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Getsinger, Amanda

Geology

MS

Tonalite-trondhjemite-granodiorite(TTG) and associated maamatism comprises up to 60% of the continental crust and provides a window into the origin of Earth's earliest crust, the role of the mantle wedge in crust development, and the thermal regimes present in both early Earth subduction zones. However, the tectonic setting in which the basaltic source material undergoes partial melting during TTG magmatism is still undetermined. Major, trace, and REE compositions of both Archean TTGs and modem adakite-like magmas have been studied in conjunction with batch melting experiments and numerical modeling to investigate source rock compositions, depths of melting, and tectonic setting, but the physical processes by which melt segregates from, and interacts with, its partially molten host may have a profound impact on the volume and composition of the segregated melt as it leaves the source region.
I tested the process of melt segregation through experiments designed to reproduce the local changes in bulk composition that are thought to occur in response to melt segregation. This process is predicted by a new numerical model. I performed piston-cylinder experiments at 1.4 GPa and at temperatures between 925°C and 1000°C on a metabasalt with conditions and compositions based on the model parameters. I added low degree hydrous partial melt compositions (5'34, lo%, and 15%) to the starting bulk metabasalt to simulate the compositional evolution of migrating melt chemically interacting and thermodynamically equilibrating with its source material. The partial melts are added in varying modal amounts (25%-50%) along with 2.5 to 4.5 wt% H₂O.
This research does not support a hypothesis that tectonically links adakite and TTG petrogenesis. The results show that introducing a low-degree partial melt into a metabasalt source material changes the geochemical system significantly. The volume of melt generated in these melt segregation equilibration (MSE) experiments was considerably greater than direct partial melting (DPM) alone. Modally, the mineralogy has changed: hornblende stability and plagioclase reduced and garnet and clinopyroxene increased as a function of temperature and melt abundance. Mg# and SiO₂ content varies as a natural response to introducing a felsic melt to a mafic source. This experiment is analogous to an influx of melt in a region of mafic rock in the lower crust. The results from these MSE experiments show that Mg#s alone cannot be used to distinguish between melt generated from the base of the crust and melt that interacted with the mantle.