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Fluids play a fundamental role in material transport in during metamorphism. One of the most important settings for metamorphic fluid-rock interaction is in subduction zones. Vein assemblages in exhumed subduction-zone complexes consist primarily ofMoreFluids play a fundamental role in material transport in during metamorphism. One of the most important settings for metamorphic fluid-rock interaction is in subduction zones. Vein assemblages in exhumed subduction-zone complexes consist primarily of Na-Al-Ca silicate and commonly contain the accessory phases apatite and rutile, which are important reservoirs for the geochemically important rare earth elements (REE) and high-field-strength elements (HFSE). The widespread occurrence of these accessory phases is problematic because they have low solubility in H2O at the conditions of vein formation. In the absence of general evidence for elevated fluid flux during vein formation, it is required that complexing agents in the fluid phase enhance accessory mineral solubility. This thesis describes results of experimental studies aimed at identifying these complexing agents.-Most previous experiments on accessory mineral solubility at high pressure (P) and temperature (T) have been conducted in pure H2O. These studies have demonstrated that it is difficult to make accurate, reproducible measurements. New experimental techniques were developed during this work allowing for accurate, reproducible solubility measurements at high P and T. These methods were employed to: (1) determine the effects of H2O-NaCl fluids on accessory mineral solubility, and (2) ascertain the degree to which dissolved Na, Al and Si, the most abundant elements in the veins, enhance the solubility of these minerals.-The results demonstrate that both H2O-NaCl and Na aluminosilicate-bearing fluid are effective in dissolving low-solubility minerals at high P and T. Both apatite and rutile solubility increase with P and T. Apatite solubility also rises with increasing XNaCl. Further, apatite and rutile solubility are enhanced by increases in Na-Al-Si concentration. The combined results represent the first systematic data set on nominally insoluble, geochemically important minerals at conditions relevant to granulites, Barrovian metamorphism and subduction zones. The solubility data on rutile and apatite can be incorporated into models of HFSE and REE mobility in theses metamorphic environments. The rutile data show that slab-derived fluids have the potential to transport more HFSE than previously thought. This observation must be considered when developing models of trace element signatures of arc magmas.