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.