and Nd isotope ratios. For the other seven samples, (206Pb/204Pb)t covers only a small range, from 17·98 to 18·11, and in Fig. 6a and b the samples define an array that roughly parallels a 143 Ma reference line. The results thus indicate that the age-corrected Pb and Nd isotope ratios are magmatic or near-magmatic values. These data reveal an overall Indian-MORB-type source (Fig. 4d–f).
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The Neyriz samples display even more coherent Indian-MORB-type isotopic characteristics (Fig. 4a–c). The value of
Nd(t) is nearly invariant (+7·8 to +8·2), and very little variation is present in age-corrected Pb isotope ratios, whereas present-day 206Pb/204Pb vs 238U/204Pb and 208Pb/204Pb vs 232Th/204Pb define a linear array that approximately parallels a 93 Ma reference line (Fig. 6c and d). The similar-age Samail ophiolite to the SE of Neyriz appears to have rather similar age-corrected Nd–Pb isotope characteristics, although Nd and Pb isotopes were measured on different samples (Chen & Pallister, 1981; McCulloch et al., 1981).
Albania
Albania hosts two Jurassic Tethyan ophiolites. Magmatic rocks of the western ophiolite are predominantly N-MORB-type in trace and major element and mineral composition (e.g. Beccaluva et al., 1994; Bortolotti et al., 2002). Boninite dikes and some later arc-type lavas are also present, however, and this ophiolite has been interpreted as a section of normal oceanic lithosphere that became incorporated into a fore-arc of the western Neotethys (e.g. Bortolotti et al., 2002). Radiolarians in chert atop pillow basalts have been dated as late Bajocian to early Callovian, 165 Ma (Marucci et al., 1994).
As with the broadly MORB-like Tibetan and Iranian rocks, the Albanian basalts show only a small range in Nd(t) (+7·1 to +7·8); (87Sr/86Sr)t again varies widely, and in one case, leaching lowered (87Sr/86Sr)t by 0·00132. Unlike the Tibetan and Iranian samples, the Pb isotope systematics of at least some of the Albanian rocks appear to have been disturbed significantly. The (206Pb/204Pb)t value varies from 17·77 to 18·51, which is a large range considering that this ophiolite covers a small area and that Nd(t) in the same samples varies by only 0·7. Also, the data do not form a trend subparallel to a 165 Ma reference line in Fig. 6e and f. Another indication of disturbance to the Pb isotope system is that one point in Fig. 4f falls slightly below or to the right of the age-adjusted Pacific–North Atlantic MORB-source field, where no MORB or OIB data are expected to lie. Not surprisingly, these rocks do not exhibit a consistently Pacific–North Atlantic- or Indian-MORB-type signature in Fig. 4d and e.
Are any of the age-corrected Pb isotope ratios representative of the original (magmatic) values? Samples AL-1 and RU-124 have (1) very low 238U/204Pb (1·94, 2·68) and 232Th/204Pb (5·35, 10·2), (2) by far the lowest present-day 206Pb/204Pb and 208Pb/204Pb, and (3) very similar age-corrected Pb isotope values. Thus, the age-corrected ratios of AL-1 and RU-124 are unlikely to be either badly under- or over-corrected and are likely to be the most representative of the original values in the Albanian suite. Ratios for sample RU-143, despite a large age correction, are close to those of AL-1 and RU-124. Moreover, for the one sample for which we measured Pb isotopes in a leached aliquant, AL-2, the age-corrected values are close to those of these samples. Taken at face value, the data for AL-1, RU-124, and RU-143 are Indian-MORB-like in Fig. 4d but marginally Pacific–North Atlantic-like in Fig. 4e. Although these characteristics could mean that the far-western Jurassic Tethys was isotopically transitional, the ambiguous nature of the Pb isotope data precludes any conclusive statement. Without reference to Pb isotopes, we can say only that the relatively low Nd(t) of the Albanian rocks is more typical of Indian N-MORB than of Pacific or North Atlantic N-MORB, and thus may weakly favor an Indian-MORB-like source.
