Evolution of Intraplate Magmas: An Experimental Approach

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Lindsley and NekvasilEAR-0003443Rocks of within-plate magmatic suites commonly show a wide range in composition, yet occur together in time and space. These suites fall into alkalic and tholeiitic types, each forming distinctive chemical trends. This work focuses specifically on three of these suite-types: 1. hy-normative alkalic rocks from both continental and ocean-island settings; 2. ocean-island tholeiites and associated rocks, and 3. continental tholeiites and their associated evolved rocks, together with the distinctive suites associated with Proterozoic massif anorthosites, ranging from high-Al gabbro through ferrodiorite (high-Fe; high Fe/Mg) to fayalite granite. Much remains unknown about the relationship among the suite-members of these three types or any possible relationship linking these suite-types. We will continue experimental simulation of step-wise fractional crystallization under variable conditions of pressure and water content for the three suite-types in order to provide constraints on their compositional evolution. The least-evolved magmas from each of the three suite-types appear to emanate from a similar tholeiitic basalt. If the three suite-types indeed share a common parentage, then the differences in trends of the more evolved members must have arisen primarily from secondary factors such as differences in pressure of fractionation, decompression history, or volatile content. Experiments will be continued that explore this possibility. The conditions leading to melts that simulate most closely the observed rock compositions within each suite will be applied to the other suite-types in attempt to divert them from their standard path and onto trends subparallel to the path most relevant to the conditions imposed. Success in this would have far-ranging implications about the processes at work in magma generation and evolution in hot-spot and early-rifting settings.