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Journal of Petrology | Volume 43 | Number 7 | Pages 1155-1176 | 2002
© Oxford University Press 2002
Between a Hotspot and a Cold Spot: Isotopic Variation in the Southeast Indian Ridge Asthenosphere, 86°E–118°E
1SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII, HONOLULU, HI 96822, USA
2COLLEGE OF OCEANIC AND ATMOSPHERIC SCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR 97331, USA
3BISHOP MUSEUM, HONOLULU, HI 96817, USA
Received July 13, 2001; Revised typescript accepted January 7, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSAMPLES AND METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Glasses from a 2600 km section of the Southeast Indian Ridge west of the Australian–Antarctic Discordance all possess Nd–Pb–Sr isotopic signatures typical of Indian Ocean ridge basalt. The boundary between Pacific- and Indian-Ocean-type ridge basalt within the Discordance thus marks the westernmost extent of shallow Pacific-type asthenosphere beneath the ridge. Along-axis He, Nd, Pb, and Sr isotopic patterns are largely independent of ridge segmentation, but a weak tendency is evident for the most strongly Indian-Ocean-type mantle to be relatively fusible and for shallower asthenosphere to have lower 3He/4He. On average,
Nd appears slightly lower than for ridges in the western Indian Ocean far from hotspots. Importantly, the regional isotopic patterns cannot be explained by a previously proposed eastward flow of Kerguelen–Heard or Amsterdam–St. Paul hotspot mantle. Nd, Pb and (to a much lesser extent) Sr isotopes correlate roughly with many incompatible element ratios, including parent–daughter ratios. If interpreted as mantle errorchrons, the latter correlations imply ‘ages’ of 200–300 Ma, significantly greater than the oldest known age of the Kerguelen–Heard hotspot (119–135 Ma) commonly postulated to have played an important role in creating the isotopic signature of Indian Ocean mantle. Rather than reflecting relatively recent mixing involving mantle from hotspots in the region, much of the observed isotopic heterogeneity may be the result of other mixing or of past intra-mantle chemical fractionation, probably associated with melting. KEY WORDS: Southeast Indian Ridge; isotopes and trace elements; mantle geochemistry; Indian Ocean hotspots
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSAMPLES AND METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Until recently, the 3200 km section of the Southeast Indian Ridge (SEIR) between the Amsterdam–St. Paul hotspot and the Australian–Antarctic Discordance (AAD) remained the largest unsampled portion of the Indian Ocean ridge system (Fig. 1). In the eastern 2600 km of this region, between 86°E and 118°E, spreading rate is nearly constant at 69–75 mm/yr and axial depth increases from west to east by more than 2300 m, indicating a gradual eastward decrease in upper mantle temperature of at least 60–110°C (e.g. Sempéré et al., 1997
). This along-axis temperature gradient has been theorized to correspond to a large-scale eastward flow (and gradual dilution) of mantle emanating from the Amsterdam–St. Paul and/or Kerguelen–Heard hotspots toward a region of unusually cool asthenosphere, sometimes termed a ‘cold spot’, beneath the AAD at 120–128°E (e.g. Marks et al., 1991; Sempéré et al., 1997). An even broader role for the Kerguelen–Heard hotspot has been proposed by several workers (e.g. Storey et al., 1989; Mahoney et al., 1992; Weis & Frey, 1996), who have suggested that this hotspot was important in the origin of the distinctive isotopic signature of mid-ocean ridge basalt (MORB) in the Indian Ocean, Indian MORB being characterized in particular by lower 206Pb/204Pb relative to 208Pb/204Pb, Nd, and 87Sr/86Sr than Pacific and North Atlantic MORB, and by comparatively low Nd and high 87Sr/86Sr (e.g. Hedge et al., 1979; Hamelin et al., 1986; Michard et al., 1986; Dosso et al., 1988; Mahoney et al., 1989, 1992; Rehkämper & Hofmann, 1997). Within the AAD near 126°E, isotopic studies have revealed the existence of an abrupt transition, occurring over <40 km along-axis, between Indian-MORB-type and Pacific-MORB-type mantle (Klein et al., 1988; Pyle et al., 1992). This boundary has been migrating westward for at least the last several million years and perhaps for the last several tens of millions of years (Pyle et al., 1992, 1995; Christie et al., 1998). However, whether this boundary represents the boundary between these two enormous domains of MORB-type mantle or only a boundary, with, for example, interfingering of the two types along-axis farther to the west, has remained unknown because of the lack of any samples from the long stretch of ridge west of the AAD.
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In 1995 and 1996, respectively, the Westward 10 and Boomerang 6 expeditions of the R.V. Melville recovered basalts from 197 sites along the SEIR between 77°E and 118°E (Christie et al., 1995; Johnson et al., 1996). In this paper we report He, Sr, Pb, and Nd isotopic results and selected elemental data for glasses from the section of ridge between 86°E and 118°E, and discuss implications for asthenospheric flow and geochemical evolution in the southeastern Indian Ocean mantle.
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SAMPLES AND METHODS |
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TOPABSTRACTINTRODUCTIONSAMPLES AND METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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With the exception of two dredge hauls from near-axis seamounts and one (WW10-116, where WW indicates Westward) from a transform-fault high between two spreading segments, the 71 glasses we analyzed isotopically were taken from, or very close to, the active spreading axis, as defined by combined bathymetric and geophysical data (Christie et al., 1995
; Johnson et al., 1996; Sempéré et al., 1997; Scheirer et al., 2000). Elemental data for a much larger number of samples (Douglas-Priebe, 1998; Christie et al., in preparation; K. T. M. Johnson, unpublished data, 2000) show that most of the lavas in the collection are normal, or N-type, MORB, although many, particularly abundant throughout the eastern two-thirds of the study area where the axial depth gradient is slightly steeper, are more enriched in highly incompatible elements relative to moderately incompatible elements than are N-MORB. Here, we refer to lavas with chondrite-normalized La/Sm, (La/Sm)n, >0·8 and/or K2O/TiO2 >0·15 as E-type MORB, although we emphasize that a continuum of compositions exists. Compositional heterogeneity at small scales is demonstrated by the fact that many of the dredge hauls containing E-MORB also contain N-MORB (Douglas-Priebe, 1998; Christie et al., in preparation).
