HIGH-PRESSURE BREAKDOWN OF ANTIGORITE TO SPINIFEX-TEXTURED OLIVINE AND ORTHO-PYROXENE, SE SPAIN

The prograde, high pressure, transition from antigorite serpentinite t o enstatite-olivine rock occurs along a tectonically undisturbed profi le at Cerro del Almirez, SE Spain. The reactant assemblage is antigori te + olivine with tremolite rimming precursor diopside. The product as semblage of tremolite + chlorite + enstatite + olivine has a spinifex- like texture with arborescent or radiating olivine elongated parallel to [001] and with radially grown enstatite. Product enstatite is very poor in Al2O3. Due to numerous oriented submicroscopic inclusions of c hromian magnetite, product olivine has a brownish pleochroism and a bu lk chromium content similar to precursor antigorite. Titanian clinohum ite with a fluorine content of 0.45-0.50 wt% persisted beyond the brea kdown of antigorite. The partitioning of iron and magnesium amongst th e silicate phases is almost identical to that at lower pressures. Aver age K-d values Mn/Mg and Ni/Mg are 0.17 and 0.70 for antigorite-olivin e pairs and 1.83 and 0.22 for orthopyroxene-olivine pairs, respectivel y. These data are useful in discriminating generations of olivine grow n on each other. From the field data a phase diagram topology for a po rtion of the system CaO-MgO-SiO2-H2O is derived. This topology forms t he basis for extrapolations into inaccessible P-T regions.