Southeast Indian Ridge Research Articles

Erratum: Corrigendum: Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge

Nature 432, 91–94 (2004). In this Letter, the quantity ɛHf in Fig. 3 and its legend should read ΔɛHf, which is the change in hafnium isotopic composition relative to the ɛNd–ɛHf mantle array.

Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge

The origin of the isotopic signature of Indian mid-ocean ridge basalts has remained enigmatic, because the geochemical composition of these basalts is consistent either with pollution from recycled, ancient altered oceanic crust and sediments, or with ancient continental crust or lithosphere. The radiogenic isotopic signature may therefore be the result of contamination of the upper mantle by plumes containing recycled altered ancient oceanic crust and sediments, detachment and dispersal of continental material into the shallow mantle during rifting and breakup of Gondwana, or contamination of the upper mantle by ancient subduction processes. The identification of a process operating on a scale large enough to affect major portions of the Indian mid-ocean ridge basalt source region has been a long-standing problem. Here we present hafnium and lead isotope data from across the Indian-Pacific mantle boundary at the Australian-Antarctic discordance region of the Southeast Indian Ridge, which demonstrate that the Pacific and Indian upper mantle basalt source domains were each affected by different mechanisms. We infer that the Indian upper-mantle isotope signature in this region is affected mainly by lower continental crust entrained during Gondwana rifting, whereas the isotope signature of the Pacific upper mantle is influenced predominantly by ocean floor subduction-related processes.

Fractionation of noble gases (He, Ar) during MORB mantle melting: a case study on the Southeast Indian Ridge

New measurements of the He, Ar and CO 2 abundances trapped in basaltic glasses from the Southeast Indian Ridge (SEIR) show that volatile concentrations in the SEIR magmas were controlled by fractional degassing. Fractionation between volatile species is consistent with their solubilities in silicate melts. As a result, there are linear relationships between (for example) ln( 4He/ 40Ar*) vs. ln[ 40Ar*] and between ln( 4He/ 40Ar*) vs. ln( 40Ar*/CO 2) (where 40Ar* is the 40Ar corrected for atmospheric contributions). The slopes of these correlations permit the relative He/Ar and Ar/CO 2 solubilities to be estimated; these are generally consistent with experimentally determined noble gas solubilities in basaltic melts. However, there are systematic differences in the degassing trajectories. For example, in a plot of ln( 4He/ 40Ar*) vs. ln( 40Ar*/CO 2), samples from the deepest portions of the ridge consistently plot at lower 4He/ 40Ar* for a given 40Ar*/CO 2, compared to shallower sections of ridge. These variations in 4He/ 40Ar* likely reflect variations in He/Ar in the primary melt, i.e. their relative abundances prior to degassing. We estimated the variation in 4He/ 40Ar* in the initial melts (i.e. the 4He/ 40Ar* prior to degassing) by extrapolating the degassing trend to a constant mantle-like 40Ar*/CO 2 ratio and assuming that the relative He–Ar–CO 2 solubilities do not vary between samples. The 4He/ 40Ar* corrected for degassing in this manner varies by a factor ≈10 and correlates positively with the 3He/ 4He ratio. It is possible that the correlation between “degassing corrected” 4He/ 40Ar* ratios and the 3He/ 4He ratio results from preferential diffusion of 3He relative to 4He and of 4He relative to 40Ar from the solid mantle into primary melts during melting. However, modeling this diffusive process fails to reproduce the comparatively large variations in 3He/ 4He found in the basalts; therefore, it seems likely that mantle heterogeneities, in combination with diffusive fractionation, resulted in coupled He isotope and He/Ar variations.

Absolute migration and the evolution of the Rodriguez triple junction since 75 Ma

The Rodriguez Triple Junction (RTJ) is a junction connecting three mid-ocean ridges in the Indian Ocean: the Southwest Indian Ridge (SWIR), the Central Indian Ridge (CIR) and the Southeast Indian Ridge (SEIR). The evolution of the RTJ has been studied extensively for the past 10 Ma and the triple junction is believed to be largely a ridge-ridge-ridge (RRR) triple junction. However, due to the scarcity of data, its configuration prior to that period is poorly understood. The migration of the RTJ in the hotspot reference frame, for the past 75 million years has been mapped, by reconstructing its traces on the three plates (Africa, Anartica and Indian) to their former positions. It is shown that the RTJ have migrated northeasterly at velocities varying from 10 cm/yr at 70 Ma to 2.6 cm/yr at 43 Ma and thereafter 3.6-3.8 cm/yr, in a fairly straight-line trajectory, suggesting a stable configuration of the RTJ since its formation. Because the RRR triple junction is the most stable configuration that is possible, it is suggested that the configuration of the RTJ has been largely RRR triple junction since its formation. Tanz. J. Sci. Vol. 28(2) 2002: 97-104

