Origin of amphibole in recrystallized and porphyroclastic mantle xenoliths from the Rhenish Massif: implications for the nature of mantle metasomatism

Details Origin of amphibole in recrystallized and porphyroclastic mantle xenoliths from the Rhenish Massif: implications for the nature of mantle metasomatism Origin of amphibole in recrystallized and porphyroclastic mantle xenoliths from the Rhenish Massif: implications for the nature of mantle metasomatism

Jan 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..91..327w&link_type=abstract

Earth and Planetary Science Letters, Volume 91, Issue 3-4, p. 327-340.

Computer Science

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Scientific paper

Pargasitic amphiboles of similar chemical composition are present in two suites of mantle peridotite from the Rhenish Massif with recrystallized and porphyroclastic textures. In both suites the formation of amphibole is associated with an enrichment of LREE over HREE. In the porphyroclastic xenoliths the amphiboles are present as neoblasts, as lamellae in orthopyroxene, and as tiny crystals intergrown with orthopyroxene and spinel in kink bands of large orthopyroxene relics. In conjunction with the temperature history it is inferred that the formation of amphibole occurred during shear deformation in a rising diapir.
The typically low TiO2 contents of the amphiboles (< 0.5%) in both suites is indicative of formation by reaction of peridotite with a low density fluid. It is proposed that the fluid was present as a stationary grain boundary phase in a rising diapir, and that this prevented strong chemical modification of the fluid. For the recrystallized suite it is shown that the quantities of K, La, and H2O present in 0.2 wt.% of a grain boundary fluid would be sufficient to account for the formation of amphibole, and the enrichment of La in this suite compared with unmetasomatized ``dry'' peridotites of similar major element bulk composition.
Amphiboles in mantle peridotite adjacent to veins filled with hornblende and phlogopite are distinctly higher in TiO2 (> 1.5%) than those from the porphyroclastic and recrystallized suites. These amphiboles reflect a different type of mantle metasomatism due to the interaction of mantle peridotite with basic melts flowing through veins in the lower lithosphere.

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