Genesis of “silicate exsolution lamellae” in chromite of the Stillwater Complex: A challenge to the high-pressure crystallization of ophiolitic chromitite

Abstract

Models for deep derivation of ophiolitic chromite from the mantle transition zone versus magmatic crystallization at lithospheric level are now highly controversial due to the discovery of ultrahigh-pressure (UHP) and super-reduced (SuR) minerals in ophiolitic chromitites and the presence of silicate mineral exsolution lamellae in chromite grains. According to experimental studies, chromite grains crystallized at low-pressures cannot exsolve silicate phases because of their low contents of CaO and SiO2. However, clinopyroxene needles in chromite grains from the Stillwater Complex, which is a typical layered mafic-ultramafic intrusion crystallized at crustal level, show morphological, compositional and crystallographic features very similar to exsolution lamellae in ophiolitic chromite grains. We propose that the Stillwater clinopyroxene ‘lamellae’ represent minute melt inclusions that were attached parallel to branches of skeletal chromite where they were trapped during continued growth of the grain. This interpretation is supported by the orientations of the lamella-like clinopyroxene and the coexistence of larger clinopyroxene inclusions and apatite crystals along with the clinopyroxene needles. Another possibility is that these lamella-like minerals precipitated from melt/fluid that filled cracks in the chromite grains. Trails of tiny inclusions preserved in some healed fractures are consistent with this mechanism, but are inconsistent with the regular orientations of most of the needle-like ‘inclusions’. The presence of such features in the Stillwater chromitites raises serious questions regarding the origin and significance of silicate lamellae in ophiolitic chromite grains and the use of such features to determine the depth of crystallization.