North Atlantic deep water in the south-western Indian Ocean

The circulation of deep water in the south-western Indian Ocean has been studied from hydrographic observations and current measurements, obtained during the Dutch–South African Agulhas Current Sources Experiment programme, and from similar public data from the World Ocean Circulation Experiment. The three major water masses involved are the saline North Atlantic deep water (NADW), its derivative in the Antarctic circumpolar current, lower circumpolar deep water (LCDW), and the aged variety of deep water, North Indian deep water (NIDW). Although bound by the shallow topography near Madagascar, about 2×10 6 m 3 /s from the upper half of the NADW core appears to flow across the sill in the Mozambique Channel into the Somali Basin, while the remaining NADW flows east at about 45°S and is transformed to LCDW by lateral and diapycnal mixing. East of Madagascar the deep circulation is dominated by the southward flow of NIDW. Northward inflow of LCDW into the Indian Ocean therefore can take place only in the eastern half of the Indian Ocean, along the Southeast Indian Ridge and the Ninetyeast Ridge.

North Atlantic deep water in the south-western Indian Ocean

The circulation of deep water in the south-western Indian Ocean has been studied from hydrographic observations and current measurements, obtained during the Dutch–South African Agulhas Current Sources Experiment programme, and from similar public data from the World Ocean Circulation Experiment. The three major water masses involved are the saline North Atlantic deep water (NADW), its derivative in the Antarctic circumpolar current, lower circumpolar deep water (LCDW), and the aged variety of deep water, North Indian deep water (NIDW). Although bound by the shallow topography near Madagascar, about 2×10 6 m 3/s from the upper half of the NADW core appears to flow across the sill in the Mozambique Channel into the Somali Basin, while the remaining NADW flows east at about 45°S and is transformed to LCDW by lateral and diapycnal mixing. East of Madagascar the deep circulation is dominated by the southward flow of NIDW. Northward inflow of LCDW into the Indian Ocean therefore can take place only in the eastern half of the Indian Ocean, along the Southeast Indian Ridge and the Ninetyeast Ridge.

Pacific-Antarctic-Australia motion and the formation of the Macquarie Plate

Magnetic anomaly and fracture zone data on the Southeast Indian Ridge (SEIR) are analysed in order to constrain the kinematic history of the Macquarie Plate, the region of the Australian Plate roughly east of 145°E and south of 52°S. Finite rotations for Australia–Antarctic motion are determined for nine chrons (2Ay, 3Ay, 5o, 6o, 8o, 10o, 12o, 13o and 17o) using data limited to the region between 88°E and 139°E. These rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the SEIR. An analysis of the synthetic flowlines shows that the Macquarie Plate region has behaved as an independent rigid plate for roughly the last 6 Myr. Finite rotations for Macquarie–Antarctic motion are determined for chrons 2Ay and 3Ay. These rotations are summed with Australia–Antarctic rotations to determine Macquarie–Australia rotations. We find that the best-fit Macquarie–Australia rotation poles lie within the zone of diffuse intraplate seismicity in the South Tasman Sea separating the Macquarie Plate from the main part of the Australian Plate. Motion of the Macquarie Plate relative to the Pacific Plate for chrons 2Ay and 3Ay is determined by summing Macquarie–Antarctic and Antarctic–Pacific rotations. The Pacific–Macquarie rotations predict a smaller rate of convergence perpendicular to the Hjort Trench than the Pacific–Australia rotations. The onset of the deformation of the South Tasman Sea and the development of the Macquarie Plate appears to have been triggered by the subduction of young, buoyant oceanic crust near the Hjort Trench and coincided with a clockwise change in Pacific–Australia motion around 6 Ma. The revised Pacific–Australia rotations also have implications for the tectonics of the Alpine Fault Zone of New Zealand. We find that changes in relative displacement along the Alpine Fault have been small over the last 20 Myr. The average rate of convergence over the last 6 Myr is about 40 per cent smaller than in previous models.

