Complex proterozoic to paleozoic history of the upper mantle recorded in the Urals lherzolite massifs by Re–Os and Sm–Nd systematics

Re-Os and Sm-Nd isotope systematics for the Nurali and the Mindyak lherzolite massifs have been determined in conjunction with their whole-rock major and trace element contents. The data suggest that the peridotites represent residues after the extraction of up to 25% of partial melt from the fertile mantle protolith. Later melt percolation and associated fluid-rock reaction events have modified the Sm/Nd, Re/Os and Pd/Os ratios of peridotites, but didn't affect significantly their major element compositions. The Re-Os and Sm-Nd isotope data strongly suggest that several magmatic events are involved in the evolution of these bodies. The mantle sections of these complexes formed during Proterozoic times, represent the first reported evidence for Precambrian peridotites in the Southern Urals. The oldest Re-Os age of 1250 +/- 80 Ma for the Nurali cumulates records the separation from the convective upper mantle. Multiple partial melting of the peridotites followed by fractional crystallisation produced layered cumulates which were subsequently stored in the sub-continental lithosphere over similar to 0.8 Ga. The age of the Nurali ophiolite coincides with the development of an epicontinental rift basin on the passive margin of the Baltica proto-continent. The younger Sm-Nd age of Mindyak peridotites (882 83 Ma) and Re-Os age of associated gabbros (804 +/- 37 Ma) record another tectonic event responsible for the separation of the Mindyak massif from convective mantle. Between similar to 850 and 650 Ma, the margins of the paleo-Asian ocean became the site of island-arc formation. This ophiolite then evolved in an intra-oceanic island-arc setting. The Mindyak lherzolite massif could be the first record of a Neoproterozoic Cadomian are in the Southern Urals. A later island-arc formation event has then affected both massifs in different ways. The Mindyak massif was incorporated into a rifting zone at similar to 500 Ma, producing a second partial melting event of peridotites, cross-cutted by mafic dykes. The Nurali massif has also been cross-cut by gabbro-diorite dykes at Devonian time during the subduction event leading to Urals island-arc formation. The combination of. Re-187-Os-187 and Sm-147-Nd-143 systematics for peridotites, mafic-ultramafic cumulates and mafic dykes reveals the complex history of ophiolite complexes, including isolation from the convective upper mantle at Proterozoic time, storage in sub-continental lithospheric mantle and re-activation during later tectonic events, such as island-arc formation. (c) 2007 Elsevier B.V. All rights reserved

Mg isotope constraints on soil pore-fluid chemistry: Evidence from Santa Cruz, California

International audienceMg isotope ratios ( 26Mg/ 24Mg) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant 26Mg/ 24Mg ratio (expressed as δ26Mg) at -0.79 ± 0.05‰, identical to seawater δ26Mg. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a δ26Mg value of 0.11‰, potentially enriched in 26Mg by up to 0.3‰ compared to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for δ26Mg, ranging from -0.99‰ near the surface to -0.43‰ at the base of the profile. Shallow pore-waters (24Mg from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering

Positive correlation between Li and Mg isotope ratios in the river waters of the Mackenzie Basin challenges the interpretation of apparent isotopic fractionation during weathering

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The fundamental role of island arc weathering in the oceanic Sr isotope budget

International audienceWe have re-assessed the Sr isotopic budget of the modern ocean taking into account the high erosion rates of volcanic islands, and especially of island arcs, emphasizing important contribution from subsurface weathering to global budgeting. We propose that intensive weathering on volcanic islands, island arcs and oceanic islands, coupled with large surface and subsurface water fluxes is the missing source of mantlederived 87Sr/86Sr (0.703) in seawater Sr isotope balance. In our approach, it represents 60% of the actual mantle-like input of Sr to the oceans, the remaining 40% supplied by ridge-crest hydrothermal activity and sea-floor low-temperature alteration of basalts. The seawater Nd isotopic budget is consistent with this interpretation and explains well the regional contributions from ridge crest and island arc activity among the oceans

Osmium, tungsten, and chromium isotopes in sediments and in Ni-rich spinel at the K-T boundary: Signature of a chondritic impactor

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