Silicate and oxide exsolution in pseudo-spinifex olivine from metaultramafic rocks of the Betic Ophiolitic Association: A TEM study

Transmission electron microscopy (TEM) has been used to study submicroscopi c particles in spinifex-like textured olivine from secondary harzbugites co llected from the Cerro del Almirez locality in the Mulhacen Complex (Betic Cordillera, SE Spain). Three main types of submicroscopic oxides have been identified: (1) equidimensional Fe-rich spinel (magnetite), with average,gr ain size in the order of 1-2 mu m; (2) elongated Cr-bearing spinels (Fe-chr omite to Cr-magnetite) with sizes ranging from 0.2 to 1 mu m long and 0.01 to 0.1 mu m thick; and (3) equidimensional Ti-rich particles from <0.01 to 0.1 mu m and compositions ranging from ilmenite to Fe-Cr-Ti oxide. Chromite and ilmenite particles form parallel, chromite-rich and ilmenite-rich band s, extending along the a-axis of the host olivine. Both phases show a fixed orientation relationship with olivine, with the approximately hexagonal cl ose-packed oxygen planes being parallel in both structures. These textural relationships indicate that both chromite and ilmenite were formed during a common exsolution process. Magnetite particles also are orientated prefere ntially relative to the olivine, but these particles are homogeneously dist ributed within the olivine, suggesting either a primary origin or an exsolu tion process that was not contemporaneous with formation of chromite and il menite. Chromite particles commonly are accompanied by lamellae of talc and/or enst atite, both showing a consistent orientation relationship with olivine. Tal c lamellae are twice as thick as the associated chromite crystals, whereas enstatite lamellae show a greater thickness and, moreover, form single enst atite particles, which consist of clino- and orthoenstatite intergrowths. T alc formation may be explained by exsolution, together with spinel, from ol ivine containing OH-groups, probably related to incomplete dehydration of s erpentine during olivine formation. On the basis of these results and previ ously reported petrological data, we have concluded that exsolution of chro mite-silicate and ilmenite occurred during the retrograde stage that follow ed the climax of the eo-Alpine metamorphic event.