ALBERT

Description: The present examination of the historical record of HED and mesosiderite basaltic meteorite falls attempts to determine whether that record is consistent with completely random fall times. Strong evidence for nonrandomness emerges, in view of a significant cluster of falls for the 1924-1939 period. The causes of the cluster appear to have acted only on basaltic meteorites, and in a way that was of only 15-year-duration.

Description: Four of the SNC meteorites of putative Martian origin are falls. Two of these fell on October 3: Chassigny in 1815 and Zagami in 1962. The probability of this coincidence arising from random fall days is approximately 1 in 60. If this coincidence is not the result of chance, it suggests that some of the SNC meteorites are derived from a meteoroid stream. In that Chassigny and Zagami span nearly the full range of SNC lithologies and histories, the coincidence of fall days is consistent with suggestions that all of the SNCs came from a single site (impact crater) on their parent planet.

Description: Almost all rock types in the SNC meteorites are cumulates, products of magma differentiation by crystal fractionation (addition or removal of crystals). If the SNC meteorites are from the surface of Mars or near sub-surface, then most of the igneous units on Mars are differentiated. Basaltic units probably experienced minor to moderate differentiation, but ultrabasic units probably experienced extreme differentiation. Products of this differentiation may include Fe-rich gabbro, pyroxenite, peridotite (and thus serpentine), and possibly massive sulfides. The SNC meteorites include ten lithologies (three in EETA79001), eight of which are crystal cumulates. The other lithologies, EETA79001 A and B are subophitic basalts. The cumulate lithologies ALHA77005 and EETA79001 C were not fully described or discussed.

Description: Almost all rock types in the SNC meteorites are cumulates, products of magma differentiation by crystal fractionation (addition or removal of crystals). If the SNC meteorites are from the surface of Mars or near subsurface, then most of the igneous units on Mars are differentiated. Basaltic units probably experienced minor to moderate differientation, but ultrabasic units probably experienced extreme differentiation. Products of this differentiation may include Fe-rich gabbro, pyroxenite, periodotite (and thus serpentine), and possibly massive sulfides. The SNC meteorites include ten lithologies (three in EETA79001), eight of which are crystal cumulates. The other lithologies, EETA79001 A and B are subophitic basalts.

Description: A Mars Pathfinder landing site in Melas Chasma (Valles Marineris) would yield significant science return, but is outside present mission constraints. In Melas Chasma, Mars Pathfinder could investigate minimally altered basaltic material, sedimentary deposits, chemical weathering, tectonic features, the highland crust, equatorial weather, and Valles mists. Critical issues include the following: (1) nature and the origin of the Valles interior layered deposits, important for understanding water as a sedimentary and chemical agent, and for the past existence of of environments favorable for life; (2) compositions of little-altered basaltic sands, important for understanding magma genesis and weathering on Mars, and the martian meteorites; and (3) structure and composition of the highland crust, important for understanding Mars' early history .

Description: The structure and chemical composition of the Lafayette meteorite were examined using several methods. The meteorite contains abundant hydrous post-magnetic alteration material consisting of ferroan smectite clays, magnetite, and ferrihydrite. The textural relations, mineralogy, and composition of these materials were examined and their preterrestrial nature was documented. Olivine, pyroxene, and glass alteration are described and the bulk compositions of the alteration veinlets is discussed. Essential features of the geochemistry of the alteration processes are described. It is suggested that the alteration of the Lafayette meteorite occurred during episodic infiltrations of small volumes of saline water. Constraints placed on water chemistry and water-rock interactions in the Martian crust are outlined.

Description: A possible mesosiderite meteorite was found in the area of the Putorana Plateau, Noril'sk district, Siberia, Russia. Although this rock resembles a mesosiderite in its hand-sample aspect and in having Ni-bearing iron metal, it is not a meteorite. This inference is based on the lack of a fusion crust, the lack of cosmogenic nuclides, oxygen with terrestrial isotope ratios, and several mineral chemical criteria. Most likely, the rock is from the iron-metal-bearing basalts of the Siberian Trap basalt sequence, which are mined for their base and platinum-group metals. Mesosiderite imposters like this may be recognized by: (1) the presence of Cu metal in hand sample or as microscopic blebs in the low-Ni metal (kamacite), (2) the absence of high-Ni metal (taenite), and (3) the presence of iron carbide (cohenite) enclosing the kamacite. Even if these macroscopic tests are inconclusive, isotopic and mineral chemical tests will also distinguish rocks like this from mesosiderites.

Description: Volatile species such as H2O, CO2, F, and Cl have significant impact in generation and differentiation of basaltic melts. Thus far experimental work has primarily focused on the effect of water and carbon dioxide on basalt crystallization, liquid-line of descent, and mantle melting [e.g., 1, 2] and the effects of halogens have received far less attention [3-4]. However, melts in the planetary interiors can have non-negligible chlorine and fluorine concentrations. Here, we explore the effects of fluorine on near-liquidus phase equilibria of basalt. We have conducted nominally anhydrous piston cylinder experiments using graphite capsules at 0.6 - 1.5 GPa on an Fe-rich model basalt composition. 1.75 wt% fluorine was added to the starting mix in the form of AgF2. Fluorine in the experimental glass was measured by SIMS and major elements of glass and minerals were analyzed by EPMA. Nominally volatile free experiments yield a liquidus temperature from 1330 C at 0.8GPa to 1400 at 1.6GPa and an olivine(Fo72)-pyroxene(En68)-liquid multiple saturation point at 1.25 GPa and 1375 C. The F-bearing experiments yield a liquiudus temperature from 1260 C at 0.6GPa to 1305 at 1.5GPa and an ol(Fo66)-pyx(En64)-MSP at 1 GPa and 1260 C. This shows that F depresses the basalt liquidus, extends the pyroxene stability field to lower pressure, and forces the liquidus phases to be more Fe-rich. KD(Fe-Mg/mineral-melt) calculated for both pyroxenes and olivines show an increase with increasing F content of the melt. Therefore, we infer that F complexes with Mg in the melt and thus increases the melt s silica activity, depressing the liquidus and changing the composition of the crystallizing minerals. Our study demonstrates that on a weight percent basis, the effect of fluorine is similar to the effect of H2O [1] and Cl [3] on freezing point depression of basalts. But on an atomic fraction basis, the effect of F on liquidus depression of basalts is xxxx compared to the effect of H. Future studies on kimberlitic and subduction zone magmas, which could have significant amount of fluorine, will need to consider the combined effects of F, Cl, and H on their stability and chemical evolution.

Description: Gregg and Greeley (1993) evaluated the mechanisms by which various magma types might have formed the long sinuous channels apparent on the surface of Venus. Among the magma types they evaluated were carbonatites, which are carbonate-rich ionic liquids. Gregg and Greeley (1993) applied thermophysical data for terrestrial carbonatite magmas to evaluate channel formation by carbonatites on Venus. Their analysis is flawed because they used incorrect values for the solidification temperatures of carbonatite magmas.

Description: Earth-based, earth-orbital, and spacecraft observations of the atmosphere and surface of Venus, thermodynamic models of atmosphere-lithosphere interactions, and where available kinetic data on relevant gas-solid reactions to place constraints on the mineralogy of the surface of Venus are used. Which minerals and mineral assemblages are stable on the surface of Venus and which, if any, of these minerals are involved in controlling the abundances of reactive gases in the atmosphere of Venus. It is concluded by identifying key issues facing us today about the mineralogy and geochemistry of the surface of Venus and suggest experimental, observational, and theoretical studies that can improve knowledge of these important questions are discussed.