Indian Ocean Mid-ocean Ridge Basalts Research Articles

Low-3He/4He sublithospheric mantle source for the most magnesian magmas of the Karoo large igneous province

The massive outpourings of Karoo and Ferrar continental flood basalts (CFBs) ∼180 Ma ago mark the initial Jurassic rifting stages of the Gondwana supercontinent. The origin and sources of these eruptions have been debated for decades, largely due to difficulties in defining their parental melt and mantle source characteristics. Recent findings of Fe- and Mg-rich dikes (depleted ferropicrite suite) from Vestfjella, western Dronning Maud Land, Antarctica, have shed light on the composition of the deep sub-Gondwanan mantle: these magmas have been connected to upper mantle sources presently sampled by the Southwest Indian Ocean mid-ocean ridge basalts (SWIR MORBs) or to high 3He/4He plume-entrained non-chondritic primitive mantle sources formed early in Earth's history. In an attempt to determine their He isotopic composition and relative contributions from magmatic, cosmogenic, and radiogenic He sources, we performed in-vacuo stepwise crushing and melting analyses of olivine mineral separates, some of which were abraded to remove the outer layer of the grains. The best estimate for the mantle isotopic composition is given by a sample with the highest amount of He released (>50%) during the first crushing step of an abraded coarse fraction. It has a 3He/4He of 7.03±0.23 (2σ) times the atmospheric ratio (Ra), which is indistinguishable from those measured from SWIR MORBs (6.3–7.3 Ra; source 3He/4He ∼6.4–7.6 Ra at 180 Ma) and notably lower than in the most primitive lavas from the North Atlantic Igneous Province (up to 50 Ra), considered to represent the epitome magmas from non-chondritic primitive mantle sources. Previously published trace element and isotopic (Sr, Nd, and Pb) compositions do not suggest a direct genetic link to any modern hotspot of Indian or southern Atlantic Oceans. Although influence of a mantle plume cannot be ruled out, the high magma temperatures and SWIR MORB-like geochemistry of the suite are best explained by supercontinent insulation of a precursory Indian Ocean upper mantle source. Such a model is also supported by the majority of the recent studies on the structure, geochronology, and petrology of the Karoo CFBs.

The Dupal isotopic anomaly in the southern Paleo-Asian Ocean: Nd–Pb isotope evidence from ophiolites in Northwest China

It has been suggested that the Dupal isotopic anomaly in the mantle can be traced in the Paleozoic ophiolites from the Neo- and Paleo-Tethyan Ocean (275–350Ma). The Karamaili ophiolite (KO) and Dalabute ophiolite (DO) in the eastern and western corners, respectively, of the Junggar basin in NW China represent remnants of the relatively older (>350Ma) Paleo-Asian Ocean (PAO) crust. Thus, these ophiolites can provide additional constraints on the long-term composition and evolution of the Paleozoic suboceanic mantle. We present new major-trace element and Sr, Nd and high-precision Pb isotope data for the basalts, gabbros and a plagioclase separate from the KO and DO. Our results indicate that the PAO crust indeed has a Dupal-like isotopic signature. In detail, all samples have relatively low εNd(t) and high 208Pb/204Pb(t) for given 206Pb/204Pb(t) ratios (i.e., positive Δ8/4 values), similar to the Dupal isotopic characteristics of Indian Ocean mid-ocean ridge basalts (MORB). The trace element signature of DO mafic rocks is similar to that of normal- and enriched-MORB whereas that of the KO is transitional between MORB and arc basalt. Therefore, the DO mantle domain reflects the PAO asthenosphere and the KO domain additionally shows the influence of the subduction process. Geochemical modeling using Th/Nd as well as Nd and Pb isotopic ratios indicates that up to 2% subduction component had been added to a depleted Indian MORB-type mantle to produce the bulk of KO rocks. The subduction component in the KO rocks consisted of variable proportions of ≤1% partial melt of unradiogenic sediment similar to modern Izu–Bonin trench sediment and hydrous fluid dehydrated from the subducted altered oceanic crust. The Devonian asthenospheric mantle beneath the southern PAO is isotopically heterogeneous, but lends support to the idea that the Dupal isotopic anomaly existed prior to the opening of the Indian Ocean. Finally, plate tectonic reconstruction indicates that the anomaly was present in the Neo- and Paleo-Tethyan oceans in the southern hemisphere and in the southern part of PAO in the northern hemisphere during the late Paleozoic.

