Sr and Nd isotopic compositions of individual olivine-hosted melt inclusions from Hawai'i and Samoa: Implications for the origin of isotopic heterogeneity in melt inclusions from OIB lavas

Geochemical investigations of mantle heterogeneity as sampled by ocean island basalts (OIB) have long relied on isotopic analyses of whole rock lavas. However recent work has shown that significant isotopic disequilibrium can exist between the phases (groundmass and phenocrysts) of a single OIB lava. In this study, we target individual olivine-hosted melt inclusions from two samples—one Samoan and one Hawai'ian—with melt inclusion 87Sr/86Sr heterogeneity previously measured using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We report 87Sr/86Sr and 143Nd/144Nd in individual melt inclusions analyzed by thermal ionization mass spectrometry (TIMS). In melt inclusions from Samoan sample AVON3-71-2, we find highly heterogeneous (935 ppm) 87Sr/86Sr (0.705193–0.705853, N = 6), consistent with previously identified 87Sr/86Sr heterogeneity (~2030 ppm) by laser ablation multi-collector ICP-MS (0.70459–0.70602, N = 12). In contrast, we find comparatively little (251 ppm) 87Sr/86Sr heterogeneity (0.703761–0.703938, N = 9) in olivine-hosted melt inclusions contained in a Hawaiian scoria clast from the Pu'u Wahi eruption (Mauna Loa). This result contrasts with a previous measurements by single-collector LA-ICP-MS that found highly heterogeneous (~8500 ppm) 87Sr/86Sr in olivine-hosted melt inclusions from the same eruption (0.7021–0.7081, N = 137). In both the AVON3-71-2 and Pu'u Wahi melt inclusions, 143Nd/144Nd is indistinguishable from their respective whole rock 143Nd/144Nd values. The isotopic measurements on the melt inclusions are paired with major and trace element concentrations to investigate the mechanisms generating 87Sr/86Sr variability in melt inclusions. The lack of significant 87Sr/86Sr variability in the Hawai'ian melt inclusions (only 251 ppm) from Pu'u Wahi suggests a relatively simple magmatic history. In contrast, for the Samoan melt inclusions, we present evidence that supports the mixing of isotopically-heterogeneous mantle-derived melts as the mechanism generating the observed 87Sr/86Sr and trace element variability in the melt inclusions from AVON3-71-2. However, brine interaction appears to have increased the Cl concentrations of some of the Samoan inclusions without significantly modifying the 87Sr/86Sr or the other elemental budgets examined in this study