The development and use of monazite and garnet geochronology with application to tectonic processes

Understanding tectonic processes in 4-D remains a fundamental target of earth scientists. This research investigates two geochronometers---garnet and monazite---and their application to tectonic processes. First, garnet geochronology determines the timing of mineral growth during the subduction and exhumation of the Western Gneiss Region, a giant ultrahigh-pressure terrane in Norway. Coupled with geochemical modeling, these data describe the major phase transformations and densification reactions that ultimately affect the amount of continental crust that subducts to high- and ultrahigh-pressures. Second, because monazite is a common accessory mineral in a broad range of compositions, it has been used extensively to date the timing of metamorphic events. The second portion of this paper presents methods that were developed to accurately date monazite with chemical abrasion (CA) thermal ionization mass spectrometry (TIMS), a high-precision geochronologic technique. Thirdly, recent advances in microbeam technologies have made the acquisition of geochronologic data far more accessible, but the methods have not been rigorously tested. The final portion of this research systematically examines two of the most common microbeam techniques---laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS). The data suggest that the accuracy and precision of microbeam techniques are only adequate for research questions where an error of +/-10% is acceptable.