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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B6, PAGES 13,831-13,849, JUNE 10, 1996 Role of the Kerguelen Plume in generating the eastern Indian Ocean seafloor Dominique Weis D6partement des Sciences de la Terre et de l'Environnement, Universit6 Libre de Bruxelles Brussels, Belgium Frederick A. Frey Department of Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology, Cambridge Abstract. Mid-ocean ridge basalts 0VIORB) in the Indian Ocean have Sr-Nd-Pb isotopic characteristics that distinguish them from seafloor basalts in the Atlantic and Pacific Oceans. These differences have important implications for mantle dynamics. We discuss the isotopic variation with eruption age of seafloor basalts recovered by deep sea drilling at 10 sites in the eastern Indian Ocean ranging in age from Eocene to Late Jurassic. Except for alkalic basalts recovered from near Christmas Island in the northeast Indian Ocean, the basement lavas are tholeiitic basalts that are characterized by a wide range in incompatible element abundance ratios, such as La/Yb and Zr/Nb. Most of the tholeiitic basalts from seven sites are geochemically similar to recent Indian Ocean MORB, but the alkalic basalts and tholeiitic lavas from two other sites have isotopic and incompatible element abundance ratios similar to lavas associated with the Kerguelen Plume. Two of these three sites, however, are not close to the track of this plume. The Dupal isotopic signature (relatively high 87Sr/86Sr and high 208pb/204pb at a given 206pb/204pb) is characteristic of lavas that have been attributed to the Kerguelen Plume, i.e., the Kerguelen Archipelago, Ninetyeast Ridge, and Kerguelen Plateau. Among eastern Indian Ocean seafloor basalts, a Dupal component is apparent in basement lavas from six of the seven drill sites in the eastern Indian Ocean that range in inferred age from -57 to 125 Ma. The oldest (-155 Ma) seafloor lavas recovered from the Indian Ocean, derived from a , A , spreading center in the Argo Abyssal Plain near northwest Australia, have high x, Nd/ß'raNd and low 87Sr/86Sr similar to the most depleted recent Indian MORB. Because the oldest volcanism on the Kerguelen Plateau (-118 Ma) is the first evidence of the activity of the Kerguelen Plume, this plume is inferred to be the source of Dupal isotopic characteristics in Indian Ocean MORBs. Some recent Indian Ocean MORB are also distinctive because many have relatively low 206pb/204pb (<17.4). Some of the oldest (110 to 155 Ma) seafloor lavas in the eastern Indian Ocean also have relatively low 206pb/204pb ratios. This low 206pb/204pb signature predates volcanism associated with the Kerguelen Plume and may reflect a significant role for continental lithosphere as a long-term source component for Indian Ocean MORB. Introduction Basalts erupted from active spreading ridge axes in the Indian Ocean define fields in Sr-Pb, Nd-Pb, and Pb-Pb isotopic space that are distinct from the fields of mid-ocean ridge basalts (MORB) erupted in the Atlantic and Pacific Oceans [e.g., Subbarao and Hedge, 1973; Duprd andAll gre, 1983; Hamelin et a/., 1985/1986; Michard et al., 1986; Price et al., 1986; Ito et al., 1987; Dosso et al., 1988; Mahoney et al., 1992]. These differences require that the basaltic Indian Ocean crust is derived from mantle sources that are unlike the sources of Atlantic and Pacific MORB. Hart [ 1988] proposed that oceanic island basalts (OIB) in the Indian Ocean are an important part of a large distinctive mantle isotopic domain (Dupal anomaly) that is Copyright 1996 by the American Geophysical Union. Paper number 96JB00410. 0148-0227/96/96JB-00410509.00 centered at -30øS and is defined by 87Sr/86Sr >0.705 and relatively high 208pb/204pb at a given 2 0 6pb/204pb. The distinctive isotopic characteristics of Indian Ocean MORB have been attributed to the influence of Dupal components from the Kerguelen Plume [Hatnelin et al., 1985/1986; Dosso et al., 1988; Storey et al., 1989], perhaps with contributions from the Crozet and Marion plumes [Mahoney et al., 1992]. An alternative explanation for the distinctive isotopic characteristics of Indian Ocean MORB, which is not mutually exclusive [Weis, 1992], is that ancient Gondwanaland continental lithosphere was dispersed and incorporated into the Indian Ocean MORB source during the breakup of Gondwanaland [Mahoney et al., 1989, 1992]. The geochemical characteristics of Indian Ocean seafloor as a function of eruption age are important in evaluating alternative interpretations for the distinctive geochemical features of recent Indian Ocean MORB. The Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) recovered basalts of variable age, up to 155 Ma, from several sites within the eastern Indian Ocean. We studied these basalts in order to assess the relative roles of components derived from depleted mantle, mantle 13,831 13,832 WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR plumes, and continental lithosphere in the sources of basalt at each DSDP site and to determine how the proportions of these components changed with eruption age. In this paper we focus on 10 sites that are not on the Ninetyeast Ridge; Prey and Weis [1995] focus on the Ninetyeast Ridge. Research Approach Excluding the Ninetyeast Ridge, the eastern Indian Ocean was sampled at nine DSDP sites and one ODP site (Figure 1). Petrographic and geochemical characteristics of basalts from these DSDP sites were reported in the initial reports for DSDP Legs 22, 26, and 27 [von der Borch et ed., 1974; Davies et ed., 1974; Veevers et ed., 1974], and a summary was given by Prey et al. [1977]. However, these previous studies did not include isotopic data for St, Nd, and Pb or precise abundance data for the incompatible trace elements, Rb, Ba, Nb, St, Zr, Hf, and Y. This paper focuses on the isotopic ratios and abundance ratios of highly incompatible elements of eastern Indian Ocean seafloor basalts because these ratios are sensitive measures of geochemical heterogeneity in the oceanic mantle; e.g., they distinguish MORB from OIB [e.g., Weaver, 1991; Hart et al., 1992]. Old ocean floor rocks have been affected by postmagmatic alteration; thus abundance data for Y, Zr, Nb, and rare earth elements (REE) are important because these elements are relatively immobile during postmagmatic alteration on the seafloor [Bienvenu et ed., 1990]. In our discussion, we use \"depleted\" to refer to basalts that have Rb/Sr, Nb/Zr, La/Yb, Ce/Y and Nd/Sm ratios less than the estimated bulk earth ratios [e.g., Sun and McDonough, 1989]; that is, these basalts (and their mantle sources) are relatively depleted in the highly incompatible elements, Rb, Nb, La, Ce, and Nd. With time these depleted sources develo 143Nd/144Nd greater than the bulk earth estimate and 87sr/g6sr less than the bulk earth estimate. Conversely, relative to bulk earth, enriched basalts have higher Rb/Sr, Nb/Zr, La/Yb, Ce/Y, Nd/Sm, and 87Sr/86Sr but lower 143Nd/144Nd. Analytical Techniques A subset of previously analyzed samples was selected to encompass the major element compositional range of these ocean floor lavas [Frey et ed., 1977]. Trace element abundances were determined by X ray fluorescence and instrumental neutron activation analysis (Table 1). Analytical procedures, and evaluation of data accuracy and precision are given by Prey et ed. [1991]. Trace element analyses were done on unleached powders, but for isotopic analysis, the samples were leached in acid to remove secondary alteration phases. We used a leaching lOON 0 o 10 ø 20\" ß 217 218 758 717 71 I 719 SINGAPORE 211 . h/HAlSTON BAS/N o 752, , 257 FREHANTLE 255 ! 