Soutenance de thèse Léna VERHOEST

Melting a heterogeneous Earth mantle under an extreme thermal gradient

vendredi 4 novembre, 14h00, au Dipartimento di Scienze Chimiche e Geologiche de l'Université de Modène (Italie).

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Abstract

The goal of this PhD thesis is to explore the lithological heterogeneity of the suboceanic mantle sourcing the Mid Ocean Ridge Basalts (MORB). Even though the heterogeneity of the mantle has been demonstrated to vary at short, kilometric scale, the composition of the basalts erupted at a single ridge axis segment usually appears relatively homogeneous. This apparent paradox is due to the relatively high degree of mantle partial melting that averages the different lithological contribution to the pooled melt, smoothing out the original variability. A way to separate the single lithological contributions is to explore the compositional variation of melts extracted from a composed mantle source that melts at different potential temperatures. Decreasing temperatures results in the preferential melting of low-solidus lithologies because of heat drained from the high-solidus components during melting. Sampling MORBs along lateral thermal gradients is therefore a promising way to collect separately the melt contributions from different lithologies.

Thermal gradients large enough to affect the melting region at the ridge axis scale can be generated by cooling edge effects where the ridge axis faces the transform fault. The cold-edge effect is usually restricted to a few km-wide region. Here I study the eastern intersection of the Romanche Transform Fault (RTF) with the Mid-Atlantic Ridge (MAR) axis in the Equatorial Atlantic Ocean. The RTF is the largest transform on Earth (950 km). It puts an old, cold and thick lithosphere (40 Ma) directly in contact with the tip of the axial segments of the MAR. This geometry results in a large lateral thermal gradient, reaching >50 km away from the transform and gives rise to peculiar tectonics and magmatism.

I studied a set of basaltic glasses, sampled along the MAR axis approaching the RTF during the PRIMAR expeditions and the SMARTIES 2019 oceanographic expedition, to which I took part. More than 180 samples have been studied for major and trace element compositions (EMPA and LA-ICP-MS) and Pb, Sr, Nd and Hf isotope ratios separated by chromatographic methods and measured by TIMS and HR-MC-ICP-MS.These MORBs show an unusual high content of K2O and Na2O, respectively up to 1.95 wt% and 4.45 wt%, plotting across the subalkaline/alkaline boundary. Overall, they vary from transitional- to highly enriched MORBs. Their compositions strictly depend on the local mantle temperature. In the colder section, both major and trace element compositions are very heterogeneous, while samples located from hotter sections have a homogeneous normal-MORB compositional patterns.

The isotopic variability in the region (87Sr/86Sr from 0.702398 to 0.703919, 206Pb/204Pb from 18.49 to 19.7, ɛNd from 2.18 to 11.52, ɛHf from -2.88 to 23.73) covers almost the whole variability observed along the MAR, including axis portions showing ridge-hotspot interactions. Explaining the observed variability requires mixing various amounts of at least three different components in addition to the depleted MORB mantle. The first, an ultra-depleted melt signature, witnesses for the involvement of a mantle component recording a very ancient depletion event (Ga). The second, a HIMU-type signature linked to the recycling of a very old oceanic crust, and the third, derived from continental material, are evidence for deep ancient mantle processes.

A group of samples shows peculiar compositions, with marked Eu and Sr positive anomalies and homogeneous DMM-type isotopic signatures. These MORBs are proposed to result from partial melting of shallow mantle portions, previously impregnated in the plagioclase stability field, during the relocation of the spreading axis in oscillatory Core Complex tectonics.

Jury

Daniele BRUNELLI, PhD advisor, Professore associato, Università degli studi di Modena e Reggio Emilia

Christophe HEMOND, PhD advisor, Enseignant-Chercheur, Université de Bretagne Occidentale

Delphine BOSCH, Examiner, Directrice de recherche CNRS, Université Montpellier II

Janne KOORNNEEF, Reviewer, Associate professor, Vrije Universiteit Amsterdam

Andrea MARZOLI, Reviewer, Professore ordinario, Università degli studi di Padova

Catherine CHAUVEL, Invited, Directrice de Recherche CNRS, Institut de physique du globe de Paris

Andreas STRACKE, Invited, Professor, University of Münster