NI ISOTOPE FRACTIONATION BY WEATHERING PROCESSES: IMPLICATIONS FOR THE NI ISOTOPE COMPOSITION OF THE OCEANS

The cycling of trace metals in the oceans has important implications for the evolution of life on Earth. Metal isotopes are a powerful tool for investigating the processes that control the biogeochemical cycles of trace metals in the oceans. The biogeochemical cycle of Ni is of interest because of the importance of this nutrient for methanogens, which may have been a key component of the biosphere on early Earth before the oxygenation of the atmosphere at ~2.4 Ga.

The release of Ni into surface and ground waters by rock weathering is one of the dominant sources of Ni to the oceans. Here we investigate Ni isotopes in deeply weathered ultramafic rocks in order to better understand the effects of the weathering processes on the isotopic composition of Ni delivered to the oceans. During deep weathering of ultramafic rocks Ni is leached from primary minerals and mobilized downward to where it is adsorbed to and precipitated in secondary minerals (laterites). Compared to the unweathered ultramafic host rocks (δ^60/58Ni ≈ 0.2‰), a sample of oxidized laterite has light Ni isotopes with a δ^60/58Ni of -0.12‰, similar to the value reported in a recent study (Gall et al. 2013). Experiments in our laboratory show that isotopically light Ni adsorbs to ferrihydrite from solution, with an offset in δ^60/58 of ~0.3‰, close to the difference in δ^60/58Ni between the host rocks and the oxidized laterite. Therefore the isotope composition of oxidized laterite likely reflects the retention of light Ni mobilized from the host rocks into residual Fe oxides.

Dissolved Ni not retained in Fe oxides moves further down the profile where some of it precipitates as Ni-Mg silicate ores. All Ni-Mg silicates measured have heavy Ni isotope compositions δ^60/58Ni (~0.3-0.9‰). These Ni-Mg silicates represent the fluid mobile Ni in the weathering system and have isotopic compositions that fall within the range measured for river water (Cameron and Vance 2013). We hypothesize that adsorption processes and retention of light Ni isotopes in weathering profiles exert a dominant control on the isotope composition of dissolved Ni in surface and ground waters and contribute to the heavy Ni isotope composition of the oceans and oceanic sediments (Cameron and Vance 2013; Gall et al. 2013).

Cameron and Vance (2013) Mineralogical Magazine 77, 811

Gall et al. (2013) EPSL, in press