Origin of Diverse Geochemical Signatures in Igneous Rocks from the West Philippine Basin: Implications for Tectonic Models

The West Philippine Basin (WPB), formed by seafloor spreading between 60 and 35 Ma, provides an excellent case study of relationships between basin tectonics and magma chemistry. At 48 Ma, the Izu-Bonin-Mariana (IBM) arc formed along the basin edge, orthogonal to the active spreading center; thus, WPB development is a key issue for this Margins Subduction Factory focus area. WPB basalts from the main spreading stage are normal to enriched mid-ocean ridge basalt (MORB) with an Indian Ocean MORB isotopic signature. Basalts from the Benham Rise and locations near the western Central Basin Spreading Center (CBSC) at 50-35 Ma are geochemically identical to oceanic island basalts. Late-stage CBSC basalts (35-26 Ma) are isotopically like main spreading-stage MORB, with widely varying and decoupled trace element enrichments. Based on basalt geochemistry, the WPB could be a trapped fragment of ancient Indian/Tethyan ocean ridge, as proposed in some models for the initiation of the IBM arc, or it could be a back-arc basin, provided plate configurations allowed replenishment of sub-Indian Ocean asthenosphere. Ocean island basalts were formed by decompression melting of an enriched source beneath the western CBSC, mixing with normal MORB sources to form enriched MORB. This was a transitory feature (15 Ma) related to spreading, rather than a deep-seated plume, and probably did not affect the early IBM arc. Magma formed in small, deep-seated batches as the extension waned. That CBSC activity continued for 22 Ma after the initiation of the IBM arc indicates that forces related to an additional subduction system influenced the WPB.

Geochemical and isotopic characteristics of volcanic rocks from the northern East China Sea shelf margin and the Okinawa Trough

Volcanic rocks both from the northern East China Sea (NECS) shelf margin and the northern Okinawa Trough are subalkaline less aluminous, and lower in High Field Strength Elements (HFSE). These rocks are higher in Large Ion Lithophile Elements (LILE), thorium and uranium contents, positive lead anomalies, negative Nb-Ta anomalies, and enrichment in Light Rare Earth Elements (LREE). Basalts from the NECS shelf margin are akin to Indian Ocean Mid-Ocean Ridge Basalt (MORB), and rhyolites from the northern Okinawa Trough have the highest (207)Pb/(208)Pb and (208)Pb/(204)Pb ratios. The NECS shelf margin basalts have lower (87)Sr/(86)Sr ratios, epsilon(Nd) and sigma(18)O than the northern Okinawa Trough silicic rocks. According to (40)K-(40)Ar isotopic ages of basalts from the NECS shelf margin, rifting of the Okinawa Trough may have been active since at least 3.65-3.86 Ma. The origin of the NECS shelf margin basalt can be explained by the interaction of melt derived from Indian Ocean MORB-like mantle with enriched subcontinental lithosphere. The basalts from both sides of the Okinawa Trough may have a similar origin during the initial rifting of the Okinawa Trough, and the formation of basaltic magmas closely relates to the thinning of continental crust. The source of the formation of the northern Okinawa Trough silicic rocks was different from that of the middle Okinawa Trough, which could have been generated by the interaction of basaltic melt with an enriched crustal component. From the Ryukyu island arc to East China, the Cenozoic basalts have apparently increasing trends of MgO contents and ratios of LREE to Heavy Rare Earth Elements (HREE), suggesting that the trace element variabilities of basalts may have been influenced by the subduction of the Philippine Sea plate, and that the effects of subduction of the Philippine Sea plate on the chemical composition of basaltic melts have had a decreasing effect from the Ryukyu island arc to East China.

Onset of the Indian Ocean isotopic signature in the Philippine Sea Plate: Hf and Pb isotope evidence from Early Cretaceous terranes