On the wind‐forcing of bottom pressure variability at Amsterdam and Kerguelen Islands, southern Indian Ocean

Bottom pressure records from Amsterdam and Kerguelen Islands (southern Indian Ocean) were collected between 1986 and 1989, and interpreted previously in terms of volume transport fluctuations between the islands [Vassie et al., 1994]. Since a substantial portion of the mean transport of the Antarctic Circumpolar Current (ACC) flows between the islands, it was assumed that ACC transport variability was the predominant signal being measured, and comparisons with ACC variability at other longitudes were made. Here we examine the relationship between atmospheric forcing and observed bottom pressure variability. We find that Kerguelen bottom pressure is forced predominantly by wind stress curl over the Kerguelen Plateau for periods between around 5 days and 1 year. A weaker correlation with circumpolar eastward winds is observed, but is seen not to be causal. Amsterdam pressure shows a similar local response to curl for high frequencies (5–10 days), but at longer periods is sensitive to curl over the sub‐basin between Kerguelen Plateau, the Southeast Indian Ridge, and Antarctica. This is the region of long‐wavelength sea level variability observed by Chao and Fu [1995], and interpreted in terms of a geostrophic resonance by Webb and de Cuevas [2002a]. Altimetric studies confirm the usefulness of the Amsterdam location in monitoring this mode. We conclude that the Amsterdam‐Kerguelen pressure difference is not a useful measure of ACC transport variability, and that previous agreement found between variability observed here and other locations is largely fortuitous. Instead, the series separately reflect different important dynamical modes in the southern Indian Ocean.

Geochemical and Hf–Pb–Sr–Nd isotopic constraints on the origin of the Amsterdam–St. Paul (Indian Ocean) hotspot basalts

The Amsterdam–St. Paul (ASP) Plateau is a recent (≤5 Ma) volcanic rise constructed along the Southeast Indian Ridge (SEIR) by the combined effects of a relatively small mantle plume and a mid-oceanic ridge. The Amsterdam and St. Paul islands are located 100 km away from each other and formed during the last 0.4 Myr; they are the only subaerial features of the ASP Plateau and the two islands are structurally separated by the presence of a SW–NE transform fault. New geochemical analyses and Hf–Pb–Sr–Nd isotopic compositions of 20 basaltic rocks from Amsterdam and St. Paul Islands constrain the nature and origin of the sources involved in the genesis of the ASP hotspot basalts. Aphyric basalts from St. Paul are mildly alkalic, incompatible element-enriched and highly fractionated; they are distinct from the tholeiitic basalts from Amsterdam, from the recently discovered Boomerang active seamount on the ASP Plateau, and from the Kerguelen Archipelago basalts on the Antarctic Plate. The St. Paul and Amsterdam basalts have very limited isotopic variations with distinct 206Pb/ 204Pb, 207Pb/ 204Pb, 208Pb/ 204Pb, and 176Hf/ 177Hf isotopic compositions (19.08±0.07, 15.61±0.02, 39.45±0.12, 0.28313±0.00003 for Amsterdam, and 18.70±0.08, 15.56±0.01, 38.87±0.05, 0.28306±0.00002 for St. Paul, respectively) that are not compatible with any direct contribution of the enriched Kerguelen plume end-member. Pb–Nd–Sr isotopic compositions of the St. Paul basalts appear consistent with simple binary mixtures between heterogeneous ambient upper mantle and a highly radiogenic Pb plume component (the ASP plume end-member), particularly expressed in the isotopic compositions of the Amsterdam basalts. However, the Amsterdam basalts have distinctly higher ϵ Hf than the St. Paul basalts (+13 and +10, respectively) for a given ϵ Nd (+4) and are inconsistent with such a simple binary mixing scenario. Isotopic systematics in the Amsterdam and St. Paul basalts indicate that the Amsterdam and St. Paul volcanoes were formed by sampling isotopically distinct zones of the ASP plume at a lateral distance of 100 km. Less than 1% variation in the proportion of recycled altered oceanic crust relative to pelagic sediment, combined with minor variations in the proportion of recycled material within the Amsterdam and St. Paul plume sources themselves relative to a peridotitic mantle source, could account for the isotopic differences between the compositions of basalts from these two islands. The particularly high 206Pb/ 204Pb component recorded in the Amsterdam and St. Paul basalts is also locally recorded at different times and locations within other Indian Ocean basalts (e.g. Ninetyeast Ridge basalts, 38 Ma) and such a component has also contaminated the source of SEIR basalts to varying degrees. The particularly high 206Pb/ 204Pb component is therefore not exclusive to the Amsterdam–St. Paul plume but is heterogeneously distributed within the Indian Ocean upper mantle. This may reflect the role of the Indian mantle plumes in dispersing recycled material within the Indian upper mantle.

