Constraints on Noble Gas Variability in OIB and MORB From Non‐Equilibrium Magmatic Degassing Models

Abstract Noble gas concentrations in ocean island basalts (OIB) and mid‐ocean ridge basalts (MORB) are affected by degassing processes, obscuring their pre‐eruptive characteristics. Degassing corrections commonly assume near‐equilibrium partitioning of noble gases between melt and vapor. However, kinetic disequilibrium has been proposed as a mechanism capable of modifying noble gas ratios. For example, kinetic effects may allow OIBs to exhibit lower 3 He contents than MORBs, even if OIB plumes sample a low 4 He/ 3 He “undegassed” primordial lower mantle—the so‐called “Helium Paradox” (Gonnermann & Mukhopadhyay, 2007, https://doi.org/10.1038/nature06240 ). It has also been suggested that kinetic effects allow OIB and MORB to share a homogeneous pre‐degassing 3 He/ 22 Ne ratio. Despite this, inferred mantle compositions may not require the presence of an “undegassed” reservoir, and there is evidence that OIB and MORB sources exhibit heterogenous 3 He/ 22 Ne ratios. Here, we model the kinetics of and noble gas exchange between bubbles and melt within a three‐stage framework for magmatic ascent to evaluate the magnitude of non‐equilibrium fractionation. The first and third stages treat the kinetics of bubble growth during closed‐system decompression, whilst the second treats gas exchange between ascending bubbles and a melt at rest within a shallow, open‐system, sub‐volcanic reservoir. Across plausible ranges of ascent rate, bubble populations, and noble gas diffusivity, our results indicate that disequilibrium cannot produce variations in 3 He contents and 3 He/ 22 Ne ratios sufficient to reproduce the range observed in OIB and MORB. Therefore, OIB and MORB sources cannot share a homogenous 3 He/ 22 Ne ratio, and disequilibrium does not resolve the “Helium Paradox.”