Assessment of Different Digestion Procedures for Mo Isotope Measurements of Black and Grey Shales Using the Double Spike Technique

Abstract

This study investigates the behavior of Mo and Mo isotopes (δ⁹⁸Mo) in shales following leaching with HCl and HNO₃ with the aim of simplifying the shale dissolution procedure. Up to 6% of the Mo was lost and the Mo isotopes were unaffected when shales were leached using 9 M HCl after ashing. Bulk sample digestion or leaching by 4 M or more concentrated HCl after ashing were all found to be acceptable and reliable approaches to the analysis of Mo isotopes in shales. After black shale (CAGS-BS) was leached with 2 M HCl, 1 M HCl, and 9 M HNO₃, the Mo concentration ([Mo]) in the leachate was lower and δ⁹⁸Mo was heavier than that obtained from bulk digestion. A Mo isotope mass-balance model showed that the δ⁹⁸Mo in the residues was lighter than the δ⁹⁸Mo from the bulk digestion of CAGS-BS and of crustal igneous rocks. No more Mo was lost, nor did Mo isotope fractionation, if the double spike was added before rather than after ashing and followed by bulk digestion or leaching with 9 M HCl. For efficiency, leaching using 4 M or more concentrated HCl after ashing is preferred for Mo isotope measurements.

Keywords

Mo isotopes / double spike / ashing / bulk digestion / leaching

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