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A series of laboratory experiments were conducted to examine how partitioning of Fe among solid reaction products and rates of H 2 generation vary as a function of temperature during serpentinization of olivine. Individual experiments were conducted at temperatures

Internal heating can cause melting, metamorphism, and crustal weakening in convergent orogens.This study evaluates the role of radiogenic heat production (RHP) in a Neoproterozoic ultrahigh-temperature metamorphic (UHTM) terrane exposed in southern Madagascar. Monazite and zircon geochronology indicates that the Paleoproterozoic Androyen and Anosyen domains (i) collided with the oceanic Vohibory Arc at~630 Ma, (ii) became incorporated into the Gondwanan collisional orogen by~580 Ma, and (iii) were exhumed during crustal thinning at 525-510 Ma. Ti-in-quartz and Zr-in-rutile thermometry reveals that UHTM occurred over >20,000 km 2 , mostly within the Anosyen domain. Assuming that U, Th, and K contents of samples from the field area are representative of the middle to lower crust during orogenesis, RHP was high enough-locally >5 μW/m 3 -to cause regional UHTM in <60 Myr. We conclude that, due in large part to the stability and insolubility of monazite at high crustal temperatures, RHP was the principal heat source responsible for UHTM, obviating the need to evoke external heat sources. Focused RHP probably thermally weakened portions of the middle crust, gravitationally destabilizing the orogen and facilitating thinning via lateral extrusion of hot crustal sections.

[1] Acid-sulfate alteration of basalt by SO 2 -bearing volcanic vapors has been proposed as one possible origin for sulfate-rich deposits on Mars. To better define mineralogical signatures of acid-sulfate alteration, laboratory experiments were performed to investigate alteration pathways and geochemical processes during reaction of basalt with sulfuric acid. Pyroclastic cinders composed of phenocrysts including plagioclase, olivine, and augite embedded in glass were reacted with sulfuric acid at 145 C for up to 137 days at a range of fluid : rock ratios. During the experiments, the phenocrysts reacted rapidly to form secondary products, while the glass was unreactive. Major products included amorphous silica, anhydrite, and Fe-rich natroalunite, along with minor iron oxides/oxyhydroxides (probably hematite) and trace levels of other sulfates. At the lowest fluid : rock ratio, hexahydrite and an unidentified Fe-silicate phase also occurred as major products. Reactionpath models indicated that formation of the products required both slow dissolution of glass and kinetic inhibitions to precipitation of a number of minerals including phyllosilicates and other aluminosilicates as well as Al-and Fe-oxides/oxyhydroxides. Similar models performed for Martian basalt compositions predict that the initial stages of acid-sulfate alteration of pyroclastic deposits on Mars should result in formation of amorphous silica, anhydrite, Fe-bearing natroalunite, and kieserite, along with relict basaltic glass. In addition, analysis of the experimental products indicates that Fe-bearing natroalunite produces a Mössbauer spectrum closely resembling that of jarosite, suggesting that it should be considered an alternative to the component in sulfate-rich bedrocks at Meridiani Planum that has previously been identified as jarosite.