Samples we analyzed isotopically were chosen to ensure a relatively uniform geographic coverage of the axis, within the limitations imposed by dredge and wax-core locations and availability of sufficient fresh glass. Additionally, we analyzed two samples from each of five individual dredge sites to assess the range of isotopic variability associated with intra-dredge-scale elemental heterogeneity. Processing and analysis of samples followed our normal procedures for MORB glasses (e.g. Pyle et al., 1992; Mahoney et al., 1994; Graham et al., 1999; Christie et al., in preparation). Isotopic fractionation corrections, standard reference values, procedural blanks, and analytical uncertainties are summarized in the Table 1 footnotes. Major and trace element data used in the figures and discussed below are presented in Table 2. A comprehensive suite of major and trace elements has been measured for a much more extensive set of 86–118°E glasses by Douglas-Priebe (1998), Christie et al. (in preparation), and K. T. M. Johnson et al. (unpublished data, 2000) using electron microprobe and inductively coupled plasma–mass spectrometric methods.
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RESULTS |
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TOPABSTRACTINTRODUCTIONSAMPLES AND METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Isotopic data
Our most fundamental result is that all of the samples analyzed are of the Indian MORB type. As with MORB from the Indian Ocean farther west, the 86–118°E data lie to the low-206Pb/204Pb, low-
Nd, high-87Sr/86Sr side of the isotopic fields defined by >95% of Pacific and North Atlantic MORB in Fig. 2a, b and d. The Indian-MORB nature of the lavas is demonstrated perhaps most graphically in the profile of 
8/4 vs longitude in Fig. 3a [8/4 is a measure of the vertical distance of a data point from a line fitted through the Pacific and North Atlantic MORB array in Fig. 2a; see Hart (1984) for the relevant equations]. For nearly all Pacific MORB 8/4 < 20, whereas 8/4 > 20 for virtually all Indian MORB (e.g. Mahoney et al., 1992, and references therein); 8/4 for the 86–118°E SEIR glasses ranges from 23 to 72.
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, E-MORB. The field for the Amsterdam–St. Paul hotspot (HS) encompasses the data of Hamelin et al. (1986)
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, data for the AAD area (Klein et al., 1988
, He-isotope data for the SEIR west of 86°E to just west of the Amsterdam–St. Paul hotspot (Graham et al., 1999
A substantial range of isotopic values is present in our dataset (Table 1); 206Pb/204Pb varies from 17·362 to 18·494, 87Sr/86Sr from 0·70271 to 0·70368, Nd from +4·2 to +9·8, and 3He/4He from 6·53 to 9·70 RA (where RA is the atmospheric ratio). Isotopic values for the few seamount glasses that have been analyzed are within the range defined by nearby axial lavas. Within-dredge Nd–Sr–Pb isotopic heterogeneity varies from almost negligible in dredges WW10-116 and -138 to a significant fraction of the total range observed in the entire 86–118°E region in dredges WW10-128 and -145, in which chemical differences are also pronounced. The two samples analyzed from WW10-128, for example, differ by 4·4 Nd units, by 0·0004 in 87Sr/86Sr, and by 0·4 in 206Pb/204Pb. In both of these dredges, the more incompatible-element-enriched glasses have the lower Nd and the higher Sr and Pb isotopic values. In the 86–118°E region overall, the Nd–Sr–Pb isotopic data for E-MORB and N-MORB overlap considerably but the N-MORB range to lower 87Sr/86Sr, whereas the E-MORB tend to have higher Pb isotope ratios. Among E- and N-MORB from adjacent sampling stations, the E-MORB have similar to lower Nd and similar to higher 87Sr/86Sr, Pb isotope ratios, and 8/4 (Fig. 3; note the reversed Nd scale in Fig. 3f). With the exception of dredge WW10-89, the E-MORB have low 3He/4He, between 6·53 and 7·36 RA, whereas the N-MORB vary over nearly the entire range observed, from 6·68 to 9·70 RA. In dredges WW10-128 and WW10-145, for each of which we analyzed He isotopes in two different samples having significantly different (La/Sm)n, the more incompatible-element-enriched glass has lower 3He/4He (by 0·57 and 0·67 RA, respectively). Overall, however, no systematic difference in He isotopes is apparent between E- and N-MORB from adjacent stations (Fig. 3d).
Our results support previous indications of large-scale isotopic provinciality within the Indian Ocean ridge system (Mahoney et al., 1989, 1992; Pyle et al., 1992). The great majority of the 86–118°E samples, for example, have 206Pb/204Pb clustering between 17·8 and 18·3. Within this same range of 206Pb/204Pb, and excluding segments near hotspots, basalts of the central–eastern Southwest Indian Ridge and the Carlsberg and Central Indian Ridge system in the western Indian Ocean have distinctly higher average Nd values (the average for each is +8·7) than do the 86–118°E SEIR lavas (average +7·1; see Fig. 4b). In contrast, lavas from the SEIR NW of Amsterdam and St. Paul islands, and from within the Indian-MORB section of the AAD to as far east as 125°E have similar Nd values to those of the 86–118°E glasses (Fig. 4a). Thus, most of the SEIR appears distinct from large portions of the more westerly ridges. This difference, if confirmed by future sampling, could be an original feature of the Indian MORB mantle domain itself, or could reflect comparatively recent differences in hotspot or other components added to, and variably mixed within, large subdomains of the Indian Ocean asthenosphere.
