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Vein-plus-Enclosing Lithospheric Mantle as Source of Alkaline Basalt: an Experimental approach

English title Vein-plus-Enclosing Lithospheric Mantle as Source of Alkaline Basalt: an Experimental approach
Applicant Pilet Sébastien
Number 109018
Funding scheme Fellowships for advanced researchers
Research institution
Division of Geology and Planetary Sciences California Institute of Technology
Institut de Minéralogie et Géochimie Université de Lausanne
Institution of higher education Institution abroad - IACH
Main discipline Geochemistry
Start/End 01.09.2005 - 31.08.2007
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All Disciplines (2)

Discipline
Geochemistry
Mineralogy

Keywords (6)

Experimental petrology; Oceanic Island Basalt; metasomatism; mantle heterogeneity; hot spot; basalt genesis

Lay Summary (English)

Lead
Lay summary
Variation of trace element and isotopic ratios in oceanic island basalt(OIB) is commonly ascribed to the recycling of ancient oceanic crustassociated with crustal or pelagic sediment assimilation. However thismodel based on geochemical arguments is in opposition with experimentalpetrology data. Partial melts of oceanic-crust lithologies produce silicasaturated liquids whereas many oceanic island rocks are characterized bysilica undersaturated compositions. Experimental data indicate that onlypartial melting of peridotite in presence of carbonate or silica deficientpyroxenite produce liquids close to the compositions of lavas observed inoceanic islands. Hence, recycling of oceanic lithosphere enriched bymetasomatic veins could represent a convincing alternative for the sourceof OIBs. However, this hypothesis does not explain the formation ofisotopic heterogeneity observed in OIBs.

Chemical variations observed in Cantal basalt (France), interpreted as theproduct of a lithospheric metasomatic mechanism, allow us to constrain thechemical evolution of a metasomatic agent within the basal lithosphere.This data demonstrates that (1) fractionation of trace element ratiosnecessary to generate, after subduction and isolation, the distinct mantlecomponents, can be explained by metasomatic process and (2) partialmelting of veins-plus-enclosing lithospheric mantle produce basalts withmajor and trace element compositions similar to those observe in OIBs.This suggests that isotopic and trace element variations observed inbasalts from individual oceanic islands is more likely the result ofmelting a heterogeneous, metasomatised, subducted oceanic lithosphererather than a mixture of chemically distinct mantle reservoirs.

The aim of this proposal is to experimentally test thevein-plus-lithospheric source model developed for Cantal basalt genesis.Piston-cylinder experiments will be performed at the Geological andPlanetary Sciences Division of the California Institute of Technology. Inorder to simulate the chemical conditions of partial melted veins, we willuse minerals separated from mantle veins as starting materials. Theseminerals, sampled from different localities in the French Pyrenees, havecompositions similar to the xenocrysts observed in Cantal basalt. Two setsof experiments are planned: (1) We will start with the determination ofthe solidus temperature of these specific veins. (2) Subsequently, meltingof layered material, composed of veins and depleted peridotite, will beperformed first of all in the presence of water and secondly with theaddition of carbonate.

The present hypothesis for the origin of alkaline basalt seems toreconcile the geochemical constrains with the experimental petrology data.Recycled metasomatised lherzolite may be the major mantle componentsampled by OIBs; recycled oceanic crust and sediment may be less common inOIB sources than is commonly assumed. This reinterpretation of the originof alkaline rocks could correspond to a fundamental advance in the fieldof petrology and the understanding of mantle evolution
Direct link to Lay Summary Last update: 21.02.2013

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