Basalts from Paleocene–recent back arc basins within the Philippine Sea Plate have the Pb and Hf–Nd isotopic characteristics of Indian Ocean mid-ocean ridge basalts (MORB). We examined the isotopic composition of rocks from Early Cretaceous terranes within the Philippine Sea Plate, in order to understand the onset and origin of the Indian MORB isotopic signal. Igneous rocks from the Huatung Basin, a fragment of Early Cretaceous oceanic lithosphere sutured to the Paleocene–Oligocene West Philippine Basin, have an Indian MORB Hf–Nd isotopic signature, but Pb isotope ratios are intermediate between those of Indian and Pacific MORB. West Philippine Basin basalts have a stronger Indian Pb isotope signature than Huatung Basin rocks. Pre-subduction mantle sources for igneous rocks from the Amami Plateau, an Early Cretaceous island arc in the northernmost Philippine Sea Plate, have isotopic characteristics of Pacific MORB, and subducted components added to the mantle sources also have isotopic characteristics of Pacific MORB, plus sediment. The Indian MORB characteristics of Early Cretaceous Huatung Basin rocks lend support to the idea that mantle sources with this signature existed prior to the opening of the present day Indian Ocean and that Tethyan oceanic basalts, now found throughout southern Eurasia, shared them. As the West Philippine Basin opened, basalts formed from mantle sources with a more pronounced Indian Pb isotope signature than those tapped by the older Huatung Basin. Results for the Amami Plateau indicate that the Philippine Sea Plate also contains Pacific-type lithosphere, perhaps scattered within subduction terranes, such as the Philippine Islands, that were disrupted by basin opening. Taken together, results indicate that the lithosphere on which the proto-Izu-Bonin-Mariana arc formed was diverse, containing Pacific and Indian oceanic lithosphere, and old, depleted and subduction-conditioned arc mantle wedge.

Hafnium Isotope and Trace Element Constraints on the Nature of Mantle Heterogeneity beneath the Central Southwest Indian Ridge (13°E to 47°E)

Hafnium isotope and incompatible trace element data are presented for a suite of mid-ocean ridge basalts (MORB) from 13 to 47 E on the Southwest Indian Ridge (SWIR), one of the slowest spreading and most isotopically heterogeneous mid-ocean ridges. Variations in Nd–Hf isotope compositions and Lu/Hf ratios clearly distinguish an Atlantic–Pacific-type MORB source, present west of 26 E, characterized by relatively low eHf values for a given eNd relative to the regression line through all Nd–Hf isotope data for oceanic basalts (termed the ‘Nd–Hf mantle array line’; the deviation from this line is termed DeHf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature, present east of 32 E, characterized by relatively high DeHf values and Lu/Hf ratios. Additionally, two localized, isotopically anomalous areas, at 13–15 E and 39–41 E, are characterized by distinctly low negative and high positive DeHf values, respectively. The low DeHf MORB from 13 to 15 E appear to reflect contamination by HIMU-type mantle from the nearby Bouvet mantle plume, whereas the trace element and isotopic compositions of MORB from 39 to 41 E are most consistent with contamination by metasomatized Archean continental lithospheric mantle. Relatively small source-melt fractionation of Lu/Hf relative to Sm/Nd, compared with MORB from faster-spreading ridges, argues against a significant role for garnet pyroxenite in the generation of most central SWIR MORB. Correlations between DeHf and Sr and Pb isotopic and trace element ratios clearly delineate a highDeHf ‘ Indian Ocean mantle component’ that can explain the isotope composition of most Indian Ocean MORB as mixtures between this component and a heterogeneous Atlantic–Pacific-type MORB source. The Hf, Nd and Sr isotope compositions of Indian Ocean MORB appear to be most consistent with the hypothesis that this component represents fragments of subduction-modified lithospheric mantle beneath Proterozoic orogenic belts that foundered into the nascent Indian Ocean upper mantle during the Mesozoic breakup of Gondwana.

Trace Element and Sr, Nd, Pb and O Isotope Variations in Medium-K and High-K Volcanic Rocks from Merapi Volcano, Central Java, Indonesia: Evidence for the Involvement of Subducted Sediments in Sunda Arc Magma Genesis

Merapi volcano (Central Java), located within the Quaternary volcanic front of the Sunda arc, is one of the most active volcanoes of the Indonesian archipelago. During the Holocene, Merapi erupted basalts and basaltic andesites of medium-K affinity during its earlier stages of activity and high-K compositions over the past ∼1900 years. Merapi lavas and pyroclastic rocks are characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and higher 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios and lower 143Nd/144Nd ratios compared with Indian Ocean mid-ocean ridge basalt (MORB). Merapi high-K series rocks are enriched in LILE and LREE and slightly depleted in heavy REE (HREE) and HFSE compared with rocks from the medium-K series. The increase in K2O is accompanied by a marked increase in 87Sr/86Sr and a general decrease in 143Nd/144Nd, but not by systematic variations in δ18O values. The low δ18O nature of the Merapi magmas, and the lack of any major shift in isotopic compositions along the evolutionary trend of the two individual series, precludes extensive crustal assimilation during magma ascent and differentiation, emphasizing the importance of subducted continental material in the genesis of Merapi magmas. The compositional contrast between medium-K and high-K series volcanics largely reflects variable contributions from subducted sediment to the mantle wedge, which was similar to a MORB-source mantle before any subduction-related modification. The temporal change in the K2O content of the magmas reflects compositional variation in the mantle wedge rather than intracrustal processes occurring within the shallow volcanic system.