A resolved mantle anomaly as the cause of the Australian‐Antarctic Discordance

We present evidence for the existence of an Australian‐Antarctic Mantle Anomaly (AAMA), which trends northwest‐southeast (NW‐SE) through the Australian‐Antarctic Discordance (AAD) on the Southeast Indian Ridge (SEIR), is confined to the upper 120 km of the mantle beneath the AAD, and dips shallowly to the west so that it extends to a depth of about 150 km west of the AAD. Average temperatures within the AAMA are depressed about 100°C relative to surrounding lithosphere and suggest very rapid cooling of newly formed lithosphere at the AAD to an effective thermal age between 20 and 30 Ma. A convective down welling beneath the AAD is not consistent with the confinement of the AAMA in the uppermost mantle. In substantial agreement with the model of Gurnis et al. [1998], we argue that the AAMA is the suspended remnant of a slab that subducted at the Gondwanaland‐Pacific convergent margin more than 100 Myr ago, foundered in the deeper mantle, and then ascended into the shallow mantle within the past 30 Myr, cutting any ties to deeper roots. The stability of the AAMA and its poor correlation with residual topography and gravity imply that it is approximately neutrally buoyant. The thermally induced density anomaly can be balanced by bulk iron depletion of less than 0.8%, consistent with the warmer conditions of formation for the Pacific than Indian lithosphere. We hypothesize that the low temperatures in the AAMA inhibit crustal formation and the AAD depth anomaly is formed at the intersection of the SEIR and the AAMA. The northward migration of the SEIR overriding the cold NW‐SE trending AAMA therefore presents a simple kinematic explanation for both the V‐shaped residual depth anomaly in the southeast Indian Ocean and the western migration of the AAD along the SEIR. Neither explanation requires the Pacific asthenospheric mantle to push westward and displace Indian asthenosphere. The AAMA may also act as a barrier to large‐scale flows in the shallow asthenosphere and may therefore define a boundary for mantle convection and between the Indian and Pacific isotopic provinces. The westward dip of the AAMA would also favor along‐axis flow from the Indian Ocean asthenosphere to the AAD that may contribute to the penetration of Indian Ocean mid‐ocean ridge basalts into the AAD.

Underthrusting at the Hjort Trench, Australian‐Pacific plate boundary: Incipient subduction?

The southernmost portion of the Macquarie Ridge Complex (MRC) comprises the Hjort Ridge, Trench, and Plateau, the topographic expression of the Australian‐Pacific plate boundary south of New Zealand. On the basis of marine geophysical (swath bathymetry/reflectivity, seismic reflection, gravity, magnetic) data, teleseismic data, and gravity modeling, we argue that the Australian plate is actively underthrusting the Pacific plate along the Hjort Trench, but self‐sustaining subduction does not yet appear to have commenced. We interpret a crustal discontinuity in the Trench as a shallowly dipping thrust splay off a sub‐vertical transform fault in the crest of the Hjort Ridge (Hjort Fault). The Hjort Fault separates oceanic crust generated at the Southeast Indian Ridge (SEIR crust), located to the south, from oceanic crust generated at the extinct Australian‐Pacific spreading center located to the north (MRC crust). Oblique convergence on the transform has initiated a significant plate boundary decollement in the Trench, but our estimate of ∼50 km of underthrusting does not support the existence of an eastwardly dipping Australian slab below ∼20 km depth. For the length of the Hjort Ridge and Trench system, oblique convergence is partitioned between the decollement in the Trench and the Hjort Fault. South of 57.5°S, the trench decollement accommodates thrusting. North of 57.5°S, the boundary‐parallel component of convergence becomes dominant, the Trench gradually shallows, and the trench decollement evolves into a low‐angle, oblique‐slip fault. The trench decollement and strike‐slip system in the Hjort Ridge are structural boundaries that contain tectonic slivers of inferred SEIR crust of the western flank of the Hjort Ridge that currently belong to neither the Australian nor the Pacific plates.

