FRACTIONAL EXTRACTION OF HELIUM BY CRUSHING OF OLIVINE AND CLINOPYROXENE PHENOCRYSTS: EFFECTS ON THE 3HE/4HE MEASURED RATIO

Abstract

A critical review is reported of the crushing technique used for the extraction of the gas content from magmatic crystals. The main objective is the evaluation of the validity and accuracy of the method for obtaining data on the helium concentration in the sample and the value of the 3He/4He isotopic ratio; particular attention is given to the analysis of the effects of prolonged crushing. The technique used is that of crushing by steps; the series of experiments reported covers a wide range of conditions representative of the nature and concentration of the helium content in phenocrysts. Data on d[4He]/dt, 3He/4He, [3He], and [4He] are reported for seven experiments on phenocrysts coming from Kauai, Maui, Mangaia (ol and cpx), Tubuai, and Raivavae. The analytical function describing the release by crushing of the gas content in the reservoir hosted by the phenocrysts as a function of time of crushing has been found to be well represented by an exponential form. In some cases the release is described by a linear combination of two exponentials, suggesting the presence of two distinct gas reservoirs in the phenocrysts; the observation of a coherent variation on the isotopic ratio points out in the same direction. The exponential time constants τ1 and τ2, which reflect physical characteristics of the sample examined and are overall determined by the crusher and the crushing procedure, cluster in two distinct groups: τ1 ranging between ~0.5 to ~3 minutes, assumed to characterize the fluid inclusions, and τ2 from ~7 to ~13 minutes for the secondary He reservoir. The results from Maui and Mangaia are examined with some detail to investigate the nature of the second reservoir rich in cosmogenic 3He for Maui and radiogenic 4He for the Mangaia cpx. The result of the research reported confirms that crushing under vacuum is at present the appropriate method to resolve the ''primary'' helium trapped in the fluid inclusions of the phenocrysts from the in situ produced radiogenic and cosmogenic helium, while melting represents the appropriate methodology to obtain a complete extraction of the helium. Protracted crushing of the phenocrysts can eventually release, together with the He of the fluid inclusions also helium contained in secondary inclusions or in the matrix. The gas extracted in later steps is actually a mixture of inclusion-trapped and ''matrix-sited helium'' (see text). To minimize this effect, the crushing phase considered should be limited to the initial steps.