The release of Pb and REE from granitoids by the dissolution of accessory phases

Yehudit Harlavan*, Yigal Erel

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

126 Scopus citations

Abstract

The release of Pb and rare earth elements (REE) during granitoid weathering was investigated through dissolution experiments of fresh granite and soil samples. Two aliquots of a granite sample from the El-Capitan Granite, Sierra Nevada, California, were leached several times using a dilute acid at pH = 1. The results of the experiment were compared with Pb and REE data from soils developed on the same rock. During the early stages of granitoid dissolution, Pb and REE were preferentially released from some of the accessory phases (i.e., allanite, sphene, and apatite). This caused higher 206Pb/207Pb and 208Pb/207Pb values and different REE patterns in solution compared with the rock values. Based on Pb isotopes and REE patterns, three stages of rock dissolution can be identified. In the first stage the dissolution of allanite dominates the release of Pb and REE from accessory phases, as 208Pb/207Pb, Ce/Pb, and chondrite-normalized Ce/Yb ratios in solution increase and approach the values of allanite. In the second stage, the dissolution of apatite and sphene become more significant. In the third stage, the isotopic ratios of Pb and the normalized-REE patterns reflect the depletion of accessory phases and the increase in the rate of feldspar dissolution. According to our estimate (based on Si release from the rock) all three stages account for the first 500 kyr of granitoid weathering. Using the isotopic ratios of Pb, major elemental compositions, and REE concentrations both in the experimental solutions and in the soil we were able to establish the following order of the weathering rates of accessory phases: allanite > apatite > sphene. In addition, we have demonstrated that biotite is significantly less resistant to weathering than hornblende under acidic conditions, and is probably dissolved completely after approximately 500 kyr of rock weathering. We also suggest that within 500 kyr of granitoid weathering K-feldspar accounts for 15% of the released K.

Original languageAmerican English
Pages (from-to)837-848
Number of pages12
JournalGeochimica et Cosmochimica Acta
Volume66
Issue number5
DOIs
StatePublished - 2002

Bibliographical note

Funding Information:
The authors wish to thank J. D. Blum for in-depth discussions, and comments on an earlier version of this paper. They thank A. Katz, L. Halicz, and N. Teutsch for laboratory analyses; K. Brown for his assistance in the field; J. van Wagtendonk for permission to collect samples in Yosemite National Park; J. D. Rimstidt and three anonymous reviewers. Funding was provided by the US-Israel BSF grant 95 to 30.

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