Abstract
The interpretation of silicon isotope data for quartz is hampered by the lack of experimentally determined fractionation factors between quartz and fluid. Further, there is a large spread in published oxygen isotope fractionation factors at low temperatures, 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 applying secondary ion mass spectrometry to directly analyze experimental products. These experiments also yield a new value for oxygen isotope fractionation. Quartz overgrowths up to 235 渭m thick were precipitated in silica鈥揌2O鈥揘aOH鈥揘aCl fluids, at pH 12鈥�13 and 250 掳C. At this temperature, 1000ln伪30Si(Qtz鈥揻luid) = 0.55 卤 0.10鈥� and 1000ln伪18O(Qtz鈥揻luid) = 10.62 卤 0.13鈥�, yielding the relations 1000ln伪30厂颈(蚕迟锄鈥揻濒耻颈诲)&苍产蝉辫;=&苍产蝉辫;(0.15&苍产蝉辫;卤&苍产蝉辫;0.03)&苍产蝉辫;*&苍产蝉辫;106/T2 and 1000ln伪18翱(蚕迟锄鈥揻濒耻颈诲)&苍产蝉辫;=&苍产蝉辫;(2.91&苍产蝉辫;卤&苍产蝉辫;0.04)&苍产蝉辫;*&苍产蝉辫;106/T2 when extended to zero fractionation at infinite temperature. Values of 未30Si(Qtz) from diagenetic cement in sandstones from the basal Cambrian Mt. Simon Formation in central North America range from 0 to 鈭� 5.4鈥�. Paired 未18O and 未30Si values from individual overgrowths preserve a record of Precambrian weathering and fluid transport. The application of the experimental quartz growth results to observations from natural sandstone samples suggests that precipitation of quartz at low temperatures in nature is dominated by kinetic, rather than equilibrium, processes.