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, both N- and E-MORB) with data for other parts of the Indian Ocean ridge system. Data for sections of ridges near hotspots are not included (e.g. the 39–41°E section of the Southwest Indian Ridge and the SEIR near Amsterdam–St. Paul). (a) Comparison with the Indian-MORB-type portion of the AAD and the western 700 km of the SEIR (Hamelin et al., 1986
As with other long sections of the Indian Ocean ridge system, correlations among Pb and Nd, and Pb and Sr isotopic ratios are poor (Fig. 2b and d). Pb–Pb isotope correlations (Fig. 2a and c) are somewhat better but still poorer than in many parts of the oceans; among the N-MORB only, 207Pb/204Pb and 206Pb/204Pb show virtually no correlation. Moreover, the general or large-scale shapes of the longitudinal patterns of Pb, Sr, Nd, and He isotopes differ significantly from each other (Fig. 3). For example, a broad peak is evident in 206Pb/204Pb, culminating at 107·5°E; this pattern is matched roughly by 208Pb/204Pb. However, it is not matched by 207Pb/204Pb (not shown) or by 8/4, which instead shows a broad overall increase from 90°E to 116·7°E. Nor is it matched by peaks or depressions in the Nd or Sr isotope patterns, although it does correspond approximately to a local minimum in 3He/4He. The Sr isotope ratio, particularly among the N-MORB, shows a modest overall decrease toward the east, reaching a minimum value at 112·4°E. Nd isotopic ratios exhibit virtually no along-axis regional gradient east of about 90°E, particularly when the E-MORB data are included.
The two WW10-145 glasses and the N-MORB from dredge WW10-128 stand out from the main group of data in Fig. 2. All three are characterized by relatively high 87Sr/86Sr for their Nd values (Fig. 2e), especially the WW10-128 (110·41°E) sample, which also has the highest Nd (+9·8) of the samples we analyzed. The only higher Nd values reported anywhere along the SEIR are for lavas from just west (zone B4) of the boundary between Indian- and Pacific-MORB mantle within the AAD near 126°E (Pyle et al., 1992; Hanan et al., 2000) (Figs 3f and 4a). The two glasses from dredge WW10-145 have by far the lowest 206Pb/204Pb values in our dataset (17·362 and 17·594). Lavas with broadly similar isotopic compositions are present at widely separated locations along the Indian Ocean ridges, particularly in the vicinity of the Indian Ocean triple junction at the western end of the SEIR and on the northern Carlsberg Ridge (Fig. 4b); even lower 206Pb/204Pb values (as low as 16·87) are found in the 39–41°E section of the Southwest Indian Ridge.
The 3He/4He ratio gradually decreases from 8·5–9·7 RA at the western end of our study area to 6·4–7·4 RA at the eastern end (Fig. 3d). All samples with 3He/4He > 7·7 RA are from west of 100°E, where the axial depth gradient is slightly shallower and where very few E-MORB were recovered. All but one sample from east of 100°E have 3He/4He < 7·4 RA and the majority of those from east of 105°E have 3He/4He < 7·0 RA. Farther east, in the AAD, all but the two samples with the highest Nd have values between 6·2 and 7·7 RA (Graham et al., 1990; D. W. Graham, unpublished data, 2000). Worldwide, values below 7·0 are unusual for MORB (e.g. Anderson, 2000); in areas far from high-3He/4He hotspots, MORB generally have 3He/4He of 7–9 RA (e.g. Graham et al., 1999, 2001, and references therein).
With the exception of several samples from the western portion of the study area, there is a weak overall tendency in the 86–118°E data for the highest-8/4, most strongly Indian-MORB-type lavas to have rather low 3He/4He (Fig. 5a). However, although the along-axis pattern for He isotopes exhibits a trough that reaches a minimum around 105–109°E, in the same portion of the ridge as the maximum in the broad peak in 206Pb/204Pb and 208Pb/204Pb, He and Pb isotope ratios do not display any simple correlation (Fig. 5b and c). Geographically, samples from west of 100°E constitute a distinctly different He isotope population from those east of 100°E (including the AAD), the two groups producing a V-shaped pattern in Fig. 5a–d. As with He–Pb, no simple correlation between He and Sr or Nd isotopes is present in the 86–118°E dataset overall but, in contrast to Pb, rough He–Sr (Fig. 5d) and He–Nd isotope correlations are seen when only the N-MORB samples are considered (Pearson correlation coefficient r = 0·75 and -0·63, respectively). They cannot obviously be explained in terms of binary mixing, however, because the low-3He/4He, low-87Sr/86Sr (Fig. 5d), high-Nd N-MORB samples, all of which are from east of 100°E, have relatively high 208Pb/204Pb and do not lie along a trend with the other N-MORB in Fig. 5b. These observations indicate that much of the mantle west of 
100°E has experienced a different geochemical history from that farther east.
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In addition to the large-scale longitudinal variations, an isotopic ‘fine structure’ is also evident along-axis. As at larger scales, the locations of small-scale excursions from the general regional patterns tend to differ for different isotope ratios. For example, the maximum 206Pb/204Pb value observed is in an E-MORB at 95·93°E (see vertical dashed line in Fig. 3); in 208Pb/204Pb this location is a local high but not a maximum; in Nd it barely stands out, and in Sr isotopes it does not stand out at all. There is a distinct local peak in 8/4 but the maximum is at 96·83°E, not 95·93°E; 3He/4He also displays a local maximum at 96·83°E, although, as noted above, with the exception of this and several other samples from the western part of the study area, the highest-8/4 lavas tend to have low 3He/4He. Likewise, a local high occurs in 87Sr/86Sr at 97·51°E; the excursion here in Nd is less marked, and absent in 206Pb/204Pb, 208Pb/204Pb, and He isotope ratios.
A notable exception to this generally poor inter-isotope correspondence is at the western end of the study area (86–90°E), where a peak is present in all isotope ratios. In detail, however, the shape of this peak varies from one isotope ratio to another and the extreme values again do not all occur at the same location: the peak in 8/4 is at 87·09°E, that in 206Pb/204Pb and 208Pb/204Pb at 89·32°E, and that in 3He/4He, 87Sr/86Sr, and Nd at 88·04°E.
