THE DISTRIBUTION OF MANTLE AND ATMOSPHERIC ARGON IN OCEANIC BASALT GLASSES

Abstract

Argon analyses by both high-resolution stepheating and stepcrushing of MORB and Loihi basalt glasses were performed to separate pristine mantle-derived Ar and contaminating atmospheric Ar. In high-vesicularity glasses (> 0.8% vesicles), most of the mantle argon resides in vesicles, from which it is released by crushing or stepheating between 600 and 900 °C. By contrast, in low vesicularity glasses (< permil vesicularity), most mantle argon is dissolved in the glass matrix, as inferred from the correlation with neutron-induced, glass-dissolved argon isotopes (39Ar, 37Ar, 38Ar from K, Ca, Cl). The distribution of mantle Ar between vesicles and glass matrix is well explained by melt-gas equilibrium partitioning at eruption according to Henry’s law, which is compatible with previously determined Henry constants of ∼(5–10) × 10−5ccSTP 40Ar mantle/g bar. Atmospheric Ar is heterogeneously distributed in all samples. Only a very minor part is dissolved in the glass matrix; a significant part correlates with vesicularity and is released by crushing, most probably from a rather small fraction of vesicles or microcracks that equilibrated with unfractionated air. Other carriers of atmospheric argon are pyroxene microlites and minor phases decomposing at intermediate temperatures that were probably contaminated upon eruption by fractionated atmospheric rare gases. Our high-resolution stepheating and stepcrushing analyses of low vesicularity samples with extraordinary high solar-like 20Ne/22Ne indicate successful discrimination of unfractionated air as a contamination source and suggest an upper mantle 40Ar/36Ar of 32,000 ± 4000 and a Hawaiian mantle plume source 40Ar/36Ar ratio close to 8000.