Intergrowth of graphite within phlogopite from Finero ultramafic complex (Italian Western Alps): implications for mantle crystallization of primary-texture mica

摘要

High Resolution Transmission and Analytical Electron Microscopy (HRTEM-AEM) of phlogopite crystals from the Finero lherzolite (Italian Western Alps) revealed nanometric intergrowths of graphite-like layers. The intergrowths consist of 4 to 6 graphite-like layers (c approximate to 3.3 Angstrom) with the basal plane parallel to (001) of the phlogopite host. The phlogopite crystals contain between 0.024 and 0.048 wt % of carbon. The carbon isotope composition varies from delta(13)C(graphite) = -16.1parts per thousand to -10.4parts per thousand (VPDB). These values are compatible with carbon phases found in other mantle-derived ultramafic rocks. A Raman analysis confirms the presence of a graphite-like phase intergrown within phlogopite. Geochemical and microtextural data point to a primary origin of the phlogopite-graphite intergrowths and suggest simultaneous crystallization of both phases from a trapped partial melt.