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Redox conditions and sulphide-oxide-silicate mineral and fluid geochemistry of subducted hydrous mantle rocks

English title Redox conditions and sulphide-oxide-silicate mineral and fluid geochemistry of subducted hydrous mantle rocks
Applicant Pettke Thomas
Number 172688
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geologie Universität Bern
Institution of higher education University of Berne - BE
Main discipline Geochemistry
Start/End 01.03.2018 - 28.02.2022
Approved amount 650'000.00
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All Disciplines (3)

Discipline
Geochemistry
Geology
Other disciplines of Earth Sciences

Keywords (15)

Subduction zones; Geochemical cycling; Redox; Hydrated mantle rocks; serpentinites; Aqueous fluids; Element distribution coefficients; Sulphide; Oxide; European Alps; Sulphur isotopes; Chalcophile elements; Geochemical cycling; LA-ICP-MS; SIMS

Lay Summary (German)

Lead
Wässrige fluide Phasen in Subduktionszonen in Tiefen bis 200 km sind der Motor der geologischen Entwicklung unseres Planeten. Diese sind zentral für Elementtransport, Lokalisierung von Deformation (Erdbeben), und liegen am Ursprung explosiver vulkanischer Aktivität an Anden-Typ Kontinentalrändern. Die Quantifizierung solch fluid-induzierter chemischer Veränderungen ist allerdings oft lückenhaft, namentlich auch betreffend Veränderungen im Oxidationszustand (Redox-Zustand) fluid-infiltrierter Gesteine in der Tiefe, was die oben genannten geologischen Phänomene nachhaltig beeinflussen dürfte.
Lay summary

Ziele des Forschungsprojektes

Serpentinite (hydratisierte Mantelgesteine von ehemaligem Ozeanboden) sind wohl das wichtigste Gestein für Wassertransport in Tiefen bis über 150 km. Basierend auf natürlichen Gesteinsproben aus den Alpen und dem fokussierten Studium Redox-sensitiver Minerale quantifizieren wir Oxidationszustandsänderungen in fluiden Phasen und deren Einfluss auf den Elementtransport in Subduktionszonen. Wir erreichen dies durch Weiterentwicklung und Anwendung diverser, hoch-technologischer Messmethoden (e.g., LA-ICP-MS = Laser Ablation komniniert mit Induktiv Gekoppelter Plasma Massenspektrometrie; SIMS = Sekundärionen Massenspektrometrie, EPMA = Elektronenstrahlsonde) auf petrologisch exzellent charakterisiertem Probenmaterial. Die Gruppe der chalkophilen Elemente und die Halogene stehen dabei im Vordergrund, sind diese doch sehr Redox-sensitiv oder ein exzellenter Indikator für Fluid-Zirkulation in der Erdtiefe.

 

Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts

Resultate dieser Grundlagenforschung haben grosses Potential, unser quantitatives Verständnis bezüglich Elementtransport ermöglicht durch wässrige fluide Phasen in und Subduktionszonen voranzubringen. Dies ist essentiell für unser fundamentales Verständnis der Prozesse, die über hunderte Millionen Jahre unsere Erde stetig modifiziert und zu dem gemacht haben, was wir heute sehen.

Direct link to Lay Summary Last update: 01.03.2018

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Associated projects

Number Title Start Funding scheme
160076 Sulphide/oxide mineral and chalcophile element geochemistry of subducted hydrous mantle rocks 01.04.2015 Project funding (Div. I-III)
157121 Element distribution and heavy stable isotope fractionation at the magmatic-hydrothermal transition 01.05.2015 Project funding (Div. I-III)

Abstract

Aqueous fluids are the driving force for mass transport and govern the geodynamic evolution of subduction zones. Quantification of this fluid-mediated chemical cycling lags behind our petrologic and geodynamic understanding of subduction zone processes. Arguably the most prominent water release from rocks during progressive subduction occurs upon antigorite decomposition in hydrated mantle rocks. This represents the trigger for prominent element transport and partial melting when this fluid percolates into more evolved slab rocks, and provokes deformation along the slab interface or within the slab via hydraulic weakening. Quantitative constraints on fluid element mobilities are limited, however, also because sulphide and oxide systematics have so far largely been ignored. Consequently, redox conditions have remained ill constrained, representing a serious gap in our understanding of aqueous fluid element mobilities, notably of chalcophile, redox-sensitive elements.This project aims at quantifying the silicate-sulphide-oxide(-carbonate) petrology and redox conditions prevailing at the antigorite-out dehydration reaction. To this end, prominent methods developments are required, most importantly LA-ICP-MS measurement protocols for sulphide and oxide minerals, and SIMS measurement of S isotope ratios in diverse sulphides. Applications are based on natural samples, with an emphasis on chalcophile element systematics as they are fluid mobile and redox-sensitive. Several ultrabasic lenses representing antigorite dehydration product rocks are hosted in the southern steep belt forming part of the Tectonic Accretion Channel (TAC) that delineates the suture of the Alpine collision. This melange unit records well over 20 million years of subduction, accretion, internal heating, and exhumation along the plate interface; however, the trace element geochemistry of ultrabasic lenses has remained essentially unconstrained.Combination of detailed field work and petrology with bulk rock and in-situ analytical techniques (LA- ICP-MS, SIMS, EPMA, Raman) will emphasize the quantification of trace elements not commonly measured such as chalcophile elements (e.g., S, As, Sb, Bi, Tl, In, Se, Cd), B, and all the halogens. Element distribution coefficients between coexisting product phases (fluid and minerals) will be determined, and variations thereof between different ultrabasic lenses are to be evaluated in the light of possible variations in redox budget and initial bulk rock composition. Oxide and sulphide sinks for specific trace elements are to be identified.Comprehensively quantifying the chemistry of the most prominent dehydration reaction in subducting slabs (antigorite-out) offers the long-needed basis for a robust interpretation of fluid element mobilities and their dependence on prevailing redox conditions. Because the field laboratory "TAC of the Alps" repre¬sents a geodynamic subduction scenario encountered in many modern (as indicated by geophysical imaging) and fossil compressional orogens, the results may aid in much better constraining the global geochemical cycles of some hitherto poorly known elements, with great potential for advancing our understanding of subduction-related geodynamics.
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