This paper reports the results of several thousand analyses of the Mg content of calcite from 31
samples from the Ubehebe Peak contact aureole, Death Valley, California. All data reported are from
metamorphic calcite formed during mixed volatile-mineral reactions in which dolomite remained in
the rock. The Mg content generally increases toward the intrusive contact and bend with increasing
temperature, but it varies strongly. Indeed, probability distributions for each sample are near Gaussian,
possess a relatively small skewness (-1.72 to 3.32), and a variance that is a multiple of the estimated
measuring uncertainty. These findings complicate direct application of the Mg content in calcite for
use as an accurate thermometer.
The purpose of the study presented in the second part of the paper is to explore the significance
of these systematic variations of Mg composition of calcite to aid the interpretation of contact metamorphic
temperatures recorded in carbonates. We developed forward models to evaluate the effect of
growth zoning, volume diffusion, and the formation of submicroscopic exsolution lamellae (<1 μm)
on the measured Mg distribution in individual calcite crystals and compared the modeling results to
the field data. Modeled Mg distributions were transformed into histograms by taking into account
intersection probabilities and random microprobe analyses. Modeling results reveal that the original
prograde Mg zoning in calcite crystal will be reset if the calcite crystal is assumed to grow slowly along
a prograde path. Original low-Mg compositions can only be preserved if the entire grain forms over
a small temperature interval, as can be expected for infiltration-driven mineral reactions. It is shown
that all three mechanisms combined give an adequate model for the Mg-content data. We demonstrate
that Mg distributions in calcite grains of the Ubehebe Peak contact aureole are the consequence of
rapid crystal growth in combination with diffusion and exsolution.