A correspondence of altimetric gravity texture to abyssal hill morphology along the flanks of the Southeast Indian Ridge

[1] We examine the relationship between altimetric gravity and abyssal hill textures along the flanks of Southeast Indian Ridge, where previous studies observed a progressive west-to-east increase in abyssal hill roughness and scales. Such a relationship is often considered unlikely because abyssal hills are typically smaller than upward continuation filter scales. However, our results, which exclude ridges, fracture zones and large pseudofaults, show that altimetric gravity roughness and characteristic scales in this region also increase from west to east and, at the largest scales, gravity texture exhibits ridge-parallel lineaments. Synthetic profiles also demonstrate how the texture parameters of a filtered field are related to those of the unfiltered field, particularly where the characteristic scale of the latter is smaller than the filter length.

Changes in strain trajectories in three different types of plate tectonic boundary deduced from earthquake focal mechanisms

Principal strain orientations (minimum horizontal compression—ex and maximum horizontal compression—ey) were established at three different types of plate tectonic boundary: two transform faults, an oceanic ridge located on the Southeast Indian Ridge and a trench located close to the South Sandwich Archipelago. To establish the strain patterns in each zone, 104 earthquake focal mechanisms (centroid-moment tensor solutions for earthquakes with mb≥4; Harvard seismology data, CMT) were examined by fault population analysis. Despite the existence of only one tectonic process that controlled deformation in these zones (divergence, convergence or passive displacement), and only one main strain tensor, several coeval strain ellipsoids were found. These differed from the main strain tensor in the location of the principal strains. In general, permutations were observed between the principal strains, i.e., interchanges between the location of the principal strain axes maintaining the strain ellipsoids in the same 3D orientation. Only in some cases were changes in the ellipsoid orientation associated with major structures.

Mechanisms of magmatic gas loss along the Southeast Indian Ridge and the Amsterdam –St. Paul Plateau

New analyses of He, Ne, Ar and CO2 trapped in basaltic glasses from the Southeast Indian Ridge (Amsterdam–St. Paul (ASP) region) show that ridge magmas degas by a Rayleigh distillation process. As a result, the absolute and relative noble gas abundances are highly fractionated with 4He/40Ar* ratios as high as 620 compared to a production ratio of ∼3 (where 40Ar* is 40Ar corrected for atmospheric contamination). There is a good correlation between 4He/40Ar* and the MgO content of the basalt, suggesting that the amount of gas lost from a particular magma is related to the degree of crystallization. Fractional crystallization forces oversaturation of CO2 because CO2 is an incompatible element. Therefore, crystallization will increase the fraction of gas lost from the magma. The He–Ar–CO2–MgO–TiO2 compositions of the ASP basalts are modeled as a combined fractional crystallization–fractional degassing process using experimentally determined noble gas and CO2 solubilities and partition coefficients at reasonable magmatic pressures (2–4 kbar). The combined fractional crystallization–degassing model reproduces the basalt compositions well, although it is not possible to rule out depth of eruption as a potential additional control on the extent of degassing. The extent of degassing determines the relative noble gas abundances (4He/40Ar*) and the 40Ar*/CO2 ratio but it cannot account for large (>factor 50) variations in He/CO2, due to the similar solubilities of He and CO2 in basaltic magmas. Instead, variations in CO2/3He (C/3He) trapped in the vesicles must reflect similar variations in the primary magma. The controls on C/3He in mid-ocean ridge basalts (MORBs) are not known. There are no obvious correlated variations between C/3He and tracers of mantle heterogeneity (3He/4He, K/Ti etc.), implying that the variations in C/3He are not likely to be a feature of the mantle source to these basalts. Mixing between MORB-like sources and more enriched, high 3He/4He sources occurs on and near the ASP plateau, resulting in variable 3He/4He and K/Ti compositions (and many other tracers). Using 4He/40Ar* to track degassing, we demonstrate that mixing systematics involving He isotopes are determined in large part by the extent of degassing. Relatively undegassed lavas (with low 4He/40Ar*) are characterized by steep 3He/4He–K/Ti mixing curves, with high He/Ti ratios in the enriched magma (relative to He/Ti in the MORB magma). Degassed samples (high 4He/40Ar*) on the other hand have roughly equal He/Ti ratios in both end-members, resulting in linear mixing trajectories involving He isotopes. Some degassing of ASP magmas must occur at depth, prior to magma mixing. As a result of degassing prior to mixing, mixing systematics of oceanic basalts that involve noble gas–lithophile pairs (e.g. 3He/4He vs. 87Sr/86Sr or 40Ar/36Ar vs. 206Pb/204Pb) are unlikely to reflect the noble gas composition of the mantle source to the basalts. Instead, the mixing curve will reflect the extent of gas loss from the magmas, which is in turn buffered by the pressure of combined crystallization–degassing and the initial CO2 content.

