Nanoscale solid-fluid interaction and amphibole formation in the lithospheric mantle.

Nanoscale amphibole nucleation and growth in the Earth’s mantle is poorly understood despite its important role in the evolution and stability of the lithosphere, along with its influence on the coexisting fluid composition. Here, we focus on the nanoscale features of a clinopyroxene-amphibole-fluid inclusion system in an amphibole-bearing upper mantle xenolith and propose a new amphibole formation mechanism at mantle depth. Our approach includes crystallographic observations and interpretations of fluid inclusion formation and molecular properties of the entrapped H2O-bearing supercritical CO2-rich mantle fluids. First, the differentiation of H2O and CO2 along the supercritical fluid-clinopyroxene interface leads to nanometre-thick hydrous monolayer formation. The disruption of the supercritical fluid-clinopyroxene interface, including the hydrous monolayer, leads to amphibole formation. Hydrous fluid-mobile elements in the hydrous monolayers support amphibole growth via diffusion along the clinopyroxene-amphibole interface enhanced by various structural misfits. Our results help to clarify the driving forces of element exchange in a supercritical fluid-solid system and present the initial stage of hydrous mineral formation and growth. This process causes fixation of H2O and common lithophile elements with relative CO2 enrichment in the residual supercritical fluid. Nanochannels can contribute to element migration in clinopyroxene-amphibole-rich, non-porous domains in the lithospheric mantle.