Nickel isotope fractionation during tropical weathering of ultramafic rocks

Abstract

International audienceAlthough 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, Goias, 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 (delta Ni-60) from the natural Ni geochemical cycle include: bedrock samples (delta Ni-60 = 0.28 +/- 0.08%), ore samples (saprolitic and lateritic, delta Ni-60 from - 0.60 to 0.30%) and soil samples (delta Ni-60 from - 0.19 to - 0.02%). An overall trend of heavier isotope depletion was observed in the solid phase during weathering (Delta Ni-60(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 (Delta Ni-60(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

Rock-type control of Ni, Cr, and Co phytoavailability in ultramafic soils

“…Δ 60 Ni soil-rock for A horizon is -0.22 ‰. This trend is previously reported in Albania (Δ 60 Ni soil-rock is up to -0.63 ‰) and at Barro Alto (Estrade et al 2015;Ratié et al 2015). The Ni isotope composition of phytoavailable fraction estimated using the DTPA-CaCl 2 solution shows that heavier Ni isotope pool is released during weathering of ultramafic rocks.…”

“…4; Gueguen et al 2013;Estrade et al 2015). The slight differences in Ni isotopic composition of parent rocks from various ultramafic massifs can be explained by variabilities of parent magma and degree of serpentinization causing a possible isotopic fractionation (Ratié et al 2015;Estrade et al 2015). In the last studies, Gall et al (2017) have suggested that serpentinization probably does not affect δ 60 Ni.…”

“…On the other hand, it is possible that during 2 h of the extraction the entire pool of Fig. 4 Comparison of Ni isotope composition between soils and rocks in this study with samples from other localities (Gall et al 2013;Gueguen et al 2013;Ratié et al 2015;Estrade et al 2015) phytoavailable Ni is not extracted because of diffusion controlled processes that may affect Ni isotopic signature.…”

“…Indonesia, Van der Ent et al 2013b). Very recently, several studies dealing with Ni isotopes composition of ultramafic rocks, soils derived from them and plants have been published (Ratié et al 2015;Estrade et al 2015;Ratié et al 2016). The range of δ…”

Geochemistry of Ophiolite Complex in North Konawe, Southeast Sulawesi

“…Various studies had conducted related to heavy metal comparison in ultramafic host rocks to the laterization product [5,16]. Equal Ni concentration in the selected samples range from 2,380 to 3,050 ppm, Mn from 959 to 1,260 ppm, Cr from 2,040 to 2,910 ppm, and Co >100 ppm.…”