The survival of tracers of primordial mantle heterogeneity investigated through 142Nd/144Nd and 3He/4He isotope decoupling in the Gorgona Island lavas

Abstract

While 142Nd/144Nd, 3He/4He, and 182W/184W isotope ratios are recognized and applied as tracers of primordial mantle heterogeneity, their distribution and reasons behind the extent of variability in the modern-day mantle remain unclear. Lavas associated with the Galapagos plume exhibit the steepest 3He/4He vs. μ182W isotope correlations and thus hold the potential to elucidate the processes involved in the preservation of isotope variability. Among the various eruptions suggested to be related to the Galapagos plume is Gorgona Island, which is geologically significant for their Phanerozoic komatiites. We report the first high precision 142Nd/144Nd isotope measurements for the Gorgona komatiites, as well as 3He/4He and major and trace element data for an array of lavas including komatiites, a picrite, a d-gabbro (“depleted”), and an e-basalt (“enriched”). The 142Nd/144Nd ratios of six komatiite samples are indistinguishable to within ±2.4 ppm (2σ) from the modern-day terrestrial mantle as sampled by most mid-ocean ridge and ocean island basalts. The 3He/4He values determined for the komatiites and a picrite are from 4.6 to 45.1 RA. Eight of the 3He/4He measurements are higher than the MORB average (8 ± 1 RA), and thus strongly point to a plume source sampling an undegassed reservoir. The RA value of 45.1 in particular ranks among the highest 3He/4He ratios measured to date at Gorgona Island and elsewhere, close to what is observed for Phanerozoic flood basalts from Baffin Island.Evaluation of the sources of the lavas using major and rare earth elements along with ε143Nd signatures suggest that the komatiites and picrites are derived from a depleted peridotite source. Meanwhile, the e-basalts are deduced to have been derived from a hybrid source consisting of roughly 8% pyroxenite to 92% peridotite. The inferred and calculated degree of pyroxenite in the sources of the different lava types align with the magnitude of 3He/4He ratios observed. Model calculations involving pyroxenite sources in the e-basalts, which are reported to exhibit 182W/184W isotope anomalies, attest to their survival, while analogous calculations with 142Nd/144Nd isotope ratios suggest homogenization occurring on a faster timescale. It is deduced that 182W/184W anomalies are likely to outlast mixing with recycled pyroxenite compared to 142Nd/144Nd and 3He/4He ratios. The decoupling of modern-day 142Nd/144Nd, 3He/4He, and 182W/184W isotope variabilities thus likely reflect the sensitivity each system has to different mantle processes that contribute to their preservation throughout time.