Mantle source versus process for the origin of geochemical heterogeneity in olivine-hosted melt inclusions from Hawaiʻi and Samoa

eScholarship, University of California 2024-01-01

Olivine-hosted melt inclusions provide a multitude of opportunities to glimpse magmatic processes at depth because olivine is an early-crystallizing mineral in mantle-derived melts, and melt inclusions often capture a period of magmatic history that is rarely recorded and often obscured by whole rock or matrix glass chemistry of lavas. My research uses geochemical analyses of olivine-hosted melt inclusions to differentiate between the influences of mantle source heterogeneity versus magmatic processes on the composition of mantle-derived magmas at oceanic hotspots.(I) Olivine-hosted melt inclusions from Puʻu Wahi, Mauna Loa, Hawaiʻi have unusually high (Sr/Ce)N ratios though lack elevated Al2O3, suggesting that plagioclase was not assimilated. This was dubbed a “ghost plagioclase” signature. The high (Sr/Ce)N ratios have been interpreted in the past to be signatures of recycled plagioclase-rich oceanic crust in the mantle source. However, the 87Sr/86Sr of individual olivine-hosted melt inclusions, whether with a high or normal (Sr/Ce)N ratio, are within the range expected for Mauna Loa lavas. Thus, the high (Sr/Ce)N ratios were likely the result of diffusive interaction with gabbro beneath Mauna Loa and not from incorporation of recycled oceanic crust in the mantle source.(II) In Chapter 2, I argue that the enriched mantle 2 (EM2) mantle source is just as damp with respect to H2O as depleted mantle (DM) or non-enriched mantle (non-EM) sources. Previous studies suggested that the EM2 source is dry with respect to H2O because lower H2O/Ce corresponded with higher 87Sr/86Sr in submarine glasses. The H2O/Ce ratios of Samoan olivine-hosted melt inclusions are similar to those of non-EM submarine glasses, suggesting that closed-system degassing, where both CO2 and H2O degas, led to lower H2O/Ce in Samoan pillow glasses compared to melt inclusions from the same seamounts. I use the correlation between CO2/Nb and (La/Sm)N in OIB and MORB glasses to estimate primary melt CO2

Geochemistry; Petrology; Geology; 87Sr/86Sr; H2O; hotspots; melt inclusions; ocean island basalt; volatiles; publication

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