Project

Global geochemical cycling; Subduction zones; Chemical composition; Pb-Nd-Sr isotopes; LA-ICP-MS; Alps; Western Gneiss Region; Almirez; Subduction zones; Global element cycling; Laser ablation ICP-MS; Serpentinite; Fluid inclusions; Eclogite; Metasomatism; Diamond trap experiments

Lead
Lay summary
Subduction zones are the site of recycling of surface materials to deep earth. Aqueous fluids drive the chemical exchange and trigger volcanic activity and earthquakes, may generate giant ore deposits, and refertilise deep mantle portions. Detailed in-situ measurements of element concentrations and Pb and Sr isotope ratios on natural samples and experimental products are employed to better constrain these fundamental chemical fluxes in subduction zones. Analytical methods developments include the Pb and Sr isotope ratio analysis of individual fluid inclusions by laser ablation ICP-MS (LA-ICP-MS), which was applied to the giant Cu-Au-Mo deposit of Bingham. The novel data reveal that the source of ore metals lies in the lithospheric mantle metasomatized by subduction fluids some 1800 million years ago. These are among the first direct data that relate ancient subduction metasomatism with much younger formation of prominent ore metal anomalies. Diamond trap experiments on the aqueous fluid chemistry in equilibrium with serpentinites at 120 - 180 km depth (i.e., in the slab beneath arc volcanism) document prominent compositional changes along with the coexisting residual mineral assemblage, demonstrating that the chemistry of fluids released from the down-going slab strongly evolves with progressive dehydration. Pathways of fluids escaping from serpentinites and invading overlying basaltic eclogites were identified in the Monviso, NW Italy, documenting serpentinite fluid metasomatism by use of vein mineral chemical signatures for the first time. At much lower depths in the forearc, ocean floor serpentinites (chrysotile/lizardite) begin to transform to antigorite serpentinite stable at higher pressure. This transformation goes along with a prominent trace element loss from the rock that may well account for chemically enriched fluids observed in serpentinite mud volcanoes at trenches. Mantle rock fluid metasomatism at up to 200km depth is recorded by garnet-omphacite-phlogopite-veins, the detailed chemical investigation of which is currently under way. A comprehensive review of the LA-ICP-MS analytical techniques applied to fluid inclusions illustrates state of the art procedures to obtaining accurate compositional and isotope ratio measurements from individual fluid inclusions.

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Last update: 21.02.2013

Active participation

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Aqueous fluids profoundly influence chemical and physical processes in subduction zones. Here, at convergent plate margins, commonly oceanic lithosphere (the slab) sinks (subducts) beneath continents, triggering many directly observable processes, including volcanic activity and earthquakes. Almost all these processes are directly or indirectly affected at some stage by a fluid phase. Properties of these liquids at depths exceeding a few tens of kilometres are only vaguely known, however, notably their chemical composition, how they move, how much mass flux they effect and over what scales in distance and time they operate. These issues probably represent the most serious impediment towards a more quantitative understanding of the chemical cycling in subduction zones.This follow-up proposal aims at further constraining the subduction cycle of hydrated oceanic mantle, serpentinites. The first aspect deals with the antigorite dehydration reaction, an essential snapshot during serpentinite subduction. Special emphasis is on the chemical and Pb-Sr-isotopic composition of the dehydration fluid captured in prograde, tiny fluid inclusions and its associated mineral assemblages (antigorite serpentinites and the devolatilization products, olivine - orthopyroxene - chlorite rocks) from Cerro del Almirez, Spain. This represents the only locality known to date where the antigorite-out reaction can be studied and sampled in the field. Bulk rock data will also be analyzed for Nd and possibly Hf isotopes in order to constrain the provenance of these serpentinites. A rigorous quantification of the chemical composition of the antigorite dehydration fluid can now be attacked in a much more comprehensive way, thanks to extensive LA-ICP-MS methods developments essential for the accurate in-situ analysis of ultra-trace element concentrations, achieved during the first stage of this project.The second aspect is the Sr-, Pb-, Nd- and possibly Hf-isotopic characterization of hydrated oceanic mantle, to characterizing the radiogenic isotope signatures that are eventually entering the subduction cycle. Modification of these radiogenic isotope signatures with progressive subduction are assessed through the analysis of antigorite-serpentinites and partly dehydrated olivine - orthopyroxene ± chlorite rocks. Only scattered such data are currently available, not allowing for a systematic assessment of the source tracer potential of these radiogenic isotope systems. Moreover, combination with extant major to trace element compositional data of the same samples (bulk rock and in-situ mineral data, in large parts available from the original project) will allow to addressing the combined chemical and isotope systematics of mantle hydration on the ocean floor and how these complementary signatures evolve with progressive subduction.The requested research will produce top-quality data efficiently and fill gaps in the chemical cycle of serpentinite subduction, including refertilization of mantle domains. The resulting comprehensive chemical and radiogenic isotope data set (original project plus follow-up proposal) is unprecedented and will allow to constraining fundamental issues on the relevance of hydrated mantle rocks in the fluid-mediated chemical cycling in subduction zones. It is generally accepted that hydrated mantle rocks essentially provide water to the subduction cycle. Which and how much of the soluble elements inventory actually originates from subducted serpentinites as opposed to subducted sediments will become much better quantified. Our views on fluid percolation paths and fluid compositional evolution will clarify. Addition of crustal components through subduction of largely devolatilized mantle rocks will better constrain deep mantle refertilization processes. Ancient subduction events most likely had similar effects. Ancient subduction-modified mantle domains can be identified through radiogenic isotope systematics and, given more quantitative constraints on modern subduction geochemical cycling, will allow assessing the effects of subduction on the evolution of Earth’s mantle and crust during the past >2 billion years.These fundamental research results will have profound implications on various key aspects of the global geochemical cycle and will inspire new initiatives in geosciences and possibly related research disciplines, well beyond the duration of this project.