Nickel isotope fractionation during tropical weathering of ultramafic rocks. | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  Nickel isotope fractionation during tropical weathering of ultramafic rocks.

Type de publication:

Journal Article

Source:

Volume 402, p.68 (Soumis)

ISBN:

00092541

URL:

http://search.ebscohost.com/login.aspx?direct=true&site=eds-live&db=a9h&AN=102455042

Mots-clés:

GEOCHEMICAL cycles, NICKEL isotopes, SHIELDS (Geology), ULTRABASIC rocks, WEATHERING

Résumé:

Although Ni isotopes have been shown to be significantly fractionated in terrestrial samples, their use in continental environmental studies has not yet been evaluated. The present study focuses on an ultramafic (UM) massif (Barro Alto, Goiás, Brazil) because such areas are naturally rich in Ni. We present developed lateritic weathering profiles. The goal of the study is to evaluate the potential of using Ni isotopes in environmental continental studies by combining its isotopic signature with mineralogy, in order to better understand the geochemical cycling of Ni in UM settings during weathering. As such, Ni isotope values were measured in samples from the Barro Alto UM complex in the main stages of the lateritic weathering profile of UM rocks, including bedrock, ores (saprolitic and lateritic samples) and soil. The mineralogical composition of the samples, with a focus on the different Ni-bearing minerals, was also determined to decipher the potential links between isotopic fractionation and weathering dynamics. Isotopic signatures (δ 60 Ni) from the natural Ni geochemical cycle include: bedrock samples (δ 60 Ni = 0.28 ± 0.08‰), ore samples (saprolitic and lateritic, δ 60 Ni from − 0.60 to 0.30‰) and soil samples (δ 60 Ni from − 0.19 to − 0.02‰). An overall trend of heavier isotope depletion was observed in the solid phase during weathering (Δ 60 Ni Soil–Bedrock = − 0.47‰). The mineralogical results were consistent with the literature and showed that the mineralogy of the lateritic part and soil was dominated by Fe-oxides, whereas clay minerals were the primary Ni phase scavengers in the saprolitic part of the profile. Thus, the formation of Ni-bearing clay minerals and Fe-oxides appeared to lead to depletion in heavier isotopes, which indicates preferential export of heavy isotopes in the dissolved phase. This result is consistent with isotopic signatures measured in the exchangeable pool of the solid phase (Δ 60 Ni exch-total up to 0.47‰), and Ni isotopes appear to be a promising tracer to better understand the biogeochemical Ni cycling on the Earth's surface. [ABSTRACT FROM AUTHOR]<i>Copyright of Chemical Geology is the property of Elsevier Science and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)