Mathematics – Logic
Scientific paper
Oct 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.4491s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 20, pp.4491-4501
Mathematics
Logic
13
Scientific paper
An empirical calibration for the quartz-calcite thermometer was derived from measured 18 O ( qz - cc ) values from greenschist-facies marbles, veins composed of cogenetic quartz and calcite and various low-grade metamorphic rocks. The 18 O ( qz - cc ) values vary systematically with independently determined formation temperature and can be fit to the expression 1000 ln ( qz - cc ) = 0.87(±0.06) × 10 6 / T 2 . In contrast, published results from direct-exchange experiments between calcite and quartz are 1000 ln ( qz - cc ) = 0.38(±0.06) × 10 6 / T 2 , far smaller than in the present study. Application of the experimental mineral-water and especially the direct-exchange calibrations to natural samples, yields unreasonably low geological temperatures. It is difficult to envision a mechanism whereby the measured fractionations in greenschist-grade marbles can be reconciled with the very low temperature estimates obtained with the direct-exchange experimental calibration. Oxygen diffusion rates in quartz are too slow to explain the discrepancy. Postmetamorphic exchange could have occurred with a hydrothermal fluid, but it is unlikely that the 18 O ( calcite ) values of all samples would be shifted by an amount that would result in a linear relationship between 1000 ln ( qz - cc ) and T -2 . More likely, the discrepancy is due to a kinetic effect in the experiments. The very small fractionations observed in the direct-exchange experiments may have been caused by diffusion-related effects during recrystallization of the quartz and calcite. The problem of recrystallization is eliminated in mineral-CO 2 exchange experiments. Combined CO 2 -calcite and CO 2 -quartz glass experiments yield the expression , in good agreement with the empirical calibration. The new empirical calibration yields reasonable temperature estimates for a wide range of samples and can be used for thermometry in rock types and over temperature intervals where other quantitative geothermometers are lacking.
Kirschner David L.
Sharp Zachary D.
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