The silicon isotope composition of the upper continental crust

Abstract

The upper continental crust (UCC) is the major source of silicon (Si) to the oceans and yet its isotopic composition is not well constrained. In an effort to investigate the degree of heterogeneity and provide a robust estimate for the average Si isotopic composition of the UCC, a representative selection of well-characterised, continentally-derived clastic sediments have been analysed using high-precision MC-ICPMS. Analyses of loess samples define a narrow range of Si isotopic compositions (δ30Si = −0.28‰ to −0.15‰). This is thought to reflect the primary igneous mineralogy and predominance of mechanical weathering in the formation of such samples. The average loess δ30Si is −0.22 ± 0.07‰ (2 s.d.), identical to average granite and felsic igneous compositions. Therefore, minor chemical weathering does not resolvably affect bulk rock δ30Si, and loess is a good proxy for the Si isotopic composition of unweathered, crystalline, continental crust. The Si isotopic compositions of shales display much more variability (δ30Si = −0.82‰ to 0.00‰). Shale Si isotope compositions do not correlate well with canonical proxies for chemical weathering, such as CIA values, but do correlate negatively with insoluble element concentrations and Al/Si ratios. This implies that more intensive or prolonged chemical weathering of a sedimentary source, with attendant desilicification, is required before resolvable negative Si isotopic fractionation occurs. Shale δ30Si values that are more positive than those of felsic igneous rocks most likely indicate the presence of marine-derived silica in such samples. Using the data gathered in this study, combined with already published granite Si isotope analyses, a weighted average composition of δ30Si = −0.25 ± 0.16‰ (2 s.d.) for the UCC has been calculated.