Relationships of isotopes to physical and chemical characteristics
Neither the general shapes nor the details of the longitudinal isotopic patterns correlate well with physical discontinuities in the ridge axis (the main offsets in the axis are indicated by tick marks at the top of panels in Fig. 3). Thus, with the present sampling, physical segmentation of the ridge in the 86–118°E region appears largely independent of mantle isotopic composition. Moreover, a relatively minor role for along-axis flow and mixing of magma within individual ridge segments is suggested by trends in one or more isotope ratios that continue across segment boundaries (e.g. at 92°E, 106·5°E, 114°E), together with the co-occurrence of E- and N-MORB within individual segments and even within some individual dredge hauls.
The closest oceanic islands to the 86–118°E section of the SEIR are Amsterdam and St. Paul, some 700 km from our westernmost station (Fig. 1). These islands sit atop the Amsterdam–St. Paul Platform, generally believed to be the expression of a near-ridge hotspot (e.g. Graham et al., 1999; Johnson et al., 2000). The next closest islands are the Kerguelen group and Heard and McDonald, which are built upon the older lithosphere of the Kerguelen Plateau. Although there has been some speculation that two hotspots might lie beneath the plateau, as the Kerguelen group is located nearly 300 km from Heard and nearby McDonald and is isotopically rather distinct from them, all are commonly attributed to a single hotspot (e.g. Weis et al., 2002) situated some 1600 km to the SW of our westernmost dredge sites (Fig. 1). Yet, unlike some hotspot-free ridge sections along which isotopic parameters bear little relationship to axial depth or major or trace element composition, such as parts of the East Pacific Rise (e.g. Bach et al., 1994; Mahoney et al., 1994), some correlations are present at 86–118°E that are superficially similar to some of those seen on ridges located much closer to hotspots (e.g. Verma et al., 1983; Schilling et al., 1999). In particular, 3He/4He (Graham et al., 2001) and 8/4 exhibit rough correlations with axial depth and/or fractionation-adjusted Na- or Fe-oxide contents (see Fig. 6a–d). On the other hand, Nd, 206Pb/204Pb, and 87Sr/86Sr do not. Many incompatible element ratios also show a crude covariation with fractionation-adjusted Na or Fe (Fig. 6e and f).
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Pb, Nd and Sr isotopes do correlate roughly with many incompatible element ratios, including ratios involving elements that possess significantly different bulk partition coefficients (e.g. U/Pb, La/Sm) and those with more similar bulk partition coefficients (e.g. Sm/Nd, Nb/La, Nd/Pb); some examples are shown in Fig. 7. All the correlations observed are poor (e.g. assuming the relationships are linear, |r| is always <0·8 and often much less). Correlations of isotope ratios and their respective parent–daughter element ratios similar to those in Fig. 7a–c and e have been reported recently for the 10–24°N and 38–41°N sections of the Mid-Atlantic Ridge (Dosso et al., 1999). In general, very poor or no correlations exist in our dataset between isotopic ratios and inter-element ratios involving two moderately incompatible elements, particularly those affected strongly by melting in the presence of garnet (e.g. Zr/Y, Hf/Lu; not shown); the same is true for some highly incompatible element pairs (Nb/U, Ba/Th). Helium isotopes display a crudely L-shaped relationship with ratios such as (La/Sm)n (Fig. 5e); this reflects the fact that, as noted above, the N-MORB are from throughout the study area and encompass nearly the entire range observed in 3He/4He whereas almost all of the E-MORB are from the eastern portion of the region, where 3He/4He values are all rather low. Correlations of element ratios with 87Sr/86Sr (of which that with Ba/Nd is one of the best; Fig. 7i) generally are substantially poorer than those with Pb or Nd isotopes, and the overall correlation between 87Sr/86Sr and Rb/Sr is essentially nonexistent (Fig. 7e). This is not just an effect of variable plagioclase fractionation on Rb/Sr (Sr being a compatible element in feldspar), because Rb/Sr correlates better with Pb and Nd isotopes than with 87Sr/86Sr (e.g. Fig. 7f). However, if samples from west of 100°E (circles in Fig. 7e and i) are excluded, 87Sr/86Sr and Rb/Sr correlate weakly (r = 0·59), and other correlations involving 87Sr/86Sr improve (e.g. that with Ba/Nd to r = 0·81).
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. The regression lines in (e) and (i) apply only to samples from east of 100°E (i.e. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONSAMPLES AND METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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Mantle flow and role of hotspot mantle
Although their total isotopic range is substantial, all SEIR samples from west of the AAD possess unequivocally Indian-MORB-type isotopic signatures (e.g. Fig. 3a). Thus, the isotopic boundary at 126°E marks the westernmost extent of Pacific-type MORB along the SEIR. No evidence exists to suggest that mantle convection west of 126°E is stirring Pacific- and Indian-MORB-source mantle together in stringers much larger than the scale of sampling (
35 km), as would be indicated by along-axis interdigitation of the two types of basalts. Nor is there any evidence that the two types of mantle are variably mixing at scales similar to or smaller than the scale of sampling, as the presence of isotopic gradients and intermediate compositions between Indian and Pacific MORB types would indicate.
