Petrology of Southern Valu Fa Ridge Lavas

Oregon State University, Corvallis OR

Investigators

Abstract

Exchange of volatiles and other chemical elements between the earth's crust, mantle, and ocean drive the composition of the atmosphere and ocean and tell us a great deal about the processes of subduction, mantle heterogeneity, and other deep earth processes. This research uses the chemistry of glassy seafloor volcanic lavas, recently collected by an Australian research vessel along the Valu Fa Ridge and its associated back arc in the western Pacific Ocean, to evaluate the nature and variability of mantle source components including arc-related fluids and parental magmas. Samples come from an area that is adjacent to the NSF-funded Integrated Study Site in the Lau Basin where subduction and mantle processes are presently under intense study. The Valu Fa Ridge lies at the southern end of the East Lau Spreading Center within the Kingdom of Tonga. The Valu Fa Ridge is important because it is rapidly propagating southward, creating new, deep-water volcanic systems that become progressively closer to the "ring-of-fire" volcanoes of the Tofua (Tonga) volcanic arc. This rapid evolution provides a unique opportunity to investigate a back-arc volcanic system, as it evolves. This research involves completing major and trace element analyses of glasses from lavas that have been dredged from the seafloor from a closely spaced transect along the Valu Fa Ridge. A wide suite of volatiles (H2O, CO2, etc.) and companion isotopes will be analyzed and interpreted. Isotope data will be analyzed and provided by international collaborators at no cost. Geochemical data will allow mapping of the distribution of key elements and element-ratios that are diagnostic of the key mantle source characteristics and components. Potential sources for this source material include deep-sea sediments and seawater-altered volcanic rocks from subducted ocean crust; water- and carbon dioxide-rich fluids or molten rock material formed in a distinct melting region that supplies the "ring-of-fire" volcanoes; and "original" mantle material that was present before subduction began. Questions addressed by this study are: (1) At what depth, temperature, and pressure are magmas forming?; (2) What is the chemical and mineralogical composition of melting mantle material in the area?; (3) How much water, carbon dioxide, and other volatile species are present when melts are formed and what is the role of volatiles in the melting process?; (4) To what extent is material derived from the subducted ocean crust in the melting region?; and (5) How do these characteristics vary along Valu Fa ridge? Broader Impacts Results from this research will be fully integrated into NSF-funded RIDGE 2000 Integrated Study Site research. The work will also support a graduate and undergraduate student, both of whom will be trained in geochemical analyses. It will also support NSF-funded analytical infrastructure at the Oregon State University. In terms of dissemination of results, material will be provided to the RIDGE 200 office for education and outreach activities focusing on the Lau Basin. All geochemical data will be submitted to the RIDGE 2000 research database. The work also engages international collaboration with scientists from Australia, Germany, Tonga and other southern Pacific island nations.