Helium isotope variations in mineral separates from Costa Rica and Nicaragua: Assessing crustal contributions, timescale variations and diffusion-related mechanisms

摘要

We report new He abundance and isotope measurements of phenocryst phases in volcanic tephra and lavas from the Nicaragua-Costa Rica section of the Central American arc, where significant variations in crustal thickness have been inferred. Helium isotope values range from 4.6R(A) to 7.5R(A), with no evidence for crustal thickness influencing measured He-3/He-4 ratios. A comparison of He abundances and isotopes measured in mafic phenocrysts from tephra vs. lavas from two separate eruptions at Cerro Negro show that both sampling media preserve phenocrysts with high He-3/He-4 values. He-3/He-4 ratios measured in phenocryst phases show good agreement with He isotope values of geothermal fluids from the same volcanoes. However, we note that the pyroxenes tend to have lower He-3/He-4 ratios (4.6-7.0R(A)) than the olivines (He-3/He-4=6.1-7.5R(A)) over a range of concentration values and are consistently lower in cogenetic phenocryst pairs at all locations sampled. In order to assess how this difference arises, we explore two alternative mechanisms: (1) diffusion-related isotopic fractionation, and (2) late-stage radiogenic He-4 additions, preferentially influencing pyroxene grains. In the first case, we reject diffusion-related firactionation of He isotopes since lower He-3/He-4 ratios are not accompanied by a decrease in He concentration values. The second scenario is evaluated on the basis of Mg numbers in cogenetic phenocryst pairs and by petrological modeling of the crystallization sequence. Mg numbers and modeling results at low pressure conditions (= 1 kbar) suggest that olivine crystallization preceded pyroxene crystallization. However, since lavas do not show evidence for extensive crustal contamination, we suggest that the best explanation for the lower He-3/He-4 ratios in pyroxenes is related to the closure temperatures of the phenocryst phases. Given its lower closure temperatures and higher He diffusion rates, we suggest that pyroxenes would be more susceptible to late-stage He exchange with a low He-3/He-4 source during ascent, presumably the surrounding crust. (c) 2005 Elsevier B.V. All rights reserved.