Uncovering the xenon isotope composition of continental rift magmas: Insight from analysis of geothermal gases at Homa Hills, Kenya

Abstract

We investigated geothermal gases from Homa Hills, a carbonatitic complex situated along an adjacent branch of the Kenyan rift system, using neon, argon, krypton, xenon and nitrogen isotopes. Large quantities of gas were sampled in Giggenbach-type bottles (Giggenbach, 1975) and analyzed by dynamic mass spectrometry to resolve isotopic variations at high precision (0.01-0.1‰; Seltzer and Bekaert, 2022; Bekaert et al., 2023; 2024). Neon and nitrogen isotope compositions are consistent with parental magmas being derived from the convecting mantle. Xenon isotopic data present ubiquitous enrichments (relative to air) of ¹²⁹Xe from the decay of extinct ¹²⁹I (T_1/2 = 15.7 Myr) and ^131-136Xe_f from fissions of ²³⁸U (T_1/2 = 4.468 Myr) and/or ²⁴⁴Pu (T_1/2 = 82 Myr). We also find slight excesses of ¹²⁸Xe (relative to ¹³⁰Xe and air), which could be due to subsurface isotopic fractionation during e.g., diffusive transport fractionation (DTF) and gravitational settling. However, the ¹²⁸Xe excesses are not accompanied by correlated Kr isotope excesses and plot off the empirical fractionation line defined from several other locations worldwide (Bekaert et al., 2023). Instead, a detailed isotope deconvolution suggests the occurrence of either chondritic Xe (with mantle ¹³⁰Xe consisting of up to 22 % of chondritic ¹³⁰Xe) or recycled Xe from the Archean atmosphere could explain the observed Xe isotope signatures. The latter possibility would have profound implications for models of mantle-surface exchange throughout Earth history.

The fission spectra indicate a predominantly ²³⁸U origin for fissiogenic Xe, with contribution of ²⁴⁴Pu-derived Xe being negligible within uncertainties, implying extensive mantle degassing during the Hadean and Archean eons. The ¹²⁹Xe*/¹³⁶Xe* ratio (where * indicates non-atmospheric excesses of Xe isotopes) of Homa Hills samples correlates with other tracers of mantle/crust contributions such as He, Ar and N isotopes. Variations in ¹²⁹Xe*/¹³⁶Xe* among the different gases sampled at Homa Hills is mainly the result of contribution from fissiogenic Xe produced in uranium-rich crustal material. Therefore, this ratio may constitute a robust tracer of mantle-crust interactions. Given available high precision data (Bekaert et al., 2023; 2024; this work) together with mantle-derived rock data, ¹²⁹Xe*/¹³⁶Xe* appears homogenous in the convecting mantle, and comparable to values observed at mantle plumes. Such homogeneity is in sharp contrast with light noble gas systematics and may call for whole mantle convection and a core origin for He and Ne..