Petrogenesis of Lava from Christmas Island, Northeast Indian Ocean: Implications for the Nature of Recycled Components in Non-Plume Intraplate Settings

Lava samples from the Christmas Island Seamount Province (CHRISP) record an extreme range in enriched mantle (EM) type Sr-Nd-Pb-Hf isotope signatures. Here we report osmium isotope data obtained on four samples from the youngest, Pliocene petit-spot phase (Upper Volcanic Series, UVS; ~4.4 Ma), and four samples from the earlier, Eocene (Lower Volcanic Series, LVS; ~40 Ma) shield building phase of Christmas Island. Osmium concentrations are low (5–82 ppt) with initial Os isotopic values (187Os/188Osi) ranging from (0.1230–0.1679). Along with additional new geochemical data (major and trace elements, Sr-Nd-Pb isotopes, olivine δ18O values), we demonstrate the following: (1) The UVS is consistent with melting of shallow Indian mid-ocean ridge basalt (MORB) mantle enriched with both lower continental crust (LCC) and subcontinental lithospheric mantle (SCLM) components; and (2) The LVS is consistent with recycling of SCLM components related to Gondwana break-up. The SCLM component has FOZO or HIMU like characteristics. One of the LVS samples has less radiogenic Os (γOs –3.4) and provides evidence for the presence of ancient SCLM in the source. The geochemistry of the Christmas Island lava series supports the idea that continental breakup causes shallow recycling of lithospheric and lower crustal components into the ambient MORB mantle.

Mineralogy and Geochemistry of Deep-Sea Sediments from the Ultraslow-Spreading Southwest Indian Ridge: Implications for Hydrothermal Input and Igneous Host Rock

Detailed mineralogical and geochemical characteristics of typical surface sediments and hydrothermal deposits collected from the ultraslow-spreading Southwest Indian Ridge (SWIR) were studied by high-resolution XRD, SEM-EDS, XRF, and ICP-MS. The SWIR marine samples can be generally classified into two main categories: surface sediment (biogenic, volcanic) and hydrothermal-derived deposit; moreover, the surface sediment can be further classified into metalliferous and non-metalliferous based on the metalliferous sediment index (MSI). The chemical composition of biogenic sediment (mainly biogenic calcite) was characterized by elevated contents of Ca, Ba, Rb, Sr, Th, and light rare earth elements (LREE), while volcanic sediment (mainly volcanogenic debris) was relatively enriched in Mn, Mg, Al, Si, Ni, Cr, and high field strength elements (HFSEs). By contrast, the hydrothermal-derived deposit (mainly pyrite-marcasite, chalcopyrite-isocubanite, and low-temperature cherts) contained significantly higher contents of Fe, Cu, Zn, Pb, Mn, Co, Mo, Ag, and U. In addition, the metalliferous surface sediment contained a higher content of Cu, Mn, Fe, Co, Mo, Ba, and As. Compared with their different host (source) rock, the basalt-hosted marine sediments contained higher contents of Ti–Al–Zr–Sc–Hf and/or Mo–Ba–Ag; In contrast, the peridotite-hosted marine sediments were typically characterized by elevated concentrations of Mg–Cu–Ni–Cr and/or Co–Sn–Au. The differences in element enrichment and mineral composition between these sediment types were closely related to their sedimentary environments (e.g., near/far away from the vent sites) and inherited from their host (source) rock. Together with combinations of certain characteristic elements (such as Al–Fe–Mn and Si–Al–Mg), relict hydrothermal products, and diagnostic mineral tracers (e.g., nontronite, SiO2(bio), olivine, serpentine, talc, sepiolite, pyroxene, zeolite, etc.), it would be more effective to differentiate the host rock of deep-sea sediments and to detect a possible hydrothermal input.

Melting of recycled ancient crust responsible for the Gutenberg discontinuity

A discontinuity in the seismic velocity associated with the lithosphere-asthenosphere interface, known as the Gutenberg discontinuity, is enigmatic in its origin. While partial mantle melts are frequently suggested to explain this discontinuity, it is not well known which factors critically regulate the melt production. Here, we report geochemical evidence showing that the melt fractions in the lithosphere-asthenosphere boundary were enhanced not only by accumulation of compacted carbonated melts related to recycled ancient marine sediments, but also by partial melting of a pyroxene-rich mantle domain related to the recycled oceanic eclogite/pyroxenites. This conclusion is derived from the first set of Mg isotope data for a suite of young petit-spot basalts erupted on the northwest Pacific plate, where a clearly defined Gutenberg discontinuity exists. Our results reveal a specific linkage between the Gutenberg discontinuity beneath the normal oceanic regions and the recycling of ancient subducted crust and carbonate through the deep Earth.

