Abstract
The present study compares the dissolution rates of plagioclase, microcline and biotite/chlorite from a bulk granite to the dissolution rates of the same minerals in mineral-rich fractions that were separated from the granite sample. The dissolution rate of plagioclase is enhanced with time as a result of exposure of its surface sites due to the removal of an iron oxide coating. Removal of the iron coating was slower in the experiment with the bulk granite than in the mineral-rich fractions due to a higher Fe concentration from biotite dissolution. As a result, the increase in plagioclase dissolution rate was initially slower in the experiment with the bulk granite. The measured steady state dissolution rates of both plagioclase (6.2 ± 1.2 ± 10-11 mol g-1 s-1) and microcline (1.6 ± 0.3 × 10-11 mol g-1 s-1) were the same in experiments conducted with the plagioclase-rich fraction, the alkali feldspar-rich fraction and the bulk granite. Based on the observed release rates of the major elements, we suggest that the biotite/chlorite-rich fraction dissolved non-congruently under near-equilibrium conditions. In contrast, the biotite and chlorite within the bulk granite sample dissolved congruently under far from equilibrium conditions. These differences result from variations in the degree of saturation of the solutions with respect to both the dissolving biotite/chlorite and to nontronite, which probably was precipitating during dissolution of the biotite and chlorite-rich fraction. Following drying of the bulk granite, the dissolution rate of biotite was significantly enhanced, whereas the dissolution rate of plagioclase decreased. The presence of coatings, wetting and drying cycles and near equilibrium conditions all significantly affect mineral dissolution rates in the field in comparison to the dissolution rate of fully wetted clean minerals under far from equilibrium laboratory conditions. To bridge the gap between the field and the laboratory mineral dissolution rates, these effects on dissolution rate should be further studied.
| Original language | English |
|---|---|
| Pages (from-to) | 607-621 |
| Number of pages | 15 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 69 |
| Issue number | 3 |
| DOIs | |
| State | Published - 1 Feb 2005 |
Bibliographical note
Funding Information:This research was supported by Grant No. 1999-076-01 from the United States-Israel Binational Science Foundation (BSF), Jerusalem, Israel. We wish to express our gratitude to A. Zanvilevich, B. Litvinovsky, M. Eyal, H. Kisch and C. Zhu for fruitful and inspiring discussions. We thank the associate editor, Eric H. Oelkers, and three anonymous reviewers for their review of the manuscript. The manuscript was partly written during a sabbatical leave of the first author at the Department of Geology and Planetary Science, University of Pittsburgh, which is acknowledged for its warm hospitality.