SYSTEMATIC ELEMENTAL FRACTIONATION OF MANTLE-DERIVED HELIUM, NEON, AND ARGON IN MID-OCEANIC RIDGE GLASSES

Abstract

Based on the proposition that the Earth contains solar-like, nucleogenic and atmosphere-derived neon components, we calculate the amounts of solar-derived 22Nes and nucleogenic 21Ne∗ present in a sample. The amounts of 22Nes and 21Ne* can then be compared with the amounts of primordial 3He and radiogenic 4He. The 3He/22Nes and 4He/21Ne* ratios observed in basaltic glasses from mid-oceanic ridges (MORBs) vary by almost 2 orders of magnitude and define a linear correlation with a slope of unity which passes through the point defined by the mean primordial 3He/22Ne ratio in the Earth (= 7.7) and the radiogenic 4He to nucleogenic 21Ne production ratio (= 2.2 × 107). This indicates that there has been significant recent elemental fractionation which has enriched MORB glasses in both primordial 3He and radiogenic 4He with respect to mantle neon. A similar enrichment of helium in MORB glasses is observed relative to mantle-derived 40Ar*.