Unusually homogeneous helium isotope composition of the Auckland Volcanic Field and its implications for the underlying mantle

Abstract

The Auckland Volcanic Field (AVF) is one of the most intensively studied monogenetic basalt fields in the world yet its origin remains enigmatic. Magmatism in the AVF occurred from ~193 ka to 500 years bp. The trace element and isotopic diversity of AVF basalts require “small-scale” compositional heterogeneity in the underlying mantle that is comparable in volume to, or slightly larger than, the scale of individual eruptions in the AVF.Olivine from tephra and lava, representing the range of AVF compositions, was crushed and analysed for 3He/4He, and He and CO2 concentrations in an attempt to further characterize and explain the significance of the AVF “end-members”. AVF basalts show a negative covariation between the amount of CO2 released by crushing of olivine and the whole rock concentrations of highly incompatible trace elements, such as Ba, Rb, Nb, Zr, Ti and K. In contrast, the amount of He released by crushing shows no simple relations with the same incompatible elements or their ratios. This leads to a significant variation in CO2/3He ratios (9.4 × 107–3.5 × 109) that may relate to differences in magma ascent dynamics, as well as to different magma sources. The measured CO2/He ratios may have been influenced by varying amounts of CO2 diffusion into vapour bubbles within melt inclusions that depend on melt composition and magma ascent rate. However, petrographically there is no evidence for systematic differences in the size or quantity of vapour bubbles in olivine-hosted melt inclusions.3He/4He ratios in 14 AVF samples studied here show a narrow range from 6.57 to 7.26 RA (mean of 7.10 ± 0.26). This may imply a dominance of the mantle helium budget by small-scale heterogeneities. Alternatively, the 3He/4He results suggest that the tectonic and magmatic history of the mantle beneath the AVF has effectively hybridized the 3He/4He ratio to a larger extent than for highly incompatible trace element ratios and Pb-Nd-Sr isotopes. The value of ~7 RA for the mantle source of AVF basalts, in light of other evidence, suggests that there is a relatively homogeneous He isotope composition for the Zealandia-Antarctic mantle domain.