Nevertheless, the broad along-axis gradients in several of the isotopic patterns in the 86–118°E region (Fig. 3) may reflect some type of large-scale mantle flow and mixing. However, contrary to predictions based upon the regional eastward decrease in asthenospheric temperature (Marks et al., 1991; Sempéré et al., 1997; West et al., 1997), a major eastward flow of hotspot mantle from either the Amsterdam–St. Paul or Kerguelen–Heard hotspot is not supported by our combined isotopic data. Going eastward, the ‘baseline’ 87Sr/86Sr value decreases slightly to as far east as 112·4°E, a trend that appears consistent with eastward flow and gradual mixing of high-87Sr/86Sr hotspot-derived mantle into relatively low-87Sr/86Sr, ambient MORB-type mantle. However, the along-axis variations in Pb isotope ratios are not consistent with such an interpretation. Both hotspots have high 208Pb/204Pb, yet 208Pb/204Pb shows a broad eastward increase over much of the region, from 90°E to 107·5°E, just the opposite of the trend expected if high-208Pb/204Pb mantle emanating from either hotspot were flowing east and mixing with normal MORB-type mantle. The same is true for 206Pb/204Pb, particularly for Amsterdam–St. Paul hotspot mantle. Likewise, both hotspots have high 8/4, yet over most of the study area, from about 90°E to 117°E, 8/4 exhibits an overall increase to the east rather than the west. Also, although both hotspots have low Nd, particularly the Kerguelen–Heard hotspot, almost no large-scale gradient in Nd is present east of about 90°E.
Moreover, the along-axis isotopic variations cannot obviously be explained by appealing to long-term temporal changes in hotspot mantle composition; that is, that the SEIR in the 86–118°E region taps ancient hotspot-derived mantle that is isotopically different from recent hotspot mantle. Cretaceous and Early Tertiary lavas of the Kerguelen Plateau and Broken Ridge (products of the ancestral Kerguelen–Heard hotspot; e.g. Duncan & Storey, 1992) and the Ninetyeast Ridge (produced by the Kerguelen–Heard hotspot and perhaps partly by the Amsterdam–St. Paul hotspot; e.g. Luyendyk & Rennick, 1977; Duncan & Storey, 1992) encompass a wider range of isotopic variation than recent ones (e.g. Davies et al., 1989; Weis et al., 1989; Saunders et al., 1991; Salters et al., 1992; Mahoney et al., 1995; Frey et al., 2002; Ingle et al., 2002; Neal et al., 2002). However, like the younger lavas, the Cretaceous and Early Tertiary lavas lack the combination of features required of any hotspot end-member that would account for the 86–118°E isotopic patterns. For example, the old hotspot lavas do not have high 8/4 together with relatively low 87Sr/86Sr (as would be required of one end-member for much of the eastern part of the study area) or vice versa (as for much of the 90–100°E portion).
He isotopes may at first seem to offer support for a general eastward flow (and aging and/or dilution) of hotspot mantle. SEIR lavas near the Amsterdam–St. Paul hotspot reach much higher 3He/4He (to 14·1 RA; Graham et al., 1999) than the 86–118°E or AAD lavas and, east of 88·0°E, 3He/4He shows an overall eastward decrease. The He isotopic ratios of Kerguelen–Heard lavas are widely variable, with 3He/4He ranging as high as 18 RA; the higher values are presumed to reflect a high-3He/4He hotspot mantle source (the lower values may result from variable interaction of hotspot magmas with the relatively old, thick Kerguelen Plateau lithosphere on which these volcanoes sit) (Hilton et al., 1995; Valbracht et al., 1996; Nicolaysen et al., 1998). However, along the SEIR, 3He/4He not only decreases to the east of 88°E but also decreases to values as low as 7·6 to the west of 88°E (Fig. 3d), a feature difficult to explain by a regional eastward flow of mantle from hotspots far to the west of our study area. Indeed, Graham et al. (1999) concluded that asthenosphere in the vicinity of the Amsterdam–St. Paul hotspot is at present flowing mostly to the NNE.
Rather than emanating from a hotspot in a relatively steady flux, hotspot-derived material conceivably might enter a general eastward mantle flow only periodically, in pulses separated by millions of years, eventually producing relatively complicated along-axis isotopic patterns that would contain more than one peak. In the 206Pb/204Pb pattern, for example, the broad peak at 107·5°E, the peak at 89·3°E, and possibly that at 95·9°E (Fig. 3c) could correspond to three such pulses. As noted above, however, peaks in one or more of the other isotope ratios fail to coincide geographically—or necessarily even in number—with those in 206Pb/204Pb. Nor are the locations of the peaks or troughs in one isotopic ratio shifted consistently to the west or east relative to those of other isotopic ratios [along-axis shifts in a consistent direction above shallow east-flowing mantle might be explicable as an effect of differential partitioning of the different elements in the melting region or, for He, just below it (Graham et al., 1992; Poreda et al., 1993; Mahoney et al., 1994; Schilling et al., 1999)]. For example, the westernmost peak in 3He/4He lies slightly west of that in 206Pb/204Pb but the easternmost He isotope peak is well to the east of that in 206Pb/204Pb. Although an eastward component of asthenospheric flow seems inevitable, given the bathymetric and thermal gradients across the 86–118°E region (Marks et al., 1991; Cochran et al., 1997; Sempéré et al., 1997; West et al., 1997) and the probable long-term eastward relative motion of the asthenosphere (Doglioni, 1990; Ricard et al., 1991; Smith & Lewis, 1999), we conclude that hotspot-derived mantle with either Amsterdam–St. Paul or Kerguelen–Heard compositions cannot be a dominant constituent of such flow.
Regardless of the nature of mantle flow and mixing, is mantle derived from either of these hotspots involved at all in generating MORB in the study area? Mixing of sufficient amounts of isotopically distinct hotspot-type mantle into moderately heterogeneous normal MORB-type mantle should produce isotopic arrays in Fig. 2 that point approximately toward the hotspot end-member’s composition. Fields for Kerguelen and Heard islands cover a wide isotopic range but are easily distinguished from the Amsterdam–St. Paul hotspot field (as represented by the volcanoes of St. Paul and Amsterdam islands), although the high-206Pb/204Pb ends of the Kerguelen and Heard fields approach the compositions seen at Amsterdam and St. Paul. Data for small subsections of the 86–118°E SEIR do not form well-defined arrays pointing to one hotspot field or the other in all isotope diagrams. Arrays defined by larger subsections, and by the 86–118°E dataset as a whole, are broad (and thus could potentially record some influence by both types of hotspot mantle), but the longest dimension of the arrays in Fig. 2a–c is oriented roughly toward the Amsterdam–St. Paul field rather than the low-206Pb/204Pb ends of the Kerguelen or Heard fields. As a group, therefore, the 86–118°E isotopic data appear to allow at least a modest role for Amsterdam–St. Paul-type or possibly high-206Pb/204Pb Heard-Island-type mantle, while permitting only a small influence, if any, of Kerguelen–Heard-type material having 206Pb/204Pb at the low end of the range observed for these islands (indeed, rather than representing hotspot mantle signatures, the Kerguelen and Heard lavas with the lowest 206Pb/204Pb may be contaminated by old continental material present in the lithosphere of the Kerguelen Plateau; Weis et al., 2002).
