[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-03] High-precision stable isotope measurements of W and Mo in single sample aliquots using mixed double spikes

*Teruhiko Kashiwabara¹, Yusuke Fukami², Ayako Watakabe¹, Minako Kurisu¹, Satoshi Tokeshi³, Tsuyoshi Iizuka⁴, Katsuhiko Suzuki¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Gakushuin University, 3.Marine Works Japan Ltd, 4.The University of Tokyo)

Keywords:tungsten, molybdenum, stable isotope, double spike method

Mass-dependent fractionation of W isotopes has been an emerging tool in recent years that can potentially constrain a wide range of geochemical processes in the crust/mantle systems and low temperature environments. In particular, this isotope system is expected to improve our understanding of the Earth system in combination with the Mo isotopes, which has successfully been established in the past two decades as a proxy, particularly for the oxygenation of Earth’s ocean and atmosphere, but also still been expanding its applications to the solid earth and other areas including ore deposit, oil, and anthropogenic tracing. Here, we establish a combined double-spike (DS) methods for W (¹⁸⁰W-¹⁸⁴W spike) and Mo (⁹⁷Mo-¹⁰⁰Mo spike) to perform simple, efficient, and robust isotope measurements of these two chemically analogous elements in single sample aliquots, which could be a robust basis for the development of the stable isotope geochemistry of W and Mo.
Based on the previous column chemistry, we optimized two-stage anion-exchange procedures to remove matrix elements, particularly the critical interferences of Ta and Hf on ¹⁸⁰W, and to collect sharply separated W and Mo fractions. The obtained recoveries are quantitative for both elements, and their purities are sufficiently high to achieve high-precision measurements comparable to previous DS measurements of individual elements. The reproducibility of our isotope measurements for in-house standard solutions (2SD) were ±0.02‰ for δ¹⁸⁶W and ±0.03‰ for δ⁹⁸Mo. We applied our method to 27 geochemical reference materials including 10 igneous rocks (AGV-2, JA-3, JR-1, JB-1, JB-1a, JB-2, JB-3, W-2a, TDB-1, WGB-1), 9 sediments (Nod-A-1, Nod-P-1, JMn-1; JMS-1, JMS-2, CRM7302-a, HISS-1, MESS-4, PAC-3), and 8 sedimentary and metasedimentary rocks (SDC-1, SDO-1, SBC-1, SCO-1, SCO-2; JSL-1, JSL-2, IOC-1) to produce a comprehensive data set. The data set confirmed the accuracy of our measurements and expands the reference materials available for interlaboratory comparisons of δ¹⁸⁶W and δ⁹⁸Mo. The data set also indicates potential pitfalls in sample preparations for particular sample types, and shows several variations of W and Mo isotopes possibly related to low-/high-temperature geochemical processes. In the presentation, we will also discuss potential applications of the combined W and Mo isotope analyses and further development of our isotope method to establish an oxybarometer of the water mass in the past.