Application of noble gas isotopic systems to identify mantle heterogeneities

Abstract

This study addresses the origin of the combined He-Ne-Ar-Xe noble gas isotopic variation in mantle derived oceanic basalts. High precision heavy noble gas compositions of basalts from the Rochambeau Rift along the Northwestern Lau Basin, the Gakkel Ridge, the Southeast Indian Ridge and the South Atlantic Ridge presented here confirms fundamental differences between the volatile constituents of the depleted mid-ocean ridge basalt (MORB) source and plume sources. We find that the depleted MORB mantle is characterized by systematically lower proportions of Pu-fission derived Xe than the Iceland plume and the Samoan-like plume sampled at the Rochambeau Rift. These two plume sources are associated with low radiogenic \(^{129}Xe/^{130}Xe\) ratios that are not attributable to recycled atmosphere and must sample ancient (4.45 Ga) volatile rich mantle reservoir that experienced a lower degree of mantle processing than the depleted MORB source. I report \(^{128}Xe/^{130}Xe\) ratios in excess with respect to the atmosphere in deeply erupting Gakkel lavas, similar to mantle derived volatiles in natural gas samples. Deconvolution of fission derived xenon isotopes indicate that natural gas samples and the Gakkel MORBs are derived from the same volatile depleted mantle reservoir. The radiogenic He isotopic composition, the low estimated ratio of Pu to U derived xenon isotopes, and the low \(^{129}Xe/^{130}Xe_E\) ratio found in the source of the West Volcanic Zone (WVZ) along the Gakkel Ridge indicates the presence of increased amounts of recycled atmospheric volatiles. I suggest that the low \(^{129}Xe/^{130}Xe\) ratio beneath the WVZ cannot be explained by sampling an ancient (>4.45 Ga) lithospheric mantle component that evolved in isolation from convection throughout Earth's history. Instead, the Xe isotopic composition may be consistent with the presence of subduction derived metasomatizing fluids with atmospheric heavy noble gases trapped in the sub-continental lithospheric mantle sampled along the ridge. Available Ne isotope MORB data and new high precision Ne data presented in this study (including basalts from the East Pacific Rise, the Galapagos Spreading Center, and the Juan de Fuca Ridge) allows me to construct a combined He-Ne isotope distribution and infer first order source mixing systematics in the upper mantle.