Isotope Geochemistry

Lavas erupted at ocean island volcanos (such as Robinson Crusoe island5, photo) have diverse chemical and isotopic compositions which constrain the evolution of the Earth's interior and atmosphere, the return of material to the Earth's interior at subduction zones, and the process of formation of volcanic rocks. I am particularly interested in the isotopes of helium, which show that the Earth's interior has experienced variable degrees of outgassing to form the atmosphere. We have recently investigated the He isotopic composition of lavas from Papua New Guinea24, the North American Basin and Range Province, Kauai (Hawaii)67, and American2 and Western Samoa14.

The heavier noble gases (neon, argon, krypton, xenon) also have an interesting story to tell about planetary evolution29. Unlike helium, which is not retained by the Earth's gravitational field, these gases have accumulated in the atmosphere from volcanic emissions throughout the history of the Earth. By comparing the composition of the atmosphere with the Earth's interior it is possible to constrain when and how the atmosphere formed. The key to this field is high precision, low-blank analyses, and this is the driving force behind the laser extraction system in my laboratory. We have measured the Ne-Ar-Xe composition of CO2-dominated fluid inclusions in some extraordinary xenoliths (pieces of the mantle entrained in lavas) from Samoa3 (photomicrograph at left).

Also in my lab Pete Burnard (now at Nancy) and Dr. David Graham at OSU recently completed on a systematic analysis of He-Ne-Ar-CO2 in mid-ocean ridge glasses from the Southeast Indian Ridge to better understand the source composition and petrogenetic behavior of the noble gases80.