MAGNETITE (U-TH)/HE GEOCHRONOLOGY: ADVANCEMENTS IN SAMPLE PREPARATION AND APPLICATION TO DATE SERPENTINIZATION

Dating the formation of magnetites resulting from the reaction of olivine with water during serpentinization has the potential to address a critical gap in the current understanding of the role of serpentinization as a thermal, mechanical and chemical process that progressively modifies the very mechanical nature of plate boundaries. Magnetite has been proven as a viable geochronometer in basaltic to intermediate volcanic rocks and is an excellent candidate for (U-Th)/He-Ne geochronology because it forms as a common mineral phase in felsic, mafic and hydrated ultramafic igneous and metamorphic rocks, has measurable amounts of U-Th, and grains can be separated by hand magnet. The present study aims to temporally constrain the timing and depth of serpentinization by dating magnetites that form during serpentinization at rifted continental margins by using (U-Th)/He and (U-Th)/Ne systematics. The use of two chronometers will add major constraints to the question of the duration of cooling, where fast cooled serpentinites are expected to have the same magnetite He and Ne ages, and slow cooled serpentinites are expected to exhibit a spread in ages, as the magnetite crossed the respective closure temperatures.

Magnetite sample screening to ensure suitability for dating in light of skeletal grain morphology and He implantation is a critical point of method development. This research incorporates high-resolution X-ray computed tomography (CT), SEM backscatter imaging with EDS and air abrasion techniques to effectively screen grains for heterogeneities and correct for adherence of outer matrix and alpha implantation. Once magnetite grains are CT-scanned, Avizo software is used to compile 3D images of the grains and map the location of any matrix or inclusions present. Blob3D software is used to quantify volume and surface area measurements to monitor air-abrasion progress and high [U] matrix removal. Grains are chosen based on homogeneity and size, and are processed at the University of Texas thermochronology laboratories.