Nanoscale hydrous silicate melt inclusions at the clinopyroxene-amphibole interface in a mantle xenolith from the Perșani Mountains Volcanic Field

Abstract

We studied amphibole lamellae and associated hydrous nano-silicate melt inclusions (NSMI) in upper mantle xenoliths from the Perșani Mountains Volcanic Field, southeastern Transylvania (Romania). Transmission Electron Microscope (TEM) results revealed submicron-scale pieces of evidence of escaped fluids in the form of nano-silicate melt inclusions along clinopyroxene-amphibole interfaces. These NSMIs consist of ∼80 vol% silicate glass and ∼ 20 vol% volatile bubble from which the former has a high SiO2 (>60 wt%) and Al2O3 (>20 wt%) and low CaO, FeO and MgO (<8 wt%) content determined by SDD-EDS analyses. We calculated the original bulk composition of the nano-silicate melt inclusions with Monte Carlo simulation, using hydrated fluid molecules and suggest that originally the nano-silicate melt inclusions had most probably low SiO2 (∼ 43.6 wt%) and high Al2O3 (∼ 15.5 wt%), Na2O (∼11.9 wt%) and H2O (∼ 30.3 wt%) contents. Using petrographic evidence, we propose that the amphibole lamellae formed as a result of post-entrapment reaction between clinopyroxene and the trapped fluid of the fluid inclusion. The presence of the hydrous nano-silicate melt inclusions suggest fluid migration along the clinopyroxene-amphibole interface, supporting the continuous growth of the studied amphibole. The association of the studied amphibole with hydrous nano-silicate melt inclusions suggests that H2O consumption from the fluid found in the micrometer-scale fluid inclusion continued at the nanoscale after a metasomatic event in the lithospheric mantle. This nanoscale process may also give information about the behavior of H2O globally in the lithospheric mantle where amphibole is stable, and along the lithosphere-asthenosphere boundary in younger oceanic and continental plates.