He–Ar–S Isotopic Compositions of Polymetallic Sulphides from Hydrothermal Vent Fields along the Ultraslow-Spreading Southwest Indian Ridge and Their Geological Implications

Noble gases have become a powerful tool to constrain the origin and evolution of ore-forming fluids in seafloor hydrothermal systems. The aim of this study was to apply these tracers to understand the genesis of newly discovered polymetallic sulphide deposits along the ultraslow-spreading Southwest Indian Ridge (SWIR). The helium, argon, and sulphur isotope compositions of metal sulphide minerals were measured for a number of active/inactive vent fields in the Indian Ocean. The helium concentrations and isotopic ratios in these ore samples are variable (4He: 0.09–2.42 × 10−8 cm3STP∙g−1; 3He: 0.06–3.28 × 10−13 cm3STP∙g−1; 3He/4He: 1.12–9.67 Ra) and generally greater than the modern atmosphere, but significantly lower than those in massive sulphides from the fast-spreading East Pacific Rise (EPR), especially for three Cu–Fe-rich samples from the ultramafic-hosted Tianzuo and Kairei vent fields. On the contrary, most of the SWIR sulphide deposits have somewhat higher 40Ar/36Ar ratios of trapped fluids (ranging from 290.6 to 303.4) when compared to the EPR ore samples. Moreover, the majority of sulphide minerals from the Indian Ocean have much higher δ34S values (3.0‰–9.8‰, ~5.9 on average, n = 49) than other basaltic-hosted active hydrothermal systems on the EPR. Overall, these He–Ar–S results are well within the range of seafloor massive sulphide deposits at global sediment-starved mid-ocean ridges (MORs), lying between those of air-saturated water (ASW) and mid-ocean ridge basalt (MORB) end members. Therefore, our study suggests that the helium was derived mainly from the MORB mantle by degassing during the high-temperature stage of hydrothermal activity, as well as from a mixture of vent fluids with variable amounts of ambient seawater during either earlier or late-stage low-temperature hydrothermal episodes, whereas the argon in ore-forming fluids trapped within sulphide minerals was predominantly derived from deep-sea water. Additionally, relatively high δ34S values exhibit a great estimated proportion (up to nearly 40%) of seawater-derived components. In summary, sub-seafloor extensive fluid circulation, pervasive low-temperature alteration, shallow seawater entrainment, and mixing processes, may make a larger contribution to the SWIR hydrothermal ore-forming systems, compared to fast-spreading centres.

Isotopic portrayal of the Earth’s upper mantle flow field

It is now well established that oceanic plates sink into the lower mantle at subduction zones, but the reverse process of replacing lost upper-mantle material is not well constrained. Even whether the return flow is strongly localized as narrow upwellings or more broadly distributed remains uncertain. Here we show that the distribution of long-lived radiogenic isotopes along the world's mid-ocean ridges can be used to map geochemical domains, which reflect contrasting refilling modes of the upper mantle. New hafnium isotopic data along the Southwest Indian Ridge delineate a sharp transition between an Indian province with a strong lower-mantle isotopic flavour and a South Atlantic province contaminated by advection of upper-mantle material beneath the lithospheric roots of the Archaean African craton. The upper mantle of both domains appears to be refilled through the seismically defined anomaly underlying South Africa and the Afar plume. Because of the viscous drag exerted by the continental keels, refilling of the upper mantle in the Atlantic and Indian domains appears to be slow and confined to localized upwellings. By contrast, in the unencumbered Pacific domain, upwellings seem comparatively much wider and more rapid.

