A window into the mantle: analyzing the geochemistry of melt inclusions from the volcanic island of Mangaia
Cabral, Rita Ann
MetadataShow full item record
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.