Silicon Isotope Investigation of Low Temperature Authigenic Siliceous Deposits and High Temperature Planetary Materials
Chen, Xinyang 1987-
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Despite the ubiquity of the element silicon (Si) in silicate rocks and minerals, there is still no clear understanding of how the Si isotopes fractionate during Earth’s surface processes (e.g., the formation of low temperature siliceous deposits) and during early So-lar System processes (e.g., nebular differentiation and metal-silicate differentiation). With recent advances in analytical techniques, the analytical precision has been greatly im-proved, which allows straightforward determination of Si isotope variations during both low and high temperature fractionations. The first part of this study investigates the petrogenesis and paleoenvironments of Phanerozoic siliceous deposits using a suite of euhedral megaquartz crystals in the Creta-ceous Edwards Formation from Central Texas. Si isotopic mapping across the megaquartz crystals show that δ30Si(NBS 28) values range from -2.72 to +2.94 ‰. The range of δ30Si values in megaquartz crystals is interpreted using a two stage model in which amorphous silica from sponge spicules is dissolved and re-precipitated as megaquartz in a closed sys-tem during diagenesis. Based on temperature estimates of 23 to 30 °C for megaquartz precipitation, the fractionation factor was determined to be between -0.95 to -1.46 ‰. The estimated average δ30Si value of the Late Cretaceous seawaters is +2.3 to +2.8 ‰, significantly higher than modern seawater. The second part of the study investigates the Si isotope compositions of a unique ungrouped achondrite NWA 7325 and a suite of other meteorite and terrestrial rock sam-ples to better understand Si isotopic variation in planetary materials and the relationship between NWA 7325 and other meteorite groups. NWA 7325 is a reduced, Mg-rich cumu-late olivine gabbro that was thought to have originated from planet Mercury. The δ30Si value of NWA 7325 is -0.45±0.05 ‰ (n=8), indistinguishable from chondrites. Bulk sili-cate Earth (BSE) and angrites have higher δ30Si values than chondrites, which is better explained by nebular fractionation rather than core formation. The difference in δ30Si val-ues between planetary materials could be caused by variable forsterite and nebular gas mixing. NWA 7325 may have accreted from materials that inherited similar proportions of early gas and condensates as chondrites, but distinct from those of the angrite and the Earth-Moon system.