264 90 ø 100 ø 110 ø 120 ø Figure 1. location map for the eastern Indian Ocean showing selected magnetic anomalies, and major bathymetric features such as the Ninetyeast and Broken Ridges. DSDP and ODP drill sites are indicated as solid circles. The 10 seafloor sites discussed in this paper are shown by larger numbers. WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 13,833 procedure comparable to Mahoney's [1987], i.e., \"cold\" acid leaching (HC1 6 N), with elimination of the fines by removing the acid immediately after 30 min in an ultrasonic bath (the time period of 30 rain is critical as it allows for a slight increase in temperature, which is necessary to strengthen the leaching effect). This leaching procedure was repeated until a colorless solution was obtained [Weis and Frey, 1991], up to 10 steps for the most altered samples. After acid leaching, the remaining powder was rinsed with quartz-distilled water at least three times. The powder was dried on a hot plate until a constant weight was achieved. The difference between this final weight and the starting weight is the weight percent loss caused by acid leaching; typically, this was between 50 and 60%. The large weight loss reflects dissolution of minerals formed during low-temperature alteration. The samples were processed following standard chemical separation procedures (i.e., HF-HC104 dissolution and anion exchange column separation of the different isotopes following the method described by Weis et al. [1987]). The blanks for the columns were below 3 ng Sr, and the total blanks for the whole procedure were below 6 ng for Sr and below 2 ng for Nd. These blanks are negligible relative to the concentrations in the samples. For Pb isotopes, the samples were processed in a clean, over pressurized (>3 mm Hg) laboratory, using reagents purified in a subboiling still. Pb was separated on artion exchange columns in a ItBr-HC1 medium, following a method derived from Manhds et a/. [1978]. Pb and U concentrations were measured on the same sample solution (aliquots were split before loading on columns ik w 235 206 e s w and sp' ed ith a U- Pb mix d pike). U as separated in a HNO3 medium. Total blank values for Pb for the whole chemical procedure were typically below 1 ng. Sr isotopic compositions were measured on single Ta filaments in the dynamic mode on a VG Sector 54 mass spectrometer. The internal precision formeasured 87Sr/86Sr is better than 1x 10 '5. The measured values are normalized to86Sr/88Sr = 0.1194. For each barrel of 20 filaments, four analyses of bIBS 987 Sr standard w 87 86 ere made. The average of Sr/ Sr over the time period the DSDP analyses were made is 0.710232 q- 8 (2c m for 18). An evaluation of between-run precision is also given by the replicate analyses reported in Table 2. Nd isotopic compositions were measured on triple Ta-Re filaments with the VG Sector 54 multicollector mass spectrometer (analyses of the Merck Nd s 143 144 + tandard yielded Nd/ Nd = 0.51173 _ 1 and 145Nd/144Nd = 0.348417 q- 5 (2c m for 12)). Nd was run as a metal, and for each run the 146, 145, 144, and 143 isotopes were measured with all values normalized to 146Nd/144Nd = 0.7219. Pb isotopic compositions and Pb and U concentrations by the isotope dilution (ID) technique were measured on single Re filaments with a Finnigan MAT 260 mass spectrometer, using the H3PO4-silica gel technique [e.g., Cameron eta/., 1969]. All the results were corrected for mass fractionation (0.13% q- 0.00% per ainu) on the basis of 72 analyses of the BIBS 981 Pb standard [Catanzaro et al., 1968] for a temperature range of 1090 ø to 1200øC. Between-run precisions are better than --0.1% for 206pb/204pb and 207pb/204pb and better than 0.15% for 208pb/204pb. The Pb and U concentrations have better than 2% precision. The evolution of isotopic ratios with time must be considered when comparing present-day radiogenic isotopic ratios in lavas of different ages. Mahoney and Spencer [1991] discussed this problem in regard to lavas from the Ontong Java plateau. They noted that the Rb/Sr, Sm/Nd, U/Pb, and Th/Pb in tholeiitic basalts are relatively low so that over 100 Myr there is relatively little change in 87 Sr/86Sr, 143Nd/144Nd ' 206pb/204 Pb, 207 pb/204pb, and 208pb/204pb. For example, in 120 Myra 238U/204pb = 20 (a relatively high value for unaltered oceanic basalts [White, 1993]) creates a change in 206pb/204pb of only 0.38. In addition, tholeiitic basalts are commonly interpreted to result from relatively high extents of melting. Assuming parent/daughter abundance ratios are not strongly affected by the partial melting process, the isotopic ratios in the sources and tholeiitic lavas evolve similarly with time. Therefore Mahoney and Spencer [1991] concluded that over 100 Myr, age corrections are relatively small, especially when compared to isotopic differences among OIB, MORB, and oceanic plateau basalts. However, postmagmatic alteration processes increase the complexity of inferring magmatic isotope ratios of old altered seafloor lavas, because postmagmatic alteration may affect isotopic ratios and parent/daughter abundance ratios. Typically, the isotopic ratios of Nd and Pb in oceanic basalts are not significantly changed during postmagmatic alteration, but formation of secondary phases with relatively high 87Sr/86Sr is common [e.g., Mahoney, 1987; Weis and Frey, 1991; Mahoney and Spencer, 1991; Staudigel et al., 1995]. As discussed earlier, in an effort to remove such secondary phases, the sampl,e.,powders were repeatedly acid-leached before determination of 8'Sr/86Sr. Although most of the samples define an inverse 143Nd/144Nd- 87Sr/86Sr trend similar to that of unaltered oceanic basalts (Figure 2), we cannot be certain that these procedures completely remove all effects of postmagmatic alteration. In contrast to unaltered tholeiitic basalts, corrections for radiogenic growth after eruption may be relatively large in highly altered lavas. This is especially true for Sr and Pb isotopic ratios, because Rb/Sr and U/Pb ratios may be significantly changed during alteration [e.g., Staudigel et al., 1995]. If alteration occurred soon after eruption, then over 100 Myr, the measured Sr and Pb isotopic ratios may differ considerably from those of the unaltered lavas at the time of eruption. In contrast, neither Sm/Nd nor 143Nd/144Nd in ocean floor basalts are usually significantly changed by postmagmatic alteration. In addition, the long half-life of 147Sm and low Sm/Nd lead to a relatively small age correction. For each DSDP site studied, Figure 2a shows fields for the present-day 87Sr/86Sr and 143Nd/144Nd values measured on the acid-leached residues. It also shows the age-corrected values calculated with the unleached whole rock Rb/Sr and Sm/Nd and ages inferred from magnetic anomalies or the age of the sediments overlying basaltic basement (Table 2). These corrections are likely to be too large for the Sr isotopic ratios because the Rb/Sr of the whole rock is probably larger than that of the acid-leached residue and the alteration processes did not occur instantaneously upon eruption. Moreover, there is an inherent uncertainty in calculating initial isotopic ratios because the age of the basalts is not precisely known. However, it is si, nificant that except for one sample from Site 212 (high 8 Sr/86Sr), age-corrected data points for 87Sr/86Sr and 143Nd/144Nd in acid-leached residues define a general inverse trend that largely overlaps the trend for recent MORB and OIB from the Indian Ocean (Figure 2b). Figure 3a shows the effects ofage correction on 206pb/200pb and 207pb/204pb based on the inferred minimum age of the basalt and the 238U/204pb measured on the acid-leached residues (Table 2). Accurate 232Th abundance data are not available (below INAA detection limit in MORB); thus no correction was made to 208pb/200pb ratios. U/ Pb ratios of Indian The 238 200 Ocean MORB are typically 5 to 10 [White, 1993], but in the acid- 13,834 WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 0.5134 0.5132 0.5130 0.5128 0.5126 0.5124 0.5122 0.5135 143Nd/144Nd 0.5133 0.5131 0.5129 0.5127 261 257 212 259 ' 215 260 211 87Sr/86Sr 0.5125 0.702 0.704 0.706 0.708 Figure 2a. The ratio of 87Sr/86Sr versus 143Nd/144Nd plotfor basalts from the eastem Indian Ocean seafloor. For each site, the two fields indicate the effects ofage corrections on 87Sr/86Sr and 143Nd/144Nd using measured Rb/Sr and Sm/Nd on unleached samples. Fields to the right are for acid-leached residues and fields to the left show age-corrected values. Ages (Table 2) were inferred from the age of the sediments overlying basaltic basement or nearby magnetic anomalies. ! 