『Abstract
Calcite is frequently cited as a source of excess Ca, Sr ad alkalinity
in solutes discharging from silicate terrains yet, no previous
effort has been made to assess systematically the overall abundance,
composition and petrogenesis of accessory calcite in granitoid
rocks. This study addresses this issue by analyzing a worldwide
distribution of more than 100 granitoid rocks. Calcite is found
to be universally present in a concentration range between 0.028
to 18.8 g kg-1 (mean = 2.52 g kg-1). Calcite
occurrences include small to large isolated anhedral grains, fracture
and cavity infillings, and sericitized cores of plagioclase. No
correlation exists between the amount of calcite present and major
rock oxide compositions, including CaO. Ion microprobe analyses
of in situ calcite grains indicate relatively low Sr (120 to 660
ppm), negligible Rb and 87Sr/86Sr ratios
equal to or higher than those of coexisting plagioclase. Solutes,
including Ca and alkalinity produced by batch leaching of the
granitoid rocks (5% CO2 in DI water for 75
d at 25℃), are dominated by the dissolution of calcite relative
to silicate minerals. The correlation of these parameters with
higher calcite concentrations decreases as leachates approach
thermodynamic saturation. In longer term column experiments (1.5
yr), reactive calcite becomes exhausted, solute Ca and Sr become
controlled by feldspar dissolution and 87Sr/86Sr
by biotite oxidation. Some accessory calcite in granitoid rocks
is related to intrusion into carbonate wall rock or produced by
later hydrothermal alteration. However, the ubiquitous occurrence
of calcite also suggests formation during late stage (subsolidus)
magmatic processes. This conclusion is supported by petrographic
observations and 87Sr/86Sr analyses. A review
of thermodynamic data indicates that at moderate pressures and
reasonable CO2 fugacities, calcite is a stable
phase at temperature of 400 to 700℃.』
1. Introduction
2. Methodology
3. Results
3.1. Distribution of calcite in granitoid rocks
3.2. Batch leachate compositions
3.3. Solute Sr and 87Sr/86Sr produced by
long-term column reaction
3.4. In situ 87Sr/86Sr in calcite determined
by SHRIMP
4. Discussion
4.1. Lack of correlation between granitoid compositions and
calcite contents
4.2. Correlations between calcite contents and solute compositions
4.3. Correlations between calcite contents, solute Sr, and 87Sr/86Sr
4.4. Petrogenesis of calcite
5. Conclusions
Acknowledgments
References