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Dehydration at subduction zones and the geochemistry of slab fluids

Science China Earth Sciences 2020年第12期63卷 1925-1937页

作者： Wancai Li Huaiwei Ni CAS Key Laboratory of Crust-Mantle Materials and Environments School of Earth and Space SciencesUniversity of Science and Technology of ChinaHefei230026China CAS Center for Excellence in Comparative Planetology Hefei230026China

Subducting oceanic slabs undergo metamorphic dehydration with the increase of temperature and pressure during subduction.Dehydration is an essential step for element recycling,and slab fluids are critical agents for mediating slab-mantle interaction.Dehydration is mainly controlled by the thermal structure of subduction zones and the stability of hydrous minerals.At fore-arc depths,slab dehydration produces aqueous fluid with dissolved salts such as NaCl.As subduction proceeds deeper,the content of silicate components increases.At sub-arc and post-arc depths,a hydrous silicate melt is likely to form,or a supercritical fluid could arise from complete miscibility between silicates and H_(2)O.The partitioning of elements between slab fluid and the residual solid rock is controlled by the type of fluid,and generally it is the supercritical fluid that is the most capable of mobilizing trace elements,being an effective carrier even for high field strength elements.Understanding the chemistry of slab fluids relies on sophisticated integration of experiments,theoretical computation and investigation of natural rock samples.This contribution focuses on the content and speciation of key volatiles,including carbon,nitrogen and sulfur,in slab fluids as well as important fluid properties such as oxygen fugacity and acidity.The properties of slab fluids show complicated variation under the control of mineral assemblages and T-P conditions.slab fluids at great depths of subductions have been inferred to be modestly alkaline and not necessarily very oxidizing as often assumed.Further progress in the research of slab dehydration and the chemistry and properties of slab fluids demands urgently the development of innovative experimental and computational technology including in situ analytical methods at high T-P.

关键词： Subduction zone Dehydration slab fluids Volatiles Oxygen fugacity Acidity

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New Caledonia Ophiolite, Marginal Rifting to Fore-arc Evolution

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Acta Geologica Sinica(English Edition) 2020年第S1期94卷 9-10页

作者： Dominique CLUZEL Jonathan AITCHISON Arianna SECCHIARI Alessandra MONTANINI Delphine BOSCH Universitéde la Nouvelle-Calédonie NouméaNew Caledonia School of Earth and Environmental Sciences University of Queensland QLD 4072Australia University of Parma Parco Area delle Scienze 157/a 43124 ParmaItaly Earth Sciences Department Géosciences Montpellier UMR-CNRS 5243-Univ.Montpellier II France

The New Caledonia Ophiolite(Peridotite Nappe), represents about one third of the island’s surface(i.e. 5 500 km2). The ophiolite is composed of harzburgites, dunites, lherzolites, minor mafic-ultramafic cumulates, and various dykes and sills. The mantle section underwent a polyphase evolution, which involved prominent depletion and re-fertilization. The oldest events are probably recorded by abyssal-type lherzolites of the northern massifs, which bear traces of moderate partial melting. Plagioclase lherzolites were formed by shallow entrapment of highly depleted MORB melt in residual spinel lherzolites. Nd isotope compositions are consistent with derivation from an asthenospheric mantle source that experienced a recent MORB-producing depletion. This evolution was most likely accomplished during the late Cretaceous breakup of the eastern Australian margin. The harzburgite-dunite association, which forms the bulk of Peridotite Nappe was probably formed through a multistage magma-producing process. Harzburgites composition may have be obtained by a first phase of ~15% dry fractional melting, followed by 15%–18% hydrous melting in a supra-subduction zone setting. Variable εNd negatively correlate with 87Sr/86Sr, while Pb isotopes cover a wide range, trending from depleted mantle towards enriched, sediment-like, compositions. Such signatures likely reflect the evolution of a highly depleted forearc mantle wedge variably modified by different fluid and melt inputs during Eocene subduction. The harzburgite-dunite set is overlain by a dunite transition zone ~300 m thick, in turn discontinuously covered by cumulate lenses consisting of layered pyroxenites, dunites, and wherlites at the base and gabbronorites/websterites on top. The mafic cumulates crystallized from primitive, ultra-depleted melts in the nascent lower fore-arc crust. In particular, FME enrichments and Nd-Pb isotopes support an origin from a refractory mantle source modified by slab fluids for the gabbronorite-forming melts. The Peridotite Nappe has been ex

关键词： ophiolite supra-subduction slab fluids metasomatism

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