White et al.(2005)による〔『The ubiquitous nature of accessory calcite in granitoid rocks: Implications for weathering, solute evolution, and petrogenesis』(1455p)から〕

『花崗岩質岩類中の副成分方解石の遍在性:風化、溶質の変化、および岩石成因との関係』


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


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