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Assessing element recycling within subduction zones based on metal isotopes

Applicant Wille Martin
Number 182508
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 01.03.2019 - 28.02.2023
Approved amount 279'009.00
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All Disciplines (3)

Discipline
Geochemistry
Geology
Mineralogy

Keywords (4)

subduction zones; molybdenum; barium; stable isotopes analysis

Lay Summary (German)

Lead
Die Bildung kontinentaler Kruste und deren chemische Zusammensetzung stehen im engen Zusammenhang mit dem Magmatismus an konvergenten Plattenrändern. Geochemische Prozesse gesteuert durch die Dehydrierung subduzierter ozeanischer Kruste und darüber liegender Sedimente haben grossen Einfluss auf die Mobilität verschiedenerer Elemente beim Materialtransport innerhalb der ganzen Subduktionszone und dem darüber liegenden vulkanischen Inselbogen in dem neue kontinentale Kruste entsteht. Die intrinsische Natur dieser Prozesse erschwert eine genaue Bestimmung der Elementtransferraten und erschwert die Rekonstruktion der sich ändernden geodynamischen und magmatischen Prozesse, die bei der Bildung kontinentaler Kruste beteiligt sind, im Laufe der Erdgeschichte.
Lay summary

Inhalt und Ziel des Forschungsprojekts
Ziel dieser Forschungsarbeit ist es die verschiedenen Prozesse zu bestimmen und zu evaluieren, die für die Mobilität und Rezyklierung der Elemente Molybdän und Barium in Subduktionszonen verantwortlich sind. Dadurch werden wir in der Lage sein die Massenbilanz zwischen Elementeintrag, Recyklierung und Transport in den unteren Mantel zu quantifizieren und den Einfluss subduzierter Elemente auf diese Massenbilanz zu untersuchen. 

Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts
Da sich die Zusammensetzung ozeanische Sedimente im Laufe der Erdgeschichte aufgrund sich ändernde klimatischen Verhältnisse geändert hat, tragen diese Erkenntnisse dazu bei die Rezyklierung von Elementen innerhalb Subduktionszonen über die Erdgeschichte zu bestimmen.

Keywords
subduction zones, Barium, Molybdenum, stable isotope analysis

Direct link to Lay Summary Last update: 21.12.2018

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Abstract

The formation of continental crust through geological history is closely linked to subduction zone magmatism at convergent margins. An intrinsic feature of subduction zones is the complex interplay between input of diverse material from the surface into the mantle and its recycling into the overlying mantle wedge by slab-derived fluids. Prograde metamorphic reactions cause dehydration and partial melting of the subducted sediments and altered oceanic crust, which can therefore be recycled and contribute to the making of new continental crust. Quantifying mass and element input fluxes into subduction zones and constraining processes that control element transfer into new continental crust is crucial to evaluate the change in the composition of the continental crust through Earth’s history. Certain incompatible elements can be enriched in oceanic sediments and altered oceanic crust and occur as authigenic mineral phases that precipitated from seawater and have a distinct isotopic composition. Although such elements are potentially suited to trace recycling of subducted material, their mobility within the sub-arc regions is dependent on parameters like redox conditions, fluid salinity, etc., which are poorly known.To untangle the influence of the different processes responsible for element cycling within arc settings, this research project will be the first comprehensive investigation that combines the redox sensitive transition metal Mo with the fluid mobile and redox insensitive element Ba. A substantial fraction of Mo and Ba in arc lavas is thought to originate from recycled subducted material. The distinct Ba isotopic composition in deep sea marine sediments compared to the igneous inventory allows identifying Ba recycling within arc lavas based on its isotopic information. Hitherto available Ba isotopes have shown measurable variations in igneous rocks are possibly the result of recycled marine Ba-sulfates in the source of the magmas. The liberation of sedimentary sulfate (i.e. S6+) influences sub-arc redox conditions and the variability of the isotopic composition of transition metals like Mo. The remoteness of the Tonga Trench from continental shelf areas favors the subduction of dominantly chemical sediments with substantial amounts of authigenic Mo and Ba and is thus an ideal location to study the recycling of both elements. The budgets and isotopic compositions of Ba and Mo will be analysed in sediments and altered oceanic crust samples obtained from drill cores in front of the trench, together with newly sampled primitive arc lavas from the Tonga volcanic arc. In order to include the perspective of a prograde metamorphic suite, the Tonga arc results will be compared with those obtained from rocks and minerals from the Zermatt-Saas ophiolite zone, which can be viewed as the exhumed equivalent of a subduction zone.The goal of this project will be to identify the processes and sub arc conditions that determine the recycling of Ba and Mo within subduction zones. This will help quantifying the mass balance between input, recycled fraction and further transport into the deep mantle and how this might change in different arc settings dependent on the geochemical composition of the sedimentary input. Ideally this can be later used to estimate how a globally changing sedimentary subduction input caused by redox changes during Earth history has influenced the recycling of redox sensitive elements within subduction zones.
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