11:15 AM - 11:30 AM
[BCG07-09] Application of acid-washing to trace element and tungsten isotope analyses of Eoarchean rocks from the Isua Supracrustal Belt
Keywords:Hf–W, Isua, acid-washing, early Earth
In this study, we applied an acid-washing method to Eoarchean metamorphosed rocks from the Isua Supracrustal Belt, with the aims to evaluate the possible effect of metasomatism on the W isotope systems, and to obtain their original W isotope compositions. We measured trace element compositions and W isotope ratios of acid-washes and residues of the metamorphosed basalts using ICP–MS.
The acid-washes show enrichments in light REE, whereas the residues exhibit enrichments in heavy REE. This suggests that acid-washing selectively removed a secondary component gained during metasomatism, given that light REE are relatively fluid-mobile as compared to heavy REE. Negative 182W anomalies were found in the washes showing light REE enrichments. By contrast, non-resolvable 182W anomaly was obtained for the residues displaying heavy REE enrichments. These findings suggest that the Isua metamorphosed basalts have a W isotope heterogeneity even in a single rock sample. Moreover, our results imply that the W isotope compositions of Isua supracrustal rocks could have been modified secondarily as a result of metasomatism. This can account for the lack of correlation between W isotopic data and other geochemical or isotopic data.