Petrogenesis and Geodynamic Implications of Late Cenozoic Basalts in North Queensland, Australia: Trace-element and Sr-Nd-Pb Isotope Evidence

Radiogenic isotopic (Sr–Nd–Pb) and trace-element compositions of late Cenozoic basalts from two discrete geographical regions in North Queensland, Australia, can be used to identify contributions from geochemically distinctive mantle source components. The North Queensland basalts have positive Δ7/4Pb and Δ8/4Pb values (relative to the Northern Hemisphere Reference Line), and high 206Pb/204Pb and 87Sr/86Sr at given eNd relative to Tertiary basalts in New South Wales, eastern Australia. The northernmost Cooktown nephelinites in North Queensland are isotopically depleted with low 87Sr/86Sr and 206Pb/204Pb and high 143Nd/144Nd compared with the more southern enriched Atherton–Nulla basalts. Sr–Nd–Pb isotopic data fit with two-component mixing between an isotopically depleted Indian Ocean mid-ocean ridge basalt source component and an enriched mantle component with an EM2 signature. The geochemical characteristics of the isotopically enriched Atherton–Nulla basalts are consistent with contributions from a subcontinental lithospheric mantle modified by subduction-related metasomatism. The isotopically depleted Cooktown nephelinites show HIMU-like incompatible element signatures that can be attributed to contributions from amphibole- and apatite-bearing assemblages in the lithospheric mantle. The low 206Pb/204Pb and high eNd of these depleted basalts are not correlated with their respective high U/Pb and low Sm/Nd parent/daughter element ratios. This decoupling implies that the formation of the metasomatic amphibole and apatite assemblages must be a close precursor of the magmatism, possibly connected with the eastward migration of the Indian Ocean asthenosphere and/or subduction at the northeastern margin of the Australian plate during early Tertiary time.

Volatile element isotopic systematics of the Rodrigues Triple Junction Indian Ocean MORB: implications for mantle heterogeneity

Mid-ocean ridge basalts (MORBs) from the Indian Ocean have been well-known to differ in Sr–Nd–Pb isotopic composition from those of the North Atlantic and the East Pacific. To understand if such heterogeneity exists also in volatile elements, we have measured elemental and isotopic composition of carbon, nitrogen, helium, and argon in trapped-gas-vesicles of the Rodrigues Triple Junction (RTJ) Indian Ocean MORBs (IOMs). The observed C–N–He–Ar isotopic ratios of the IOMs generally agree with average values of North Atlantic MORBs (NAMs) and East Pacific MORBs (EPMs). In contrast, the N 2/ 40Ar, C/N, and CO 2/ 3He ratios in fluid inclusions of the IOMs are quite unusual: current estimates of the N 2/ 40Ar, C/N, and CO 2/ 3He trapped in vesicles from the NAMs and EPMs are 91±20, 215∼4600, and (1.5±0.5)×10 9, respectively, whereas those of the IOMs are 120±36, 26∼190, and (0.3±0.1)×10 9, respectively. The 40Ar/ 36Ar and the 4He/ 40Ar * ratios indicate that the unusual N 2/ 40Ar, C/N, and CO 2/ 3He ratios of IOMs in this study have not been produced by atmospheric contamination or progressive fractional degassing, respectively, but more likely by mantle heterogeneity. The IOMs analyzed in this study seem to be depleted in the recycled carbon than the previous reported NAMs and EPMs. Assuming that values of IOMs in this study as those of the primitive (juvenile) mantle carbon, it is derived that most (65∼95%) carbon in the previously reported NAMs and EPMs is the recycled carbon (carbonate and organic matter) in origin, from a simple mass balance calculation. This may be one clue of the carbon recycling in the mantle.

