T199 The Role of Silica in the Earth System: From Organisms to Global Biogeochemical Cycles

Tuesday, 24 October 2017
8:00 AM - 12:00 PM
Room 606

THEORETICAL CALIBRATION OF THE QUARTZ-WATER TRIPLE OXYGEN ISOTOPE THERMOMETER

Paper number: T199-7
Program: Technical Sessions
Time: 9:30 AM – 9:45 AM

Authors

Abstract

Of the three stable isotopes of oxygen (16O, 17O, and 18O), only two (18O and 16O) have been widely used for isotope geothermometry. Ratios of 18O to 16O can be used to determine mineral formation temperatures provided two preconditions are met, namely, (1) growth occurred at equilibrium and (2) the isotopic composition of the source fluid is known. However, verifying these preconditions continues to be a challenge in the rock record. For most terrestrial processes (i.e. mass-dependent processes) the 17O-16O relationship scales with the 18O-16O relationship and is thought to grant little new information. However, theory predicts a small temperature-dependence of the equilibrium triple oxygen isotope composition that offers an independent constraint on mineral growth conditions. New advances in analytical precision have, for the first time, made these small variations useful. For example, if both the 18O/16O ratios and triple-isotope variations imply the same set of possible formation temperatures, then the mineral likely grew at equilibrium. Here, we present new theoretical triple-isotope geothermometer calibrations for the quartz-H2O(liq.) system based on statistical thermodynamics and density functional theory. The new theoretical results compare well with previous experimental results for 18O/16O and 17O/16O equilibrium fractionation where data is available. In addition, we report new isotopic analyses of the ~3.5Ga Buck Reef Chert which has previously been determined to be pristine and marine in origin. The measured oxygen isotope composition of the cherts are found to be incompatible with a seawater which has a modern-like oxygen isotope composition. Our results indicate that the seawater in which those cherts formed had a δ’18O value less than ‒9.8‰±0.3‰ and was cooler than 68±2°C. These values are consistent with previous results from coupled 18O/16O and D/H analyses of cherts in the same formation as well as previous modeling of seawater oxygen isotope compositions.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-304717

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