Magnesium isotopes record the origin of stratiform chromitite

Description

(1) First report of whole-rock Mg isotopes spanning the stratigraphy of the Stillwater; (2) Homogeneous and light Mg isotopic compositions in samples of Banded Series; (3) Lighter Mg isotopes in chromitite seams and heavy Mg isotopes in surrounding silicate cumulates of Ultramafic Series; (4) Settle down the debate on “chromite-silicate cotectic model” or “solely chromite saturated model” for stratiform chromitite origin.

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Sample digestion and chemical separation The procedures for sample dissolution, column chemistry, and instrumental analysis closely follow those reported in previous studies (e.g., Teng et al., 2010, 2015; Ke et al., 2016), and are briefly summarized here. Silicate cumulates were crushed to ~200 mesh and dissolved overnight in a mixture of HF, HCl, and HNO3 at 130 °C in capped Savillex beakers. Chromitite samples were ground to a very fine powder and dissolved in concentrated 3:1 HF-HNO3 in a microwave oven, with temperatures increasing to 180, 210, 220, and 225 °C at half-hour intervals. All solutions were dried on a hot plate and sequentially treated by aqua regia and concentrated HNO3. The samples were then dried and re-dissolved in 1N HNO3 in preparation for chromatographic separation. Mg was purified by cation exchange chromatography with Bio-Rad AG50W-X8 (200-400 mesh) resin at the Isotope Geochemistry Lab, China University of Geosciences (Beijing), following established procedures (e.g., Dauphas et al., 2010; Teng et al., 2010, 2015; Xiao et al., 2016; Ke et al., 2016). The purification procedure was performed twice to ensure the purity of sample Mg obtained. Routine recovery was >99%, and the whole procedure blank was monitored to be < 10 ng, which is negligible compared to the ~20 μg of Mg processed. Mass spectrometry The Mg isotopic measurements were performed on Thermo Finnigan Neptune Plus multi-collector inductively coupled plasma mass spectrometers using the sample-standard bracketing method. Each measurement consisted of a 3-s idle time and 40 cycles of 4.194 s integration times. A 3% HNO3 blank was measured at the beginning of each analytical session for on-peak-zero correction. Isotopic data are expressed in standard δ notation as per-mil deviations relative to the DSM-3 standards as: δ26Mg [‰] = [(26Mg/24Mg)sample / (26Mg/24Mg)DSM-3 – 1] × 1000. To enhance the reproducibility and accuracy of the measurements, the standard-sample sequences were repeated four times. The isotopic compositions reported herein are the average values of the repeated analyses, and uncertainties are reported as two standard deviations (2SD) for δ26Mg. Two international rocky standards, BHVO-2 and JP-1, were processed along with unknowns to monitor accuracy throughout the column chemistry and isotopic analyses (Measured by the critical mixture double spike technique; He et al., 2022), and yielded δ26Mg values (BHVO-2: δ26Mg = -0.19 ± 0.04‰ (2SD); JP-1: δ26Mg = -0.23 ± 0.13‰ (2SD)) consistent with their recommended values (BHVO-2: δ26Mg = -0.20 ± 0.07‰ (2SD); JP-1: δ26Mg = -0.25 ± 0.02‰ (2SD) within quoted errors (Teng et al., 2015; Ke et al., 2016).

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