Additional insight is provided by an evaluation of incompatible element ratios; specifically, whether they are consistent with mixing between hotspot mantle and typical, moderately heterogeneous MORB-type mantle. In general, hotspot-type mantle will have relative enrichments in the highly incompatible elements compared with MORB-source mantle. Only a few combined isotopic and trace element data have been published for the moderately evolved lavas making up the islands of Amsterdam and St. Paul; many more data exist for the Kerguelen Archipelago and Heard Island, but most are for lavas considerably more alkalic (basalts, basanites, nephelinites, and phonolites) than the tholeiitic basalts erupted along the SEIR, and probably represent rather small fractions of partial melting (e.g. Barling et al., 1994; Weis et al., 1998). Also, in some of the island samples, abundances of mobile elements such as Rb, K, and possibly Ba may have been modified by subaerial weathering. Hence, not all of the measured incompatible element ratios in the island lavas are close to the values in their mantle sources. However, ratios of alteration-resistant, highly incompatible elements with closely similar bulk distribution coefficients, such as Nb and La, should not be changed much by either moderate amounts of alteration or even rather small amounts of partial melting [Nb tends to be slightly more incompatible than La (e.g. Niu & Batiza, 1997), so small-degree melts may have slightly higher Nb/La than their sources]. Figure 7j shows that the available data for both St. Paul and Kerguelen–Heard lavas have markedly lower Nb/La than required to explain the 86–118°E SEIR trend; that is, they do not lie along either a linear or hyperbolic extension of it. More trace element data are needed for Amsterdam–St. Paul hotspot lavas before any conclusive statement can be made, but the existing results provide no evidence that material derived from either hotspot plays more than a minor role in the 86–118°E SEIR source mantle. [As an aside, we also note that anciently subducted, recycled marine sedimentary material, whether supplied by hotspots or not, appears not to be a suitable mixing end-member for producing the elongate SEIR array, because such material would be characterized by high 208Pb/204Pb yet low, not high, Nb/La values (probably 0·3, on average; e.g. Rehkämper & Hofmann, 1997)].
The one place in the entire region where isotopic variations appear broadly consistent with the presence of a moderate amount of hotspot-derived mantle is at the western end of the study area, at 86–90°E (Fig. 3). Intriguingly, this is close to the location where, on the basis of satellite-derived gravity data, Small & Sandwell (1994) and Small (1995) proposed that an asthenospheric jet or channel some 1600 km long may carry mantle from the Kerguelen–Heard hotspot to the SEIR. However, according to the plume source–ridge sink model (Schilling, 1985), in which a residual connection remains between a hotspot and a spreading ridge after the ridge has overridden the hotspot, the predicted width of an axial geochemical anomaly 1600 km from a hotspot would be much smaller (<50 km) than the isotopic peak at 86–90°E. The geochemical evidence for a Kerguelen–Heard hotspot influence is weak, for samples from the western part of the study area do not define arrays that consistently point toward Kerguelen–Heard isotopic or isotope–element ratio space, and all of the samples from west of 95·8°E are incompatible-element-depleted N-MORB. In any case, if a flow channel from the Kerguelen–Heard hotspot does exist, our data indicate that its along-axis isotopic signal to the east has disappeared to ‘background’ values by about 90°E (Fig. 3).
Alternatively, Graham et al. (2001) recently suggested that the asthenosphere between St. Paul and the AAD may be vertically zoned, and that the local highs in 3He/4He along the SEIR, such as that at 88·0°E, may mark sites where comparatively deep mantle with relatively high 3He/4He is being tapped. Information on the relative depths and amounts of mantle melting is provided by the depth of the ridge axis and fractionation-adjusted major-element characteristics such as glass-group average Fe- and Na-oxide (with lower Fe corresponding to lower mean pressures and higher Na to smaller mean fractions of partial melting) (e.g. Klein & Langmuir, 1987). Rough correlations of the He isotope ratio with these parameters suggest that portions of SEIR mantle melting at relatively shallow depths tend to have lower 3He/4He values [Fig. 6a and b, and Graham et al. (2001)]. A weak tendency also is evident for the highest-8/4, most strongly Indian-MORB-type mantle to be associated with greater axial depths and lower amounts of partial melting (Fig. 6c and d) and, by implication, with relatively fusible mantle and perhaps a shallower mean depth of melting. Thus, the rough correlations in Fig. 6 are consistent with at least modest vertical compositional zoning of the sub-ridge mantle. However, the rather poor correspondence of the along-axis He isotope peaks to the peaks in other isotopic ratios (see above) would appear to require that if deeper, higher-3He/4He mantle is welling up in places, it and the shallower asthenosphere it invades must be heterogeneous from one place to another in Sr, Nd, and Pb isotopes.
Mantle isochrons?
