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Understanding osmium (Os) recycling during subduction zone metamorphism

Applicant Saintilan Nicolas
Number 180133
Funding scheme Ambizione
Research institution Institut für Geochemie und Petrologie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geochemistry
Start/End 01.11.2018 - 31.10.2022
Approved amount 735'660.00
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All Disciplines (2)

Discipline
Geochemistry
Geochronology

Keywords (12)

antigorite; sulfide; osmium; subduction zone; recycling ; geodynamics; metamorphic rock; mantle; eclogite; serpentinite; blueschist; isotope geochemistry

Lay Summary (French)

Lead
L’osmium est un métal utilisé comme traceur chimique des hétérogénéités et de la dynamique du manteau terrestre. De telles hétérogénéités seraient liées au recyclage des roches de la lithosphère dans les zones de subduction. Les modifications minéralogiques, chimiques et de déshydratation lors du métamorphisme des roches de zone de subduction sont à l’origine des processus de volcanisme d’arc ainsi que la source probable des fluides à l’origine des porphyres géants de Cu-Au. Toutefois, en dépit de son potentiel de traceur de tels processus, le comportement de l’osmium reste relativement méconnu lors de ces modifications dans les zones de subduction.
Lay summary

Contenu et objectifs du travail de recherche

Nous voulons produire une carte détaillée de la composition isotopique en osmium d’une zone de subduction fossile dans le contexte de la chaîne orogénique alpine. En détail, nous étudierons la composition isotopique en rhénium et osmium (Re-Os) d’une suite exhumée de roches métamorphiques de haute et très haute pression ainsi que les sulfures qu’elles contiennent. Ces roches proviendront de localités clés pour lesquelles l’histoire métamorphique « pression-température-temps » est connue. Les sulfures, qui sont les principaux hôtes de l’osmium chalcophile, seront aussi caractérisés (i) pétrographiquement, (ii) en termes de composition chimique détaillée, et (iii) de composition isotopique en soufre. 

Contexte scientifique et social du projet de recherche

Cette étude contribuera des informations essentielles quant au rôle des réactions de déshydratation des roches de zone de subduction sur la mobilité des métaux (Os, Re, Cu, Au). Nous pourrons documenter le degré d’oxydation des fluides aqueux ainsi que leur origine, probablement due à la réaction majeure de déshydratation des roches du manteau lithosphérique.

Keywords

osmium, metamorphic rock, subduction zone, recycling, sulfide, blueschist, eclogite, serpentine, mantle, geodynamics, antigorite, metals

Direct link to Lay Summary Last update: 06.09.2018

Responsible applicant and co-applicants

Employees

Knowledge transfer events

Active participation

Title Type of contribution Date Place Persons involved
17th Swiss Geoscience Meeting Talk 22.11.2019 Fribourg, Switzerland Saintilan Nicolas;
Biennial Meeting of the Society for Geology Applied to Mineral Deposits (SGA) Talk 27.08.2019 Glasgow, Great Britain and Northern Ireland Saintilan Nicolas;
Goldschmidt Conference 2019 Talk 18.08.2019 Barcelona, Spain Saintilan Nicolas;


Associated projects

Number Title Start Funding scheme
171496 Part 1: Solving the controversy on the genesis of the world’s largest sediment-hosted copper reserves via detailed sulfide Re-Os geochronology: implications for genetic models and mineral exploration programs, and, Part 2: Further development of the 01.03.2017 Advanced Postdoc.Mobility
162075 Development of Re-Os systematics for Cu-Co sulfides: isochron and model age determination, accuracy and reproducibility. Application to the origin of the Cu- and Co-Cu mineralization in the Spar Lake deposit and the Idaho cobalt belt 01.09.2015 Early Postdoc.Mobility

Abstract

The oceanic and continental crust and underlying lithospheric mantle are recycled into the Earth’s deeper mantle in subduction zones. The subducted rocks in the slab undergo mineralogical and chemical changes with increasing pressure and temperature, including dehydration and metasomatism. The aqueous fluids released from the slab are the driving force for mass transport and govern the geodynamic evolution of subduction zones. Thus, the mantle wedge above the slab is metasomatised and may melt partially, thereby initiating a cascade of events including arc volcanism and related porphyry-type Cu-Au mineralization. Finally, with slab break-off, the chemical composition of the mantle is inevitably modified.Osmium (Os), which is a Platinum Group Element (PGE), is redox-sensitive and highly compatible in the mantle. Rhenium (Re), which is a similarly siderophile and chalcophile element considered with the PGEs, is mildly incompatible. As a consequence, melts have higher Re/Os ratios than their mantle source. In addition, Re and Os participate in the 187Re-187Os radioactive decay scheme. Therefore, any evolved crustal rock with elevated Re/Os ratio will have significantly higher 187Os/188Os ratios than the mantle. This rationale has motivated the use of Os isotopes as chemical tracer of mantle dynamics. Osmium isotopic heterogeneities identified in mantle-derived rocks were interpreted as being caused by the recycling of crustal material (approximated as sediments and basalts) in the deeper mantle. However, the behavior of Os during subduction zone metamorphism and the knowledge of the Os isotopic composition of the actual rocks composing the slab (i.e., high pressure - HP blueschists & eclogites, ultra-high-pressure - UHP rocks, serpentinites) lags behind the systematic use made of Os as tracer of mantle geodynamics. This project aims at drawing the first detailed map of the Os isotopic composition of a paleosubduction zone (<100km) by studying a suite of exhumed HP-UHP metamorphic rocks and their accessory sulfides in the framework of the Alpine orogenic belt in Western Europe. To this end, I will use natural samples from key localities where the petrography, geochemistry, and P-T-time paths of the rocks have been thoroughly characterized by experts in metamorphic geology. The samples include mafic and ultramafic rocks representing the protoliths and the products of the lawsonite-breakdown or antigorite-breakdown reactions. My project will constrain the Re-Os isotope geochemistry of the various HP-UHP rocks and their accessory sulfides. These sulfides, which will be characterized through sulfide petrology, quantitative major and trace element mineral chemistry, and bulk 34S VCDT analysis, will be recovered as monophase sulfide mineral separates produced from the rocks by adapting my methodology originally developed for ore samples. The comprehensive characterization of the Os isotopic composition of the HP-UHP metamorphic rocks and their accessory sulfides should show that the recycling of Os during slab dehydration and into the deeper mantle is linked to the open/close-system behavior of the Re-Os system in the sulfides. The concurrent study of the Os and S isotope systematics of these sulfides can give insights into the oxidation state of the fluids released from the subducted crust. Thus, Os may serve as proxy for the mobility of other chalcophile elements (e.g., Re, Cu, Au). It is suspected that the antigorite-out dehydration reaction may be a significant stage of mobilization of redox-sensitive Os in aqueous fluids. Re-Os isochron dates of the sulfides in antigorite serpentinite may identify the age of the protolith or of serpentinisation.
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