Abstract
The interpretation of silicon isotope data for quartz is hampered by the lack of experimentally determined fractionation factors between quartz and fluid and there is a large spread in published oxygen isotope fractionation factors, primarily due to extrapolation from experimental calibrations at high temperature. We present the first measurements of silicon isotope ratios from experimentally precipitated quartz and estimate the equilibrium fractionation vs. dissolved silica using a novel in situ analysis technique using secondary ion mass spectrometry to directly measure experimental products. These experiments also yield a new value of oxygen isotope fractionation. Quartz overgrowths 235 µm thick were precipitated in silica-H2O-NaOH-NaCl fluids, at pH 12-13 and 250ºC. At 250ºC, ∆30Si(Qtz-fluid) =0.55 ± 0.10‰ and ∆18O(Qtz-fluid) =10.62 ± 0.13‰. Values of 30Si(Qtz) from diagenetic cement in sandstones from the basal Cambrian Mt. Simon Formation in central North America range from 0 to -5.4‰. Paired 18O and 30Si values from individual overgrowths preserve a record of Precambrian weathering and fluid transport.
