首页|Copper and gallium isotopic behavior in highly weathered soils
Copper and gallium isotopic behavior in highly weathered soils
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NSTL
Elsevier
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 (delta Cu-65/63 = +0.07 +/- 0.04 parts per thousand relative to NIST 976), soil samples are both enriched and depleted in the heavy isotopes of Cu (delta Cu-65/63(bulk) = -0.29 to +0.24 parts per thousand). A positive correlation (r = +0.74) of delta Cu-65/63(bulk) with tau(Cu,Th) [= [(C-Cu/C-Th)sample / (C-Cu/C-Th)(parent) - 1] x 100%] reflects preferential leaching of heavier Cu isotopes out of the weathering profile. Lighter Cu isotopes were retained in the silicate-hosted fraction (delta Cu-65/63(sil) = -0.84 to -0.11 parts per thousand), and heavy Cu isotopes are enriched in the adsorbed fraction (delta Cu-65/63(ads) = +0.27 to +1.59 parts per thousand) relative to the exchangeable fraction (delta Cu-65/63(exch) = +0.06 to +0.44 parts per thousand), which consist mainly of highly crystalline and poorly crystalline Fe-hydroxides, respectively. Relative to bedrock (delta Ga-71/69 = 0.00 +/- 0.05 parts per thousand relative to the Ga-IPGP standard), soil samples are slightly enriched in the heavy isotopes of Ga (delta Ga-71/69(bulk) = +0.01 to +0.20 parts per thousand). In contrast to Cu, weathering preferentially releases the light isotopes of Ga, yielding higher delta Ga-71/69 values for the exchangeable (+0.54 to +1.18 parts per thousand) and adsorbed fractions (-0.01 to +0.55 parts per thousand). 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.
NSTL
Elsevier
