Strengths and Weaknesses of the Analytical Techniques Used for Measuring Low Mercury Mass Fractions (< 10 ng g-1) in Crystalline Rocks: Direct Mercury Analyser Versus Cold Vapour-Atomic Fluorescence Spectroscopy

Elemental mercury (Hg) is routinely determined in crystalline rocks with mass fractions lower than 10 ng g^-1 by thermal decomposition using Direct Mercury Analyzer (DMA-80) or Lumex RA-915+ (equipped with a PYRO-915+ attachment) instruments, both based on atomic absorption spectroscopy. However, 223 analyses over the course of one year with DMA-80 and cold vapour-atomic fluorescence spectroscopy (CV-AFS) on three reference materials (RMs) and six crystalline rocks (granite, diorite, gabbro, spinel peridotite, phlogopite-rich peridotite, and sulfide-rich orthogneiss) from the exposed transcrustal section of the Ivrea-Verbano Zone and upper crustal Serie dei Laghi unit (western Alps, Italy) reveal that rock analyses using the DMA-80 are variably affected by different internal and external biases when Hg mass fractions are below 10 ng g^-1. Conversely, CV-AFS analyses are more precise, providing homogenous and repeatable results, even at ultra-low Hg mass fractions (< 1 pg g^-1). Furthermore, CV-AFS analyses show that gabbro and spinel peridotite powders roasted for analysis by DMA-80 still contain ~ 0.6 to ~ 1.4 ng g^-1 of Hg, implying inefficient release of Hg from basic/ultrabasic lithologies. Therefore, we recommend the use of CV-AFS for Hg measurements in crystalline rocks. We also propose a new Hg reference value of 3.9 ± 1.5 ng g^-1 for the GSJ granodiorite reference material JG-1a.