1 431 d/1 44Nd ß i - i ß i ß i ß N-MORB Ridges .)61 St. Paul 213 257 Ninetyeast Ridge Q '212 216 EM II 259 Kerguelen ( 211 215 Plume Christmas Isl. Kerguelen Archipelago Sediments //' ß O EMI ( 1 87 Sr/ lP6 ts , . , 87Sr/8.6Sr = 0.715 own to 0.702 0.703 0.704 0.705 0.706 0.707 0.708 143Nd/144Nd = 0.5120 Figure 2b. The ratio of 87Sr/86Sr versus 143Nd/144Nd plot comparing age-corrected fieldsfor old eastern Indian Ocean crust (this paper) to lavas from Ninetyeast Ridge drill sites with data for Site 216 shown as a distinct field [Mahoney etal., 1983; Hart, 1988; Saunders eta/., 1991; Weis and Frey, 1991; Frey and Weis, 1995], fields for recent Indian Ocean MORB (see references in works by Weis et al. [1992] and Le Roex et al. [1983], Hamelin et al. [ 1985/86], Michard eta/. [ 1986], Price eta/. [ 1986], Dosso eta/. [1988], and Mahoney eta/. [ 1992]), the mantle components normal MORB (NMORB), enriched mantle 1 (EM1), and EM2 [Zindler and Hart, 1986] and fields for the Kerguelen Archipelago [Gautier eta/., 1990; Weis eta/., 1993a, b], Christmas [Hart, 1988; Falloon et al., 1989], and St. Paul Islands [Dosso etal., 1988]. The Kerguelen Plume composition is as defined by Weis et al. [ 1993a]. The Banda Sea sediment field is from Vroon eta/. [1993]. Compared to the present-day fields for Indian Ridges and Ker u elen Archipelago, the fields of most DSDP sites (e.g., Site 259) are offset to lower 143Nd/144Nd at a given 87Sr/ø6Sr. This offset reflects aging ofthe mantle source; e.g., the arrow labeled \"200 Ma trend\" shows aging of a MORB-like source for 200 Ma (parent/daughter ratios for this source are averages fordepleted MORB, Rb/Sr =0.011 and Sm/Nd =0.336 [Hofmann, 1988]; because of changes caused by melting these are maximum and minimum ratios, respectively. WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 13,835 15.7 15.6 15.5 15.4 207pb/204pb 259 21 l ) 212 260 257 15.3 , , i , I , , ß , ......... ß 7.0 17.5 18.0 18.5 19.o Figure 3a. The ratio 207pb/204pb versus 206pb/204pb plot for old eastern Indian Ocean basalts showing the effects of age corrections for in situ U decay (measured fields on right and age- corrected fields on left; U/Pb measured on acid-leached residues). The age-corrections are especially large for Site 257 lavas, and the spread in 206pb/204pb is diminished by the age correction. Ages used for each site are given in Table 2. The true magmatic ratios are intermediate between the measured and age-corrected ratios (see discussion in text). leached residues of these DSDP samples, 238U/204pb ranges from 4.8 to 163 and only 9 of 18 samples have 238U/204pb below 30. Consequently, in Figure 3a the 206pb/204pb age corrections for some samples are significant. As with the age- corrected 87Sr/86Sr, we infer that the true magmatic 206pb/204pb ratios at the time of formation are intermediate between the measured and age-corrected ratios. An important result is that the age-corrected 206pb/204pb ratios for these DSDP basalts are within the range defined by young MORB and O113 from the Indian Ocean (Figures 3c and 3d). Because of the low abundance of 235U the age corrections for 207pb/204pb are less significant (Figure 3a). Results: General Although these basalts have been affected by postmagmatic alteration, major element analyses of the least altered lavas, and in some cases fresh glasses, show that the basalts are tholeiitic at eight of the nine DSDP sites studied [Frey et al., 1977]. Alkalic basalts containing titanaugite, amphibole, and small amounts of biotite and high abundances of relatively immobile incompatible elements (Figure 4) were recovered only at Site 211 [Frey et al., 1977]. Abundances of Zr and Ce are positively correlated with Nb abundance and reflect magmatic characteristics (Figure 4). Abundances of Rb and Ba are not as well correlated with Nb abundance which probably reflects the effects of postmagmatic alteration; the scatter shows the difficulty in making a reliable age correction on the basis of measured Rb/Sr. The Sr, Nd, and Pb isotopic ratios in these DSDP basalts range widely, but they generally overlap with the range defined by Indian Ocean MORB, lavas from the Ninetyeast Ridge, and lavas from the Kerguelen Archipelago (Figures 2b, 3c and 3d). In the following section, we discuss each site proceeding from west to east across the eastern Indian Ocean. Results: Specific Site 215 (8ø7.30'S, 84ø47.50'E) This site was drilled 240 km west of the Ninetyeast Ridge and is off the ridge at >5000m water depth (Figure 1). Approximately 25 m of tholeiitic basalt, composed of at least 14 pillowed flows, were penetrated beneath -59-60 Ma sediments [Hekinian, 1974]. Unaltered glass is abundant in this core, and the high K20 (-1%) and P20 5 (-0.25%) contents of these glasses indicate that these basalts are enriched in incompatible elements relative to depleted MORB [Melson et al., 1975; Frey et a/., 1977]. These glasses (15 samples) are similar in composition, and our study of four additional whole rocks from different core sections confirms the geochemical homogeneity of lavas at this site (Table 1). Site 215 basalts are more enriched in incompatible elements (i.e., higher Ce/Y and La/Yb and lower Zr/Nb) than basaltic lavas recovered from the Ninetyeast Ridge (Figure 5), a linear volcanic ridge (Figure 1) that is interpreted to be the trace of the Kerguelen Plume on the Indian Plate [e.g., Weis et al., 1992]. Relative to the transitional basalts of the Kerguelen Archipelago [Gautier et al., 1990], the Site 215 basalts have similar Ce/Y but tend to lower La/Yb and Zr/Nb (Figure 5). In addition, Site 215 basalts have 87Sr/86Sr, 143Nd/144Nd, 207pb/204pb and 208pb/204pb similar to these transitional Kerguelen basalts (Figures 2b, 3c and the 206 204 3d). Although calculated initial Pb/ Pb (17.94-17.99) are lower than in lavas from the Kerguelen Archipelago, the relatively low measured 206pb/204pb (18.10-18.15)overlap with those of the upper Miocene alkaline lavas of the southeast Kerguelen Archipelago, which have been interpreted by Weis et a/. [1993a] to be representative of the Kerguelen Plume (Figures 3c and 3d). Therefore basalts from Site 215 have the high A7/4 (8-9) and A8/4 (81-85) that define the Dupal isotopic anomaly and characterize the Kerguelen Plume [e.g., Gautier et al., 1990; Weis et al., 1989a, b], where A7/4 =[(207pb/204pb)sampl e - (207pb/204pb)NHRL] x 100, with the NHRL equation being 207pb/204pb = 0.1084 (206pb/204pb) + 13.491) and A8/4 = [(208pb/204pb)sample- (208pb/204pb)NHRL] x 100, with the 37.5 208pb/204pb 211256 213 215 37.0 ß 7.0 17.5 18.0 t . 212 259 257 206pb/204pb 18.5 19.0 Figure 3b. The ratio 208pb/204pb versus 206pb/204pb plot showing effects of age corrections on 206pb/204pb. (See Figure 3a caption.). No corrections areindicated for 208pb/204pb as no accurate 232Th abundance data are available. 13,836 WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 15.70 15.65 15.60 15.55 15.50 15.45 15.40 15.35 ß ! ß ! ß ! ß I - ! ß ß i ß ß ! ß 207 pb/204pb Kerguelen 2'12 E Plume 3 259 256 Track sedime( 1-2-3 260 216 Paul Christmas Isl. Indian Ridges I N-MORB Ninetyeast Ridge 211 Kerguelen Archipelago 206pb/204pb 15.3o . , ß , ß , . i , i , i , i , i , i , 17.2 17.4 17.6 17.8 18.0 18.2 18.4 18.6 18.8 19.0 19.2 Figure 3c. The ratio 207pb/204pb versus 206pb/204pb plotcomparing age-corrected fields for old Indian Ocean crust basalts (Figure 3a) to fields for the Ninetyeast Ridge (data for Site 216 are shown as a separate field), recent Indian Ocean MORB, mantle components, and the Kerguelen, Christmas and St. Paul Islands. Data sources are as given for Figure 2b. One sample of Site 260 (20-1, 16-18 cm) has a206pb/204pb > 20and has not been plotted. Track 1-2-3 sediment fields are data for sediments collected from three transects in the Banda Sea [Vroon et al., 1993]. 40.0 39.5 39.0 37.5 208pb/204pb EMI 257 K erg uelen S e di me n t s Track 256 Archipelago EM II 1-2-3 Kerguelen Plume 216 & 254 215 261 211 Indian Ridges N-MORB 213 259 Ninetyeast Ridge Christmas Isl. 206pb/204pb 37.o 17.2 17.4 17.6 17.8 18.o 18.2 18.4 18.6 18.8 19.o 19.2 Figure 3d. The ratio 208pb/204pb versus 206pb/204pb plot (only 206pb/204pb data have been age corrected, see Figure 3b). Fields are as in Figure 3c, except that data for the two Ninetyeast Ridge sites (216 and 254) with relatively low 206pb/204pb are indicated. WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 13,837 400 30O 200 100 8O 6O 4O 256 215 211 ß ß ß ß ß o 20 ß 0 ß I I , I , I ß I ß I , I , 0 10 20 30 40 50 60 70 80 Nb 100 4o ß I ' I ' 20 0 ' I ' I ' 500 400 300 200 100 q- 0 - ,Oi , , 0 l0 20 I ' I ' I ' I ' ß ß 211 $ 212 + 213 \" 215 x 256 . , [] 257 ß 259 a 260 o 261 I , I , I , I ' I ' I ' I , I I ß I . I . 