Age constraints on crustal recycling to the mantle beneath the southern Chile Ridge: He‐Pb‐Sr‐Nd isotope systematics

Basalts from the four southernmost segments of the subducting Chile Ridge (numbered 1–4 stepping away from the trench) display large variations in Sr, Nd, Pb, and He isotope and trace element compositions. Klein and Karsten [1995] showed that segments 1 and 3 display clear trace element evidence for recycled material in their source (e.g., low Ce/Pb). The uniformly mid‐ocean ridge basalt (MORB)‐like 3He/4He and modest variations in Pb, Sr, and Nd isotopes of segment 1 (nearest the trench) suggest recent (<20 Ma) introduction of a contaminant into its source, consistent with recycling of material from the adjacent subduction zone. In contrast, segment 3 lavas display a dramatic southward increase in enrichment, extending to highly radiogenic Pb and Sr isotopic compositions (e.g., 206Pb/204Pb = 19.5) and the lowest 3He/4He yet measured in MORB (3.5RA). The segment 3 variations are most readily explained by ancient (∼2 Ga) recycling of terrigenous sediment and altered crust, but we cannot rule out more recent recycling of material derived from a distant continental source. The similarity in isotopic signatures of segment 4 lavas to Indian Ocean MORB extends the Dupal anomaly to the Chile Ridge. Like Indian Ocean MORB, the segment 4 isotopic variations are consistent with contamination by anciently recycled pelagic sediment and altered crust and require a complex history involving at least three stages of evolution and possibly a more recent enrichment event. Southern Chile Ridge MORB reflect the extensive degree of heterogeneity that is introduced into the depleted upper mantle by diverse processes associated with recycling. These heterogeneities occur on a scale of ∼50–100 km, corresponding to transform‐ and propagating‐rift‐bounded segmentation, and attest to the presence of distinct chemical domains in the mantle often bounded by surficial tectonic features that maintain their integrity on the scale sampled by melting.

Isotopic characteristics of subduction fluids in an intra-oceanic setting, Izu–Bonin Arc, Japan

New radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active Izu–Bonin arc, the Oligocene Izu arc, and their associated rift basins. As with many intra-oceanic island arcs, the Pliocene–Recent Izu–Bonin frontal-arc lavas are highly depleted in Zr, Nb and the rare-earth elements relative to typical mid-ocean ridge basalt (MORB), indicating that the mantle wedge source has undergone a previous episode of melting. Ratios between these elements (such as Nb/Zr and La/Sm), as well as 143Nd/ 144Nd, do not vary significantly along the length of the frontal-arc. These parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge. However, Ba/Zr, Ba/Rb and 87Sr/ 86Sr increase along the frontal-arc to the north. This leads us to propose that a variable enrichment in Ba and radiogenic Sr is superimposed on the mantle wedge. Sr–Nd and Pb–Nd isotope variation indicate that both Sr and Pb become more radiogenic after fluid addition. However, Pb isotope ratios do not correlate with increases in Pb concentration or ratios such as Ba/Zr and Nb/Pb. In other words, the Pb isotopic composition of the arc lavas appears to be independent of the amount of Pb introduced by subduction fluids into the mantle source. This buffering of Pb isotopes along the frontal-arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid. Isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities. Despite this, we are able to determine that the mantle wedge has isotopic characteristics similar to Indian Ocean MORB, and that the subduction-fluid solute is primarily derived from subducted oceanic basalt with a <2% contribution from subducted sediment. Lavas in the Oligocene Izu arc and fore-arc basin were derived from a mantle wedge of similar composition to the active arc. Despite levels of Pb enrichment comparable to those of the modern arc, the Pb isotopes of the Oligocene volcanics indicate a lower sediment input into the melting region.

Recycled ocean crust and sediment in Indian Ocean MORB

Mid-ocean ridge basalts (MORB) from the Indian Ocean are characterized by higher 87Sr/ 86Sr and lower 206Pb/ 204Pb, on average, than Atlantic and Pacific MORB. These distinct isotopic characteristics are thought to be the result of large-scale mantle contamination, either by ancient recycled sediments or delaminated subcontinental lithosphere. To constrain the upper mantle mixing end-members, we have analyzed the concentrations of Nb, U, Th, Pb and 17 other incompatible elements (by HPLC and ID-TIMS methods), as well as Sr, Nd, and Pb isotope ratios in nine hand picked MORB glasses from the Central Indian and Carlsberg Ridge. Nb/U and Ce/Pb ratios are sensitive indicators of crustal contaminants in the mantle, and such contaminations have been identified in EM-type ocean island basalts. Here we show the first case of highly significant correlations of Nb/U, Ba/Nb and Ba/Th with 87Sr/ 86Sr for Indian Ocean MORB and we confirm and strengthen a previously detected correlation with Ce/Pb (or Nd/Pb). These correlations are interpreted as residual heterogeneities that have not been completely rehomogenized by convective stirring of the mantle. The results are explained by a quantitative model involving contamination of an Atlantic-Pacific type upper mantle with a heterogeneous, recycled component consisting of ∼ 1.5 Ga old altered oceanic crust with small, but variable, amounts of ancient pelagic sediment added. This contamination causes higher 87Sr/ 86Sr, lower 143Nd/ 144Nd, generally low but variable 206Pb/ 204Pb and the distinct trace element correlations seen in the Indian Ocean MORB data. Mass balance calculations show that only small amounts of pelagic sediment are necessary to satisfy the criteria of the contamination model, and these can be subducted within as little as 200 Ma. Lead isotopic systematics constrain the recycled sediment component to be older than about 1.25 Ga. Apart from this, the model does not constrain the age of the contamination event since the results are equally valid for (1) “slow” mixing of the recycled component subsequent to subduction at ∼ 1.5 Ga or (2) storage of the material after subduction in a mantle boundary layer followed by relatively recent reinjection into the MORB source and “fast” mixing.

Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)

This paper is devoted to an exploratory geochemical study (trace elements, SrNd isotopes) of a suite of cumulate features cropping out in the mantle harzburgites of Oman. The cumulates are concentrically distributed around a fossil mantle diapir. From the centre to the periphery of the diapir, their mineralogy becomes more and more evolved, from troctolite to olivine gabbro and gabbronorite, and their textural and field characteristics point to injection in a cooler environment. Nd isotopic data are consistent with a mantle origin for all these cumulates. The mantle source is heterogeneous on a small scale (6.09 < ϵ Nd < 1015) but this heterogeneity and the average ϵ rmNd (≈8) are comparable to those of Indian Ocean mid-ocean ridge basalts (MORB's). An origin of this cumulate suite, largely by fractional crystallization from liquids with MORB characteristics, is supported by major and compatible trace element variations. However compatible elements show that it is unlikely that all these cumulates are derived by different degrees of fractional crystallization from the same liquid. This conclusion is corroborated by incompatible trace element data. The calculated liquids in equilibrium with these cumulates have MORB-like REE patterns. However, variations in REE patterns from one lithological group to the next, and within each lithological group, imply a pronounced heterogeneity in the REE content of the equilibrium liquids that clearly does not result from a simple fractional crystallization process. Liquids in equilibrium with the troctolites have a higher range in [La] n/[Yb] n than liquids in equilibrium with the olivine gabbros. The most evolved cumulates (gabbronorites) have REE patterns that might imply a more complex origin for their parental melt involving mixing of MORB-like liquids with melts from a LREE-depleted source.

Role of the Kerguelen Plume in generating the eastern Indian Ocean seafloor

Mid‐ocean ridge basalts (MORB) 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 spreading center in the Argo Abyssal Plain near northwest Australia, have high 143Nd/144Nd and low 87/86Sr 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 (&lt;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.

Source characteristics of the basement rocks from the Sulu and Celebes Basins (Western Pacific): chemical and isotopic evidence

New Sr- Nd- and Pb-isotopic and trace element data are presented on basalts from the Sulu and Celebes Basins, and the submerged Cagayan Ridge Arc (Western Pacific), recently sampled during Ocean Drilling Program Leg 124. Drilling has shown that the Sulu Basin developed about 18 Ma ago as a backarc basin, associated with the now submerged Cagayan Ridge Arc, whereas the Celebes Basin was generated about 43 Ma ago, contemporaneous with a general plate reorganisation in the Western Pacific, subsequently developing as an open ocean receiving pelagic sediments until the middle Miocene. In both basins, a late middle Miocene collision phase and the onset of volcanic activity on adjacent arcs in the late Miocene are recorded. Covariations between 87Sr/86Sr and 143Nd/144Nd show that the seafloor basalts from both the Sulu and Celebes Basins are isotopically similar to depleted Indian mid-ocean ridge basalts (MORB), and distinct from East Pacific Rise MORB, defining a single negative correlation. The Cagayan Arc volcanics are different, in that they have distinctly lower ɛNd(T) for a given ɛSr(T), compared to Sulu and Celebes basalts. In the 207Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, the Celebes, Sulu and Cagayan rocks all plot distinctly above the Northern Hemisphere Reference Line, with high Δ7/4 Pb (5.3–9.3) and D8/4 Pb (46.3–68.1) values. They define a single trend of radiogenic lead enrichment from Celebes through Sulu to Cagayan Ridge, within the Indian Ocean MORB data field. The data suggest that the overall chemical and isotopic features of the Sulu, Cagayan and Celebes rocks may be explained by partial melting of a depleted asthenospheric N-MORB-type (“normal”) mantle source with isotopic characteristics similar to those of the Indian Ocean MORB source. This asthenospheric source was slightly heterogeneous, giving rise to the Sr-Nd isotopic differences between the Celebes and Sulu basalts, and the Cagayan Ridge volcanics. In addition, a probably slab-derived component enriched in LILE and LREE is required to generate the elemental characteristics and low Nd(T) of the Cagayan Ridge island arc tholeiitic and calcalkaline lavas, and to contribute to a small extent in the backarc basalts of the Sulu Sea. The results of this study confirm and extend the widespread Indian Ocean MORB signature in the Western Pacific region. This signature could have been inherited by the Indian Ocean mantle itself during the rupture of Gondwanaland, when fragments of this mantle could have migrated towards the present position of the Celebes, Sulu and Cagayan sources.

