Copper and gallium isotopic behavior in highly weathered soils

Abstract

The trace elements copper (Cu) and gallium (Ga) are of potential utility in geological studies related to weathering and pedogenic processes, as well as to paleoclimate changes. However, the behavior of Cu and Ga and their isotopic fractionation mechanisms during chemical weathering are not well understood at present. To address this issue, coupled Cu and Ga isotopes were measured in 17 samples from a sedimentologically and geochemically well-characterized weathering profile developed on Cenozoic basalts in Hainan Island, South China. Each sample was subjected to a sequential extraction procedure yielding four fractions of Cu and Ga (i.e., exchangeable, adsorbed, organic-bound, and silicate-hosted), for which isotopic compositions were separately determined in addition to analyses of the bulk sample.Relative to bedrock (δ65/63Cu = +0.07 ± 0.04‰ relative to NIST 976), soil samples are both enriched and depleted in the heavy isotopes of Cu (δ65/63Cubulk = −0.29 to +0.24‰). A positive correlation (r = +0.74) of δ65/63Cubulk with τCu,Th [= [(CCu/CTh)sample / (CCu/CTh)parent – 1] × 100%] reflects preferential leaching of heavier Cu isotopes out of the weathering profile. Lighter Cu isotopes were retained in the silicate-hosted fraction (δ65/63Cusil = −0.84 to −0.11‰), and heavy Cu isotopes are enriched in the adsorbed fraction (δ65/63Cuads = +0.27 to +1.59‰) relative to the exchangeable fraction (δ65/63Cuexch = +0.06 to +0.44‰), which consist mainly of highly crystalline and poorly crystalline Fe-hydroxides, respectively. Relative to bedrock (δ71/69Ga = 0.00 ± 0.05‰ relative to the Ga-IPGP standard), soil samples are slightly enriched in the heavy isotopes of Ga (δ71/69Gabulk = +0.01 to +0.20‰). In contrast to Cu, weathering preferentially releases the light isotopes of Ga, yielding higher δ71/69Ga values for the exchangeable (+0.54 to +1.18‰) and adsorbed fractions (−0.01 to +0.55‰). Reactions with Fe-hydroxides are the dominant control on Cu and Ga isotopic variation, with preferential uptake of heavier isotopes during adsorption and release of lighter isotopes during desorption. Ga substitution for Fe in tetrahedral and octahedral sites led to the preferential release of isotopically lighter Ga from primary minerals. These results advance our understanding of the isotopic fractionation mechanisms of Cu and Ga during chemical weathering and the potential utility of these isotopic systems as paleo-weathering proxies.