30 40 50 60 70 Nb Figure 4. Abundance of various incompatible elements (inppm) versus Nb content (in ppm) in basalts from the. eastern Indian Ocean seafloor. The highest abundances are in the alkalic basalts from Site 211. Except for Rb and Ba, which have been affected by postmagmatic alteration, the abundances of incompatible elements are positively correlated. 8O NHRL equation being 208pb/204pb = 1.209 (206pb/204pb) + 15.627 [Han, 1984]. Site 213 (10ø12.71'S, 93ø53.77'E) This site is in the westem Wharton Basin 500 km east of the Ninetyeast Ridge and separated from the Ninetyeast Ridge by the Ninetyeast Fracture Zone. It is near the east-west trending Chron 25; if these basalts formed at a spreading ridge axis, they erupted at the east-west striking spreading center that became extinct in the middle Eocene (Figure 1). Eighteen meters of pillow basalt were recovered beneath 56-58 Ma sediments. In contrast to Site 215, glasses from this site have low K20 (0.06%) and P205 (0.09%) contents [Melson et al., 1975]. Frey et al. [1977] concluded that Site 213 basalts are depleted MORB. Our study of five samples confirms that most of the basalts in this core are depleted MORB in composition (Figures 4 and 5) and in Sr and Nd isotopic ratios (Figure 2b). However, the lowermost basalt studied from this core is geochemically distinct. It is relatively enriched in the incompatible elements Ba, Nb, St, Zr, and light REE (Table 1 and Figure 4)and has higher 87Sr/86Sr and lower 143Nd/144Nd than the sample with lower incompatible element abundances. However, its Sr and Nd isotopic ratios are within the Indian Ocean MORB field (Table 2 and Figure 2b). Compared to Indian Ocean MORB, this sample has anomalously high Pb isotopic ratios, e.g., 207pb/204pb = 15.65, which is also higher than those of lavas from the Kerguelen Archipelago (Table 2 and Figures 3c and 3d). Site 212 (19ø1134'S, 99ø17.84'E) This site was drilled in the deepest part of the Wharton Basin (6233 m) at the southern end of a long linear topographic high, the Investigator Ridge, and near the east-west trending Chron 34 (Figure 1). The sediment overlying the basalt lacks fossils, and the age of the basement is poorly constrained to be ~100 Ma [Sclater et al., 1974]; Powell et al. [1988] used magnetic anomalies to infer a basement age of 90 Ma. Five meters of pillow basalt were penetrated. These basalts are very altered, typically 5-10% weight loss on ignition [Hekinian, 1974]. Analysis of a single glass chip and whole rocks shows that these basalts have very low TiO 2 and Zr abundances [Melson et al., 1975; Frey eta/., 1977]. Our data for six basalts show that they have very high Cr abundances (~800 ppm, Table 1) which is consistent with the high MgO (9.0%) and CaO (13.5%) of the glass. The whole rocks also have low abundances of 13,838 WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR Table 1. Abundances of Trace Elements in DSDP Basalts From the Eastern Indian Ocean Site 215 18-2 18-2 18-3 47-53 106-110 110-112 Rb 15.5 14.5 10.6 Sr 284 284 282 Ba 285 269 262 19-2 145-150 12.7 294 274 V 213 187 174 192 Cr 258 237 299 215 Ni 105 101 100 92 Zn 72 64 58 64 Ga 17.2 18.4 17.6 17.7 Y 27.4 26.6 27.0 27.3 Zr 134 127 118 122 Nb 22.5 21.6 20.2 20.6 Hf 2.82 3.1 - La 14.5 16.2 Ce 47 32.6 36 Nd 16.8 20.1 Sm 4.17 4.5 Eu 1.41 1.5 Tb 0.75 0.8 Yb 2.56 2.5 Lu 0.37 0.51 43 Rb Sr Ba Site 213 17-2 17-3 18-2 19-2 19-2 108-110 90-99 115-117 54-56 127-130 12.3 10.5 6.5 13.3 13.3 127 136 121 122 260 7 15 14 176 V 230 256 236 248 219 Cr 327 319 330 354 225 Ni 90 104 82 101 71 Zn 106 84 151 132 146 Ga 15.8 17.0 16.2 16.5 18.6 Y 22.5 26.7 23.6 26.7 29.0 Zr 61 62 64 68 135 Nb 1.6 2.0 1.1 2.1 22 Hf 3.2 2.2 18.8 7.6 6.3 30.8 6.2 16.4 2.18 4.23 0.85 1.65 0.64 0.74 2.55 2.75 0.41 0.39 Ca Ce Nd Sm Eu Tb Yb Lu 3 19 incompatible elements such as Y, Zr, Nb, and REE; the erratic and high contents of Rb and Ba reflect the high extent of alteration (Table 1 and Figure 4). However, these low abundances are not accompanied by MORB-like Zr/Nb, Ce/Y, and La/Yb, which are more similar to the ratios in lavas from the Ninetyeast Ridge (Figure 5). One sample from Site 212 was analyzed for its isotopic compositions. It has a 143Nd/144Nd typical of depleted MORB, but even the acid-leached residue has still a very high 87Sr/86Sr (0.70755) that must reflect the extremely altered nature of these basalts [Hekinian, 1974]. Apparently, some of the postmagmatic phases were not removed by the acid leaching. With respect to Pb isotopic ratios, this sample is similar to the lowermost basalt studied at Site 213; that is, it has Pb isotopic ratios higher than Indian Ocean MORB and its 207pb/204pb (15.66) is higher than lavas from the Kerguelen Archipelago, comparable to those of Indian Ocean sediments [Vroon et al., 1993]. Hence it has a high A7/4 of 14.4, although its 208pb/204pb falls within the Kerguelen field (Figures 3c and 3d). Site 211 (9ø46.53'S, 102ø41.95'E) This site was drilled in deep water (5525 m) west of Christmas Island. Christmas Island is composed of alkaline basalts ranging WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 13,839 Table 1. (continued) Rb Sr Ba Site 212 39-1 39-1 39-2 39-2 39-2 39-3 134-136 146-149 0-14 39-42 60-64 145-147 39.0 368 24.9 18.7 20.2 31.1 75 112 72 67 68 93 153 11 9 116 V 224 189 240 206 221 206 Cr 820 782 881 764 786 872 Ni 144 138 148 144 141 160 Zn 86 78 93 71 84 76 Ga - 11.9 15.0 13.6 13.9 12.6 Y 16 13.8 11.9 15.2 16.9 15.7 Zr 45 31 30 24 26 32 Nb 3 2.1 1.2 1.3 1.3 2.1 Hf 0.64 La Ce Nd Sm Eu Tb Yb Lu , , 1.9 5 3.4 1.08 0.47 0.32 1.17 0.18 Rb Sr Ba 7.6 4.2 9.6 7.3 site 211 2.39 6.2 3.5 1.29 0.49 0.30 1.72 0.28 Diabase Sill Basement 12-1 12-1 12-2 14-2 15-2 15-2 15-3 15-3 15-3 23-25 143-145 100-102 55-61 14-16 95-97 40-46 40-45 67-70 42.0 41.0 45.1 42.0 18.9 70.4 61.7 59.3 91.4 447 540 514 447 265 535 595 482 530 469 534 488 489 151 501 526 432 508 v 154 Cr 299 Ni 116 Zn 94 Ga 19.9 15-4 70-73 52.4 411 426 Hf 4.12 Zr 193 Nb 54 Hf 4.12 147 165 215 163 119 118 102 146 137 274 222 92 94 53 57 58 72 85 136 101 107 103 68 63 79 118 111 82 90 113 111 87 80 107 101 85 18.0 18.2 19.9 20.1 20.0 20.5 20.0 21.2 19.0 183 201 342 368 323 304 338 333 52 57 70 76 67 63 70 68 84 75 81 74 La 35.3 Ce 70.7 Nd 29.3 Sm 6.01 Eu 1.93 Tb 0.69 Yb 1.75 Lu 0.26 75 65 84 86 5.82 300 61 5.82 37.3 77.3 32.4 7.04 2.29 0.78 2.28 0.33 in age from Eocene to Miocene [Smith and Mountain, 1925; Falloon eta/., 1989]. These basalts have relatively radiogenic Pb isotopic compositions with 206pb/204pb > 18.8 [Hart, 1988]. At Site 211 a 10-m-thick diabase sill (40Ar/39Ar age of 71 Ma [McDougall, 1974]) occurs 18 m above an amphibole-bearing basaltic basement that is inferred to be >76 Ma. Although the sill has significantly lower abundances of Nb and Zr than the basement lavas (Table 1 and Figure 4), all of the lavas from this site are alkalic basalts that are very enriched in incompatible elements relative to the lavas from the other sites (Figure 4). In terms of La/Yb, Ce/Y and Zr/Nb, Site 211 lavas are similar to the mildly alkaline suite of the Kerguelen Archipelago (Figure 5). These are the only alkalic lavas recovered from the eastern Indian Ocean sea floor. The Sr and Nd isotopic ratios of a basement lava from Site 211 are close to those of Christmas Island lavas (Figure 2b). Compared to lavas from Christmas Island, this Site 211 basalt has lower 206pb/204pb and 207pb/204pb (Figures 3cand 3d), but its 206pb/204pb ratio is nevertheless the highest value measured on samples of the northeastern Indian Ocean seafloor (except for an anomalous sample from Site 260, Table 2). The Pb isotopic ratios of the sill sample overlap with the upper Miocene alkalic 13,840 WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR Table 1. {continued Rb Sr Ba Site 256 9-3 9-3 9-3 10-1 10-4 10-4 11-1 11-3 15-17 52-54 138-140 141-143 62-64 114-116 27-30 148-150 5.1 2.8 5.1 6.6 1.8 2.6 4.4 3.6 191 168 176 172 186 172 166 170 60 59 64 56 28 46 51 V 545 424 426 486 416 410 426 Cr 116 110 100 120 130 155 102 Ni 73 78 73 76 85 90 77 Zn 134 136 127 139 134 141 127 Ga 21.3 20.5 20.5 21.2 21.9 20.0 19.0 19.9 Y 33.7 34.3 35.2 34.8 33.8 34.7 33.7 35.0 Zr 164 140 146 154 137 133 143 Nb 17.0 13.6 14.8 15.3 13.3 12.5 14.4 La Ce Nd Sm Eu Tb Yb Lu Rb Sr Ba 28 9.0 10.1 10.4 9.8 33 29 29 28 30 30 17 18 17 18 4.31 4.86 4.73 4.85 1.37 1.55 1.59 1.57 1.1 0.83 1.1 1.1 3.0 3.4 3.4 3.3 0.47 0.65 0.54 0.53 30 Site 257 11-1 11-3 14-5 16-1 16-3 17-1 17-1 17-5 122-124 25-27 25-27 65-67 31-33 