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We present a double-spike technique for precise determination of mass-dependent fractionation of tungsten (W) stable isotopes. Instrumental mass bias effects and isotope fractionation during W separation are corrected for by means of a W-180-W-183 double-spike added prior to sample dissolution. The separation of W from the matrix is achieved via three-step anion-exchange chromatography. Tungsten stable isotopic compositions were determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Results are expressed relative to the NIST SRM 3136 W standard as the permil difference in W-186/W-184 (delta W-186). The external reproducibility of the NIST SRM 3163 W standard is +/- 0.05 parts per thousand with an average composition of -0.01 parts per thousand on W-186/W-184 (2s.d., n = 171). Tungsten stable isotope compositions for USGS standard reference materials are as follows: AGV-2 (andesite; delta W-186 +/- 0.18 +/- 0.05 parts per thousand, 2s.d., n = 6), SDC-1 (mica schist; delta W-186 = 0.36 +/- 0.05 parts per thousand, 2s.d., n = 6), SDO-1 (shale; delta W-186 = 0.26 +/- 0.06 parts per thousand, 2s.d., n = 6), and manganese nodules NOD-A-1 (delta W-186 = 0.07 +/- 0.05 parts per thousand, 2s.d., n = 10) and NOD-P-1 (delta W-186 = 0.31 +/- 0.04 parts per thousand, 2s.d., n = 7). The long-term external reproducibility (2s.d.) for these rock standards is +/- 0.05 parts per thousand.

The Ninetyeast Ridge is an similar to 5500 km long, north-south-oriented, submarine volcanic ridge in the eastern Indian Ocean that formed from magmatism associated with the deep-seated Kerguelen mantle plume as the Indian plate drifted rapidly northward during the Late Cretaceous. Basalts recovered along the ridge have the characteristic Dupal geochemical signature of Indian Ocean basalts, but debate concerning the nature and number of components in their mantle source persists. New multiple collector inductively coupled plasma mass spectrometry (Pb, Hf) and thermal ionization mass spectrometry (Sr, Nd) isotopic analyses were obtained for tholeiites representative of the similar to 180 m of basaltic basement recovered from three drill sites (Site 758, 82 Ma; Site 757, 58 Ma; Site 756, 43 Ma) along the Ninetyeast Ridge during Ocean Drilling Program Leg 121. No systematic isotopic variation is observed along the ridge, which is inconsistent with the hypothesis of an aging mantle plume origin for the ridge. The isotopic compositions are generally intermediate between those of the volcanic products of the Kerguelen and Amsterdam-St. Paul hotspots and define mixing trends between components with relatively enriched and depleted signatures. At least three, possibly four, source components are required to explain the observed isotopic variability along the Ninetyeast Ridge. The unradiogenic signatures of some Ninetyeast Ridge basalts (e.g. Sr-87/Sr-86 = 0 center dot 70381-0 center dot 70438) are not related to the source of Indian MORB and indicate the presence of a relatively depleted component in a deep mantle source. A similar source component is also identified in other Indian Ocean island basalts (e.g. Crozet, Reunion) not related to magmatic activity of the Kerguelen hotspot. The Pb-Hf-Sr-Nd isotopic compositions of the Ninetyeast Ridge basalts are consistent with the presence of a mixture of recycled sediments and lower continental crust together with altered ocean, AuthorCount:4

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At Elan Bank of the Kerguelen Plateau in the southeast Indian Ocean, Leg 183 of the Ocean Drilling Program recovered clasts of garnet-biotite gneiss in a fluvial conglomerate intercalated with basalt flows. U-Pb and Pb-Pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 Ma, which is much older than the surrounding Indian Ocean seafloor. These dates show that old continental crust resides in the shallow crust of the oceanic Kerguelen Plateau and that the breakup of Gondwana dispersed continental fragments into the nascent Indian Ocean lithosphere., SCOPUS: ar.j, info:eu-repo/semantics/published

