Abstract. Major element, multispectral, and magnetic properties data were obtained at Ares Vallis during the Mars Pathfinder mission. To understand the compositional, mineralogical, and process implications of these data, we obtained major element, mineralogical, and magnetic data for well-crystalline and nanophase ferric minerals, terrestrial analogue samples with known geologic context, and SNC meteorites. Analogue samples include unaltered, palagonitic, and sulfatetic tephra from Mauna Kea Volcano (hydrolytic and acid-sulfate alteration), steam vent material from Kilauea Volcano (hydrolytic alteration), and impactites from Meteor Crater

Director: Don Winston "Molar tooth" is a sedimentary structure of interconnecting, thin sheets and small spheroids, composed of uniform, blocky calcite crystals 5-15 um in diameter, and is common in argillitic dolomite of the Middle Proterozoic Belt and Purcell Supergroups of the Rocky Mountains. Experiments that closely replicate "molar tooth" structures suggest that "molar tooth" formed as gas generated voids that filled with blocky calcite. Experimental models using mud, yeast and sugar in glass aquaria produced bubbles and gas expansion cracks which closely mimic shapes of "molar tooth" structures. Mixing CaCl2 and Na2C03 solutions precipitated finely crystalline blocky calcite, similar to "molar tooth" calcite. Comparisons of the Belt with the modern Dead Sea, where bacterial decomposition of gypsum is inferred to produce H2S and CO2 gases suggests a similar origin for "molar tooth" structures and calcite. "Molar tooth" structures in the Belt commonly forms a repeated sequence of shapes in fining upward si 1 iciclastic-to-dolomite cycles. Microfossils and amorphous organic debris in the calcite and host rock suggests "molar tooth" calcite structures resulted from consumption of evaporite minerals such as gypsum by bacterial metabolism and formation of gas bubbles and cracks which were filled by H2S and CO2 gas which in turn generated pyrite and calcite. ACKNOWLEDGMENTS I thank my advisors Dr. Charles Miller, Dr. Graham Thompson, and Dr. Don Winston for their encouragement, suggestions, and careful, critical review of this manuscript. I extend my gratitude to Don for his infectious enthusiasm for the Belt and all aspects of this project. Thanks to John Rittel for marvelous laboratory ingenuity and skilled 5EM techniques. Thanks to John for his talented experimental mind and for the many laughs we shared. Thanks to Jeff Moe for x-ray analysis. Thanks to Don Winston and John Cup1 in for drafting the map. Thanks to Patti Furniss for constant encouragement.

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