Instead of variable mixing between an incompatible-element-depleted MORB mantle end-member and mantle derived from extant hotspots in the eastern Indian Ocean, mixing involving such an end-member and relatively high-206Pb/204Pb, incompatible-element-enriched material similar to the ‘C’ component proposed by Hanan & Graham (1996) to variably affect MORB worldwide may be invoked to account for the elongation of the arrays in Figs 2 and 7. C-type compositions are assumed to represent old recycled oceanic lithosphere, probably delivered to the MORB source mantle over time by multiple mantle plumes. Similarly, involvement of ‘low-6/4’ or ‘EM1’-like, ultimately continentally derived, material with low 206Pb/204Pb and Nd, and relatively high 87Sr/86Sr, 208Pb/204Pb, and 207Pb/204Pb may partly explain the breadth of the SEIR arrays in Figs 2 and 7, particularly for the samples from west of 100°E. Unfortunately, such a mixing hypothesis is very difficult to test because it is so general, but would be strengthened if other potential explanations for the 86–118°E data could be ruled out.
Rather than mixing, a possible alternative explanation for the observed relationships among isotopes and between isotopic and incompatible element ratios is that they preserve a record of ancient fractionation (melt enrichment or depletion) in the mantle now feeding the SEIR. If so, the rough correlations between isotopic ratios and their respective parent–daughter element ratios (Fig. 7a–e) could have broad age significance (e.g. Brooks et al., 1976). If these correlations are interpreted as mantle ‘errorchrons’, then ages calculated from the slopes of lines fitted through the data vary from about 200 to 300 Ma. Although the uncertainty on each of these ‘ages’ is very large, the values increase in the order 232Th–208Pb 238U–206Pb < 147Sm–143Nd 235U–207Pb. As noted above, no significant 87Sr/86Sr–87Rb/86Sr correlation is present if all the data are included; but if the samples from the western part of the study area are excluded, a Rb–Sr ‘age’ similar to that for Sm–Nd and 235U–207Pb is obtained.
Qualitatively, the differences among the three Pb-isotope ‘ages’ are reminiscent of those commonly seen in systems that, after a period of closed-system decay and ingrowth, have experienced relatively recent open-system modification of parent–daughter ratios. The analogy in the SEIR case presumably would be the modification of mantle U/Pb and Th/Pb ratios by melting and crystallization during sub-ridge magma production and differentiation, with Pb behaving as a less incompatible element than Th or U. More generally, the changes in parent–daughter ratios caused by magma genesis will tend to rotate and degrade any pre-existing isochronal relationships. That the Sm–Nd and Rb–Sr ‘ages’ are near the older end of the range of those for Pb isotopes is consistent with the relative fractionation (during recent melting and magma formation) of Sm/Nd and Rb/Sr being less than for U/Pb or Th/Pb. Assuming the ages implied by Fig. 7a–e correspond to a single event, the considerable scatter in the correlations (and ages) would reflect (1) differences in bulk distribution coefficients among the different parent–daughter pairs, (2) variations in melt fraction and magmatic differentiation, plus (3) intrinsic isotopic and elemental heterogeneity in the mantle source (see Dosso et al., 1999) not removed by and/or unrelated to any single past enrichment or depletion event or to the recent magmatism producing the SEIR basalts. An additional important factor for 207Pb/204Pb, in particular, is the comparatively great analytical uncertainty (±0·010) relative to the observed total range of variation in this ratio (0·093). Substantial intrinsic source heterogeneity in fact appears to be required by (1) the sizeable isotopic ranges present at low values of Th/Pb, U/Pb, and Rb/Sr, (2) the lack of any meaningful correlation between 87Rb/86Sr and 87Sr/86Sr when the data for samples from the western part of the study area are included and, for example, (3) the low-206Pb/204Pb WW10-145 samples and the 86–90°E samples, which fall off many of the correlations defined by the majority of samples. Furthermore, (4) within some individual dredge hauls the isotopic differences between samples, such as the 4·4 Nd unit difference between WW10-128-1 and -18, are much larger than can be accounted for solely by radiogenic ingrowth in 200–300 Myr with the measured—or any plausible—parent–daughter ratios.
A poorly defined 207Pb–206Pb ‘age’ of 815 ± 245 Ma can be derived from the slope of the data array in Fig. 2c (r = 0·56). This value, although significantly greater than the 200–300 Ma values estimated from the correlations in Fig. 7a–e, is less than the 1–2·5 Ga 207Pb–206Pb ‘ages’ found for many MORB datasets (e.g. Church & Tatsumoto, 1975; Sun, 1980; Galer, 1999), which, at large spatial scales, are commonly interpreted to represent an integrated or mean evolution age of source mantle that has experienced a fairly complicated open-system, several-billion-year history (e.g. White, 1993; but see also Albarède, 2001). In the 86–118°E SEIR mantle source, any 200–300 Ma fractionation event consistent with the correlations in Fig. 7a–e would probably modify preexisting 207Pb/204Pb–206Pb/204Pb relationships only rather modestly because, as just noted, such an event would only partially eradicate preexisting isotopic heterogeneity. As an illustration, Fig. 8 shows estimated 250 Ma source values, calculated assuming the source U/Pb ratios are half those measured in the respective SEIR lavas [i.e. assuming, simplistically, that recent magmatism has in all cases produced lavas with twice their source’s U/Pb value (see White, 1993; Dosso et al., 1999)]. The estimated 250 Ma source field is substantially smaller than the present-day field (mainly in its range of 206Pb/204Pb) but still sizeable.
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, mantle-source values 250 Myr ago, calculated assuming the U/Pb ratio in the source is half the value measured in the lava. Data for the two WW10-145 samples are not shown. Error ellipse is for measured values.
Assuming the correlations in Fig. 7a–e do have age significance, we note that the inferred ages bracket the long period of continental stretching and rifting that preceded breakup in East Gondwana, which was followed by fully developed seafloor spreading 160–170 Myr ago along the paleo-Southwest Indian Ridge (e.g. Coffin & Rabinowitz, 1988; Lawver & Gahagan, 1993). Therefore, one possibility is that the mantle now beneath the SEIR at 86–118°E may preserve a record of small amounts of asthenospheric melting and mantle veining associated with decompression via extension of the overlying lithosphere when this mantle was located beneath the East African–Madagascan–Antarctic region of Gondwana some 200–300 Myr ago. In this context, we note that MORB of the western Southwest Indian Ridge also define a rough 200 Ma Sm–Nd errorchron (le Roex et al., 1983), and that broadly similar values are indicated by available Sm–Nd isotopic data for the Central Indian, Carlsberg, and central–eastern Southwest Indian ridges (excluding segments nearest the Marion and Réunion hotspots) (Mahoney et al., 1989, 1992; Rehkämper & Hofmann, 1997). (Whether Pb and Sr isotope systematics for these ridges yield similar results is unclear because many of the analyzed samples are not glasses but crystalline basalts affected by variable amounts of seawater alteration, and few U and Th abundance data are as yet available.)
