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Metasomatism of the oceanic lithosphere and implications for intra-plate magmatism

English title Metasomatism of the oceanic lithosphere and implications for intra-plate magmatism
Applicant Pilet Sébastien
Number 140494
Funding scheme Project funding (Div. I-III)
Research institution Institut de Minéralogie et Géochimie Université de Lausanne
Institution of higher education University of Lausanne - LA
Main discipline Geology
Start/End 01.07.2012 - 31.10.2016
Approved amount 287'158.00
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All Disciplines (3)

Discipline
Geology
Mineralogy
Geochemistry

Keywords (5)

Metasomatism; basalt; Volcanism; Intraplate; Lithosphere

Lay Summary (English)

Lead
Lay summary

Deep mantle plumes are considered as the main “engine” for the formation of intra-plate volcanoes. The discovery of Petit-spot volcanoes off Japan which occur in response to plate flexure near subduction zones have demonstrated that intra-plate magma could also be produced by shallow tectonic processes. The goal of this project is to constrain the mechanism of alkaline magma generation in Petit-spot setting and test if this mechanism is similar to the one recorded in oceanic islands. For this study, we have selected a series of alkaline sills from Santa Rosa (Santa Elena peninsula, Costa Rica); sills interpreted as an accreted Petit-spot volcano in the north of Costa Rica.

The chemical and isotopic composition of Santa Rosa sills will be used to test the different hypotheses suggest for the formation of alkaline basalts. Alkaline basalts are “classically” interpreted as low degree melts from an enriched peridotitic source in presence of CO2, but an alternative suggests that alkaline magmas could be produced by the melting of veined/metasomatized lithosphere.

A second goal of this project is to characterize the nature of the oceanic lithosphere. Oceanic lithosphere is generally considerate as the residue of melt extraction at mid-ocean ridges. But, some authors suggest that this lithospheric mantle could be re-enriched by the percolation of melt originating from the low velocity zone. Xenoliths from oceanic islands could not use to test this hypothesis because their compositions are modified by plume activity. The study of xenoliths and xenocrystals carrying by Petit-spot lava from Santa Rosa will provide new information on the nature of the oceanic lithosphere and allow to evaluate the chemical composition of the oceanic lithospheric mantle before its subduction and recycling into the convecting mantle.

Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Name Institute

Publications

Publication
Garnet xenocryst from petit-spot lavas as an indicator for off-axis mantle refertilization at intermediate spreading ridges
Rochat L., Pilet S., Müntener O., Duretz T., Baumgartner L., Abe N., Hirano N. (2017), Garnet xenocryst from petit-spot lavas as an indicator for off-axis mantle refertilization at intermediate spreading ridges, in Geology, 45(12), 1091-1094.
Pre-subduction metasomatic enrichment of the oceanic lithosphere induced by plate flexure
Pilet Sebastien Abe Natsue Rochat Laetitia et al (2016), Pre-subduction metasomatic enrichment of the oceanic lithosphere induced by plate flexure, in Nature Geoscience, 9(12), 898-903.
Thermal erosion of cratonic lithosphere as a trigger for mass-extinction.
Guex Jean Pilet Sebastien Muntener Othmar et al (2016), Thermal erosion of cratonic lithosphere as a trigger for mass-extinction., in Scientific Report, 6(23168), 1-9.
Generation of low-silica alkaline lavas: Petrological constraints, models, and thermal implications
Pilet Sebastien (2015), Generation of low-silica alkaline lavas: Petrological constraints, models, and thermal implications, in Foulger Gillian Lustrino Michele King Scott (ed.), Geological Society of America and American Geophysical Union, Boulder, Colorado, USA, 281-304.
Low-volume intraplate volcanism in the Early/Middle Jurassic Pacific basin documented by accreted sequences in Costa Rica
Buchs David Pilet Sebastien Baumgartner Peter O. Cosca Michael Flores Kennet Bandini Alexandre (2013), Low-volume intraplate volcanism in the Early/Middle Jurassic Pacific basin documented by accreted sequences in Costa Rica, in Geochemistry, Geophysics, Geosystems., 14(5), 1552-1568.

