Abstract
Flow-through dissolution experiments were carried out on crushed granitoid rock (the Elat Granite) and three mineral separates (plagioclase, perthite, and biotite + chlorite) from this rock at pH 1 and 25°C. Major element concentrations were combined with Pb and Sr isotopic analyses of starting materials and output solutions and together enabled us to elucidate several important mechanisms related to granitoid rock weathering. We observed an initial stage of rock dissolution (<200 hours of reaction) that was characterized by elemental release from traces of calcite and/or apatite and to a lesser extent from the interlayer sites of biotite. Dissolution in the interval of 200 to 400 h was dominated by the release of elements from the interlayer sites of biotite, and at 400 to 1000 h of reaction the chemistry of output solutions was dominated by the release of elements from tetrahedral and octahedral sites of biotite as well as from plagioclase. After 1000 h, the dissolution of plagioclase, and to a lesser extent biotite, dominated the composition of elements released by the rock. We demonstrate that Pb and Sr isotope ratios in the output solutions can be used to identify each of these stages of dissolution. By comparing our experimental results on the release of Pb and Sr isotopes with field measurements of Pb and Sr isotopes in soil chronosequences from the Wind River and the Sierra Nevada Mountains (USA), we are able to show that similar isotopic patterns appear in both the pH 1 experiments and in soils formed under natural conditions at higher pH. By combining these experimental results with previous field studies, we are able to estimate the duration of most of these stages of granitoid weathering under natural conditions in temperate climates. In soils older than a few hundred years and younger than 10,000 yr the release of elements from interlayer sites of biotite controls the weathering flux. Soils between 10,000 and 100,000 yr old are dominated by biotite and plagioclase weathering, with biotite weathering controlling the first part of this period and plagioclase dominating the later part. After more than 100,000 yr, plagioclase, and to a lesser degree biotite, dominate the weathering flux within these granitoid soils.
| Original language | English |
|---|---|
| Pages (from-to) | 4649-4663 |
| Number of pages | 15 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 68 |
| Issue number | 22 |
| DOIs | |
| State | Published - 15 Nov 2004 |
Bibliographical note
Funding Information:The authors wish to thank L. Halicz, I. Segal, S. Ehrlich, and O. Yoffe of the Geological Survey of Israel, A. Katz of the Hebrew University, and A. Klaue and B. Klaue of the University of Michigan for assistance in Pb and Sr isotope, trace metal, and major element analyses. We also wish to thank A. Zanvilevich, B. Litvinovsky, M. Eyal, and H. Kisch of Ben Gurion University for their assistance with the petrography of the Elat Granite. The authors thank K. Nagi, L. Stillings, and two anonymous reviewers for their useful comments. This research was supported by a United States–Israel Binational Science Foundation (BSF) grant 1999-076-01.