THE RECORD OF MAGMATIC VOLATILES IN ACCESSORY MINERALS

Volatiles elements such as H, C, F, Cl, and S play a prominent role in the evolution of magmas and behavior of volcanic systems. Much of our understanding of magmatic volatiles comes from studies of melt inclusions trapped in phenocrysts. However, the inherent limitations in the use of melt inclusions for understanding magmatic volatiles (size, relative age, susceptibility to modification) demand the development of companion techniques. Recent developments include the study of hydrogen in major, nominally anhydrous minerals such as pyroxene (Wade et al., 2008, O'Leary et al., 2010), olivine (Demouchy et al., 2006), and plagioclase (Johnson and Rossman, 2004), but accessory minerals have been largely ignored. This is unfortunate, because accessory minerals such as apatite and zircon do incorporate measurable amounts of some volatile elements, in addition to other trace elements used for elemental/isotopic tracing and chronometry. For apatite, volatile elements are essential structural components, with total concentrations exceeding ~1 wt%, whereas the amount of hydrogen in non-metamict zircon is typically less than ~100 ppm.

The technique most amenable to measuring that range of concentrations in small volumes of material is secondary ion mass spectrometry (SIMS). However, the ability to measure small amounts of volatiles in minerals both accurately and precisely is a recent development that requires adherence to strict sample preparation and analytical protocols, including the development of robust calibrations based on well characterized standards. We will review recent developments in said protocols, recent applications to magmatic systems on Earth and the Moon, and potential future applications.