Collaborative Research: Volatile Contents of Komatiites

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

Intellectual merit. Komatiites provide key, but controversial, constraints on the thermal and compositional evolution of the Earth. Of primary importance are the water contents of komatiite magmas - at low water contents formation of komatiites require mantle potential temperatures >500 C above modern values, whereas wet melting requires more modest increases. Present estimates of komatiite water contents are variable and hotly debated. Further, relatively little is known about the abundances of other volatile elements (S, F, Cl) in komatiite magmas and their mantle sources, although these have the potential to provide primary constraints for models of geochemical cycling and degassing. We propose a collaborative study of volatile contents in komatiites. This will primarily be through measurements of melt inclusions trapped within silicate and oxide phases in well-preserved komatiite samples from the Belingwe Greenstone Belt in Zimbabwe, but will also involve examination and preliminary measurements of komatiites from a selection of other low and medium metamorphic grade terranes. The study will use a combination of petrographic observations and microanalytical techniques: Secondary Ion Mass Spectrometry (SIMS); Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Electron Microprobe Analysis (EMPA). Previous data reported from melt inclusions in Belingwe samples are not definitive in terms of estimates of komatiite water contents and we propose a careful analytical scheme to rectify this. We will also measure abundances of other volatile species (CO2, F, S, Cl) as well as major and trace elements, S and H speciation and H isotope composition. This additional data will help define the effects of degassing, hydrogen diffusion, crustal assimilation, alteration and other processes that have the potential to alter primary water contents, but will also provide new insights into the compositions of komatiite magmas and their mantle sources. Preliminary data demonstrate that melt inclusions in samples we have in hand contain significant and measurable abundances of H2O, CO2, S and Cl. Overall our study will provide some of the first direct constraints on the volatile contents of komatiite magmas and Archean mantle and komatiites source regions. Broader impacts. This work will address several broader impacts. The work will support two PI's, foster collaboration between two research institutions, Oregon State University and the Carnegie Institute of Washington, and contribute to development and maintenance of geochemical analysis facilities in both places. The work will also provide fundamental information about the chemical and thermal evolution of the earth and is part of a long-term project to investigate the volatile structure and evolution of the mantle. A particularly important impact of this work will be to foster undergraduate research and to engage talented undergraduates in state-of-the-art research and use of research facilities at OSU and CIW. Two undergraduates from OSU will perform senior thesis research related to the project and this will involve visits to both CIW and to field outcrops of komatiite and associated rocks in the Abitibi Greenstone Belt in Canada. Both PI's are active in fostering undergraduate research. PI-Kent is coordinating undergraduate research for the OSU petrology group and will co-teach a course in undergraduate research for juniors and seniors in Fall 2006. Undergraduate research at Oregon State is currently being reinvigorated through these activities and via recent employment of an undergraduate research coordinator.