Calcium isotope fractionation during tropical weathering of granites

Abstract

Calcium (Ca) is a highly soluble and mobile element during silicate weathering, and its isotopes could be a promising tracer for continental weathering processes. However, it remains controversial whether Ca isotopes fractionate during granitoids’ weathering. Here we investigated Ca isotope fractionation (measured by δ44/42Ca) behaviours on the weathering profile of the late middle Jurassic Fogang granite (165 ± 2 Ma) under a tropical climate with the help of radiogenic anomaly data (i.e., ε40/44Ca) (both relative to the NIST standard SRM 915a), as well as its implications for the Ca isotopic composition in the hydrosphere. A total of 15 saprolite samples, 2 fresh parent rocks, 3 fresh individual minerals of one granite weathering profile from Fogang county, South China, and 8 stream water samples nearby the profile have been analysed for their Ca isotopic compositions by thermal ionisation mass spectrometry using a double spike method. Results reveal extreme radiogenic and stable Ca isotopic variations among saprolites. The saprolites have ε40/44Ca ranging from 0.8 to 55.6, whereas fresh granite rocks exhibit uniform ε40/44Ca close to 0. The large variation of ε40/44Ca in the saprolites can be attributed to the preferential dissolution of plagioclase and exsolved albite in K-feldspar or the variation of K/Ca molar ratios in K-feldspar. In general, saprolites are enriched in lighter Ca isotopes compared to the parent rocks (0.39 ± 0.04 ‰) and have δ44/42Ca values ranging from −0.45 ‰ to 0.43 ‰. This suggests that newly formed clay minerals preferentially absorb the lighter Ca isotopes (e.g., 40Ca). However, the δ44/42Ca of stream waters draining highly weathered granite catchments reflect δ44/42Ca of bedrock, rather than δ44/42Ca of the saprolites. One possible reason is that too little Ca was adsorbed by secondary clay minerals in the saprolite to shift the Ca isotope composition of dissolved Ca from the bedrock. Our study shows that a combination of radiogenic and stable Ca isotope data is a promising tracer for the sources, migration, and transformation processes of this key element during continental weathering processes. This research contributes to a broader understanding of Ca isotopes as a tracer for tropical weathering and their implications for global biogeochemical cycles.