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theses

Source lithology of mantle plumes and other volcanic sources has been inferred from isotopic data for decades, however these studies cannot determine if the geochemical signature of pyroxenitic source is due to the presence of a pyroxenite, or if the geochemical signature had been imprinted upon peridotite by some metosomatic process (e.g. refertilization). Mn, Ca and Ni concentrations in olivines are used to infer the source lithology of primitive basalts. Secular cooling of the Galápagos hotspot has been established by petrologic modeling. An increase in the entrainment of relatively dense pyroxenite in the Galápagos plume over time may explain this trend in decreasing Tp with time, as pyroxenite entrainment would have a negative effect on the buoyancy of the plume. Analyses of olivines from Curacao indicates that pyroxenite was absent in the source of these Cretaceous age Galápagos plume lavas. Magmatism preserved at Quepos marks the transition of the Galápagos plume from the head (LIP) stage to the tail (hotspot track) stage. Major and trace element analyses of Quepos samples show a geochemical signal of a pyroxenite. The presence of pyroxenite as a distinct lithology in the source of these lavas is confirmed by olivine analyses. The extreme enrichment of radiogenic lead found in the lavas of Mangaia have long been interpreted as reflecting a component of ancient recycled crust and is representative of the isotopic endmember HIMU. Olivine analyses of samples from Mangaia indicate that the source of these lavas is predominantly peridotite. If the source of Mangaia had a substantial pyroxenitic component, as inferred from isotopic data, then it is likely that it had its lithological identity destroyed. This may have been the result of the refertilization of mantle periditote by the injection of silicic, pyroxenite source melt. Ni excess observed in the chemistry of olivines from various LIPs and hotspots contrast their Mn and Ca concentrations which are indicative of a peridotite source. High 3He/4He found in samples for which He analysis has been done, suggest a deep lower mantle source. The Ni excess may be due to the interaction between the lower mantle and the core.

ETD_5937

Ph.D.

Includes bibliographical references

by Christopher A. Vidito

doi:10.7282/T3MG7R42

ETD doctoral

The author owns the copyright to this work.