Temporal evolution of the kerguelen plume: Geochemical evidence from 38 to 82 ma lavas forming the Ninetyeast ridge

Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have206pb/204Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher206Pb/204Pb, such as that expressed in lavas from St. Paul Island.

Sr‐Nd‐Pb isotope systematics of the Banda Arc, Indonesia: Combined subduction and assimilation of continental material

We present Sr, Nd, and Pb isotope results and SiO2, Rb, Sr, Sm, Nd, U, Th, and Pb data for six active volcanoes and one extinct volcanic island distributed over the whole length of the Banda Arc. Rock types range from low‐K tholeiitic in the NE to high‐K calc‐alkaline in the SW. The volcanoes in the NE have “normal” arc signatures (87Sr/86Sr = 0.7045–0.7055, 143Nd/144Nd = 0.51273–0.51291, and 206Pb/204Pb = 18.66–18.75), whereas those in the SW have extreme values (87Sr/86Sr = 0.7065–0.7083, 143Nd/144Nd = 0.51252–0.51267, and 206Pb/204Pb = 19.28–19.43). Serua, situated in the central part, is the most anomalous volcano with regard to its Sr and Nd isotopic composition (87Sr/86Sr = 0.7075–0.7095 and 143Nd/144Nd = 0.51240–0.51260) but not with regard to Pb isotopes (206Pb/204Pb = 19.02–19.08). The inactive island of Romang in the SW overlaps the Serua trends. The volcanoes display variable within‐suite ranges in 87Sr/86Sr and 143Nd/144Nd. Large ranges (e.g., at Nila) are consistent with assimilation (10–20%) of carbonate‐bearing sediments from the arc crust. Despite the evidence for assimilation, it cannot explain all of the Sr‐Nd isotopic trends found, and Banda Arc magmas must have already obtained a “continental” signature at depth before they reached the arc crust. Within‐suite trends of Pb isotopes are virtually absent. We found an extreme range in the volcanics along the arc which coincide with a similar trend in sediments in front of the arc and consider this as strong evidence for the contribution of subducted continent‐derived material to magma sources. Bulk addition of 0.1–2% of local sediment in the NE Banda Arc, and of 1–3% in the SW Banda Arc, to an Indian Ocean mid‐ocean ridge basalt (I‐MORB) source can explain the isotopic trends; both Serua and Romang require &gt; 5% sediment. The Pb isotopes (e.g., 207Pb/204Pb ‐ 208Pb/204Pb) also suggest changes in the mantle end‐member from I‐MORB to oceanic island basalt (OIB) source type. The latter becomes more conspicuous toward the SW and has the high 208Pb/ 204Pb characteristic of Indian Ocean (Dupal) OIBs. We hypothesize that mixing of magmas in the mantle wedge and/or in the arc crust was an important mechanism by which mantle and subducted end‐members were incorporated in the final products.

Contamination of Indian Ocean asthenosphere by the Kerguelen–Heard mantle plume

THE mid-ocean-ridge basalts (MORBs) from the Indian Ocean are isotopically distinctive. They have higher 87Sr/86Sr, 208Pb/206Pb and 207Pb/206Pb ratios than Pacific andAtlantic counterparts1,2 (see Fig. 1), suggesting that the upper mantle beneath the Indian Ocean has remained a partially isolated system2. The Australian–Antarctic discordance, a bathymetrically depressed segment of the South-East Indian Ridge, may represent a fundamental boundary between the Indian and Pacific mantle domains3. What processes are responsible for such domains? We argue here that the Kerguelen–Heard plume, which is producing Dupal-type basalts on Kerguelen and Heard Islands in the southern Indian Ocean, has contaminated large volumes of the Indian Ocean asthenosphere. This has produced the distinctive composition of Indian Ocean MORBs.

