ZAGAMI - PRODUCT OF A 2-STAGE MAGMATIC HISTORY

Large specimens of the Zagami shergottite show highly varied grain sizes and mineral abundances on a cm scale and preferred alignment of pyroxene laths. We document the presence of whitlockite and of melt veins and pockets of shock origin. Pyroxene crystals have homogeneous Mg-rich pigeonite and augite cores, overgrown by Fe-rich, zoned pyroxene (mostly pigeonite) rims. Amphibole-bearing magmatic (melt) inclusions occur exclusively in the cores. We conclude that Zagami experienced a two-stage crystallization history. The first stage occurred in a deep-seated, slowly cooling magma chamber. There, the homogeneous Mg-rich cores of the pyroxenes crystallized during relatively slow cooling. The deep-seated origin of the cores is also indicated by the presence of amphibole within them, which requires pressures of formation equivalent to crystallization at depths > 7.5 km on Mars. Modest abundance of homogeneous Mg-rich cores (15-20%) indicates that crystal settling did not play a significant role in this part of the magma chamber. During the second stage, the Mg-rich pyroxenes were entrained into a magma that either intruded to the near-surface and cooled in a relatively thin dike or sill, or extruded to the surface and crystallized in a lava flow > 10 m thick, again without indications for crystal settling. This scenario is suggested by estimates of cooling rates of 0.1-0.5-degrees-C/h, based on sizes of the plagioclase (maskelynite) crystals, and of approximately 0.02-degrees-C/h, based on the width of pyroxene exsolution lamellae (BREARLEY, 1991 ). Irregular shapes of the Mg-rich cores result from resorption and possible solid-state diffusion, and the shapes and sizes of the pyroxenes after crystallization of Fe-rich pigeonite rims onto the cores were strongly controlled by the shapes and sizes of the cores. The finer-grained areas of the rock inherited smaller and more numerous Mg-rich pyroxene cores from the first stage than did the coarser-grained areas. Crystallization of both augite and pigeonite cores at depth, but mostly pigeonite rims in the near-surface environment, may be the result of a phase-boundary shift and expansion of the pigeonite stability field at lower pressures. The estimated depth of the magma chamber for Zagami of > 7.5 km and thickness of the putative lava flow of > 10 m are consistent with calculations and observations of volcanic constructs and flows in the Tharsis region of Mars.