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Metasomatism and physical properties of the oceanic lithosphere

English title Metasomatism and physical properties of the oceanic lithosphere
Applicant Pilet Sébastien
Number 179046
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 Geology
Start/End 01.07.2018 - 30.06.2022
Approved amount 526'648.00
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All Disciplines (2)

Discipline
Geology
Geochemistry

Keywords (4)

oceanic lithosphere; metasomatism; water transport; mantle geochemistry

Lay Summary (French)

Lead
Dr S. Pilet en collaboration avec le docteur Dr N.Abe (Jamstec, Japon) et le professeur N. Hirano (Université de Tohoku, Japon)
Lay summary

Des études géophysiques ont montré que la limite entre la lithosphère et l’asthénosphère est associée à des anomalies sismiques et électriques. L’interprétation de ces anomalies est critique, car cette zone découple les plaques lithosphériques rigides de la partie convective du manteau. S’agit-il d’une limite rhéologique ou d’une limite compositionnelle associée à la présence de faibles quantités de magma ?

Le manteau lithosphérique océanique est formé au niveau des rides médio-océaniques et correspond au manteau résiduel appauvri après extraction des basaltes formant le plancher océanique. Cependant, une étude récente a montré que de faibles quantités de magma peuvent percoler à travers la base de ce manteau lithosphérique. Ce mécanisme dit métasomatique s’observe par une modification chimique des péridotites, mais également par l’addition de minéraux hydratés sous forme de veines.

Le but de ce projet est de préciser l’effet de ce métasomatisme sur la composition du manteau lithosphérique et de tester si cette modification peut expliquer certaines anomalies géophysiques observées sous les océans, remettant en cause la localisation et la nature de la limite lithosphère-asthénosphère. Pour contraindre ces processus, premièrement, une série d’enclaves mantelliques provenant de la plaque Pacifique va être étudiée. Ces enclaves uniques sont observées dans des laves de type petit-spot, laves produites par l’extraction de faibles quantités de magma de la base du manteau lithosphérique en réponse à la flexure de la plaque océanique en front de subduction. Deuxièmement, une étude des veines métasomatiques incluses dans des lamproïtes du Wyoming est planifiée afin d’évaluer le type de veines produites à des profondeurs correspondant à la base du manteau lithosphérique océanique.
Direct link to Lay Summary Last update: 12.04.2018

Responsible applicant and co-applicants

Employees

Project partner

Associated projects

Number Title Start Funding scheme
140494 Metasomatism of the oceanic lithosphere and implications for intra-plate magmatism 01.07.2012 Project funding (Div. I-III)

Abstract

The lithosphere-asthenosphere boundary (LAB) decouples a rigid and cool lithospheric plate from the underlying weak and hot asthenosphere. The location and the physical parameters controlling the change in seismic and electric properties at the LAB have been debated for years, and the presence of either water in nominally anhydrous minerals or low melt fractions at the top of the asthenospheric mantle is hypothesized to explain the decrease in seismic velocities. The properties of the LAB are currently constrained by geophysical studies combined with laboratory experiments, but our recent studies on xenoliths and xenocrysts sampled by petit-spot lavas provide direct information via the rock record on the deep processes affecting the oceanic lithosphere. First, we have shown that extensional stresses in the oceanic lithosphere allow low degree melts from the seismic low velocity zone to percolate, interact, and metasomatize the base of oceanic lithospheric mantle. Secondly, the presence of a garnet xenocryst characterized by a chemical signature supporting formation via subsolidus cooling from a plagioclase-bearing cumulate indicates that melts produced off-axis could refertilize the base of the cooling lithosphere drifting off the mid-ocean-ridge. Our studies provide evidence for melt migration, which seems critical to reconcile the petrological constraints on mantle melting and the presence of low-degree melts at 60-80 km depth below old oceanic lithosphere, as documented by geophysical studies. Crystallization of small melt fractions produces metasomatic cumulates and leads to cryptic metasomatism in peridotite. We will test the hypothesis that some of the geophysical anomalies observed at the LAB could represent metasomatized lithosphere rather than low degree melts at the top of the asthenosphere, as usually assumed. A similar hypothesis has been proposed to explain the mid-lithosphere seismic discontinuity in continental lithosphere, but such a hypothesis is not considered for oceanic settings.This project is a collaboration between Japanese (Dr N. Abe, and Prof. N. Hirano), and Swiss researchers to better understand the nature of the LAB and the role of metasomatism in modifying the physical properties of the lithosphere. Water in the mantle is critical for understanding the rheological and seismic properties of peridotitic mantle. The first part of the project (Project A - Characterizing the Pacific lithospheric mantle - a systematic study of petit-spot xenoliths) is dedicated to the study of mantle xenoliths from Japan in order to evaluate the chemical signature (depleted versus refertilized) and the volatile budget of the Pacific lithosphere.The second part of the project (Project B -Petrogenesis of metasomatic cumulates and their implications for the formation of chemical and physical heterogeneities in the lithosphere) will develop a petrological model for metasomatism in the continental lithosphere. The differentiation mechanism of metasomatic melts at 60 to 80 km depth and the distribution of such melt within the mantle will be constrained using metasomatic xenoliths sampled by lamproites from Leucite Hills in Wyoming (USA). These xenoliths consist of various ultramafic lithologies including harzburgite, pyroxenites and various phlogopite-bearing assemblages. The presence of metasomatic phlogopite-rich rocks has been linked to the mid-lithosphere seismic discontinuity observed at 60 to 80 km depth across the Wyoming craton. Water, major and trace elements will be determined in the different minerals composing the Leucite Hills xenoliths. These data will provide the necessary chemical parameters to constrain the importance of metasomatism for modifying the H2O content and chemical enrichment of the lithosphere, parameters which are critical to predict the effet of metasomatism on the physical properties of the mantle. The results from both projects will shed new light on the geophysical properties of the lithosphere and will test whether the presence of metasomatic domains within the lithosphere may explain the seismic and electrical anomalies observed at 60-80 km depth in both oceanic or continentale settings.
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