HR: 0800h
AN: P51A-0900    [Abstracts]
TI: Modeling the Chemical Composition of the Fluid that Formed the ALH84001 Carbonates
AU: * Niles, P B
EM: paulniles@yahoo.com
AF: Astromaterials Research and Exploration Science Office, NASA Johnson Space Center, Houston, TX 77058 United States
AU: Leshin, L
EM: laurie.leshin@nasa.gov
AF: Goddard Space Flight Center, Code 600, Building 6, Greenbelt, MD 20771 United States
AB: The character of aqueous systems on Mars can provide us with important information regarding the history of water and the possibilities for the presence of life on Mars. Evidence of these aqueous systems has been preserved in carbonates found in the martian meteorite ALH84001 whose crystallization age of 4.5 Ga indicates that it has experienced almost all of Mars' history. In addition, the 3.9 Ga age of the carbonates places their formation at a critical time that has been argued to have been `warm and wet' by many studies. The carbonates in the ALH84001 meteorite provide the best opportunity, among all of the martian meteorites, to understand the details of an ancient aqueous system on Mars. Their unique chemical, isotopic and mineralogical composition provides the opportunity to make conclusive statements about the geological conditions in which they formed including the temperature, association with the atmosphere, chemistry of the fluids, and the presence or absence of life. This study uses an empirical model to understand the attributes of the formation fluid based on the unique chemical compositions of the carbonates. This requires the assumption that the ALH84001 carbonate globules formed from a single fluid whose chemical composition changed due to the precipitation of carbonates more calcium rich than the overall fluid composition. The model consists of a simple stepwise stoichiometric calculation of the precipitation of the ALH84001 carbonates from a hypothetical solution. From extensive measurements of the chemical composition of the globules and their abundance in the rock, one can calculate the total amount of magnesium, calcium, and iron removed from the formation fluid as the carbonates precipitated. The unique zoned nature of the ALH84001 carbonates provides a real constraint on the possible fluid compositions consistent with their precipitation. Our results indicate that the fluid that formed the ALH84001 carbonates had an Mg/Ca ratio that was greater than about 4, and an Fe/Ca ratio that was greater than about 1. These elemental ratios are consistent with a fluid that had interacted with Mg-rich ultramafic rock and was reducing enough to retain significant amounts of dissolved iron. ALH84001 is a magnesium rich ultramafic rock, however, it shows no sign of having undergone any kind of dissolution or weathering besides the precipitation of the carbonates. Therefore, it is possible that the fluid interacted with a host-rock of similar composition to ALH84001 located elsewhere prior to precipitating the carbonates in ALH84001.
DE: 1000 GEOCHEMISTRY
DE: 1009 Geochemical modeling (3610, 8410)
DE: 5220 Hydrothermal systems and weathering on other planets
DE: 5415 Erosion and weathering
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: Fall Meeting 2005