The evolution of MORB and plume mantle volatile budgets: C onstraints from fission X e isotopes in S outhwest I ndian R idge basalts

We present high‐precision measurements of the fission isotopes of xenon (Xe) in basalts from the Southwest Indian Ridge (SWIR) between 16°E and 25°E. Corrections for syn‐ to post‐eruptive atmospheric contamination yield the Xe isotopic compositions of SWIR mantle sources. We solve for the proportions of mantle Xe derived from the primordial mantle Xe budget, recycling of atmospheric Xe, decay of short‐lived 129 I, fission of extinct 244 Pu, and fission of extant 238 U. Xe isotope systematics evident in SWIR basalts and other mantle‐derived samples provide new insights into the integrated history of mantle source degassing and regassing. We find that recycled atmospheric Xe dominates the Xe inventories of the SWIR Western and Eastern Orthogonal Supersegment mantle sources (∼80–90% of 132 Xe is recycled in origin), consistent with results from studies of plume‐influenced basalts from Iceland and the Rochambeau Rift. While significant regassing of the mantle is evident, we also find differences in the extent of degassing of the MORB and plume sources. MORB sources are consistently characterized by a lower fraction of fission Xe derived from Pu‐fission, indicating a greater extent of degassing relative to the plume source. The prevalence of recycled atmospheric Xe in mantle sources indicates incorporation of depleted recycled material even into mantle sources with primitive He and Ne isotopic compositions. Consequently, depleted lithophile isotopic compositions in mantle sources with primitive He and Ne cannot be interpreted as evidence for a nonchondritic bulk silicate Earth.