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

This study investigates the behavior of Mo and Mo isotopes (δ 98 Mo) in shales following leaching with HCl and HNO 3 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 HNO 3 , the Mo concentration ([Mo]) in the leachate was lower and δ 98 Mo was heavier than that obtained from bulk digestion. A Mo isotope mass-balance model showed that the δ 98 Mo in the residues was lighter than the δ 98 Mo 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.