70-72 97-99 67-69 16.1 9.5 13.0 6.3 0.5 31 50 0.5 204 76 76 93 80 80 87 83 229 114 11 41 12 11 33 34 V 217 285 325 288 256 243 274 264 Cr 425 492 183 365 435 179 232 226 Ni 83 117 87 116 100 77 121 88 Zn 75 92 93 81 70 79 82 83 Ga 14.0 13.3 17.0 16.0 15.7 14.9 15.0 15.7 Y 18.5 15.9 24.3 17.6 21.8 17.8 22.0 21.4 Zr 82 39 52 51 43 43 48 47 Nb 7.6 1.6 2.1 2.0 1.3 2.5 2.9 2.6 Hf 1.89 1.09 1.78 La 6.11 1.60 1.78 1.41 Ce 16.1 5.4 6.6 2.7 4.9 Nd 10.5 4.5 5.6 4.9 Sm 2.75 1.64 2.06 1.60 Eu 1.03 0.64 0.81 0.71 Tb 0.54 0.37 0.49 0.36 Yb 1.93 1.95 2.84 2.5 Lu 0.29 0.30 0.43 0.42 4.9 3.1 lavas of the Kerguelen Archipelago. Therefore among all lavas recovered by drilling from the eastern Indian Ocean seafloor, these Site 211 alkalic basalts are geochemically the most similar to recent lavas erupted in the Kerguelen Archipelago [Weis et al., 1993a, b]. However, Site 211 is not close to the track of the Kerguelen Plume or any other recognized plume. It is likely that lavas from this site are related to the volcanism that created the northeast trending bathymetric highs that form the Cocos-Keeling Plateau-Christmas Island complex (Figure 1). Site 256 (23ø27.35'S, 100ø46.46'E) This site is in the southern Wharton Basin. Although drilled in deep water (5361 m), the site is near a trend of bathymetric highs extending northeast from Broken Ridge, i.e., Golden Draak knoll, Batavia knoll, and Zeewk knoll (Figure 1) (see Figure 5 of Powell et al. [1988] for details). Based on fossils, the minimum basement age is 102 Ma; using magnetic anomalies, Powell et al. [ 1988] inferred a basement age of 125 Ma. Basement penetration of 19 m recovered Fe-Ti rich tholeiitic basalts. Both the major element and incompatible element abundances of Site 256 basalts are similar tobasalts recovered from the Ninetyeast Ridge qFi ure 5 and Frey et al. [1977]. Moreover, 87Sr/86Sr, 143Nd/t4'*Nd, 206pb/204 b a 207 204 6 a P ,nd Pb/ Pb in Site 25 1 vas overlap with the range of Ninetyeast Ridge basalts (Figures 2b, 3c and 3d). Site 257 (39ø59'S, 108ø21'E) DSDP Sites 257, 259, and 260 are near the western coast of Australia (Figure 1). This basaltic seafloor is inferred to have WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 13,841 Table 1. (continued) Site 259 Site 260 Site 261 38-1 41-1 18-2 20-1 33-1 34-1 35-2 36-1 39-1 65-67 101-104 140-142 16-18 101-105 75-77 120-123 60-63 11-13 Rb 7.2 12.1 3.6 3.1 20.3 10.9 14.2 24.4 6.1 Sr 91 99 129 112 81 87 98 91 85 Ba 7 64 35 20 12 V 270 260 479 391 333 323 431 539 321 Cr 148 185 125 106 172 181 38 22 165 Ni 59 45 210 51 85 76 55 53 78 Zn 112 132 178 151 104 92 213 166 94 Ga 16.8 17.0 21.9 19.8 17.1 17.5 21.1 23.2 18.0 Y 29.9 33.1 31.8 37.1 30.6 33.1 61.8 47.3 32.4 Zr 72 74 138 119 72 77 205 176 78 Nb 2.1 1.3 5.3 5.5 0.9 1.1 7.5 7.1 2.0 Hf 1.92 2.04 3.14 1.83 5.75 La 5.07 4.39 7.60 6.0 1.99 2.34 8.35 2.45 Ce 10.6 12.8 14.9 17 7.7 11 29.6 17.9 9 Nd 8.9 8.9 10.1 9.7 7.1 7.1 23.5 7.8 Sm 3.09 3.20 3.95 3.75 2.76 3.11 7.84 3.16 Eu 1.03 1.17 1.46 1.21 1.04 1.03 2.62 1.17 Tb 0.63 0.83 0.79 - 0.70 0.73 1.63 0.79 Yb 3.04 3.27 3.31 4.1 3.45 3.4 5.96 3.90 Lu 0.43 0.50 0.47 0.65 0.53 0.62 0.85 0.60 In parts per million. Sample designation indicates core and section number followed by an interval in centimeters. Data for REE (Lu through Lu) and Hf are by instrumental neutron activation at MIT; data for other elements are determined by X ray fluorescence at University of Massachusetts, Amherst. When Ce abundances are not accompanied by other REE data, Ce was determined by XRF. For discussion of precision and accuracy, see Frey eta/. [ 1991]. formed at the northeast-southwest oriented spreading ridge that separated Greater India from Australia [e.g., Markl, 1974; Veevers et al., 1974; Rundle et al., 1974; Fullerton et al., 1989]. At Site 257, Middle Albian -106-110 Ma sediments occur 13 m above the basaltic basement. Basement penetration was 64.5 m. Although basalts from core 1 ! have -100 Ma K/Ar ages roughly consistent with the age of the overlying sediments, much older K/Ar ages (157 to 196 Ma) were obtained from basalts lower in the core [Rundle et al., 1974]. Based on extrapolation of magnetic anomalies, inferred basement ages range from 110 to 130 Ma (Powell et al. [1988] and Luyendyk and Davies [1974], respectively). Most of the basalts from this core have incompatible element abundance ratios intermediate between SEIR MORB and lavas from the Ninetyeast Ridge (Figure 5). Their isotopic ratios overlap with the Indian Ocean MORB field (Figures 2b, 3c and 3d). In contrast, the uppermost basalts (core 11, section 1, Table 1) have La/Yb, Ce/Y and Zr/Nb similar to basalts from Site 256 and lavas from the Ninetyeast Ridge (see Fleet et al. [1976] and Figure 5). The uppermost lava is also similar to Ninetyeast Ridge lavas in Sr and Nd isotopic ratios, although its combination of Sr-Nd is not within Ninetyeast Ridge field (Table 2 and Figure 2b). It also has higher Pb isotopic ratios than the depleted lavas from Site 257 (Table 2). Although the accuracy of the K-Ar ages is unknown, it is intriguing that the K- Ar age (-100 Ma [Rundle et al., 1974]) of the uppermost basalts is also similar to the minimum age inferred for the enriched basalts at Site 256. However, like other Site 257 lavas, the enriched basalt has relatively low 206pb/204pb (17.57). This is much lower than that found in lavas from the Ninetyeast Ridge and Indian 0]]3. Thus the youngest Indian ocean crust sampled at Site 257 is geochemically enriched, similar to lavas subsequently erupted on the Ninetyeast Ridge. However, all basalts at Site 257 plot within the Indian Ridges field in Pb-Pb diagrams (Figures 3c and 3d) and have the low 206pb/204pb that is characteristic of many Indian Ocean MORB and which has not been found in lavas related to the Kerguelen Plume. Site 259 (29ø37'S, 112ø42'E) Based on the age of overlying earliest Aptian sediments, basement at this site is older than ~112 Ma; Powell et al. [1988] used magnetic anomalies to infer a basement age of 125 Ma. As at Site 257, the oceanic crust at Site 259 is presumed to have formed at a northeast-southwest oriented ridge. The Site 259 lavas have La/Yb, Ce/Y, and Zr/Nb typical of depleted SEIR MORB (Figure 5). The two analyzed samples are geochemically similar, except for a difference in Zr/Nb which probably reflects analytical error at these low Nb contents (--2 + 0.6 ppm, Table 1 [Rhodes et al., 1990]). The 87Sr/86Sr and 143Nd/144Nd overlap with the high 87Sr/86Sr end of the recent Indian Ocean MORB field; the offset of the age-corrected values to lower 143Nd/144Nd and 87Sr/86Sr (Figure 2b) is consistent with aging of a depleted mantle (MORB) source. In Pb isotopes, both lavas have 206 b 204p a P / b t the high end of the Indian Ocean MORB field. However, like the depleted basalts from Site 212 and the enriched basalt from Site 213, these Site 259 lavas have 207pb/204pb greater than any lava from the Kerguelen Archipelago (Figures 3c and 3d), with A7/4 of 14.0-14.3 and A8/4 of 32.7-60.7. Site 260 (16ø9'S, 110ø18'E) Like the basement at Sites 257 and 259, oceanic crust at Site 260 in the northeast Indian Ocean (Figure 1) is inferred to have 13,842 WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 50 40 30 20 10 I ' I ' I ' I ' I ' I ' I ' I Kerguelen Archipelago i © 212 I I + 213 I I e57 I I' 2591 I ø 26al I ß 765 I SEIR , n a : , . , . , . [ I I ß , :--, : -, . o Do . Kerguelen Archipelago NER SEIR q- o 0 !0 20 30 40 50 60 70 80 90 Zr/Nb Figure 5. Ce/Y and La/Yb versus Zr/Nb in basalts from the eastern Indian Ocean seafloor (data from Table 1 and average for Site 765 lavas from Ishiwatari [1992] compared to fields defined by lavas from the Kerguelen Archipelago [Storey et al., 1988; Gautier et al., 1990; Weis et al., 1993a], Ninetyeast Ridge [Frey et al., 1991], and Southeast Indian Ridge (SEIR MOP, B) [Price et al., 1986]; the range for Kerguelen Archipelago lavas from low to high La/Yb and CePs r reflects the evolution from older, --25 Ma, transitional basalts to younger, <10 Ma, highly alkaline lavas. We use these elements because they are relatively unaffected by postmagmatic processes. The selected ratios involve elements of different incompatibility and they illustrate the diversity of these DSDP lavas. These ratios clearly distinguish depleted MORB from O11t and enriched MOP, B; La/Yb indicates the slope of a chondrite-normalized REE plot (La/Yb