Geochemical measurements of OIB (ocean island basalt) samples have demonstrated that the Earth's mantle is compositionally heterogeneous, but the cause of this heterogeneity is a point of debate within the scientific community. One such OIB location is Mangaia, Cook Islands (Polynesia). Its lead isotopic composition defines the HIMU (high μ = high ^238U/^204Pb) mantle endmember, which many attribute to recycled oceanic crust being present in the mantle source. If true, this endmember represents an important vehicle for returning surface material to the mantle and an opportunity to study volatile element cycling through the mantle. Sulfur isotopic measurements were made on sulfides hosted in melt inclusions from Mangaia. Prior to 2.45 Ga, the Earth's atmosphere was not oxygenated, allowing photochemical cycles to fractionate sulfur isotopes. This form of fractionation results in a mass independently fractionated (MIF) sulfur isotopic signature in surface materials containing sulfur. We have found such a signal in sulfide inclusions from Mangaia, indicating that the material erupted at this young (~19 Ma) ocean island was once at the surface over 2.45 Ga. This finding confirms the recycled origin hypothesis for the generation of the HIMU mantle endmember. Lead isotopes and major elements were measured in olivine hosted melt inclusions from the island. Previous studies by Saal et al. (1998) and Yurimoto et al. (2004) have revealed large lead isotopic variability, spanning half of the global range for OIBs. A more recent study by Paul et al. (2011) has shown much reduced lead isotopic variability using a different analysis technique. We find the lead isotopic variability in glassy melt inclusions to be less than previously found and attribute much of the earlier observed variability to contaminant lead. Volatile and trace elements were measured in the same olivine hosted melt inclusions, providing the first ever coupled lead isotopes, major, trace, and volatile elements in glassy melt inclusions from the island. We observe some of the highest water and carbon dioxide contents found in OIBs globally. This allows us to constrain volatile abundances in the HIMU mantle source as well as volatile cycling in the mantle, from subduction zones to hotspots.

Mafic magma is the most common magma erupted at the surface of the earth. It is generated from partial melting of the mantle, which has been subdivided into end-members based on unique geochemical signatures. One reason these end members, or heterogeneities, exist is subduction of lithospheric plates back into the mantle. The amount of elements, such as Cl and K, removed during subduction and recycled into the deep mantle, is poorly constrained. Additionally, the amount of volatiles, such as Cl, that are recycled into the deep mantle will strongly affect the behavior of the system. I have looked at Cl and K in HIMU source melts to see how it varies. Cl/Nb and K/Nb suggest that elevated Cl/K ratios are the result of depletion of K rather than increased Cl recycled into the deep mantle. After the mantle has partially melted and mafic melt has migrated to the surface, it usually erupts effusively or with low explosivity because of its low viscosity, but it is possible for larger eruptions to occur. These larger, Plinian eruptions, are not well understood in mafic systems. It is generally thought that basalt has a viscosity that is too low to allow for such an eruption to occur. Plinian eruptions require fragmentation to occur, which means the melt must undergo brittle failure. This may occur if the melt ascends rapidly enough to allow pressure to build in bubbles without the bubbles expanding. To test this, I have done decompression experiments to try to bracket the ascent rate for two Plinian eruptions. One eruption has a fast ascent, faster than those seen in more silicic melts, whereas the other eruption is unable to be reproduced in the lab, however it began with a increased viscosity in the partly crystallized magma. After fragmentation and eruption, it is generally thought that tephra do not continue to crystallize. We have found that crystallinity increases from rim to core in two basaltic pumice. Textural data along with a cooling model has allowed us to estimate growth rates in a natural system, which are similar to experimental data. / text

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