Northwestern Australian margin
A continental block (Argo Land) rifted in the Jurassic from the portion of Gondwana that is now northwestern Australia (e.g. Audley-Charles et al., 1988). Breakup was followed, slightly before 155 Ma, by seafloor spreading along a new branch of the Tethyan spreading system. At about 135 Ma, rearrangement of this system led to the formation of a new seaway between Australia and Greater India that eventually developed into the northeastern Indian Ocean (e.g. Powell et al., 1988). The breakup events are recorded in extensive volcanic sequences along the northwestern margin of Australia. Farther offshore, a sizeable remnant of the post-breakup Tethyan Ocean crust is preserved in the Argo Abyssal Basin.
Dredge samples
Breakup-related lavas were dredged along the lower slopes of the Australian margin (Fig. 1) during cruises 95 and 96 of the Rig Seismic. On the basis of trace and major element data, the rocks range from E-MORB to N-MORB (Crawford & von Rad, 1994). Dredge haul 95/10 recovered E-MORB of Oxfordian–Callovian age (160 Ma). Dredges 96/08, 23, and 24 yielded slightly less incompatible-element-enriched Oxfordian–Callovian E-MORB, and dredge 96/34 contained Valanginian (134 Ma) T- to N-MORB. We analyzed isotope ratios of eight samples from these dredges and, for five samples, measured both unleached and leached splits. Leaching again variably lowered (87Sr/86Sr)t, whereas Pb isotope ratios and Nd(t) in the leached and unleached pairs were similar.
The E-MORB from dredges 96/23 and 96/24, on the flank of the Scott Plateau, have similar age-corrected values that fall in or near the age-adjusted Re–Cr–Am source field in Figs 4 and 5. Similar Nd(t) and (206Pb/204Pb)t values were obtained for the two E-MORB from dredge 95/10 on Rowley Terrace, the 95/10 rocks differing from those of 96/23 and 24 in having slightly lower (207Pb/204Pb)t and (87Sr/86Sr)t.
Three T- and N-MORB-type samples from dredge 96/34 at the foot of the Exmouth Plateau have higher Nd and lower Pb isotope ratios [e.g. Nd(t) = +5·8 to +6·9] than the E-MORB. Sr isotopes were measured only for unleached splits, but only modest alteration of (87Sr/86Sr)t is indicated by the position of the data within or close to the MORB-source field in Fig. 5. Most importantly, the 96/34 data display a consistently Indian-MORB-type signature in Fig. 4a–c.
The dredge 96/08 sample is distinct, with very high (207Pb/204Pb)t (15·69), low Nd(t) (+0·9), and high (87Sr/86Sr)t (0·70670). In combination, these characteristics suggest an influence from continental crust or lithospheric mantle. Consistent with this interpretation, the incompatible element pattern of this rifted-margin basalt, although generally E-MORB-like, shows a depletion in Nb, with (Th/Nb)n = 3·3 (Crawford & von Rad, 1994).
Drill-core samples
The Argo Abyssal Basin was drilled at DSDP Site 261 and Ocean Drilling Program (ODP) Site 765 (Shipboard Scientific Party, 1974, 1990). Site 765 is on Tethyan crust with a magnetic (chron M26) and radiometric age of 155 Ma (Ludden, 1992), whereas Site 261 is on 152 Ma (chron M24A) crust. At Site 261, a 10 m thick basalt sill and two lava units were cored. At Site 765, two basalt units were identified in Hole 765C, and about 30 m away, 22 units in Hole 765D. All of these rocks are chemically N-MORB (Robinson & Whitford, 1974; Ludden & Dionne, 1992; Weis & Frey, 1996).
Ludden & Dionne (1992) measured present-day isotope ratios of several Site 765 basalts. For Site 261, a sample from the sill and one from the upper lava unit were analyzed by Weis & Frey (1996), yielding Nd(t) of +11·1 and +14·5, respectively, and (206Pb/204Pb)t of 17·72 and 17·89. This combination of values lies in or to the high-Nd(t) side of the Pacific–North Atlantic MORB-source field in Fig. 4d. However, 143Nd/144Nd was measured on leached splits, whereas Sm and Nd concentrations used for the age correction were determined on unleached splits. Because leaching preferentially removes more-soluble minerals, Sm/Nd can be significantly different in leached splits than in unleached splits [e.g. see data for 261-35R-4 (98–100) in Table 1]; therefore this approach can result in under- or over-correction of Nd(t). Age-corrected (208Pb/204Pb)t was not reported, but (207Pb/204Pb)t was higher, at 15·50 and 15·59, than for Pacific–North Atlantic MORB mantle at the same (206Pb/204Pb)t; the 15·59 value is also higher than for Indian MORB mantle. Weis & Frey (1996) suspected that such high (207Pb/204Pb)t values were caused by small amounts of sample contamination by Pb derived from marine sediment.
