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The mineral and fluid chemistry of prograde serpentinite dehydration

English title The mineral and fluid chemistry of prograde serpentinite dehydration
Applicant Pettke Thomas
Number 137686
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 01.08.2012 - 31.03.2015
Approved amount 184'440.00
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All Disciplines (2)

Discipline
Geochemistry
Geology

Keywords (8)

geochemistry; subduction; laser-ablation ICP-MS; isotopes; mantle rocks; serpentinites; metasomatism; global element cycling

Lay Summary (German)

Lead
Subduktionszonen sind Orte, wo Oberflächenmaterial tief in die Erde rezykliert wird, und Serpentinite vom Ozeanboden spielen dabei eine zentrale Rolle. Das Studium natürlicher Proben wird es erlauben, diese Wasser-katalysierten chemischen Veränderungen zu quantifizieren, was einen fundamentalen Beitrag liefert zum besseren Verständnis der geochemischen Entwicklung der Erde, inklusive Vulkanausbrüche, Erdbeben, oder die Bildung riesiger Erzrohstoffquellen.
Lay summary

Während der Subduktion katalysieren wässrige Lösungen reich an gelösten Gesteinskomponenten chemische Veränderungen in der Erdtiefe, welche unter anderem Vulkaneruptionen, Erdbeben, oder auch die Bildung riesiger Erzlagerstätten auslösen und die chemische Zusammensetzung in der Erdtiefe nachhaltig verändern. Das Dissertationsprojekt von Annette Bretscher untersucht natürliches Probenmaterial, welches die Entwicklung von Serpentiniten und deren Umwandlungsprodukte bei der Subduktion in rund 60 km Tiefe abdeckt. Die petrographische Charakterisierung von Gesteinsproben von Almirez, Spanien, und die Bestimmung der Zusammensetzung (Haupt- und Spurenelemente sowie Isotopenverhältnisse) von Gesamtgesteinproben und von gesteinsbildenden Mineralien steht im Fokus. Diese Daten erlauben es, Element-Verteilungskoeffizienten zwischen koexistierenden Mineralen zu berechnen. Solche Daten identifizieren welche Elemente vorab in welchen Mineralen eingebaut werden, und die Stabilität dieser Minerale mit progressiver Metamorphose bestimmt dann, wann gewisse Elemente freigesetzt werden können. Analytisch messen wir diese Daten in situ mit der Elektronensonde, mit Laser Ablation ICP-MS und mit Sekundärer Ionen-Massenspektrometrie.

Weil Serpentinite einen wichtigen Anteil der subduzierten ozeanischen Platte ausmachen und den wohl bedeutendsten Transporter von Oberflächenwasser in die sonst trockenen Mantelgesteine darstellen, und weil Wasser in der Erdtiefe eine Reihe fundamentaler geologischer Prozesse auslöst (siehe oben), sind unsere Resultate essentiell für:
(1) Das quantitativeres Verständnis betreffend welche Elemente und wie viel davon durch Serpentinite in Subduktionszonen rezykliert wird,
(2) Die Identifikation des geochemischen Fingerabdrucks von diesem rezyklierten Material in vulkanischen Produkten an kollidierenden Plattenrändern, und
(3) Eine Abschätzung wie viel von diesem subduzierten Material tief in der Erde "verschwindet".
Letzteres beeinflusst fundamental die langzeitige Entwicklung der geochemischen Domänen der Erde und verspricht, die Funktionsweise des dynamischen Systems Erde besser zu verstehen.

Dank der quantifizierten Element-Verteilungskoeffizienten können nun geochemische Modellierungen betreffend der Elementrezyklierung an Subduktionszonen verfeinert werden.

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.
Melting of metasomatized peridotite at 4–6 GPa and up to 1200 °C: an experimental approach
Kessel R., Pettke T., Fumagalli P. (2015), Melting of metasomatized peridotite at 4–6 GPa and up to 1200 °C: an experimental approach, in Contrib. Mineral. Petrol. , 169, 37.
Channelized fluid flow and eclogite-facies metasomatism along the subduction shear zone
Angiboust S., Pettke T., DeHoog J. C. M., Caron B., Oncken O. (2014), Channelized fluid flow and eclogite-facies metasomatism along the subduction shear zone, in J. Petrol. , 55, 883-916.
Experimental study of trace element release during ultrahigh-pressure serpentinite dehydration
Spandler C., Pettke T., Hermann J. (2014), Experimental study of trace element release during ultrahigh-pressure serpentinite dehydration, in Earth Planet Sci. Lett. , 391, 296-306.
Petrology and trace element budgets of high-pressure peridotites indicate subduction dehydration of pristine serpentinites (Cima di Gagnone, central Alps, Switzerland)
Scambelluri M., Pettke T., Rampone E., Godard M., Reusser E. (2014), Petrology and trace element budgets of high-pressure peridotites indicate subduction dehydration of pristine serpentinites (Cima di Gagnone, central Alps, Switzerland), in J. Petrol. , 55, 459-498.
Halogen and noble gas systematics of ocean floor and ophiolitic serpentinites: implications for global volatile recycling
Kendrick M. A., Honda M., Pettke T., Scambelluri M., Phillips D. (2013), Halogen and noble gas systematics of ocean floor and ophiolitic serpentinites: implications for global volatile recycling, in Earth Planet Sci. Lett. , 365, 86-96.