On the other hand, errorchrons in this same general age range also have been observed for ridges far outside the Indian Ocean. The most recent and well-documented example is the correlation of 206Pb/204Pb, 208Pb/204Pb, and Nd and Sr isotopes with their respective parent–daughter ratios reported by Dosso et al. (1999) for the 10–24°N and 38–41°N sections of the Mid-Atlantic Ridge. The corresponding ages overlap those inferred from the 86–118°E SEIR data and, in this particular case, they may be ascribed to the earliest stages of breakup of Pangea (Dosso et al., 1999). However, errorchron ages (mostly Sm–Nd) in the same general range also have been reported for oceanic areas that have had no known relation to continental breakup for many hundreds of millions or even more than a billion years; for example, for East Pacific Rise seamounts (Zindler et al., 1984; Graham et al., 1988; Niu et al., 1996). Thus, a common interpretation of the ages implied by correlations of isotopic and parent–daughter ratios in MORB is that they represent a typical ‘turnover’, ‘replenishment’, or ‘recycling’ time of MORB asthenosphere in an open-system mantle. However, they may more likely just be a consequence of chemical equilibration times in a system undergoing mixing between different mantle reservoirs; if so, they have no particular chronological significance (Albarède, 2001). At present, we consider this explanation to be as viable for the 86–118°E SEIR as a Gondwanan-breakup connection.
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CONCLUDING REMARKS |
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TOPABSTRACTINTRODUCTIONSAMPLES AND METHODSRESULTSDISCUSSIONCONCLUDING REMARKSREFERENCES |
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The distinctive Pb–Nd–Sr isotopic character of Indian MORB is generally attributed to the past incorporation and dispersal in the asthenosphere of material derived from old continental lithosphere and/or anciently subducted sediments (possibly, but not necessarily, delivered to the Indian MORB source region by mantle plumes) (e.g. see Mahoney et al., 1992
; Rehkämper & Hofmann, 1997). Among sampled parts of the Indian Ocean ridge system far from currently active hotspots, the comparatively low Nd of much of the SEIR (Fig. 4) suggests that the eastern Indian Ocean asthenosphere may contain—or retain—slightly more such material, on average, than that beneath ridges farther west. Alternatively, the material that affected the sub-SEIR mantle had slightly lower average Nd than that which affected the sources of sampled portions of the Central and Southwest Indian Ridges.
The Indian MORB isotopic signature has been proposed to be both a comparatively youthful feature, perhaps not much older than the early stages of the breakup of Gondwana (e.g. Storey et al., 1988, 1989; Mahoney et al., 1989, 1992; Weis & Frey, 1996), and a very ancient one (i.e. >1 Ga) (e.g. Hart, 1984; Dosso et al., 1988; Crawford et al., 1995). Studies of old Indian MORB and of Tethyan crust formed to the north of East Gondwana (Pyle et al., 1995; Weis & Frey, 1996; Mahoney et al., 1998; Zhang et al., 2000) reveal that Indian-MORB-type isotopic compositions existed in places as long ago as the Late Jurassic in the Indian and southern Neo-Tethyan oceans, although they appear to have been distributed less uniformly than at present because some of the old basalts also lack Indian-MORB-type signatures, unlike the MORB in the main part of the Indian Ocean today. Regardless of whether the correlations in Fig. 7a–e reflect some type of mantle mixing or a widespread asthenospheric fractionation event associated with pre-breakup extension of Gondwanan lithosphere, the implied 200–300 Ma ages provide no evidence for an isotopic signature of truly great age in the mantle now beneath the 86–118°E SEIR. However, if the correlations do have any real time-significance, their y-intercepts suggest a source composition that would already have been slightly to moderately Indian-MORB-like 200–300 Myr ago. In turn, this would imply an age for the Indian-MORB-type signature in this portion of the mantle significantly greater than the maximum known age of the Kerguelen–Heard hotspot [119 Ma (Pringle & Duncan, 2000) or possibly 135 Ma (Frey et al., 1996)], which several workers have proposed to have been wholly or partly responsible for the introduction of the Indian MORB isotopic signature into the eastern Indian Ocean mantle (e.g. Storey et al., 1988, 1989; Mahoney et al., 1992; Weis & Frey, 1996). Both this consideration and the general lack of either a major Kerguelen–Heard or Amsterdam–St. Paul influence in 86–118°E MORB suggest that the effect of material derived from these hotspots on the composition of asthenosphere in the southeastern Indian Ocean is much smaller than believed previously.
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ACKNOWLEDGEMENTS |
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We are grateful to K. Spencer, D. Pyle, Z. P. Yang, and N. Hulbirt for help with various aspects of the work onshore, and to the marine technicians and crew of the R.V. Melville, whose competence and dedication ensured the success of sampling operations at sea. We thank F. Frey for helpful comments, and J. Lupton for insightful discussions and access to the helium isotope laboratory (supported by the NOAA Vents program). B. Hanan, D. Weis, and an anonymous referee provided valuable critical reviews. W. White kindly permitted use of his unpublished data for Amsterdam and St. Paul. This study was supported by NSF-OCE grants to the four senior authors.
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FOOTNOTES |
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*Corresponding author. Telephone: 1-808-956-8705. Fax: 1-808-956-5512. E-mail: jmahoney{at}soest.hawaii.edu
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