We measured four samples from Site 261, two from the upper and lower parts of the sill and one from each lava unit, plus one sample each from Holes 765C and 765D. All have closely similar Nd(t), between +8·3 and +8·8. The range of (87Sr/86Sr)t is also relatively small (0·70288–0·70323; Fig. 5), and (207Pb/204Pb)t is between 15·42 and 15·48, with one exception (see below). Again with one exception, the age-corrected Nd and Pb isotope ratios plot within the Indian MORB mantle field (or in one case slightly to the left of it in Fig. 4d and e).
Although Indian-MORB-like, the range of (206Pb/204Pb)t is substantial, from 17·05 to 17·86 for the unleached splits. This spread of (206Pb/204Pb)t at nearly constant Nd(t) strongly suggests that the Pb isotope systematics of some of the samples are disturbed. Not surprisingly, the data are scattered about a 155 Ma reference line in a 206Pb/204Pb vs 238U/204Pb diagram (not shown). Most suspect are samples whose Pb isotope ratios are the most sensitive to even minor disturbance; that is, those with large parent–daughter ratios, for which the age correction is greatest. These are the Site 765C basalt and sample 35R-4 (98–100) from the upper lava unit of Site 261, which have very high 238U/204Pb (110 and 108). The Site 765C sample has by far the lowest age-corrected (206Pb/204Pb)t, its data point actually falling outside (to the left of) the Indian MORB-source field in Fig. 4d and e. Also, we leached a split of 35R-4 (98–100), and the age-corrected (206Pb/204Pb)t and (207Pb/204Pb)t values for the two splits are drastically different (17·60 and 15·49 vs 19·89 and 15·62), the data for the leached split plotting far out to the right of Pacific–North Atlantic MORB-source array in Fig. 4d–f.
In contrast, the leached and unleached splits of the sill sample that we leached [35R-2 (87–89)] have almost identical values (17·34, 15·45 and 17·34, 15·44), which are also similar to those of the other (unleached) sill sample (17·44, 15·45). These results suggest that the data for the sill represent near-magmatic values despite a significant age correction. Likewise, almost identical (206Pb/204Pb)t and (207Pb/204Pb)t were obtained for the leached (17·86, 15·48) and unleached (17·83, 15·47) splits of the lower lava unit of Site 261. For this sample and the Hole 765D basalt, which has very similar values (17·81, 15·46), the age correction is relatively small, as 238U/204Pb is relatively low (12·7–15·0). The age-corrected Pb isotope values for these samples too are likely to be near-magmatic.
Eastern Himalayan syntaxis
In China, Burma, and extreme northeastern India, fragments of Paleotethyan lithosphere ranging from Early Carboniferous to Permian are exposed in several ophiolite zones (Fig. 1) in which the magmatic rocks predominantly have N-, T-, or E-MORB-type trace and major element compositions (e.g. Mo et al., 1993, 1994, 1998; Wu et al., 1995; Shen et al., 2002a, 2002b; Ghosh & Ray, 2003). Xu & Castillo (2004) analyzed N-MORB-type gabbro of the Early Carboniferous Shuanggou ophiolite in the Ailaoshan ophiolite zone of southwestern China, and Early Permian T-MORB-type basalt from two sites in the Jinsha River ophiolite north of the Ailaoshan zone. Their data suggest sources with broadly Indian-MORB-like Nd–Pb isotopic characteristics but, like our results for the Albanian and Site 261 and 765 basalts, and those of Xu et al. (2002) for the Carboniferous northern Mian–Lue ophiolite of central China, the data do not consistently all fall inside the Indian MORB-source field in Fig. 7.