is =1.48 in chondrites); Ce/Y (=0.39 in chondrites) is also plotted because there are more data for these elements (Table 1). These incompatible element ratios show that most of the basement sites in the eastern Indian Ocean have recovered basalts which are intermediate between depleted MORB and Ninetyeast Ridge lavas. Only Site 211 and Site 215 basalts are within the field of lavas from the Kerguelen Archipelago. formed from the northeast-southwest spreading center that separated Greater India from Australia. At Site 260, the recovered basalt is interpreted to be a sill that is overlain by 105 Ma sediments. Although 9 m of basalt was penetrated, only 0.5 m of core was recovered. Similar to basalts from Site 257, Ce/Y, Zr/Nb and La/Yb in the Site 260 lava are intermediate between SEIR MORB and lavas from the Ninetyeast Ridge (Figure 5). Consistent with this result, measured 8'Sr/86Sr and WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 13,843 Table 2. St, Nd, and Pb Isotopic Data and Pb and U Concentrations by Isotope Dilution in DSDP Basalts From the Eastern Indian Ocean Leg Site Sample Age, Ma 22 213 18-2 115-117 57 22 213 19-2 127-130 57 22 215 18-2 106-110 60 22 215 18-3 110-112 60 22 211 12-1 23-25 76 22 211 15-4 70-73 76 22 212 39-1 134-136 90 27 260 20-1 16-18 105 27 260 18-2 140-142 105 87Rb/86Sr 87Sr/86sra 2C m 87Sff86Sr 143Nd/144Nda 2Om end 143Nd/144Nd Initial b Initial b 0.155 (0.702696) (10) 0.70257 (0.513043) (13) 7.9 0.51296 0.702809 33 0.702611 14 0.148 (0.703082) (7) 0.70296 (0.512977) (14) 6.6 0.51292 0.703172 8 0.148 0.703109 7 0.70299 0.703173 6 0.148 (0.704451) (6) 0.70433 (0.512738) (9) 2.0 0.51268 0.704461 7 0.109 (0.704462) (6) 0.70437 (0.512723) (8) 1.7 0.51267 0.704420 8 0.272 (0.704194) (6) 0.70390 (0.512656) (24) 0.4 0.51259 0.704167 22 , 0.369 (0.704096) (7) 0.70370 (0.512824) (18) 3.6 0.51276 0.704118 21 1.51 (0.707548) (42) 0.70562 (0.513055) (34) 8.1 0.51292 0.080 (0.703702) (8) 0.70359 (0.512910) (10) 5.3 0.51276 0.703768 7 0.081 (0.703805) (6) 0.70369 (0.512872) (17) 4.6 0.51272 0.512853 21 4.2 0.0181 (0.703232) (7) 0.70320 (0.513128) (30) 9.6 0.51299 0.703194 10 0.703233 8 26 257 11-3 25-27 110 0.362 (0.703761) O) 0.70320 0.513104 d 168 d 9.1 0.51294 26 257 16-3 31-33 110 0.703721 8 0.228 (0.704114) (5) 0.70376 0.111 (0.704188) (8) 0.70399 0.704151 9 0.704227 9 0.0437 (0.704203) (8) 0.70412 0.704259 9 0.229 (0.704205) (11) 0.70380 0.704198 11 0.704257 11 0.704240 24 0.354 (0.703802) (5) 0.70317 26 257 11-1 122-124 110 (0.512835) (19) 3.8 0.51272 26 256 10-1 141-143 125 (0.512903) (16) 5.2 0.51277 26 256 10-4 114-116 125 27 259 38-1 65-67 125 (0.512855) (7) 4.2 0.51272 0.512860 25 4.3 (0.512885) (33) 4.8 0.51271 27 259 41-1 101-104 125 0.512886 35 4.8 (0.512938) (31) 5.9 0.51276 0.512965 49 6.4 27 261 33-1101-105 152 0.73 (0.703982) (12) 0.70242 (0.513208) (22) 11.1 0.51297 0.703979 18 27 261 35-2120-123 152 0.419 (0.703340) (10) 0.70243 (0.513380) (50) 14.5 0.51318 0.703341 20 0.703328 25 a The different numbers correspond to duplicate analysis on the VG54 mass spectrometer and show the between-run reproducibility. The number in parentheses is therun with the better precision and stability and is the one used in this 1 o r. ' c 87 147 238 \"Initial\" values, i.e., measured ratios orrected for in situ decay of Rb, Sin, U, and 235U, respectively, for the age given See analytical section in the text for discussion. c eN d calculated for the \"initial\" values and relative to bulk earth values at the age given for each sample (BE(O): 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967). a Very low intensity analysis, poor precision. This 143 Nd/144Nd value isused in the plots because it isnot significantly different from the value from another sample at the same site (Leg 26, Site 257, 16-3 31-33). end Initial c 7.8 6.9 2.3 2.1 1.1 4.3 7.8 4.8 4.1 9.6 8.7 4.4 5.6 4.7 4.6 5.5 10.4 14.4 143Nd/144Nd overlap with the enriched end of the MORB field i ure 2b) and like the Site 259 lavas, the offset of age corrected 144 87 86 leld for t B Nd/ Nd and Sr/ Sr from the fi recen MOR (Figure 2b) is consistent with aging of a depleted mantle source. The Pb isotopic characteristics of these two Site 260 samples are anusua! (Figure 3), one sample has relatively low initial 206pb/204pb (-17.5) and a high 207pb/204pb (15.5), whereas the other has unusually high initial 206pb/204pb (20.3) and 207 pb/204 Pb ( 15.8) (not plotted on Figure 3 ). o t Site 261 (12ø57'S, 117 54 E) This site in the northern Argo Abyssal Plain penetrated 47 m of basalt below sediments of 152 Ma (Figure 1). Thus these basalts are the oldest recovered by DSDP in the eastern Indian Ocean, and they are similar in age to the 155 Ma basalts recovered at ODP Site 765 in the southern Argo Abyssal Plain [Ludden and Dionne, 1992]. The basaltic core can be divided into three units [Robinson and Whi ord, 1974]. The uppermost unit A is a 10-m 13,844 WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR Table 2. (continued) Pb ppm lJ ppm 2381j/204pb 206pb/204pb 207pb/204pb 208pb/204pb 206pb/204pb 207pb/204pb Initial b Initial b 0.07 0.01 9.0 18.17 15.491 38.01 18.09 15.49 Ce/Pb 108.6 0.29 0.08 17.7 18.66 15.66 38.84 18.50 15.65 21.7 0.37 0.1 0.76 0.21 17.5 18.15 15.548 38.42 17.99 15.54 42.9 1.02 0.28 17.3 18.10 15.533 38.32 17.94 15.53 35.3 0.94 0.29 19.7 18.48 15.582 38.77 18.25 15.57 75.2 1.2 0.09 4.80 18.65 15.557 38.90 18.59 15.55 64.4 0.17 0.06 22.6 18.80 15.673 38.77 18.48 15.66 29.4 0.18 0.44 163 22.85 15.911 38.07 20.29 15.79 94.4 0.28 0.15 33.6 18.05 15.579 37.96 17.52 15.55 53.2 0.3 0.15 31.3 18.03 15.544 37.92 17.54 15.52 0.06 0.01 14.3 17.72 15.386 37.35 17.47 15.37 81.7 0.12 0.07 36.3 18.06 15.439 37.47 17.44 15.41 45.0 0.76 0.95 80 19.19 15.627 38.01 17.82 15.56 21.2 0.13 0.12 61 19.17 15.621 39.33 17.98 15.56 223.1 0.21 0.11 33.9 19.05 15.58 39.2 18.39 15.55 142.9 0.41 0.05 7.7 18.26 15.61 38.31 18.11 15.60 25.8 0.33 0.03 4.82 18.58 15.648 38.42 18.49 15.64 38.8 0.33 0.03 4.82 18.58 15.635 38.44 18.50 15.63 0.12 0.06 30.9 18.46 15.537 37.62 17.72 15.50 64.2 0.7 0.38 34.6 18.72 15.634 38.39 17.89 15.59 42.3 coarse-grained sill with a highl depleted MORB composition 87 86 143 144 e (Figure 5) whose Sr/ Sr and Nd/ Nd are at the depl ted end of the Indian Ocean MORB field (Figure 2b). Although the older, underlying units B and C are less depleted in incompatible element abundances than unit A, the s,a le from unit B has equally low 87Sr/86Sr and even higher 14ONd/144Nd (Table 2 and Figure 2b). These Site 261 lavas have Sr and Nd isotopic signatures similar to those of Site 765 basalts [Ludden and Dionne, 1992], and they have lower 87Sr/86Sr and higher 143Nd/144Nd than basalts from the other eastern Indian Ocean sites studied in this paper (Figure 2b). Site 261 lavas have the low 206pb/204pb ratios typical of some Indian Ocean MORB, but they have relatively high 207pb/204pb (15.50 to 15.59), although not as high as in depleted MORB samples from Sites 212 and 259 (Figure 3c). Discussion Occurrence of Enriched MORB At three of the studied DSDP sites, the recovered basalts are highly enriched in incompatible elements relative to MORB. At Sites 215 and 256, the tholeiitic lavas have isotopic and incompatible element ratios similar to lavas associated with the Kerguelen Plume (Figures 2b, 3c, 3d and 5). Because the Ninetyeast Ridge, which is interpreted to be a hotspot track related to the Kerguelen Plume [e.g., Weis eta/., 1992], is only 240 km east of Site 215, it is conceivable that the >60 Ma basaltic basement at Site 215 is related to the Ninetyeast Ridge. Site 256 is located on a northeast trending series of bathymetric highs emanating from Broken Ridge (Figure 1), which formed as the northern portion of the Kerguelen Plateau. This very large WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 13,845 plateau is also interpreted to be a manifestation of the Kerguelen Plume [e.g., Davies et al., 1989; Weis et al., 1989a; Salters et al., 1991; Storey et al., 1992; Miiller et al., 1993]. Ages for lavas from the Kerguelen Plateau range from 85 to 118 Ma [Leclaire et a/., 1987; Whitechurch et al., 1992; Pringle et al., 1994], whereas lavas from Broken Ridge have ages ranging from 63 to 89 Ma [Duncan, 1991]. Because of their geochemical similarities with lavas related to the Kerguelen Plume and their minimum -102 Ma and maximum 125 Ma age, Site 256 lavas may represent