Metasomatism in the sub-continental lithospheric mantle beneath the south French Massif Central: Constraints from trace elements, Li and H in peridotite minerals

Abstract

Mantle metasomatism by percolating melts/fluids can significantly modify the geochemical and mineralogical compositions of the sub-continental lithospheric mantle (SCLM). We present a detailed study of these modifications in Li concentrations, δ 7 Li and water contents in mantle minerals from a suite of peridotite xenoliths entrained by a Cenozoic strombolian volcano in the south of the French Massif Central (FMC). The trace element compositions in clinopyroxenes and amphiboles were first investigated to track metasomatism. The wide spectrum of trace element distribution in clinopyroxenes (e.g., (La/Yb) N from 0.25 to 22.21; Ti/Eu ratios from 453 to 4892) suggests that the SCLM have undergone metasomatism by carbonatitic melts/fluids or melts/fluids related to subducted materials. But two amphibole-bearing samples exhibit equilibrated trace element partitioning between clinopyroxenes and coexisting amphiboles, with depletion of LREE ((La/Yb) N =0.26 and 0.30), indicating that amphiboles have grown in another modally metasomatic process predating to the cryptic metasomatism accounting for LREE enrichment and HFSE negative anomalies in other samples. Li concentrations in minerals are similar to those in the normal mantle, with inter-mineral Li partitioning nearly equilibrated and intragranular Li distribution nearly homogeneous. However, the negative δ 7 Li values of clinopyroxenes and orthopyroxenes in some samples (as low as-8.8 ‰ in clinopyroxene of sample MC38) can be attributed to exchange with a small-volume melt of moderate Li concentration and light Li isotopic compositions, originally associated with a recycled component. The preservation of currently observed inter-mineral large Li isotopic difference indicates that the percolation of the melt should have occurred shortly before the entrainment of peridotite xenoliths by the host magma. Water contents in minerals vary from 41 to 428 ppm in clinopyroxenes, from 28 to 152 ppm in orthopyroxenes, and their roughly negative co-variation with Fo contents in coexisting olivines implies that partial melting has mainly controlled their variations among most samples. The different behaviors of OH in different metasomatized samples with LREE enrichment point to the involvement of metasomatic agents of different origins. The aqueous agent responsible for generation of amphiboles in two samples has not lead to a notable increase of water content.

Mantle metasomatic influence on water contents in continental lithosphere: New constraints from garnet pyroxenite xenoliths (France & Cameroon volcanic provinces)

“…10), consistent with recent evidence of carbonatitic metasomatism in the FMC (e.g. Gu et al, 2018). The negative correlations between clinopyroxene and whole-rock La N /Sm N ratios and water concentrations (Fig.…”

“…the northern domain was metasomatized by carbonatitic melts (Lenoir et al, 2000;Wittig et al, 2007), whereas the southern domain was metasomatized by both hydrous silicate melts J o u r n a l P r e -p r o o f and carbonatitic melts (Wittig et al, 2007;Gu et al, 2018). The FMC samples studied herein show a robust correlation between La N /Sm N and Ti/Eu (R 2 = 0.88, Fig.…”

“…Based on REE data, Wittig et al (2007) interpreted that the northern and southern domains were metasomatized by carbonatitic fluids and silicate melts, respectively. However, Gu et al (2016Gu et al ( , 2018 recently showed that the southern domain was also metasomatized by either fluid with carbonatitic affinities or melt/fluid interactions related to subducted materials.…”

Petrogenesis and Geological Implications of the Oligocene Mingze monzodiorites, Southern Lhasa

“…We suggest, therefore, that the parent magma of Mingze monzodiorites was metasomatized by an older source material within a subduction setting. Metasomatism in lithospheric mantle could possibly have been triggered by both subduction and thickened crustal delamination [50,51]. However, the coeval Chengba-Chongmuda (~30 Ma) and Linzi (~28 Ma) adakitic intrusions have been interpreted as a partial melting of the lower crust that contained a certain amount of juvenile mantle-derived basaltic melts [7,52].…”

Can destabilization rims of hydrous minerals be used to constrain magma ascent kinetics at lava dome volcanoes?

“…The CdP is a 30-km-long north-south alignment of around a hundred monogenic cones, domes, and maars emplaced between ~100 and 6.7 ± 0.1 ka ( Figure 1; Juvigne 1992;Boivin et al 2009). French Massif Central magmatism is related to asthenospheric upwelling (Hoernle et al 1995;Granet et al 1995) and melting of a strongly metasomatized HIMU-type mantle (Lustrino & Wilson 2007;Hamelin et al 2009;France et al 2015;Gu et al 2016Gu et al , 2018; the HIMU isotopic signature likely documenting the influence of a recycled component in the mantle source, Zindler & Hart, 1986). Volatile-and silica-rich magmas, like the trachyte lavas forming the Sarcoui edifice studied herein (~12.5 ± 0.5 ka;…”