Collaboration

Group / person Country
Types of collaboration
Dr D. Buchs / IFM-GEOMAR, Kiel Germany (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
Prof. O. Muntener /University of Lausanne Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
Prof. P.O. Baumgartner / University of Lausanne Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication

Communication with the public

Communication Title Media Place Year
Media relations: print media, online media Les bouleversement de la vie sur Terre 24 heures Western Switzerland 2016
Media relations: radio, television Petit-spot volcanoes RTS la 1er Western Switzerland 2016

Associated projects

Number Title Start Funding scheme
179046 Metasomatism and physical properties of the oceanic lithosphere 01.07.2018 Project funding (Div. I-III)

Abstract

Deep mantle plumes are considered as the main “engine” for the formation of intra-plate volcanoes. Nevertheless the surface expression of intraplate magmatism varies from large islands chain like Hawaii- Emperor to countless small seamounts. Regarding the global distribution of these seamounts, it is unlikely that all these seamounts are plume related. An alternative to plumes suggests that the generation of some oceanic islands and seamounts could be related to shallow (i.e. lithospheric) processes. An example of such processes is the formation of Petit-spot volcanoes recently discovered off Japan and Tonga islands, which occur in response to plate flexure near subduction zones. The goal of this project which requires the founding for two PhD students is to compare the petrogenesis of alkaline rocks produce in two distinct geodynamic contexts, i.e. in plume versus non-plume processes, to document the differences of rock composition between shallow versus deep origin. We propose to perform detailed petrological and geochemical studies of: (1) alkaline lavas from Fogo and Brava islands (Cape Verde Archipelago), islands which are interpreted as the surface expression of a deep mantle plume; and (2) alkaline sills emplaced in radiolarites from Santa Rosa (Santa Elena peninsula, Costa Rica), sills which are interpreted as an accreted Petit-spot volcano in the north of Costa Rica. These two complementary studies will allow to test the different hypotheses for the formation of alkaline rocks and to clarify the potential role of the oceanic lithosphere in the petrogenesis of these rocks. Alkaline lavas are “classically” interpreted as low degree melts produced by partial melting of an enriched peridotitic source in presence of CO2 at high pressures. An alternative hypothesis suggests that alkaline magmas are produced by melting of veined/metasomatized lithosphere in situ or after recycling into the convecting mantle by subduction or delamination. Since the metasomatic hypothesis implies that alkaline magmas are typically produced by high degrees of melting of the metasomatic veins, the process responsible for the metasomatic enrichment of the oceanic lithosphere is an important component of the model. The metasomatic veins are interpreted as cumulate formed during the percolation and cooling of low degree melts within the lithospheric mantle. These low degree melts are interpreted as derived from the low velocity zone (LVZ) present at the base of the oceanic lithosphere. Nevertheless, a major issue about this hypothesis was the lack of direct evidence for global metasomatism in the oceanic lithosphere. Mantle xenoliths carried by oceanic islands basalts could provide information about the nature and composition of the oceanic lithosphere, but the metasomatism recorded by these xenoliths is interpreted as the interaction of alkaline magma produced by deep plumes and lithospheric mantle rather then the interaction of low degree melts from the LVZ with the lithosphere. An alternative way to obtain information about the nature of the oceanic lithosphere not “altered” by plume activity was to study xenoliths or xenocrysts from Petit-spot volcanoes; this is the second goal of this project. Preliminary results from Santa Rosa alkaline sills indicate the presence of cpx-xenocrysts. The compositions of these xenocrysts suggest that this cpx could correspond to a relic of metasomatic veins present in the oceanic lithosphere. If this hypthesis is correct, it would imply (the first direct evidence?) that the oceanic lithosphere was metasomatized before subduction and recycling into the convecting mantle. Moreover, similar cpx-xenocrysts associated with small mantle xenoliths are observed in Fogo alkaline basalts. So we plan to study in detail theses xenocrysts and xenoliths from both locations using EMPA, LA-ICP-MS, LA-MC-ICP-MS and SIMS to characterize their formation and the underlying petrological and geochemical mechanism. We expect to be able to characterize the metasomatism of the oceanic lithosphere in different tectonic settings.
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