Submarine Basalts of the Northern Kerguelen Plateau: Interaction Between the Kerguelen Plume and the Southeast Indian Ridge Revealed at ODP Site 1140

During Ocean Drilling Program Leg 183, basaltic cores were retrieved from the Northern Kerguelen Plateau (NKP) at Site 1140 on the extreme north of the plateau, ∼270 km north of the Kerguelen Archipelago. Amongst the six basement units recovered, five were pillow basalts with fresh glassy rims together with flow lobes. This is the first evidence for submarine eruption anywhere on the Kerguelen Plateau. Each flow unit has distinct geochemical and isotopic characteristics that span the range from Southeast Indian Ridge (SEIR) mid-ocean ridge basalt to tholeiitic–transitional basalts derived from the Kerguelen mantle plume. Relationships between element abundance ratios involving incompatible elements and isotopic ratios reflect mixing of near-primary melts from an SEIR source and the Kerguelen plume with an increasing role for an incompatible element-rich end-member in the order: Unit 1 < 6 ≤ 5 < 2 < 3; that is, there is no systematic downcore geochemical variation. Modeling results show that the depleted SEIR component is dominant, ranging from 99–90% in Unit 1 to 76–63% in Unit 3. Comparison with Kerguelen Archipelago flood basalts with similar geochemical characteristics indicates that in the northern part of the archipelago, where 29–28 Ma tholeiitic–transitional basalts are present, incorporation of asthenospheric mantle by the plume on its way to the surface may have been significant. At 34 Ma, when Site 1140 basalts were formed, the SEIR was only ∼50 km away. Apparently, the proximity of the spreading center and plume allowed mixing of geochemically diverse magmas. In contrast to older Cretaceous parts of the Kerguelen Plateau, the Cenozoic Northern Kerguelen Plateau appears oceanic in origin and there is no evidence for a component derived from continental crust in Site 1140 basalts.

The Depleted Mantle Component in Kerguelen Archipelago Basalts: Petrogenesis of Tholeiitic-Transitional Basalts From the Loranchet Peninsula

A geochemical study of 28 Ma tholeiitic to transitional basalts from the Kerguelen Archipelago (Mont des Ruches and Mont Fontaine) indicates that three distinct magma types erupted within ∼1 Myr. Low-MgO basalts (∼4–6 wt %) in both sections are overlain by high-MgO basalts (∼7–13 wt %), mostly present in Mont Fontaine. Both high- and low-MgO basalts have nearly identical low 87Sr/86Sr and high 143Nd/144Nd and formed from similar parental magmas that represent mixtures between a depleted mantle component and the Kerguelen plume. The third magma type, predominant in Mont des Ruches, is represented by high-MgO basalts that are isotopically heterogeneous with isotopic ratios that are intermediate between those of the stratigraphically lower basalts and the Kerguelen plume compositions; this third magma type may have formed by mixing of similar material, but with a higher contribution from the Kerguelen plume. The depleted component involved in all three magma types is similar to the source for Southeast Indian Ridge basalts and is present in Kerguelen Archipelago basalts older than 26 Ma, which erupted when the ridge axis was <500 km away from the Kerguelen hotspot. Depleted heterogeneities intrinsic to the plume and entrainment of depleted mantle during plume ascent do not explain the marked cut-off in the presence of a depleted component in the archipelago basalts within a time interval of 1 Myr after 26 Ma. Mixing of depleted asthenosphere with the plume in sublithospheric channels during migration of the Southeast Indian Ridge axis away from the Kerguelen hotspot is proposed as a suitable explanation to account for the temporal distribution of the depleted component in basalts from the Northern Kerguelen Plateau and the >26 Ma Kerguelen Archipelago flood basalts; cessation of plume–ridge interactions may explain the absence of depleted basalts in the youngest sections that erupted further away from the ridge axis.

