Ultrahigh-precision noble gas isotope analyses reveal pervasive subsurface fractionation in hydrothermal systems

Abstract

Mantle-derived noble gases in volcanic gases are powerful tracers of terrestrial volatile evolution, as they contain mixtures of both primordial (from Earth’s accretion) and secondary (e.g., radiogenic) isotope signals that characterize the composition of deep Earth. However, volcanic gases emitted through subaerial hydrothermal systems also contain contributions from shallow reservoirs (groundwater, crust, atmosphere). Deconvolving deep and shallow source signals is critical for robust interpretations of mantle-derived signals. Here, we use a novel dynamic mass spectrometry technique to measure argon, krypton, and xenon isotopes in volcanic gas with ultrahigh precision. Data from Iceland, Germany, United States (Yellowstone, Salton Sea), Costa Rica, and Chile show that subsurface isotope fractionation within hydrothermal systems is a globally pervasive and previously unrecognized process causing substantial nonradiogenic Ar-Kr-Xe isotope variations. Quantitatively accounting for this process is vital for accurately interpreting mantle-derived volatile (e.g., noble gas and nitrogen) signals, with profound implications for our understanding of terrestrial volatile evolution.

Primordial noble gas isotopes from immoderate crushing of an Icelandic basalt glass

“…Small symbols are individual data, while larger symbols are averages. (a) Mega-crush and regular crush step 136 Xe/ 130 Xe vs. 129 Xe/ 130 Xe data (1σ error bars) are consistent with prior Xe measurements in Iceland samples(Mukhopadhyay, 2012;Péron et al, 2021) and plume-influenced samples from Rochambeau Rift (Samoan plume), Galápagos, and Yellowstone(Pet ő et al, 2013;Broadley et al, 2020;Bekaert et al, 2023). (b) The error weighted average of mega-crush step 128…”

“…Cite this letter as: Xe/ 130 Xe vs. 129 Xe/ 130 Xe data (1σ error bars), along with average or most mantle-like compositions from plume and upper mantle samples (Péron and Moreira, 2018;Caffee et al, 1999;Holland and Ballentine, 2006;Bekaert et al, 2023;see Fig. S-6 for details).…”

“…Fits forced through atmosphere and a mixing line between the MiðfellRP09 average and atmosphere are shown. The slope of the plume fit is strongly affected by the precisely determined Yellowstone 4B average, which may reflect some mass dependent fractionation in the hydrothermal system (Bekaert et al, 2023). While individual measurements for DG2017 (Péron et al, 2021) are shown, only the average was used to compute the best plume slope and its uncertainty.…”

“…However, wide coverage of upper mantle and ocean island heterogeneity is yet to be achieved. Furthermore, mantle compositions for Kr and the rarest Xe isotopes ( 124 Xe, 126 Xe, and 128 Xe) are limited to unusually gas-rich basalt samples (Moreira et al, 1998), continental well gases (Caffee et al, 1999;Holland and Ballentine, 2006;Caracausi et al, 2016;Bekaert et al, 2019) and volcanic gases (Broadley et al, 2020;Bekaert et al, 2023), where large quantities of gas are available for analysis.…”