Pyroxene exsolution topotaxy in majoritic garnet from 250 to 300 km depth

Abstract

We report here the detailed microstructure and chemistry of pyroxene exsolution from a polycrystalline garnet porphyroblast of the Western Gneiss Region (WGR) garnet peridotite, Otrøy, Norway. For both clinopyroxene (Cpx) and orthopyroxene (Opx), the same basic crystallographic relationship is found with the host garnet: (100)py//{112}grt, (010)px//{110}grt and (001)px//{111}grt for the majority (>90%) of its intracrystalline pyroxene rods. In addition, this pattern is also exhibited by some interstitial Opx and a subpopulation of both pyroxenes shows a different pattern or no discernible pattern. The results provide quantitative microstructural evidence demonstrating an exsolution (precipitation) origin of both the intracrystalline Cpx and Opx and the small interstitial Opx crystals. The reconstructed precursor majoritic garnet, taking into account both the intracrystalline pyroxenes and interstitial Opx, was characterized by Si = ∼3.07 cation per formula unit that corresponds to a minimum pressure of 7.7 GPa (∼250 km depth). We also deduce from the observation of Opx being the majority of intracrystalline precipitates and 100% of the interstitial ones that the precursor majoritic garnet probably originated from a pressure less than ∼10 GPa (∼300 km depth). A multistage decomposition hypothesis is proposed for this WGR majoritic garnet during exhumation of the peridotite from 250 to 300 km depth to explain the topotaxy and chemistry of the exsolved pyroxenes.