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Here, we report isotopic results for six Paleotethyan basalts: two each from the Early Carboniferous Changning–Menglian ophiolite of southwestern China and the Permian Mayodia ophiolite of northeastern India, and one sample each from the Early Carboniferous Laowangzai area of the Ailaoshan ophiolite and the Early Permian Lancang River zone of southwestern China. Alteration-resistant trace and major elements indicate N-MORB-type compositions for all (Mo et al., 1998
; Shen et al., 2002a, 2002b; Ghosh & Ray, 2003).
Two samples, LZK1-3 from Laowangzai and HN-4 from the Lancang River zone, are isotopically unlike either Indian or Pacific–North Atlantic MORB, the data falling at the upper edge of the estimated 300 Ma Pacific–North Atlantic MORB-source field in Fig. 7a, within the Indian MORB-source field in Fig. 7b, and above both fields in Fig. 7c. We infer that these rocks contain some Pb derived from continental material or marine sediment, probably an overprint introduced during hydrothermal alteration or ophiolite obduction. Further evidence of such an overprint is seen in their high (87Sr/86Sr)t (0·70984, 0·70585), the value for LZK1-3 being greater than Carboniferous-to-present seawater values (e.g. Burke et al., 1982).
In contrast, the samples from the Changning–Menglian and Mayodia ophiolites all exhibit consistently Indian MORB-type Pb–Pb and Nd–Pb isotope signatures; (206Pb/204Pb)t is relatively low for their Nd(t) and (208Pb/204Pb)t and, importantly, (207Pb/204Pb)t is not anomalously high but well within the Indian MORB-source field in Fig. 7c. These basalts also have much lower (87Sr/86Sr)t for their Nd(t) than LZK1-3 and HN-4.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODS AND SAMPLESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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A widespread Indian-Ocean-type isotopic signature in the Tethyan mantle
Rocks from two locations in Iran, a 1300 km stretch of Tibet, and off northwestern Australia demonstrate that mantle sources very similar to those feeding the Indian Ocean spreading centers and hotspots today were present in a wide region of the Neotethys (Fig. 8) in the Early Cretaceous and Jurassic. The Jurassic rocks, in particular, indicate that such mantle existed in much of the central and eastern Neotethyan region before and during the earliest stages of Indian Ocean opening. Whether Indian-Ocean-like mantle also underlay the Jurassic western Neotethys is unclear; the Albanian data certainly do not rule out this possibility, and samples from the Troodos ophiolite of Cyprus, SE of and about 60 Myr younger than the Albanian ophiolite, possess broadly Indian-MORB-type Nd–Pb isotope characteristics (Xu & Castillo, 2004
). Farther east, the only Jurassic or Early Cretaceous rocks studied to date that lack an Indian-Ocean-like signature are the 150 Ma N- and T-MORB suite from Masirah (Mahoney et al., 1998), the crust of which was formed near the southern boundary of the Tethyan Ocean in a setting similar to that of the Gulf of Sheba today.
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We emphasize that although some of the MORB-like Tethyan rocks may have been formed in intra-oceanic back-arc or even fore-arc environments rather than at mid-ocean ridges (see above), contamination of asthenosphere with marine sedimentary material subducted shortly before (or even a few hundred million years before; see Moores et al., 2000
) volcanism cannot be a general explanation for their Indian-Ocean-type characteristics, particularly their low relative 206Pb/204Pb. Marine sediments in general have high 207Pb/204Pb, 208Pb/204Pb, and 87Sr/86Sr and low Nd relative to Pacific–North Atlantic MORB, but they also have rather high 206Pb/204Pb (e.g. Ben Othman et al., 1989). Sediment entering subduction zones today has average 206Pb/204Pb = 18·91 (Plank & Langmuir, 1998), and this value would have been only about 0·8 lower at the beginning of the Phanerozoic (e.g. Stacey & Kramers, 1975).