Collaboration

Group / person Country
Types of collaboration
Dr. C. Spandler, James Cook University, Townsville Australia (Oceania)
- in-depth/constructive exchanges on approaches, methods or results
Prof. M. Scambelluri, University of Genova, DIPTERIS Italy (Europe)
- in-depth/constructive exchanges on approaches, methods or results
Prof R. Kessel, The Hebrew University, Jerusalem Israel (Asia)
- in-depth/constructive exchanges on approaches, methods or results

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
Swiss Geoscience Meeting 2013 Talk given at a conference The mineral and fluid chemistry of high-P serpentinite dehydration 15.11.2013 Lausanne, Switzerland Bretscher Annette; Pettke Thomas;
DPMS Spring Day 2013 Talk given at a conference The mineral and fluid chemistry of high-P serpentinite dehydration 26.04.2013 Bern, Switzerland Pettke Thomas; Bretscher Annette;


Knowledge transfer events

Active participation

Title Type of contribution Date Place Persons involved
Nationaler Zukunftstag Workshop 14.11.2013 Bern, Switzerland Bretscher Annette; Pettke Thomas;
Workshop GEO — LOGISCH Workshop 15.03.2013 Bern, Switzerland Bretscher Annette;


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)
124370 Fluids in subduction zones II 01.10.2009 SNSF Professorships
157121 Element distribution and heavy stable isotope fractionation at the magmatic-hydrothermal transition 01.05.2015 Project funding (Div. I-III)

Abstract

Aqueous fluids profoundly influence chemical and physical processes in subduction zones. Serpentinite dehydration is considered to represent the major water supply that eventually triggers magmatism at convergent plate margins, and residual rocks may return back to the mantle and thus recycle exogenous chemical components to the deep earth. Surprisingly, quantitative constraints from natural samples on the chemical composition of fluids released at specific serpentinite dehydration reactions at forearc and subarc depths have remained only scarce to date. Which and how much of the soluble elements inventory of arc magmas interpreted to originate from slab dehydration actually originates from subducted serpentinites as opposed to subducted sediments or altered oceanic crust has thus remained largely speculative.This PhD thesis aims at quantifying the chemistry of the brucite and antigorite dehydration reactions, including that of the liberated aqueous fluid phase, based on two prime field localities. Cerro del Almirez, Spain, records the antigorite-out reaction. Erro Tobbio, NW Italy, records the brucite-out reaction. The chosen approach emphasizes in-situ analytical techniques to analyze all reaction educt and product phases, most importantly Laser ablation ICP-MS, EPMA and SIMS, and is complemented by RAMAN identification of serpentine minerals in lower-grade rocks. LA-ICP-MS will be used to analyze bulk, high-P fluid inclusions trapped in eclogitic reaction product minerals. At an advanced stage of the project, SIMS will be employed for the quantification of "anionic" elements, notably Cl, S, F and possibly Br and I, to address the long-standing debate on the importance of halides for trace element mobility in the otherwise aqueous silicate fluids released at forearc to subarc depths.The Cerro del Almirez locality offers the investigation of another fundamental question, namely whether antigorite dehydration was triggered by external fluid influx (likely equilibrated with metasediments of the type cropping out nearby), or whether it represent an isograd and the prograde oli¬vine+orthopyroxene+ chlorite grew "in its own juice". Strontium and Pb isotope ratios of bulk rocks and prograde fluid inclusion fractions (bulk inclusion samples) will be employed to test the "fluid-from-sediment-infiltration" hypothesis, the answer to which has profound consequences for our understanding of what chemical signal may be released upon antigorite dehydration and, thus, what serpentinites may contribute to the slab signal at subarc depth.Chlorite in progressively subducted serpentinites is a relevant water carrier and may be the last hydrous phase to decompose along common subduction P-T gradients. Specific emphasis will be put on the thorough chemical characterization of chlorite for all the localities investigated to obtaining the trace element inventory fixed in chlorite with progressive subduction and to constraining the possible fluid fingerprint of chlorite devolatilization at subarc depth.The requested research will produce top-quality data efficiently and fill gaps in our understanding of the chemical cycle of serpentinite subduction, including refertilization of deep mantle domains. This should allow to much better constraining the impact of serpentinite dehydration in the fluid chemical cycling at subduction zones.
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