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The Bjerkreim-Sokndal layered intrusion forms part of the Proterozoic South Rogaland Igneous Complex of southwest Norway. The intrusion has been divided into two parts; an anorthositic to gabbro-noritic Layered Series, and a generally more massive mangeritic, quartz-mangeritic to charnockitic upper part. On the basis of repeated lithological sequences, the Layered Series has been divided into a number of megacyclic units which are interpreted as reflecting magma chamber replenishment. Initial (940 Ma) Sr, Nd and Pb isotope ratios for plagioclases separated from samples crossing the transition between megacyclic unit III and megacyclic unit IV range from 87Sr/86Sr(i) = 0.7048, 143Nd/144Nd(i) = 0.51156 (ε(Nd) = +2.6) and 206Pb/204Pb(i) = 17.50; to 87Sr/86Sr(i) = 0.7061, 143Nd/144Nd(i) = 0.51143 (ε(Nd) = +0.1) and 206Pb/204Pb(i) = 17.73. The isotopic compositions vary systematically with stratigraphic height, and the interplay between the three isotopic systems is used to divide the stratigraphic profile into two parts: replenishment and post-replenishment. Across the replenishment interval isotopic variations are consistent with mixing between a resident and a replenishing magma. The replenishing magma was characterized by 87Sr/86Sr(i) = 0.7048, 143Nd/144Nd(i) = 0.51156 (ε(Nd) = +2.6) and 206Pb/204Pb(i) = 17.50. The positive ε(Nd) of this magma indicates that its source region was either in the mantle or was itself recently derived from the mantle. In contrast the resident magma had 87Sr/86Sr(i) = 0.7061; 143Nd/144Nd(i) = 0.51143 (ε(Nd) = +0.1) and 206Pb/204Pb(i) = 17.73 indicating that it had assimilated crustal material. In addition, the isotopic correlations across the replenishment interval indicate that prior to mixing the concentrations of Nd and Pb in the resident magma had been strongly reduced, probably by fractionation of apatite. Above the replenishment interval, Nd and Pb isotopic variations are apparently decoupled from Sr isotopic variations. Two mechani, SCOPUS: ar.j, info:eu-repo/semantics/published

Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998-99) and four from Legs 119 and 120 (1987-88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at ~ 110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatilerich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet-biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet-biotite gneiss are the, SCOPUS: re.j, info:eu-repo/semantics/published

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ODP Leg 153 drilled five sites on the western wall of the Mid-Atlantic Ridge between 23°20′N and 23°30′N, at distances of 5-35 km south of the Kane Transform (Figure 1). The targets of Leg 153 were extensive exposures of serpentinized peridotites and gabbroic rocks on the western median valley wall. The rock cores collected reveal major compositional and structural variations in both the serpentinites and gabbroic rocks that complement foregoing submersible observations. The samples also provide a sense of the complex igneous, structural, and metamorphic evolution of volumes of these major rock units beyond the scale of hand specimens. -from Author, SCOPUS: NotDefined.j, info:eu-repo/semantics/published

THE bimodal distribution of helium isotope ratios in ocean-island lavas has provided powerful constraints on the composition and evolution of the Earth’s mantle1-3. ‘High-3He hotspot’ ratios (3He/4He greater than in mid-ocean-ridge basalt, MORB) found on Hawaii4 and Iceland5 are thought to trace pristine plumes from the deep(er) mantle, whereas ‘low-3He hotspot’ values (3He/ 3He≤MORB value) at Tristan da Cunha6 and St Helena7 are considered to characterize plumes composed (in part) of recycled oceanic or continental crust. Here we report the observation of both ‘high-3He’ and 6low-3He’ characteristics8 in lavas from a single ocean island–Heard Island, in the Indian Ocean. Whereas the high-3He lavas provide unambiguous evidence for the involvement of a deep-seated plume in their genesis, we argue that the low 3He/4He ratios in other lavas result from shallow-level contamination by radiogenic helium before eruption. These observations call into question the presumed association between low-3He ratios (at Heard Island and elsewhere) and ancient crustal material recycled back into the mantle. © 1995 Nature Publishing Group., SCOPUS: le.j, info:eu-repo/semantics/published

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