The picture that emerges is thus that the Jurassic and Early Cretaceous mantle of the central and eastern Neotethyan Ocean was isotopically equivalent to Indian Ocean mantle, except for (very?) rare pockets or stringers of broadly Pacific–North Atlantic-like composition. In contrast, the sources of all pre-120 Ma Pacific MORB yet analyzed were isotopically indistinguishable from those of modern Pacific MORB (Janney & Castillo, 1996, 1997; Mahoney et al., 1998, 2004). One possibility is that both Indian Ocean and Tethyan mantle were physically separate domains but acquired similar isotopic characteristics as a result of the same process(es). However, given that the two basins were interconnected throughout the period of growth of the Indian Ocean, the most straightforward interpretation is that the present Indian Ocean domain is inherited from the Tethyan mantle domain; that is, that the crust of both oceans was derived from the same extensive region of mantle. Furthermore, although the Paleotethyan rocks provide somewhat mixed signals, the majority of results support an isotopically Indian MORB-type mantle beneath at least parts of the Paleotethyan Ocean as long ago as the Early Carboniferous (Fig. 7 and Xu et al., 2002; Xu & Castillo, 2004).
Intriguingly, the Jurassic E-MORB from the Australian margin and the Early Cretaceous OIB-type rocks at Dajiweng and Bar in Tibet are isotopically similar to the recent Indian OIB of Réunion, Mauritius, Crozet, and Amsterdam. So too are Early Cretaceous (120 Ma) OIB-type lavas at Masirah (Fig. 4a–c) (Mahoney et al., 1998). Thus, hotspot-like mantle with a broadly Réunion-type isotopic composition appears to have been rather common in the Jurassic and Early Cretaceous, just as it is in far-flung parts of the Indian Ocean today.
Origin of the Indian and Tethyan MORB mantle domain
The Indian MORB mantle domain is commonly suggested to be a result of addition of low-206Pb/204Pb, low-Nd, high-87Sr/86Sr material to the asthenosphere. This material is often postulated to be derived from (1) anciently subducted (>1 Ga), low-U/Pb marine sediments (plus a basaltic slab component, usually) (e.g. Dupré & Allègre, 1983; Hamelin et al., 1986; Michard et al., 1986; Price et al., 1986; le Roex et al., 1989; Rehkämper & Hofmann, 1997; Chauvel & Blichert-Toft, 2001), or (2) mobilization of old low-U/Pb continental lithospheric mantle and/or lower crust during ancient continental collisions, impingement of plume heads beneath Gondwana, Gondwanan or pre-Gondwanan rifting events, and/or subduction erosion (e.g. McKenzie & O'Nions, 1983; Mahoney et al., 1989, 1992; Tatsumoto & Nakamura, 1991; Douglass et al., 1999; Flower et al., 2001; le Roux et al., 2002; Escrig et al., 2004; Hanan et al., 2004). A combination of (1) and (2) also has been proposed (le Roux et al., 2002). In contrast, evolution of an extensive asthenospheric region isolated from plume input and from mixing with the rest of the asthenosphere since early in Earth history has been suggested (e.g. Dosso et al., 1988; Xu et al., 2002).
With regard to mobilization of continental lithosphere, it is noteworthy that both the Neotethys and Paleotethys largely grew by repeated rifting of portions of the supercontinent of Gondwana (e.g. Stampfli & Borel, 2002), and that large volumes of Gondwanan lower crust and lithospheric mantle indeed appear to be missing today, presumably removed at various times by one or more of the above mechanisms (e.g. Ballard & Pollack, 1988; Ashwal & Burke, 1989; Black & Liegeois, 1993; Poudjom Djomani et al., 2001; Zheng et al., 2001; Gao et al., 2002). Regarding hypotheses involving sediment recycling, ancient marine sediments theoretically could have been introduced into the asthenosphere in three ways: they could have been mixed in directly, the hybrid mantle then evolving isotopically over >1 Gyr; they could have been subducted deeply (660 km or deeper), stored for long periods, and returned relatively recently to the upper mantle, probably by plumes; or they could have been accreted to the continental lithosphere (e.g. Von Huene & Scholl, 1991) and stored for long periods in isolation from the convecting mantle prior to entry into the asthenosphere.
