Ultra-high precision analysis of mantle-derived noble gases by dynamic mass spectrometry: analytical developments.

The original discovery of "excess" 3He in mantle-derived rocks1 constituted the first evidence that primordial volatiles are still escaping from Earths interior. A wealth of research over subsequent decades has demonstrated the power of noble gas geochemistry in documenting Earths accretion history and geodynamical evolution2. Incremental improvements in sampling procedures, analytical methods, as well as accuracy and precision of static vacuum mass spectrometers have provided significant insights into the origin and evolution of terrestrial volatiles [3-5]. However, some fundamental questions remain unanswered regarding the timing and heterogeneous nature of Earth's volatile accretion2,6 due to inherent limitations in the analytical capability of static vacuum mass spectrometers (at about the permil level, ). Over the last several decades, the development of noble gas analysis by dynamic mass spectrometry has enabled orders of magnitude better precision (down to ~0.01 for stable Ar, Kr, and Xe isotopes7) versus static vacuum mass spectrometry, on noble gases in groundwater and ice core samples7-9. The key to this approach is the continuous toggling between sample and reference gas, which requires large amounts of sample gas to be available for analysis. Here, we present recent analytical developments aimed at measuring large amounts of mantle-derived noble gases (Ar,Kr,Xe) sampled in Giggenbach bottles8 by dynamic mass spectrometry, in the Seltzer Lab recently set up at WHOI. We present the capability, challenges, and potential limitations of this approach, and discuss preliminary data obtained for magmatic gas samples collected in a number of diverse geological settings. We show that ultra-high precision measurements not only give unprecedented access to mantle-derived signatures, but also provide a wealth of information regarding the physical processes occurring during magmatic CO2 bubbling through hydrogeological systems, including long-term distillation effects of noble gas stripping from groundwater that was originally near saturation with atmospheric air. [1] Lupton & Craig 1975 [2] Mukhopadhyay & Parai 2019 [3] Holland & Ballentine 2006 [4] Mukhopadhyay 2012 [5] Péron & Moreira 2018 [6] Bekaert et al. 2019 [7] Seltzer et al. 2019 [8] Sowers et al. 1989 [9] Severinghaus et al. 2003.