IN-SITU MICRO-BADDELEYITE U-PB SIMS DATING OF MAFIC ROCKS: PROGRESS REPORT AND SUCCESSES; AN AGE FROM EVERY DIKE

Over the past four years, we have been developing and refining methods to date micro-baddeleyite grains in-situ whose exposed dimensions are less than 20 µm in length using secondary ion mass spectrometry (SIMS) U-Pb isotopic analysis. The method has been successfully employed for grains as small as 3 microns and is ideal for samples that contain baddeleyite crystals that are too small to separate physically, such as fine-grained tholeiitic dikes and lavas, or samples that are too rare and precious to crush for mineral separation, such as meteorites, drill cores and samples from remote regions. Baddeleyite is common as a magmatic phase in tholeiitic and (less commonly) alkaline mafic rocks where it is not known to nucleate metamorphically. Therefore baddeleyite ages can generally be regarded as magmatic. We have occasionally documented zircon overgrowths on baddeleyite, however. The method requires only portions of polished thin sections and is relatively non-destructive, as it preserves the analyzed sections largely intact and still suitable for additional types of analyses. X-ray mapping, energy-dispersive spectrometry and backscattered electron imaging are used to locate and image the grains and have added benefits of identifying alteration-free grains, armoring relationships, and mineral growth mechanisms. Coexisting zircons can also be dated in the same sections, if present, and often yield metamorphic ages. Analytical precision is practically limited by presence of common Pb (e.g., present at grain boundaries or crystal imperfections), and can be enhanced by averaging multiple crystal analyses. From our experience, precisions obtained by averaging 8 to 10 spot analyses range from 0.5 to 1% for ²⁰⁷Pb/²⁰⁶Pb dates from rocks that are >1000 Ma, and 3 to 8% for ²³⁸U/²⁰⁶Pb dates from Paleozoic to Mesozoic rocks. Recent case studies include lunar, Martian and eucritic meteorites, tholeiitic dikes and sills from 2.7 Ga to 0.4 Ga, Mesoproterozoic volcanic rocks and Caribbean intrusive rocks.