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 fluid inclusion formation, crystallographic observations and molecular properties of the entrapped H 2 O-bearing supercritical CO 2 -rich mantle fluids. First, the differentiation of H 2 O and CO 2 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 H 2 O and common lithophile elements with relative CO 2 enrichment in the residual supercritical fluid. Nanochannels can contribute to element migration in clinopyroxene-amphibole-rich, non-porous domains in the lithospheric mantle.
Lange et al. (Tue,) studied this question.