Volatiles in Submarine Basaltic Glasses from the Northern Kerguelen Plateau (ODP Site 1140): Implications for Source Region Compositions, Magmatic Processes, and Plateau Subsidence

Submarine pillow basalts (34 Ma) recovered from the Northern INTRODUCTION Kerguelen Plateau at ODP Site 1140 contain abundant unaltered The Kerguelen Plateau and Broken Ridge formed mostly glass, providing the first opportunity to measure the volatile contents during the Cretaceous as a single, giant oceanic plateau of tholeiitic basaltic magmas related to the Kerguelen mantle plume. that is interpreted to represent voluminous magmatism The glasses have La/Sm and Nb/Zr ratios that vary from values associated with emplacement of a mantle plume head similar to Southeast Indian Ridge (SEIR) MORB (Unit 1), to below young Indian Ocean lithosphere (Duncan & slightly more enriched (Unit 6), to values transitional between Storey, 1992; Weis et al., 1992). During Ocean Drilling SEIR MORB and basaltic magmas formed by melting of the Program (ODP) Leg 183, igneous basement rock and Kerguelen plume (Units 2 and 3). Volatile contents for glasses in sediment cores were obtained from five sites on the Units 1 and 6 are similar to depleted mid-ocean ridge basalt Kerguelen Plateau and two on Broken Ridge (Fig. 1). The (MORB) values (0·25–0·27 wt % H2O, 1240–1450 ppm S, results, combined with subsequent radiometric dating, 42–54 ppm Cl). In contrast, H2O contents are higher for the demonstrate that from south to north the age of the enriched glasses (Unit 2, 0·44 wt % H2O; Unit 3, 0·69 wt %), uppermost crust decreases from 120–110 Ma in the as are S (1500 ppm) and Cl (146–206 ppm). Cl/K ratios Southern Kerguelen Plateau, to >110 to 95 Ma in the for all glasses are relatively low (0·03–0·04), indicating that Central Kerguelen Plateau, Broken Ridge and Elan Bank, assimilation of hydrothermally altered material did not occur during to Ζ35 Ma in the Northern Kerguelen Plateau (Coffin shallow-level crystallization. H2O/Ce for the enriched glasses et al., 2000; Pringle & Duncan, 2000; Duncan, 2002). At (Units 2 and 3) is significantly lower than Pacific and South six of seven basement sites, as well as at four sites drilled Atlantic MORB values, suggesting that low H2O/Ce may be an on earlier ODP cruises, subaerially erupted basalt flows inherent characteristic of the Kerguelen plume source. Vapor saturation were recovered, indicating that the growth rate of the pressures calculated using the H2O and CO2 contents of the glasses Kerguelen large igneous province (LIP) was sufficient to indicate that >1700 m of subsidence has occurred on this part of form a subaerial landmass. However, a sequence of the plateau since eruption of the basalts at 34 Ma. Subsidence submarine-erupted pillow basalt flows of 90 m thickness estimates for Site 1140 and other ODP drill sites indicate that the was drilled at Site 1140 on the Northern Kerguelen various parts of the Kerguelen Plateau subsided at a rate comparable Plateau (Coffin et al., 2000). with that for normal Indian Ocean lithosphere. The Site 1140 submarine glasses provide the first

Hf Isotope Evidence for a Miocene Change in the Kerguelen Mantle Plume Composition

