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Sulphide/oxide mineral and chalcophile element geochemistry of subducted hydrous mantle rocks

English title Sulphide/oxide mineral and chalcophile element geochemistry of subducted hydrous mantle rocks
Applicant Pettke Thomas
Number 160076
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 01.04.2015 - 31.07.2016
Approved amount 165'824.00
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All Disciplines (2)

Discipline
Geochemistry
Geology

Keywords (8)

Serpentinite; fluid inclusions; chalcophile elements; Almirez; element distribution coefficients; hydrated mantle rocks; analytical geochemistry; LA-ICP-MS

Lay Summary (German)

Lead
Wässrige fluide Phasen sind zentral für die geochemischen Prozesse assoziiert mit Subduktionszonen und steuern Magmatismus inklusive Vulkanismus oder beeinflussen die Seismik (Erdbeben) an konvergierenden Plattengrenzen. Serpentinite sind wasserhaltige Mantelgesteine, welche subduziert werden und in grosser Tiefe solches Wasser freisetzen können. 0.02 mm grosse Flüssigkeitseinschlüsse sind Zeuge davon; gemessen mit LA-ICP-MS zeigen sie spezifische Elementkonzentrationen und Anreicherungen. Somit können wir nun chemische Hinweise auf die initialen, Wasser-katalysierten Prozesse in subduzierten Gesteinen erkennen und also die auslösenden Prozesse von Vulkanismus an Subduktionszonen besser verstehen.
Lay summary

Freie, wässrige, fluide Phasen in grosser Tiefe sind zentral für den geochemischen Massentransport und steuern die geodynamische Entwicklung in Subduktionszonen. Kenntnisse betreffend chemischer Zusammensetzung solcher Fluide hinken unserem Verständnis der Petrologie und der geodynamischen Prozesse allerdings nach. Serpentinite (Mantelgesteine mit bis >10 % Wasser in Mineralen gebunden) sind dabei zentral, setzen diese doch solches Fluid sukzessive frei bei Subduktion.

Dieses Projekt, die Beendigung einer Dissertation, beinhaltet (1) die umfassende geochemische Quantifizierung der Entwässerungsreaktion von Serpentinit zu Chlorit-Harzburgit am Paradebeispiel von Almirez, Spanien, und (2) es werden mittels Kompilation globaler Geochemiedaten (a) Elementsignaturen identifiziert, welche das geologische Milieu der Serpentinitbildung aufzeigen, und (b) wie sich ein solch charakteristischer chemischer Fingerabdruck mit progressiver Subduktion verändert.

Prominente Anreicherungen an B, As, Sb, Pb, Bi, Cs, Rb, Ba, Sr, S, Cl, und Br charakterisieren das in 60 km Tiefe freigesetzte Wasser, gemessen als 0.02 mm grosse Flüssigkeitseinschlüsse mittels LA-ICP-MS in Olivinkristallen. Dieser Datensatz, bisher einzigartig in seiner Art, zeigt Elementanreicherungsmuster, die Geologen in Vulkanen an konvergierenden Plattengrenzen wieder finden. Zudem hinterlassen diese Elementmuster einen Fingerabdruck in Gesteinen der subduzierten Platte, welche durch solche wässrigen Lösungen chemisch überprägt wurden und also Wasserwanderpfade in grosser Tiefe identifizieren. Elementmuster der nach dem Wasserverlust übrig bleibenden Gesteine haben noch eine Erinnerung an das Milieu, in welchem die Serpentinite damals am Ozeanboden entstanden sind.

Zusammen ermöglichen diese neuen Resultate zur Geochemie der wässrigen Phasen tief in Subduktionszonen nun ein besseres Verständnis der Prozesse, die am Ursprung von Vulkanismus, Erdbeben, oder auch der Bildung gigantischer Erzlagerstätten wirken.
Direct link to Lay Summary Last update: 11.11.2016

