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Equilibria in the Sift (Silica-Ilmenite-Rutile-Ferrosilite) system: SiO 2 4-(Mg, Fe)TiO3 = TiOz + (Mg, Fe)SiO3 have been calibrated in the range 800 1100 ~ C and 12 26 kbar using a piston-cylinder apparatus to assess the potential of the equilibria for geobarometry in granulite facies assemblages that lack garnet. Thermodynamic calculations indicate that the two end-member equilibria involving quartz + geikielite = rutile + enstatite, and quartz + ilmenite = futile + ferrosilite, are metastable. We therefore reversed equilibria over the compositional range Fs4o 70, using AgsoPd2o capsules with fo2 buffered at or near iron-wfistite. Ilmenite compositions coexisting with orthopyroxene are X~'rio3 of 0.06 to 0.

Pedogenic dolomite and high magnesium calcite (HMC) occur as micritic matrix and void filling microspar in soils developed on semi-arid 350 Ka basalt surfaces on Hawai'i Island. The void filling carbonate presents an unusual appearance of alternating concentric < 5 urn to -30 um layers ofHMC and dolomite at shallow (100 cm) depths and similar layers that are exclusively dolomite between depths of 170 em and 350 em. Elevated Mg/Ca ratios in soil water, caused by rapid basalt weathering, likely provided geochemical conditions that permitted evaporative precipitation of magnesian carbonates. Variations in the amount of soil water, soil water chemistry, and/or crystallization rate probably controlled the precipitation of HMC vs. dolomite at 100 em, whereas increased residence time of soil water and more stable soil water chemistry below the 170 em depth led to the exclusive formation of dolomite.

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