early volcanism related to the Kerguelen Plume. In contrast, the enriched alkalic lavas at Site 211 in the northern Wharton Basin cannot be directly related to the Kerguelen Plume. Site 211 is located on a series of northeast trending bathymetric highs whose origin is unknown, and their strike is not consistent with the trace of a known hotspot. The alkalic basalts at this site, however, have compositional similarities to the much younger basalts forming the Kerguelen Archipelago (Figure 5). Although lavas from nearby Christmas Island have higher 206pb/204pb than Kerguelen lavas, the Pb isotopic fields defined by lavas from Site 211 overlap the Kerguelen field (Figures 3b and 3c). In addition, a basement sample from this site has Sr and Nd isotopic ratios close to the low 87Sr/86Sr-high 143Nd/144Nd end of the range defined by lavas from the Ninetyeast Ridge and Kerguelen Archipelago (Figure 2b). Therefore lavas with geochemical characteristics similar to lavas associated with the Kerguelen Plume have erupted in locations where the volcanism cannot be directly related to the Kerguelen Plume. Lavas at two sites (257 and 213) range widely in ratios of incompatible elements but have Sr and Nd isotopic ratios close to or within the Indian MORB field. At Site 257, close to the southwest coast of Australia (Figure 1), the uppermost lavas are compositionally very similar to the enriched lavas at site 256. An important difference is that all of the relatively old Site 257 basalts have low initial 206pb/204pb (<17.8). This feature is a distinctive characteristic of some recent Indian Ocean MORB, which has not been observed in lavas from the Kerguelen Archipelago (Figures 3c and 3d). Site 213 is located in a region where the east-west magnetic lineations are remarkably clear (Figure 1), and the basement is inferred to have been formed from an east-west spreading ridge axis well north of the Kerguelen Plume [e.g., Royer et al., 1991]. Consistent with this interpretation, most of the Site 213 core is depleted MORB. However, the lowermost basalt in this core is enriched in incompatible elements (Table I and Figures 4 and 5) and has anomalous Pb isotope ratios that are much higher than Indian Ocean MORB. In fact, 207pb/204pb even exceeds that measured in lavas from the Kerguelen Archipelago (Figure 3c and Table 2). Occurrence of Depleted MORB Most of the basalts from six DSDP sites (Sites 212, 213, 257, 259, 260, and 261) and ODP Site 765, ranging in inferred ages from -56 to 155 Ma, are geochemically similar to Indian Ocean MORB. They have relatively low La/Yb and Ce/Y, (87Sr/86Sr)i <0.7038 (the Site 212 sample is an exception) and (143Nd/144Nd)i > 0.5127 and (eNd)i > 4.1(Figures 2 and 5). These basalts, however, have diverse Pb isotopic characteristics (Figure 3). Lavas associated with the Kerguelen Plume, i.e., those formin the Kerguelen Archipelago and Ninetyeast Ridge, have initial 206pb/204pb of 17.67 to 18.71 (Figure 6a). Enriched and depleted lavas from DSDP sites with ages of <125 Ma (Sites 213, 215, 211, 212, 256, and 259) have 206pb/204pb within this range (Figure 6a). These lavas contrast with some recent Indian Ocean MORB, some lavas recovered from the Kerguelen Plateau, and some of the lavas from the oldest eastern Indian Ocean drill sites, Sites 257 and 765, which range to much lower 206pb/204pb (to 17.30, Figure 6a). Thus unusually low 206pb/204pb is characteristic of both recent Indian Ocean MORB and relatively old seafloor in the eastern Indian Ocean, but it is not characteristic of <82 Ma lavas associated with the Kerguelen Plume (Figure 6a). Similarly, relatively high A8/4 values are associated with lavas from the Kerguelen Plume and lavas at the seven DSDP sites ranging in age from -56 to 125 Ma (Sites 213, 215, 211, 212, 260, 256, and 259). In contrast, lavas from DSDP Sites 257 and 261 have much lower A8/4 (Figure 6b). Lavas from DSDP Site 261 and ODP Site 765 are the oldest Indian Ocean seafloor studied, and they also have higher (143Nd/144Nd), >0.51285, than lavas associated with the Kerguelen Plume; thus they are similar to recent Indian MORB (Figure 6c). Source ofAnomalously High 207pb/204pb At three of the DSDP sites studied, some of the basalts have anomalously high 207pb/204pb (Sites 212, 213, and 259), i.e., higher than Indian Ocean MORB and lavas from the Kerguelen Archipelago (Figure 3c). Conversely, in the 208pb/204pb versus 206pb/204pb diagram (Figure 3d), most of these samples plot within the Kerguelen and Ninetyeast Ridge fields. In an oceanic environment, only sediments have such high 207pb/204pb. The Site 212 basalt has also ahigh age-corrected 87Sr/86Sr (0.70741) indicating that some of the postmagmatic phases were not removed by acid leaching. In Pb-Pb diagrams (Figure 3c), the samples with anomalous 207pb/204pb plot on trends between the Indian MORB field and the field for Indian Ocean sediments [Ben Othman et al., 1989; Vroon et al., 1993]. In general, abundance ratios of Ce/Pb are unusually high in these eastern Indian Ocean basalts (12 of 18 samples have Ce/Pb >40, Table 2, compared to a typical MORB ratio of -25 [Hofmann et al., 1986]). Samples with anomalously high 207pb/204pb, however, have lower Ce/Pb, <40 (Table 2). Mixing calculations by Ben Othman eta/. [ 1989] in their study of sediment recycling into the mantle indicate that the addition of only 1% sediment to the mantle leads to low Ce/Pb and anomalously high 207pb/204pb. We conclude that these samples may contain small amounts of sediment that were not removed by acid leaching. Therefore we do not use these high 207pb/204pb values in our discussion of source components. Origin of the Dupal Anomaly Following numerous previous studies starting with Subbarao and Hedge [1973], Duprg and Alldgre [1983] showed that many oceanic island basalts in the Indian Ocean have distinctive St, Nd, and Pb isotopic ratios, which Hart [1984] termed the Dupal anomaly. This large distinctive isotopic domain is centered at -30øS and is defined by 87Sr/86Sr >0.705 and A8/4 > +60 [Hart, 1988]. Hart [1988] noted that only three localities in the northern hemisphere have lavas with a Dupal signature. The distinctive isotopic characteristics of Indian Ocean MORB in comparison to Atlantic and Pacific MORB are interpreted to result from a Dupal component incorporated into the Indian Ocean asthenosphere. This isotopically distinctive asthenosphere has also been inferred to be the source for lavas erupted in the marginal basins of the western Pacific. Hickey-Vargas eta/. [ 1995] conclude that Indian Ocean asthenosphere, perhaps flowing in along the northern 19.25 18.75 18.25 17.75 17.25 0 20O 150 100 (206pb!204pb)i St. Paul Kerguelen Archipelago 0.5134 0.5132 Kerguelen Plume ß SWIR ITJ N inetyeast Ridge 211 ! Delta 8/4 Kerguelen Plume 215 St. Paul ITJ 212 Kerguelen Plateau drilled -- dredged - 257 259 256 20 40 60 80 1 O0 120 140 215 260 212 D 211 [] Kerguelen Archipelago Ninetyeast Ridge 213 257 Kerguelen Plateau drilled dredged 259 256 0 0 20 40 60 80 100 120 140 * ! 143Nd/144Nd ITJ 213 Ninetyeast o.513o SEIR Kerguelen Ridge 212 Archi D o.5128 765 261 !,[=8 !,[=20 160 Age Ma 765 261 0.5128 0.5124 SWlR Kerguelen Plume 215 211 257 256 259 Rajmahal 16o Age Ma ß C, 261 o.5122 , I , I , I I a I I I 0 20 40 60 80 100 120 140 160 Age Ma Figure 6. (opposite) Isotopic parameters (206pb/204pb)i, A8/4and (143Nd/144Nd)i versus age for eastern Indian 206 204 os ec Ocean seafloor. (a) All Pb/ Pb rati are age-corr ted. The upward arrow for Site 260 indicates that the ratio for a sill in this core is off-scale at 20.3. Because U and Pb data are not available for all lavas from Site 765, corrected ratios are shown for two assumed 238U/204pb ratios (u - 8 and 20). (b) The A values for the older basalts are maximum values because 206pb/204pb ratios have been age corrected, butthe 208pb/204pb ratios have not been corrected because precise Th abundance data are not available. (c) All 143Nd/144Nd ratios are age- corrected. ITJ, SWIR, and SEIR indicate Indian Ocean triple junction, Southwest Indian Ridge, and Southeast Indian Ridge, respectively. Dashed lines in Ninetyeast Ridge field (Figures 6a and 6b) are defined by data from DSDP Site 216 [Frey and Weis, 1995]. Field labeled Kerguelen Plume is from Weis et al. [1993a]. Data for Site 765 are revised from Ludden and Dionne [ 1992; J.N. Ludden, personal communication, 