Characterization of the HIMU Mantle Source from Noble Gas Isotopes in Volcanic Gas Emissions in São Tomé (Cameroon Volcanic Line)

The source of volcanism in the Cameroon volcanic line remains unresolved. To better constrain the mantle components involved, we measure light noble gas (He-Ne-Ar) isotopes and, for the first time, heavy noble gas (Kr-Xe) isotopes in mantle-derived gases from São Tomé, using high-precision dynamic mass spectrometry (DMS). Our data show that the source of São Tomé volcanism is a plume-like mantle reservoir, which is enriched in ²²Ne relative to nucleogenic ²¹Ne when compared to samples originating from the convecting upper mantle, as represented by Mid-Ocean Ridge Basalts (MORBs). In contrast, the isotopic compositions of helium and xenon appear similar to those of the MORB mantle. DMS analysis of Xe isotopes reveals that volcanic gases are affected by diffusive transport fractionation in the subsurface, causing small enrichment in the light isotopes (e.g., ¹²⁸Xe). After correction for this secondary fractionation, we observe a slight, yet discernible, ¹³⁶Xe excess relative to the MORB mantle, attributable to spontaneous fission of ²³⁸U. We hypothesize that this excess fissiogenic Xe may arise from the ²³⁸U associated with recycled crustal component(s) in the São Tomé mantle source, potentially related to the influence of the HIMU-type mantle. The consistent ¹²⁹Xe/¹³⁶Xe (relative to air) observed in both MORB and plume-influenced mantle components is difficult to reconcile with the commonly accepted view that ¹³⁶Xe excesses originate from two distinct sources²³⁸U and ²⁴⁴Pu fission, respectivelyin these separate mantle reservoirs. Further investigation is needed to determine whether the apparent homogeneity in ¹²⁹Xe/¹³⁶Xe excess across the mantle, which contrasts with the distinct sources and evolutionary histories of other volatile elements (e.g., He, Ne, and N₂), is coincidental or indicative of an underlying process.