NAVIGATING COMPLEXITY: IN-SITU AND SINGLE-MINERAL STUDIES OF INTERMEDIATE MAGMAS

Andesitic and other intermediate volcanic rocks are an important and characteristic component of subduction zone magmatism and represent a significant proportion of the global eruptive output of convergent margins – particularly in continental subduction systems. However the origins of intermediate magmas are complex and remain a source of widespread debate. In part this is a direct consequence of the petrographic complexity of many of these magmas, including disequilibrium mineral assemblages, textural variations and complex zonation within mineral phases; and often contain mineral populations demonstrably derived from a number of different sources. This complexity limits the utility of traditional whole-rock chemical and isotopic analyses, and as a result mineral-based studies are particularly important for studying the sources and processes involved in the formation of intermediate magmas.

In this presentation I will discuss insights into the genesis of intermediate magmas from application of in-situ trace and isotopic analyses of minerals. Important issues include the representativeness of results from individual minerals, methods of combining textural and compositional data, and the integration of multiple data sources. Examples of the utility of this approach include identification of mineral populations and assemblages, identification of the compositions and crystallization conditions of parental magmas, the use of diffusion modeling to constrain residence times and thermal histories and the use of mineral compositions to test models for andesite genesis.