The O, Sr, Nd and Pb isotope geochemistry of MORB

We report analyses of O, Sr, Nd and Pb isotope ratios in 52 fresh glasses of mid-ocean ridge basalt (MORB). δ 18O-values in basalts range from +5.35 to +6.05‰ and do not correlate with indices of fractional crystallization or with 87 Sr 86 Sr . Values up to +6.47‰ occur in andesites and dacites from the propagating rift at 95°W on the Galápagos Spreading Center. Neither fractional crystallization nor assimilation of altered material appear to be dominant controls on the δ 18O of basalts. A weak, but statistically significant correlation was found between δ 18O and Nd and Pb isotope ratios in Pacific MORB which warrants further investigation. Otherwise, no statistically significant correlations between δ 18O and radiogenic isotope ratios were found, indicating that any O isotope variations in the depleted upper mantle are largely unrelated to incompatible-element enrichments and depletions. There are important and significant differences in variances of radiogenic isotopes in MORB from different ocean basins, but these differences do not appear to be related to spreading rate. We conclude that they reflect real differences in the homogeneity of the upper mantle beneath ocean ridges and that efficacy of mixing during magmatic processes is not a dominant control of the degree of heterogeneity observed in MORB. Pacific MORB has more uniform isotope ratios than MORB from other ocean basins. Radiogenic isotope ratios are most strongly correlated among Pacific MORB and least correlated among Indian Ocean MORB. Means of radiogenic isotope ratios of MORB from various ridges also vary considerably. However, there is no consistent relationship between means of various isotope ratios: means of both 87 Sr 86 Sr and 143 Nd 144 Nd are lower in Pacific than in Atlantic MORB; Pb isotope ratios in MORB from the two oceans are similar. Indian Ocean MORB has higher 87 Sr 86 Sr and lower 143 Nd 144 Nd than MORB from the Atlantic and Pacific but lower 206 Pb 204 Pb . Our data strongly reinforce the view that the depleted upper mantle consists of a number of distinct subreservoirs produced by a variety of processes. Although the Pb presently in the upper mantle may not have long resided there, it could not have been derived from a primitive reservoir. A positive correlation between 206 Pb 204 Pb and 208 Pb ★ 206 Pb ★ suggests the Pb in the mantle beneath the Pacific and Atlantic is derived from an enriched reservoir. The Pb isotope ratios in Indian Ocean MORB are suggestive of the presence of a recycled component.

Geochemistry of basalts from the Indian Ocean triple junction: implications for the generation and evolution of Indian Ocean ridge basalts

Basalts dredged from ridge axes within 70 km of the Indian Ocean triple junction in the western Indian Ocean have many geochemical and petrologic characteristics in common with depleted mid-ocean ridge basalts (MORBs) from the Atlantic and Pacific. For example there is overlap in major and trace element abundances, and in diagnostic ratios such as K/Rb (700–925) and La/Sm (less than chondritic). Also, glass inclusions in calcic plagioclase (An 89–90) provide evidence for a primitive high Mg/Fe, low TiO 2 melt. In contrast, basalts dredged from 250 to 400 km southwest of the triple junction on the Southwest Indian Ridge are compositionally distinct from depleted MORB. They are nepheline-normative or slightly hypersthene normative and have higher alkali metal and incompatible element abundances than depleted MORBs with similar MgO contents. All of these Indian Ocean basalts have Sr, Nd and Pb isotope ratios which corroborate previous studies showing that relative to depleted Atlantic and Pacific MORB, many Indian Ocean MORBs have low 206Pb/ 204Pb and high 87Sr/ 86Sr. However, individual Indian Ocean ridges have different radiogenic isotope characteristics, and basalts from the vicinity of the triple junction have unusually high 87Sr/ 86Sr (∼ 0.7032) at low 206Pb/ 204Pb ratios (17.3–18.2). Moreover, the shallow axial region of the Central Indian Ridge from ∼ 12°S to the triple junction (26°S) has high 87Sr/ 86Sr (> 0.7030). Apparently, the depleted component of Indian Ocean MORBs has been contaminated by an isotopically unusual component which does not occur in Pacific and Atlantic MORBs, and is not dominant in basalts from many Indian Ocean islands. The degree of this contamination is not uniform in western Indian Ocean MORB; the most contaminated basalts occur from 12°S on the Central Indian Ridge to the triple junction (∼ 26°S) and easterly along the Southeast Indian Ridge to ∼ 72°E.