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Back-arc magmatism: relating subduction zone fluxes to the record in mantle xenoliths and basalt chemistry

Applicant Müntener Othmar
Number 162666
Funding scheme Project funding (Div. I-III)
Research institution Institut des sciences de la Terre Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geochemistry
Start/End 01.10.2015 - 31.01.2021
Approved amount 496'990.00
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All Disciplines (2)

Discipline
Geochemistry
Mineralogy

Keywords (5)

Diffusion; Subduction zones; Back-arc magmatism; Mantle xenoliths; Time scales

Lay Summary (German)

Lead
Back arc magmatism: relating subduction zone fluxes to the record in mantle xenoliths and basalt chemistryZusammenhänge zwischen Materialtransport in Subduktionszonen und Magmatismus im Hinterland von InselbögenRelations entre le transport de matière dans les zones de subduction et le magmatisme arrière arc.Subduktionszonen befinden sich entlang konvergenten Plattengrenzen auf der Erde und sind durch weit verbreitete magmatische Aktivität gekennzeichnet. Eine Kombination aus Material aus der subduzierten Platte und Schmelzen des Erdmantels kann die Vielfalt der vulkanischen und plutonischen Gesteine erklären. Die systematische Änderung der Zusammensetzung der Magmen als Funktion der Distanz zum Inselbogen ist zwar bekannt, aber ob diese Änderung von den Mantelquellen, dem Fluss von Material aus der Subduktionszone oder durch konvektive Prozesse unterhalb des vulkanischen Inselbogens verursacht wird ist noch wenig bekannt.
Lay summary

In diesem Projekt testen wir einen alternativen Mechanismus, um die Variation der Magmachemie  als Funktion der Distanz zum aktiven Inselbogen zu verstehen - dichtes Material an der Basis der kontinentalen Kruste wird in den Erdmantel absinken und wieder aufschmelzen. Kann dieser Prozess in den vulkanischen Produkten an der Oberfläche nachvollzogen werden? Unsere wichtigsten Ziele in diesem Projekt sind (i) ob Mantelxenolithe und insbesondere das wichtigste Mineral Olivin eine systematische Veränderung der Gehalte an flüchtigen Bestandteilen wie Wasser zeigt, (ii) ob die Änderung der Basaltchemie mit dem Absinken und Schmelzen von dichtem Material erklärt werden kann, und (iii) ob dieser Prozess vor allem in Zeiten erhöhter Magmatischer Aktivität und erhöhten Konvergenzraten zwischen tektonischen Platten auftritt. 

 

 

Direct link to Lay Summary Last update: 25.09.2015

Responsible applicant and co-applicants

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Associated projects

Number Title Start Funding scheme
163991 A FEG-Electronprobe Microanalyzer for analysis of Earth materials at the sub-micron scale 01.04.2016 R'EQUIP

Abstract

Here we study the mechanism of the temporal evolution of subalkaline to alkaline magmas in the back-arc of subduction zones to get constraints on the processes of mantle melting (e.g. decompression melting versus delamination melting) and how these processes interact in space and time. Models on arc fluxes, and experimental constraints indicate that dense cumulates at the base of arc crust formed during intense activity of arc magmatism, and geodynamic models and seismic data show that such dense cumulates can delaminate on the 1 to 10 my time scale. Since amphibole-rich cumulates will melt at temperatures >1050°C and pressures < 2.5-3 GPa, and melt productivity for amphibole-rich rocks is very high in the temperature interval between 1050 and 1100°C, alkaline magmas produced by melting of amphibole-bearing arc cumulates must form a significant proportion of the overall magma budget of island arcs. This is an alternative to ‘plum pudding’ mantle melting of old material that has been commonly proposed to explain variability in arc back-arc systems. We will investigate the trace element and H2O content of olivine and opx in mantle xenoliths, and olivine phenocrysts to test the hypothesis of ‘wet melting’ of delaminated cumulates. Plutonic enclaves in selected basalts should help to constrain crustal thickness and nature of the underlying crust. Finally, quantification of delamination magmatism will help to refine models of mass fluxes in subduction zones.In order to achieve these ambitious research goals, we will conduct field and analytical work on selected back-arc areas in South America and compare these data to other areas to identify the spatial and temporal variability of the transition arc to back arc magmatism. The goal is to derive a detailed, time resolved section of back-arc volcanic chemistry to identify potential periods of magmatism related to arc cumulate recycling. Field geometries of the different basaltic units will be explored to constrain the temporal evolution of arc to back-arc magmatism. Laser Ablation ICP-MS and FTIR measurements on selected trace elements in olivine will be used for diffusion modeling, in order to test whether ‘primary’ H2O contents are preserved in olivine from mantle xenoliths, with the ancillary goal to determine time-scales of mineral growth for the different eruptive products and to determine the ‘background’ H2O content in the underlying mantle. One of the largest back-arc magmatic provinces (Southern Argentina) will be investigated to understand the spatial distribution of variations in magma chemistry and the time-scales involved. Current knowledge based on published data suggested that the formation of back-arc alkaline magmas in Patagonia is related to openings of ‘slab windows’ as a consequence of mid-ocean ridge (Chile ridge) subduction. The data will be compared to the chemical variability in the SW Japan and Sardinia-Aeolian island subduction system with a different geodynamic context. We will use these examples to monitor various sources and temporal changes of calc-alkaline and alkaline magmas, and to develop a major and trace element model to link arc and back-arc magmatism.
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