Osmium isotopic constraints on the nature of the DUPAL anomaly from Indian mid-ocean-ridge basalts

The isotopic compositions of mid-ocean-ridge basalts (MORB) from the Indian Ocean have led to the identification of a large-scale isotopic anomaly relative to Pacific and Atlantic ocean MORB. Constraining the origin of this so-called DUPAL anomaly may lead to a better understanding of the genesis of upper-mantle heterogeneity. Previous isotopic studies have proposed recycling of ancient subcontinental lithospheric mantle or sediments with oceanic crust to be responsible for the DUPAL signature. Here we report Os, Pb, Sr and Nd isotopic compositions of Indian MORB from the Central Indian ridge, the Rodriguez triple junction and the South West Indian ridge. All measured samples have higher (187)Os/(188)Os ratios than the depleted upper-mantle value and Pb, Sr and Nd isotopic compositions that imply the involvement of at least two distinct enriched components in the Indian upper-mantle. Using isotopic and geodynamical arguments, we reject both subcontinental lithospheric mantle and recycled sediments with oceanic crust as the cause of the DUPAL anomaly. Instead, we argue that delamination of lower continental crust may explain the DUPAL isotopic signature of Indian MORB.

An ultraslow-spreading class of ocean ridge

New investigations of the Southwest Indian and Arctic ridges reveal an ultraslow-spreading class of ocean ridge that is characterized by intermittent volcanism and a lack of transform faults. We find that the mantle beneath such ridges is emplaced continuously to the seafloor over large regions. The differences between ultraslow- and slow-spreading ridges are as great as those between slow- and fast-spreading ridges. The ultraslow-spreading ridges usually form at full spreading rates less than about 12 mm yr(-1), though their characteristics are commonly found at rates up to approximately 20 mm yr(-1). The ultraslow-spreading ridges consist of linked magmatic and amagmatic accretionary ridge segments. The amagmatic segments are a previously unrecognized class of accretionary plate boundary structure and can assume any orientation, with angles relative to the spreading direction ranging from orthogonal to acute. These amagmatic segments sometimes coexist with magmatic ridge segments for millions of years to form stable plate boundaries, or may displace or be displaced by transforms and magmatic ridge segments as spreading rate, mantle thermal structure and ridge geometry change.

Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites

Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the (143)Nd/(144)Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges.

Geochemical tracing of Pacific-to-Atlantic upper-mantle flow through the Drake passage

The Earth's convecting upper mantle can be viewed as comprising three main reservoirs, beneath the Pacific, Atlantic and Indian oceans. Because of the uneven global distribution and migration of ridges and subduction zones, the surface area of the Pacific reservoir is at present contracting at about 0.6 km2 x y(r-1), while the Atlantic and Indian reservoirs are growing at about 0.45 km2 x yr(-1) and 0.15 km2 x yr(-1), respectively. Garfunkel and others have argued that there must accordingly be net mantle flow from the Pacific to the Atlantic and Indian reservoirs (in order to maintain mass balance), and Alvarez further predicted that this flow should be restricted to the few parts of the Pacific rim (here termed 'gateways') where there are no continental roots or subduction zones that might act as barriers to shallow mantle flow. The main Pacific gateways are, according to Alvarez, the southeast Indian Ocean, the Caribbean Sea and the Drake passage. Here we report geochemical data which confirm that there has been some outflow of Pacific mantle into the Drake passage--but probably in response to regional tectonic constraints, rather than global mass-balance requirements. We also show that a mantle domain boundary, equivalent to the Australian-Antarctic discordance, must lie between the Drake passage and the east Scotia Sea.

Oxygen-isotope evidence for recycled crust in the sources of mid-ocean-ridge basalts

Mid-ocean-ridge basalts (MORBs) are the most abundant terrestrial magmas and are believed to form by partial melting of a globally extensive reservoir of ultramafic rocks in the upper mantle. MORBs vary in their abundances of incompatible elements (that is, those that partition into silicate liquids during partial melting) and in the isotopic ratios of several radiogenic isotope systems. These variations define a spectrum between 'depleted' and 'enriched' compositions, characterized by respectively low and high abundances of incompatible elements. Compositional variations in the sources of MORBs could reflect recycling of subducted crustal materials into the source reservoir, or any of a number of processes of intramantle differentiations. Variations in (18)O/(16)O (principally sensitive to the interaction of rocks with the Earth's hydrosphere) offer a test of these alternatives. Here we show that (18)O/(16)O ratios of MORBs are correlated with aspects of their incompatible-element chemistry. These correlations are consistent with control of the oxygen-isotope and incompatible-element geochemistry of MORBs by a component of recycled crust that is variably distributed throughout their upper mantle sources.