1995]; data from Heard Island are from Barling et al. [1994]; all other data sources are as indicated in caption for Figure 2b. WEIS AND FREY: EASTERN INDIAN OCEAN SEAFLOOR 13,847 boundary of Australia prior to -50 Ma, was an important source component for these western Pacific lavas. Of the 10 drill sites that recovered basalts from the eastern Indian Ocean, lavas at seven sites (<125 Ma) have A8/4 >33 (range 33 to 85). In contrast, the presence of a Dupal component is not obvious in the oldest lavas from the eastern Indian Ocean seafloor; for example, lavas from Sites 261 and 765 (-152 to 155 Ma) have very high 143Nd/144Nd (Figure 6c), and low 87Sr/86Sr (Figure 2b and Ludden and Dionne [1992]) and lavas from Site 261 have very low A8/4 (Figure 6b). Therefore, in the eastern Indian Ocean seafloor, there is evidence for a Dupal component that has persisted since --125 Ma. We emphasize that this 125 Ma age is not rigorously constrained because the ages of basement lavas at these DSDP sites have not been reliably determined. The earliest manifestation of Dupal characteristics in dated lavas is in the oldest lavas associated with the Kerguelen Plume; i.e., the 110- 118 Ma lavas forming the Kerguelen Plateau. Because =118 Ma corresponds to the first unambiguous evidence of activity of the Kerguelen Plume, we infer that the Dupal anomaly is carried by the Kerguelen Plume and that the source of this anomaly is deep within the mantle [e.g., Castillo, 1988; Weis et al., 1989a]. The origin of the geochemical characteristics of the Dupal anomaly [Duprd and Alldgre, 1983; Hart, 1984], with its subequatorial concentration, is an unresolved problem [Hart, 1988]. Most geochemical models require a long isolation time (1 to 2 Gyr) to develop the isotopic characteristics of Oltl. A commonly proposed origin for the Dupal anomaly is recycling of subcontinental lithosphere into the mantle by a delamination process [Hart et al., 1986; Hawkesworth et al., 1986; Har 1988; Sun and McDonough, 1989]. We propose that the concentration of the Dupal anomaly to the subequatorial southern hemisphere may be directly connected to the nearly fixed location of the African continent (and by extension the Gondwana supercontinent). This would allow either for a thermal blanketing [Anderson, 1982] or an underplating (thickening) of the lithosphere, which would favor delamination. An important aspect of the Indian Ocean and the eastern Atlantic Ocean is the occurrence of major flood basalt provinces on the surrounding continents [White and McKenzie, 1989]. Because continental flood basalt provinces reflect a large magmatic output in a relatively short interval of time, they could generate a thickened lithospheric mantle. This thickened lithospheric mantle could subsequently delaminate cold lithosphere into the asthenospheric mantle through a process of thermal erosion or delamination. The oldest flood basalts in this region are the Karoo and Ferrar, which erupted in the middle to early Jurassic. Delamination resulting from these volcanic events preceded the first manifestations of the Kerguelen Plume by <100 Myr. Source of Anomalously Low 206pb/204pb A distinctive feature of some recent Indian Ocean MORB is et al., 1996] have unusually low 206pb/204pb; (2) some of the oldest basalts (110 to 155 Ma) in the eastern Indian Ocean have lower 206pb/204pb than lavas associated with Indian Ocean plumes (e.g., Figure 6a and Mahoney et al. [1995]) and (c) the low 206pb/204pb component in Indian Ocean MORB has been present since the initial formation of the ocean, and, although minor in volume, it is widely distributed on each of the Indian Ocean ridge systems. Summary Eastern Indian Ocean seafloor basalts ranging in age from Eocene to late Jurassic are tholeiitic basalts, except at one site near Christmas Island where alkalic basalts were recovered. Both enriched and depleted MORB have been recovered. The isotopic characteristics of basalts younger than 125 Ma indicate the presence of a Dupal component (lavas from Site 257 are an exception) that is absent in the oldest (155 Ma) seafloor samples. The first evidence of activity of the Kerguelen Plume is at 118 Ma with volcanism on the Kerguelen Plateau. This leads us to conclude that the Kerguelen Plume is the carrier of the Dupal anomaly in the Indian Ocean. In addition, we propose that the concentration of this anomaly in the southern hemisphere is related to the nearly fixed location of the African continent above the mantle. This situation favored recycling of continental lithosphere into the mantle via delamination. Delamination resulted either from thermal blanketing or underplating. Some of the oldest seafloor lavas which predate volcanism associated with the Kerguelen Plume have the low 206pb/204pb values that are characteristic of some recent Indian Ocean MORB. Relatively low 206pb/204pb is typical of continental basalts in Madagascar and western Australia; therefore we infer that widely di?ersed continental lithosphere is the source ofthe low 206pb/20'*pb inIndian Ocean MORB. Relatively old, >45 Ma, eastern Indian Ocean seafloor resulted from the activity of three different spreading systems which have been active at different time periods; in order of decreasing age these are a nearly east-west striking ridge in the Argo Abyssal plain bordering Northwest Australia, a northeast-southwest ridge bordering southwest Australia, and an east-west spreading system in the Wharton Basin (Figure 1). Basalts from the oldest spreading center, sampled at Sites 261 and 765 in the Argo Abyssal Plain, are depleted MORB that are very much like lavas erupted along the active Southeast Indian Ridge. In contrast basalts from the Wharton Basin, sampled at Sites 213, 212, and 256, have isotopic ratios indicating the presence of a Dupal component which was derived from the Kerguelen Plume. Acknowledgments. We thank J. M. Rhodes for use of the XRF facility at the University of Massachusetts, Amherst; P. Ila for assistance in data acquisition; J.-P. Mennessier for help with the isotope chemistry; relatively low 206pb/204pb, e.g., lavas from the Triple Junction J.Scoates for editorial assistance; and H.-J. Yang for graphics. Wealso and portions ofthe SWIR (Figure 6a). Lavas from Sites 257 and thank J.Ryan, the JGR Associate Editor and an anonymous reviewer for 765 also range tolow 206pb/204pb (Figure 6a). The origin of the constructive reviews, and we have benefited from discussions with M. mantle component withlow 206pb/20d:pb is uncertain. Mahoney Coffin and J. Veevers. This research wassupported by U.S. NSF grants eta/. [1992] discussed two alternative possibilities: 1) Gondwana OCE-8823028 and EAR-9303535 and Belgian FRFC grant 2.9002.90. Precambrian lithosphere or (2) mantle plumes. The 206pb/204pb values of Indian Ocean basalts as a function of eruption age are References important in evaluating these alternatives. Evidence against a Anderson, D.L., Isotopic evolution of themantle: A model, Earth Planet. plume origin isthat recent lavas related toIndian Ocean plumes $ci. Lett., 57, 13-24, 1982. do not have similarly low206pb/204pb [Mahoney et' al.,1992]. Barling, J., S.L. Goldstein, and I.A. 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McKenzie, Magmatism at rift zones: the generation of volcanic continental margins and flood basalts, J. Geophys. Res., 94, 7685-7729, 1989. White, W.M., 238U/204pb in MORB and open system evolution of the depleted mantle, Earth Planet. ScL Lett., 115, 211-226, 1993. Whitechurch H., R. Montigny, J. Sevigny, M. Storey, and V. Salters, 40Ar/39A ages ofcentral Kerguelen Plateau basalts, Proc. Ocean Drill. Program $ci. Results, 120, 71-78, 1992. Zindler, A., and S.R. Hart, Chemical geodynamics, Annu. Rev. Earth Planet. Scœ, 14, 493-571, 1986. F. A. Frey, 54-1226, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139- 4307. (e-mail: fafrey@mit. edu) D. Weis, Maitre de Recherches FNRS, Dtpartement des Sciences de la Terre et de l'Environnement, CP 160/02, Universit6 Libre de Bruxelles, Avenue F.D. Roosevelt, 50, B-1050 Brussels, Belgium. (e-mail: dweis @resulb.ulb.ac.be) (Received March 1, 1995; revised January 22, 1996; accepted January 31, 1996.)
UBC Faculty Research and Publications
Role of the Kerguelen Plume in generating the eastern Indian Ocean seafloor. Weis, Dominique 1996-12-31
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