Chromium isotope fractionation during subduction-related metamorphism, black shale weathering, and hydrothermal alteration

Abstract

ABSTRACT. Chromium (Cr) isotopes are an emerging proxy for redox processes at Earth's surface. 16 However, many geological reservoirs and isotope fractionation processes are still not well 17 understood. The purpose of this contribution is to move forward our understanding of (1) Earth's 18 high temperature Cr isotope inventory and (2)

High‐Precision Measurement of Stable Cr Isotopes in Geological Reference Materials by a Double‐Spike TIMS Method

“…In this study, two basalt reference materials from the USGS, BHVO‐1 (−0.120 ± 0.029‰, 2 s ) and BHVO‐2 (−0.129 ± 0.032‰, 2 s ), gave slightly lower δ 53/52 Cr NIST SRM 979 values than that of peridotite reference material JP‐1 from JSG (−0.088 ± 0.034‰; 2 s ) with the mean results over a period of two years. The data obtained in this study for these basalt and peridotite reference materials are consistent with the δ 53/52 Cr NIST SRM 979 values reported by previous studies within the precision quoted (Schoenberg et al , , Wang et al , Li et al , , Gueguen et al , Bonnand et al , b, Wu et al , Xia et al , Zhu et al ) (Figure b).…”

“…Comparison of geological reference material data analysed in this study (red symbols) with literature values (black symbols) (Schoenberg et al , , Gueguen et al . , Wang et al , Li et al , , Bonnand et al , b, Xia et al , Wu et al , Zhu et al ). [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…The 50 Cr‐ 54 Cr double spike is usually used to correct the mass dependence fractionation generated during sample processing as well as by instrumental mass bias effects for the Cr isotope ratio of samples (Ellis and Bullen , Schoenberg et al , Frei et al , Zink et al , Qin et al , Bonnand et al , Farkaš et al , Planavsky et al , Wang et al , Cole et al , Li et al , Xia et al , Zhu et al ). The mass‐dependent isotopic fractionation in the laboratory, from the column chemistry and from isotopic fractionation by TIMS, was corrected by a 50 Cr‐ 54 Cr double‐spike technique.…”

Chromium isotope stratigraphy of Ediacaran cap dolostones, Doushantuo Formation, South China

“…In anoxic deep waters of the Precambrian, hydrothermally-derived Cr(III) would be rapidly hydrolyzed at circumneutral pH (Ball and Nordstrom, 1998) and would consequently precipitate in the vicinity of vents without being dispersed into the global ocean (Crowe et al, submitted for publicatrion). This scenario would render negatively fractionated Cr(III) from hydrothermal vents a less significant source for Cr in the cap dolostones analyzed herein and is consistent with a likely insignificant influence of hydrothermally-derived Cr to Earth's surficial Cr isotope cycle based on mass balance calculations recently reported by Wang et al (2016). Also, a lack of positive (Eu/Eu*) SN anomalies in the studied cap dolostones (Hohl et al, 2015), otherwise typical of subaqueous hydrothermal vent fluids, suggests that mixing of seawater with ocean-ridge derived hydrothermal fluids is a rather unlikely scenario to explain the isotopically light Cr analyzed in the cap dolostones herein.…”

“…oxidized during weathering processes on land (Oze et al, 2007;Izbicki et al, 2008;Frei et al, 2009Frei et al, , 2011Crowe et al, 2013;Frei and Polat, 2013;Berger and Frei, 2014;Paulukat et al, 2015), can be assumed to result from the accumulation of for example weathering-resistant Cr-bearing spinel and/or Cr(III)-bearing silicate minerals (Rai et al, 1989;Oze et al, 2004;Garnier et al, 2008Garnier et al, , 2013Schoenberg et al, 2008;Kelepertzis et al, 2013;Morrison et al, 2015;Wang et al, 2016). Dilution of seawater with freshwater from slowly melting icebergs is indicated for the studied cap dolostones by typical seawater-like REE + Y patterns with generally superchondritic but less than modern marine Y/Ho ratios (Bau and Dulski, 1994;Lawrence and Kamber, 2006), by enhanced ΣREE contents and by a negative correlation of Y/Ho ratios with whole rock Al concentrations (Hohl et al, 2015).…”