Fall 2010 Colloquium Abstracts - Gordon Moore

October 20, 2010

Messengers From the Deep: What Can Primitive Magmas Tell Us About the Mantle?

Gordon Moore (Arizona State University)

Primitive lavas are defined as lavas that have the observed geochemical characteristics thought to be indicative of primary mantle magmas (i.e. mantle melts that have not undergone any differentiation or modification since melting). Understanding the chemistry of such melts, and the mantle xenoliths they sometimes carry to the surface, represents one of only a few ways earth scientists have of constraining the chemical nature of the mantle. Interpretation of the observed petrology and chemistry of any lava erupted at the surface faces a series of challenges however, due to the potential modification of such observables during transport, storage, and eruption. This is particularly true for inferring mantle volatile contents such as H2O and CO2 from volcanic observations (e.g. surface gas and/or melt inclusion measurements). Therefore we must first understand how these modifying processes affect mantle melts as they progress to the surface before inferring anything about the mantle they originated in. High pressure and temperature experiments that define parameters such as melt/volatile solubility behavior, volatile-rich phase equilibria, and element partitioning between crystal/melt and melt/vapor, place constraints on the significance of these modifying processes. This in turn allows us to better interpret the observed characteristics of any lava, and to better define the nature of a primary mantle melt. These experiments also place constraints on the physical and chemical magmatic parameters necessary to understanding their shallow eruptive behavior. Examples are given of ongoing experimental studies of the Peridot Mesa basanite (alkalic, CO2-rich) and high-Mg andesites (calc-alkaline, H2O-rich).