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Title: Experimental calibration of silicon and oxygen isotope fractionations between quartz and water at 250°C by in situ microanalysis of experimental products and application to zoned low δ30Si quartz overgrowths

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 report 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 μm thick were precipitated in silica–H2O–NaOH–NaCl fluids, at pH 12–13 and 250 °C. At this temperature, 1000lnα30Si(Qtz–fluid) = 0.55 ± 0.10‰ and 1000lnα18O(Qtz–fluid) = 10.62 ± 0.13‰, yielding the relations 1000lnα30Si(Qtz–fluid) = (0.15 ± 0.03) * 106/T2 and 1000lnα18O(Qtz–fluid) = (2.91 ± 0.04) * 106/T2 when extended to zero fractionation at infinite temperature. Values of δ30Si(Qtz) from diagenetic cement in sandstones from the basal Cambrian Mt. Simon Formation in central North America range from 0 to ₋5.4‰. Paired δ18O and δ30Si values from individual overgrowths preserve a record of Precambrian weathering and fluid transport. In conclusion, the application ofmore » 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.« less
Authors:
 [1] ;  [2] ;  [3] ;  [4] ;  [2] ;  [2]
  1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Univ. of Wisconsin, Madison, WI (United States)
  2. Univ. of Wisconsin, Madison, WI (United States)
  3. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  4. Trent Univ., Peterborough, ON (Canada)
Publication Date:
OSTI Identifier:
1255066
Report Number(s):
LA-UR--15-24608
Journal ID: ISSN 0009-2541; PII: S0009254115301005
Grant/Contract Number:
AC52-06NA25396; 93ER14389; EAR-1053466; 1355590
Type:
Accepted Manuscript
Journal Name:
Chemical Geology
Additional Journal Information:
Journal Volume: 421; Journal Issue: C; Journal ID: ISSN 0009-2541
Publisher:
Elsevier
Research Org:
Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES isotope fractionation; SIMS; experimental calibration; Precambrian weathering; d³⁰Si, d¹⁸O