The last possibility removes the distinction between sediment-recycling and continental lithosphere-mobilization hypotheses. The alternative idea that large amounts of low-206Pb/204Pb material (whether of sedimentary origin or not) have been introduced to the asthenosphere via mantle plumes is not supported as a general explanation for Indian and Tethyan MORB mantle by either present-day Indian Ocean hotspots (e.g. Mahoney et al., 1992) or by the OIB- or E-MORB-like Tethyan rocks, all of which lack sufficiently low 206Pb/204Pb (see Fig. 4). On the other hand, ancient mixing of sediments directly into the asthenosphere requires that the resulting hybrid mantle remain isolated from the rest of the asthenosphere for a very long time (>1 Gyr). For as much as a few hundred million years, isolation might be accomplished by slab ‘curtains’ at long-lived subduction zones, such as those that appear to have surrounded much of the Tethys and Gondwana from the Middle Paleozoic to Mesozoic (e.g. Stampfli & Borel, 2002). However, given the complexity of plate motions and ever-changing plate boundaries, isolation of any large region of asthenosphere by slab curtains for much longer times seems implausible. Indeed, the problem of maintaining convective isolation is a major challenge to any hypothesis involving a truly ancient origin for Indian and Tethyan MORB asthenosphere.
Rehkämper & Hofmann (1997) estimated that <10 wt % of contamination of (originally) Pacific–North Atlantic-type asthenosphere by 1·5 Ga subducted slab material containing <10 wt % of marine sediment could account for much of the isotopic variation in Indian MORB. For the Indian MORB domain as a whole, they estimated that the amount of subducted sediment required would be significantly less than the amount of mobilized ex-continental lithosphere (mantle only, not crust) needed to achieve the same results. For their model sediment, they assumed much higher concentrations of Pb (55 ppm) and several other elements than in average subducting marine sediment (Pb 19·9 ppm; Plank & Langmuir, 1998); thus, their estimate of the amount of sediment contamination required is best viewed as a minimum. More importantly, recent work shows that profound chemical changes occur during dehydration of sediment and basaltic crust in subduction zones. Large proportions of several trace elements, particularly Pb, are removed with the fluid phase during this process (e.g. Kogiso et al., 1997; Aizawa et al., 1999; Johnson & Plank, 1999; Becker et al., 2000). Uranium (and Th) appears to be much less mobile than Pb during dehydration, causing 238U/204Pb in the residue of both sediment and basalt to increase significantly, probably by factors of 3–5. Therefore, unlike non-dehydrated marine sediment (which has low 238U/204Pb averaging around 4–6; Ben Othman et al., 1989; Plank & Langmuir, 1998), 206Pb/204Pb in subducted, dehydrated sediment should increase rather rapidly to values higher than found in any modern MORB (and the increase will be even more rapid in dehydrated ocean crust). Aizawa et al. (1999), for example, estimated that 206Pb/204Pb would increase by about 2·2 per Gyr in average dehydrated, subducted marine sediment. Thus, we conclude that normal, anciently subducted marine sediment cannot be the low-206Pb/204Pb material in the Tethyan and Indian Ocean asthenosphere. Moreover, admixture of small amounts of un-dehydrated sediment plus altered ocean crust into MORB-source mantle produces incompatible-element patterns with pronounced Pb peaks that are lacking in Indian MORB (Fig. 9a).
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Relatively recent addition of low-206Pb/204Pb lower mantle to the Indian Ocean asthenosphere, either via plumes or by a general upward ‘leakage’, has been suggested by Kamber & Collerson (1999)
. In this view, the lower mantle does not contain significant amounts of subducted material but has evolved largely in isolation from the rest of the mantle for at least 3 Gyr and is only slightly modified from a primitive composition. A similar origin has been explored for the source of the world's largest oceanic plateau, the Ontong Java in the Pacific (Tejada et al., 2004). A serious problem with this hypothesis for explaining the Indian MORB domain is that it predicts that the lower mantle is only slightly degassed and has very high 3He/4He (90 times the atmospheric ratio). Thus, Indian MORB should have higher 3He/4He than other MORB. However, 3He/4He values of Indian MORB are not higher than for MORB globally; rather, the longest hotspot-free sections of the Indian Ocean ridge system have lower than average values (Graham et al., 1999; Mahoney et al., 2002; Georgen et al., 2003).
