Ralph P. Harvey

Petrography, mineral chemistry, and petrogenesis of Antarctic Shergottite LEW88516

Abstract LEW88516 (LEW), like its near-twin ALH77005 (ALH), is a maskelynite-bearing harzburgite and a member of the SNC family of achondrites. LEW is texturally heterogenous, exhibiting both poikilitic (pyroxene oikocrysts enclosing subhedral olivine and euhedral chromite) and non-poikilitic (cumulus framework of olivine and minor pyroxene, with interstitial maskelynite and secondary minerals) textures. Pyroxenes in LEW appear to be slightly more iron-rich and more varied in composition than those in ALH. LEW olivines have distinctly higher Fe contents; most of the olivine grains in LEW have compositions more iron-rich than any ALH olivine observed. LEW olivine in non-poikilitic areas is substantially more iron-rich than olivine armored by pyroxene. Trace element and minor element patterns of LEW and ALH minerals are essentially identical and are consistent with closed-system, large-volume crystallization of the major phases, followed by development of later phases after assembly of the crystal pile. Major element compositions show the effects of moderate near-solidus re-equilibration in both meteorites. The compositions of possible LEW parent magmas, calculated from mass-balance methods using the components of magmatic inclusions in large olivine grains, are very similar to those proposed earlier for ALH and the EETA79001 shergottite. While the subtle differences between LEW and ALH suggest they are not part of the same fall, it is likely that they are derived from the same igneous processes.

Outgassed Water on Mars: Constraints from Melt Inclusions in SNC Meteorites

The SNC (shergottite-nakhlite-chassignite) meteorites, thought to be igneous rocks from Mars, contain melt inclusions trapped at depth in early-formed crystals. Determination of the pre-eruptive water contents of SNC parental magmas from calculations of the solidification histories of these amphibole-bearing inclusions indicates that martian magmas commonly contained 1.4 percent water by weight. When combined with an estimate of the volume of igneous materials on Mars, this information suggests that the total amount of water outgassed since 3.9 billion years ago corresponds to global depths on the order of 200 meters. This value is significantly higher than previous geochemical estimates but lower than estimates based on erosion by floods. These results imply a wetter Mars interior than has been previously thought and support suggestions of significant outgassing before formation of a stable crust or heterogeneous accretion of a veneer of cometary matter.

The parent magma of the nakhlite meteorites - Clues from melt inclusions

Abstract Several forms of trapped liquid found within nakhlite meteorites have been examined, including interstitial melt and magmatic inclusions within the cores of large olivine grains. Differences in the mineralogy and texture between two types of trapped melt inclusions, and between these inclusions and the mesostasis, indicate that vitrophyric inclusions are most appropriate for estimating the composition of a nakhlite parental magma in equilibrium with early-forming olivine and augite. Parent liquids were calculated from the mineralogy of large inclusions in Nakhla and Governador Valadares using a system of mass-balance equations solved by linear regression methods. The chosen parental liquids were cosaturated in olivine and augite and had Mg/Fe values consistent with measured augite/liquid Kds. These parental magma compositions are similar to other published compositions for Nakhla, Chassigny and Shergotty parental melts, and may correspond to a significant magma type on Mars.

Petrogenesis of the nakhlite meteorites - Evidence from cumulate mineral zoning

Abstract A simple igneous petrogenesis for the meteorite Nakhla has previously been called into question because Mg Fe ratios in olivine indicate substantial disequilibrium between the predominant cumulus minerals (olivine and augite). Comparative analyses of simulated diffusive zoning and the observed cumulus mineral zoning for all three nakhlites (Nakhla, Governador Valadares, and Lafayette) show that their current compositions do not necessarily reflect parental magma compositions. Instead, diffusion has altered primary cumulus compositions to varying degrees, Nakhla being the least affected, and Lafayette being almost completely re-equilibrated. Only the cores of augite grains in Nakhla and Governador Valadares appear to have preserved their original compositions. Mineral zoning in each meteorite is strongly concentric around mesostasis areas, suggesting that reaction with intercumulus liquid has controlled the observed zoning. The presence of pigeonite and orthopyroxene overgrowths in Nakhla and Governador Valadares, and of poikilitic orthopyroxene enclosing olivine relicts in Lafayette, indicate substantial late-magmatic reactions. Two-pyroxene geothermometry for Lafayette indicates temperatures around 950°C, suggesting subsolidus equilibration as well. The nakhlites appear to be a series of relatively simple cumulate rocks which have undergone various amounts of late-magmatic and subsolidus diffusion, possibly reflecting their relative positions in a cooling cumulate pile.