Responsible applicant and co-applicants

Employees

Name Institute

Publications

Publication
The influence of oceanic oxidation on serpentinite dehydration during subduction
Bretscher Annette, Hermann Joerg, Pettke Thomas (2018), The influence of oceanic oxidation on serpentinite dehydration during subduction, in Earth and Planetary Science Letters, 499, 173-184.
Fluid-mobile elements in serpentinites: Constraints on serpentinisation environments and element cycling in subduction zones
Peters Daniel, Bretscher Annette, John Timm, Scambelluri Marco, Pettke Thomas (2017), Fluid-mobile elements in serpentinites: Constraints on serpentinisation environments and element cycling in subduction zones, in Chemical Geology, 466, 654-666.
The behaviour of incompatible elements during hydrous melting of metasomatized peridotite at 4–6 GPa and 1000 °C–1200 °C
Kessel R., Fumagalli P., Pettke T. (2015), The behaviour of incompatible elements during hydrous melting of metasomatized peridotite at 4–6 GPa and 1000 °C–1200 °C, in Lithos, 236-237, 141-155.

Collaboration

Group / person Country
Types of collaboration
Prof. M. Scambelluri, DISTAV - Università di Genova Italy (Europe)
- Publication
Marie Curie Action: ITN - ZIP France (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication

Associated projects

Number Title Start Funding scheme
172688 Redox conditions and sulphide-oxide-silicate mineral and fluid geochemistry of subducted hydrous mantle rocks 01.03.2018 Project funding (Div. I-III)
137686 The mineral and fluid chemistry of prograde serpentinite dehydration 01.08.2012 Project funding (Div. I-III)

Abstract

Aqueous fluids are the driving force for mass transport and govern the geodynamic evolution of subduction zones to depths shallower than first melting. Fluid-rich matter lost from the down-going slab triggers arc magmatism, and residual rocks return back to the mantle and thus recycle exogenous components to the deep Earth. Quantification of this fluid-mediated chemical cycling lags behind our petrologic and geodynamic understanding of subduction zone processes. This project represents the termination of the PhD thesis started in a previous SNF project (The mineral and fluid chemistry of prograde serpentinite dehydration). Antigorite dehydration is considered to represent the major water supply from subducting slabs. The Chl-harzburgites form Cerro del Almirez (S-Spain) are a spectacular natural record of subduction related high pressure breakdown of Atg-serpentinites to Ol+Opx+Chl assemblages occurring at ca. 60 km depth. Having constrained the geochemistry of rocks and minerals across the antigorite dehydration reaction in the previous SNF project, this final part deals with (1) quantification of the dehydration fluid composition via measurement of individual fluid inclusion relics hosted in prograde olivine from chlorite harzburgites, and (2) assessing the potential of fluid mobile element systematics on a global scale to identifying the environment of peridotite hydration on the ocean floor and how such diagnostic trace element signatures evolve with progressive subduction.The chemistry of the antigorite dehydration fluid is characterised by concentrations of B, As, Sb, Pb, Bi, Cs, Rb, Ba, Sr, S, Cl, and Br largely exceeding primitive mantle values (As up to 520 PM, Sb up to 1440 PM, Bi up to 90 PM, Br up to 1155 PM). Fluid/mineral distribution coefficients of these trace elements are above 1 for all coexisting prograde minerals (olivine, orthopyroxene, and chlorite); however, they cluster around 1 for B and Li. Ratios of Cl/Br of the polyphase inclusions cluster around 0.0011 ±0.0003, identical to the ratio previously determined for bulk samples by Kendrick et al. (2011). These novel data demonstrate prominent fluid mobile trace element mobility triggered by antigorite dehydration, traces of which are now to be identified in fluid-metasomatized slab rocks and in arc magmatism.(2) Evaluation of a new compilation of published ocean floor serpentinite data has allowed to identify trace element abundance ratios involving U and alkali elements that discriminate between mid ocean ridge and forearc serpentinisation environments. Employing data from subducted serpentinites (Erro Tobbio and Almirez) demonstrates that trace element abundance signatures are little modified during brucite dehydration, while significant changes are documented at antigorite dehydration. However, the trace element flavour of the original site of serpentinisation persists. Together these novel data quantify the hydrous mantle rock specific trace element systematics and the chemistry of the fluid liberated from dehydrating serpentinites at subarc depths, chemical signals that can now be searched for in rocks that were formed in response to such fluid infiltration, metasomatic slab rocks and arc magmas alike.
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