We suggest that other voluminous, potential low-206Pb/204Pb, sources are the pyroxenitic lower-crustal restite formed in ancient arcs by extraction of andesitic magma and/or the primary, basaltic lower crust formed in such arcs prior to andesitic magmatism. For example, Tatsumi (2000) estimated the composition of Archean pyroxenitic restite generated by melting of basaltic lower-arc crust to produce andesite (e.g. Kay & Kay, 1988; Turcotte, 1989), and showed that restite retaining a few percent of frozen andesitic melt would evolve isotopically to a low-206Pb/204Pb, low-Nd, relatively high-87Sr/86Sr composition similar to that found today in the Pitcairn, Hawaiian, and other EM-1-type hotspots. He argued that because of its high density, ancient restite, once detached from the lithosphere, would tend to sink deep into the lower mantle, re-emerging in Late Phanerozoic plumes to produce low-206Pb/204Pb OIB. As noted above, the lack of low 206Pb/204Pb in Indian Ocean islands or the Tethyan OIB- and E-MORB-type rocks suggests that plume-driven contamination is not the source of low-206Pb/204Pb Indian and Tethyan MORB-type compositions. We suggest that alternative mechanisms involving restite might be (1) long-term storage in the largely arc-generated continental lithosphere, followed by eventual detachment and mixing into the asthenosphere, (2) storage of detached restite in a zone of neutral buoyancy in the mantle Transition Zone around 660 km (e.g. Yasuda et al., 1997), or (3) relatively ancient mixing of detached restite into convecting MORB-source mantle, followed by isotopic evolution of the hybrid asthenosphere. Long-term lithospheric storage avoids the need for prolonged convective isolation of a large region of asthenosphere; the same may or may not be true of storage in the Transition Zone, depending on the amount of convective coupling between it and the upper mantle. In the case of lithospheric storage, assuming sufficient amounts of ancient restite are in fact preserved in the continents, detachment presumably would occur at different times during plume-head, continental collision, and/or extension events (e.g. Meissner & Mooney, 1998; Jull & Kelemen, 2001).
Our simple mass-balance modeling using a lower-crustal restite consisting of pyroxenite plus 5 wt % retained andesite (Table 4) suggests that <10 wt % of contamination of Pacific–North Atlantic-type asthenosphere by 1–3 Ga restite can account for most low-206Pb/204Pb Indian MORB (Fig. 10a–c). The great majority of Indian MORB 206Pb/204Pb, 207Pb/204Pb, 143Nd/144Nd, and 87Sr/86Sr values can be explained by mixtures involving <3 wt % of a restite component. Although Fig. 10 illustrates a case involving mixing between ancient restite and average modern Pacific–North Atlantic MORB mantle, such mixing would actually involve MORB mantle with a range of isotopic compositions; also, this type of model does not place strong bounds on when mixing occurs. Figure 9b shows that addition of 1–5 wt % of restite to average Pacific MORB-source mantle yields basalts with incompatible element concentrations in the general range of those for average Indian MORB, but that sizeable Pb peaks are again generated. Further, restite with the Th/U composition used in Table 4 is not completely successful in accounting for the high relative 208Pb/204Pb of many Indian MORB (Fig. 10d). Also, a total restite volume of 4 x 108 km3 would be required beneath the greater Indian Ocean region, assuming an average of only 1 wt % restite in a 550 km thick asthenosphere (i.e. 660 km minus the thickness of mature oceanic lithosphere). A restite-rich layer in the Transition Zone may be able to provide such a quantity (corresponding to a 2000 km x 2000 km slab 100 km thick), but continental lithosphere seems a less likely source because, even though large amounts of Gondwanan lower crust and mantle lithosphere may have been removed in the past, pyroxenitic restite probably comprises a rather small fraction of the total. Although not shown in the figures, we obtained similar results to those for pyroxenitic restite using Tatsumi's (2000) ancient primary basaltic lower-arc crust.
