Xenon isotope composition of the La Palma mantle plume (Canary Islands): assessing upper mantle versus plume mantle source evolution

Determining the xenon (Xe) isotope composition of Earth's deep-mantle reservoirs is key to constraining early terrestrial volatile acquisition and subsequent evolution. Here, we employ dynamic mass spectrometry to characterize the Xe isotope composition of the Dos Aguas CO 2 -rich, cold mineral spring (La Palma, Canary Islands), previously shown to exhibit the highest 3 He/ 4 He reported from the Canarian archipelago (10.85 Ra). Although the plume mantle-derived Xe contribution is small (≤0.8%), its signature matches that of the convecting upper mantle (e.g., δ 129 Xe*/δ 136 Xe* ≈ 1; i.e., with comparable 129 Xe* and 136 Xe* excesses relative to air). This result aligns with the growing evidence that plume mantle- and upper mantle-derived sources are colinear in δ 129 Xe vs. δ 136 Xe and δ 129 Xe*/δ 136 Xe* vs. δ 130 Xe*/ 136 Xe* spaces. For a decade, however, the latter space was considered a powerful means to discriminate between both mantle sources. Here, we use numerical modelling of mantle Xe evolution to answer a simple question: could the distinct evolutions of the highly degassed, convecting upper mantle (primary source of fissiogenic 136 Xe*: extant 238 U) and the primitive, minimally degassed plume mantle (primary source of 136 Xe*: extinct 244 Pu) have coincidentally converged towards a unique δ 129 Xe*/δ 136 Xe* ≈ 1. Our model shows that, despite a two orders of magnitude difference in the total extents of degassing, such a coincidence cannot be discarded. An alternative scenario would be that deep, primitive plume mantle sources akin to La Palma may also be dominated by extant 238 U fission rather than extinct 244 Pu, potentially requiring deep mantle sources to be far more degassed than previously considered.