Influence of H₂O, CO₂, and eclogite on reactions at the subcontinental lithosphere–asthenosphere interface

Tolotti, CDK; Conceicao, RV; Klemme, S; Romeiro, MAT; Queiroga, GN; Secchi, DK; Souza, MR; Drago, SM

Zusammenfassung

This experimental study investigates interactions between the subcontinental lithosphere and an eclogite-rich asthenosphere source under water- and carbon-bearing conditions, simulating processes across variable geothermal gradients and depths. High-pressure experiments conducted at 2.5 and 4.5 GPa and 900–1300 ◦C yielded mineral assemblages comprising olivine, orthopyroxene, clinopyroxene, garnet, amphibole, and magnesite, with phlogopite occurring as a late-stage phase associated with the glass. These glasses range from ultrabasic to alkaline and silica-rich compositions. Reaction textures indicate significant alkali mobility and C–O–H fluid activity, characterized by the formation of orthopyroxene- and amphibole-enriched domains around 1100 ◦C. Notably, amphibole exhibits unexpected stability up to ~1100 ◦C at both 2.5 and 4.5 GPa. In contrast, carbonate stability is more restricted, with magnesite observed only at 1000 ◦C and 4.5 GPa. Above 1100 ◦C, amphibole breakdown enriches the melt in alkalis and H₂O. The residual assemblage at 1200–1300 ◦C consists of olivine and metasomatic clinopyroxene. The results indicate enhanced amphibole stability in enriched environments and demonstrate that H₂O inhibits CO₂ dissolution in the melt, promoting the generation of silica- enriched melts when both volatiles coexist.