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Consideration of slab dewatering suggests to us a different potential mechanism for generating Indian and Tethyan MORB-type compositions, through past addition to the asthenosphere of low-U/Pb fluids derived from dehydration of marine sediment and basaltic crust. Such fluids would metasomatize or react with their host asthenosphere, which would then evolve with relatively low U/Pb. Partial convective isolation might be promoted by downward transport of the hybrid asthenosphere to Transition-Zone depths (via initial viscous coupling to the slab; Hieronymus & Baker, 2004
). Table 4 lists estimated average concentrations of several key trace elements in model fluids, calculated using Kogiso et al.'s (1997) and Aizawa et al.'s (1999) experimentally derived mobility factors and assuming the fluids are produced from average subducting sediment and altered oceanic crust. Figure 11 shows the estimated present-day isotopic ratios of hybrid asthenosphere formed at different times in the past (0·5–2 Ga) by adding 0·1, 0·2, and 0·5 wt % of slab-derived fluid to average Pacific–North Atlantic-type MORB mantle; the sediment component of the fluid is assumed to be 10% and the oceanic crust component to be 90%. Figure 11a–c shows that hybrid mantle formed at ages less than about 1·5 Ga can account for much of the isotopic range observed for Indian MORB (it also may help account for Hf–Nd isotope systematics; Kempton et al., 2002). Similar to the restite example, however, the model fluid does not yield high enough present-day 208Pb/204Pb to explain a sizeable subset of the Indian MORB data (Fig. 11d). Further, the incompatible element patterns of basalts formed from model fluid-affected asthenosphere have marked Pb peaks; troughs at Nb are also evident (Fig. 9c). Thus, with the present model parameters this mechanism appears only partly successful as a means of generating Indian and Tethyan MORB mantle. Nevertheless, we feel it warrants further study, in view of the difficulties with all other proposed origins, and given that slab dewatering has been a major process throughout much of Earth history.
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), 0·2 wt % (
), or 0·5 wt % (
) of model slab-derived fluid at different times in the past between 2 and 0 Ga (curves connect compositions of mantle contaminated at the time labeled). The proportion of the fluid derived from dehydration of marine sediment is assumed to be 10% and that from dehydration of altered basaltic crust 90% (see |
CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMETHODS AND SAMPLESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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The collective Nd–Pb and Pb–Pb isotopic data for Tethyan magmatic rocks suggest that Indian MORB mantle is largely ‘inherited’ from Tethyan mantle; that is, that the crust of both oceans was formed from the same mantle domain, and that the age of this domain is substantially greater than the age of the earliest Indian Ocean crust. Besides Indian-MORB-type mantle, OIB-like mantle broadly similar to that producing the widely separated modern Indian Ocean islands of Réunion, Mauritius, Crozet, and Amsterdam was also present in the Jurassic and Early Cretaceous Tethys.
In addition to previously proposed mechanisms for the origin of the Indian MORB mantle domain, two other processes that can produce basalts with a number of the key isotopic characteristics of Indian MORB are introduction of ancient pyroxenitic lower-crustal restite (or basaltic lower-arc crust) into originally Pacific–North Atlantic-type mantle, and past addition of low-U/Pb fluids derived from dewatering of subducted sediment and ocean crust to originally Pacific–North Atlantic-type mantle. At present, however, both of these processes also appear to have significant shortcomings, as do all previously proposed mechanisms.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONMETHODS AND SAMPLESRESULTSDISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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We thank H. Babaie, L. Beccaluva, J.-Z. Li, J. Ray, S.-Y. Sheng, and Q.-W. Zhu for supplying samples, elemental data, and/or petrographic data, and K. Spencer, D. Vonderhaar, R. Carmody, S. Zhou, Badengzhu, Z. Lei, Qiongda, and N. Hulbirt for help with other aspects of the work. A. Hassanipak provided valuable field expertise in Iran. The Site 261 and 765 samples were provided by the Ocean Drilling Program. P. Janney, an anonymous referee, and P. Castillo provided insightful reviews. Principal funding came from US NSF grant EAR-9805318, Chinese NSF grants 49772107 and 49802005, and Chinese National Key Project G1998040800.
* Corresponding author. Telephone: 808-956-8705. Fax: 808-956-5512. Email: jmahoney{at}hawaii.edu
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