Trace Metals in Amphibole from Mount St. Helens, Mt. Hood, Shiveluch, and Mount Pinatubo: Insight into Metal Mobility in Volcanic Systems

Arc magmas associated with subduction zones are often linked to the formation of some magmatic ore deposits. Observing the processes associated with metal mobility and enrichment in active arc volcanoes can elucidate the controls that lead to the formation of such ore deposits. In addition, volatile element mobility in a volcanic system may be related to the timing and style of volcanic eruptions. We have undertaken a study of trace metal abundances (Li, Cu, Zn, Pb, Sn, Mo, others) in amphibole from a number of intermediate volcanic systems to constrain the timing of trace metal mobility in arc magmas. Individual volcanoes show variable behavior. At Mount St. Helens, most analyzed metals show systematic variations similar to lithophile elements that can be explained with magma mixing and/or fractional crystallization. Lithium and Cu, however, are clearly decoupled from other trace elements, and concentrations vary by sample and date of eruption. For example, Li and Cu concentrations in amphibole are 3-5x higher in samples from the May 18 cryptodome than the May 18 pumice, in otherwise chemically identical phenocrysts. No significant zoning of Li or Cu has been observed. These two elements are likely transported in volatile fluids and rapidly equilibrated with phenocrysts in the region of volatile enrichment. In the case of the May 18 eruption, the cryptodome was fluxed with Li and Cu-bearing volatiles while it stalled at the top of a volatile-saturated magma chamber. Over the summer of 1980, variations in Li and Cu concentrations suggest Cu and Li were reconcentrated in magmas, as concentrations of Cu and Li exceeding that of the cryptodome occur in samples from the June 12 eruption, and the lowest concentrations are recorded from the July 22 and August 7 eruption. At Mt. Hood, Li and Cu concentrations correlate with each other but are decoupled from lithophile trace elements. Although two distinct depths of amphibole crystallization have been established for this system, no clear difference can be seen in Cu and Li or other metals between these populations. This suggests Li and Cu may rapidly equilibrate in days to weeks during mixing and eruption. While Li and Cu show evidence for volatile mobility, we have not seen equivalent variations in other mobile metals (Mo, Sn etc.) at Shiveluch, Mt. Hood or Mt. St. Helens, despite the fact that these metals are typically enriched along with Cu in many magmatic ore deposits. Further studies of Mount Pinatubo will explore a high S system.