An alternative isochron method for measuring cosmogenic ^3He in lava flows

Abstract

This study evaluates an alternative isochron method designed to improve the precision and the accuracy of the measurement of cosmogenic 3He (3Hec) and the calibration of 3Hec production rates in lava flows. While the accuracy of the standard two-steps routine ((i) crushing, (ii) melting) may be affected by possible crushing-induced release of matrix-sited 3Hec, this new isochron technique does not involve the preliminary crushing step. Such advance is thus potentially able to improve the accuracy of 3Hec determination. Moreover this method has the advantage to yield cosmogenic 3He concentrations using only one type of extraction technique. Cosmogenic 3He isochrons can be built successfully measuring the 3He and 4He concentrations of several phenocrysts replicates fused in vacuo, provided that these aliquots have identical magmatic 3He/4He ratios, identical 3Hec concentrations and variable magmatic helium contents. When plotted in the 3He/4He vs 1/4He space, such a suite of data define a straight line whose intercept yield the magmatic (3He/4He)mag ratio, and whose slope is the cosmogenic 3He concentration. The frequent addition of radiogenic 4He (4Helow asterisk) may be easily accounted taking advantage of a property that is characteristic of non-eroded lava flows: the 3Hec/4Helow asterisk ratio is not time-dependent. This radiogenic correction of the isochron slope involves the R-factor, whose calculation requires that the U and Th abundances in the phenocrysts and the host lava be known, as well as the diameter and the shape of the analyzed minerals. This correction also requires that all the analyzed aliquots have identical radiogenic 4He concentration. The isochron method was tested on surface samples of three Quaternary basalt flows from Mount Etna and Mauna Loa (aged of 6 ±1, 8.2 ± 0.1 and 41 ± 3 ka). The obtained data set yielded statistically robust isochrons and cosmogenic 3He concentrations that are not significantly different from those obtained with the standard method. Importantly, for samples with high magmatic/cosmogenic 3He ratio, the isochron technique offers both better accuracy and precision than the standard two-steps method. This alternative technique should thus open new perspectives for the particularly important task of 3Hec production rate calibrations from lava flow surfaces.

Additional details