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

While Nd-142/Nd-144, He-3/He-4, and W-182/W-184 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 He-3/He-4 vs. mu W-182 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 Nd-142/Nd-144 isotope measurements for the Gorgona komatiites, as well as He-3/He-4 and major and trace element data for an array of lavas including komatiites, a picrite, a dgabbro ("depleted"), and an e-basalt ("enriched"). The Nd-142/Nd-144 ratios of six komatiite samples are indistinguishable to within +/- 2.4 ppm (2 sigma) from the modern-day terrestrial mantle as sampled by most mid-ocean ridge and ocean island basalts. The He-3/He-4 values determined for the komatiites and a picrite are from 4.6 to 45.1 RA. Eight of the He-3/He-4 measurements are higher than the MORB average (8 +/- 1 RA), and thus strongly point to a plume source sampling an undegassed reservoir. The RAvalue of 45.1 in particular ranks among the highest He-3/He-4 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 epsilon Nd-143 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 He-3/He-4 ratios observed. Model calculations involving pyroxenite sources in the e-basalts, which are reported to exhibit W-182/W-184 isotope anomalies, attest to their survival, while analogous calculations with Nd-142/Nd-144 isotope ratios suggest homogenization occurring on a faster timescale. It is deduced that W-182/W-184 anomalies are likely to outlast mixing with recycled pyroxenite compared to Nd-142/Nd-144 and He-3/He-4 ratios. The decoupling of modern-day Nd-142/Nd-144, He-3/He-4, and W-182/W-184 isotope variabilities thus likely reflect the sensitivity each system has to different mantle processes that contribute to their preservation throughout time. (c) 2023 Elsevier B.V. All rights reserved.