We report high-precision multiple collector inductively coupled plasma INTRODUCTION mass spectrometry (MC-ICP-MS) analyses of Hf isotopic comSpatial and temporal isotope variations in basalts can positions for 39 Kerguelen Archipelago volcanic rocks that add a provide important constraints on the origin of the mantle new perspective to the temporal evolution of the Kerguelen mantle source of hotspots, plume dynamics and the interaction plume. Samples cover the entire range of chemical and Sr, Nd and of plumes with the oceanic lithosphere. The relationships Pb isotopic variations reported for the archipelago, and vary in age between ocean island basalts (OIB) and mantle geofrom 29 to 0·1 Ma. Their Hf isotopic compositions show the chemistry have been investigated in recent publications largest variation for an individual oceanic island and Hf correlates on both ridge-axis hotspots (Iceland; Hanan & Schilling, negatively with alkalinity. The Hf–Nd isotopic compositions of the 1997; Chauvel & Hemond, 2000; Kempton et al., 2000), pre-Miocene flood basalt group (>25 Ma) are distinct from those and within-plate hotspots (Hawaii; e.g. Lassiter et al., of the 25 Ma group 1986), and are therefore critical for delimiting the range of flood basalts require interaction between a shallow reservoir of of enriched isotopic variations for OIB, and (2) during asthenospheric mantle or oceanic lithosphere and the Kerguelen plume formation of the Kerguelen Archipelago the environment core, best represented by the Mt. Crozier basalts on the archipelago. of the hotspot changed from ridge-centred at >40 Ma This scenario is consistent with the relative proximity of the Kerguelen to the present intra-plate position. The archipelago has Archipelago to the Southeast Indian Ridge (>300–400 km) when been centred on or near the hotspot because the Antarctic the >25 Ma flood basalts erupted. The younger alkaline volcanic plate has remained essentially stationary with respect to rocks were generated by low-degree melting in the plume envelope, the hotspot reference frame since the Eocene (Duncan an area of enriched mantle material, different from the plume core, & Richards, 1991). The recent extensive and systematic and presumably formed by different combinations of old recycled sampling of volcanic stratigraphic sections from the Keroceanic crust and plateaux. The enriched geochemical character of guelen Archipelago provides a new framework within the Kerguelen plume core supports the hypothesis that the EM Iwhich to study the evolution of the Kerguelen plume. type plume source includes recycled oceanic plateaux. The Kerguelen Archipelago formed on the northernmost part of the Kerguelen Plateau on the Antarctic plate, far from continental margins. In contrast to the

Between a Hotspot and a Cold Spot: Isotopic Variation in the Southeast Indian Ridge Asthenosphere, 86degreesE-118degreesE

Glasses from a 2600 km section of the Southeast Indian Ridge west of the Australian–Antarctic Discordance all possess Nd–Pb–Sr isotopic signatures typical of Indian Ocean ridge basalt. The boundary between Pacific- and Indian-Ocean-type ridge basalt within the Discordance thus marks the westernmost extent of shallow Pacific-type asthenosphere beneath the ridge. Along-axis He, Nd, Pb, and Sr isotopic patterns are largely independent of ridge segmentation, but a weak tendency is evident for the most strongly Indian-Ocean-type mantle to be relatively fusible and for shallower asthenosphere to have lower 3He/4He. On average, εNd appears slightly lower than for ridges in the western Indian Ocean far from hotspots. Importantly, the regional isotopic patterns cannot be explained by a previously proposed eastward flow of Kerguelen–Heard or Amsterdam–St. Paul hotspot mantle. Nd, Pb and (to a much lesser extent) Sr isotopes correlate roughly with many incompatible element ratios, including parent–daughter ratios. If interpreted as mantle errorchrons, the latter correlations imply ‘ages’ of 200–300 Ma, significantly greater than the oldest known age of the Kerguelen–Heard hotspot (119–135 Ma) commonly postulated to have played an important role in creating the isotopic signature of Indian Ocean mantle. Rather than reflecting relatively recent mixing involving mantle from hotspots in the region, much of the observed isotopic heterogeneity may be the result of other mixing or of past intra-mantle chemical fractionation, probably associated with melting.

Magnesium, calcium and strontium in waters of the southern Tasman Sea at the confluence of the Indian, Pacific and Southern Oceans

Data are presented for magnesium (Mg), calcium (Ca) and strontium (Sr) analyses for surface samples and conductivity–temperature–depth profiles of waters along three main transects across the Subtropical Front (STF) and the Subantarctic Front (SAF) in a region between Tasmania and New Zealand down to 57�S. This area is at the confluence of the Indian, Pacific and Southern Oceans where the Subantarctic Mode Water (SAMW) is formed and which is characterized by well-mixed and thick (down to 1000 m) thermostads with uniform salinity and temperature. Such thermostads form at the end of winter, and two of such mode water-forming areas were sampled in October–November 1998. Magnesium and Ca are proving to be quasi-conservative elements down the water column, but there is systematic depletion for Mg compared with profiles measured in the North and South Atlantic Oceans. This offset is interpreted to be the result of hydrothermal activity in the region at the south-east Indian Ridge, which scavenges some Mg. The Circumpolar Deep Water (CDW) intersects this Ridge and upwells south of the SAF and eventually is transformed in the upper 1000 m of the water column into the SAMW. This explains the uniform Mg depletion throughout the entire depth profiles. Strontium, on the other hand, displays significant shifts near the ocean surface at many locations, and this is interpreted as the result of biogenic productivity, which engenders the precipitation of celestite microcrystals.