INTEGRATED LASER ABLATION AND RAMAN MICROPROBE U/PB, (U-TH)/HE, AND RADIATION DAMAGE CHRONOLOGY OF DETRITAL MINERALS (Invited Presentation)

Recent advances in the use of ultraviolet laser ablation microprobes for (U-Th)/He dating of accessory minerals such as rutile, titanite, monazite, xenotime, and zircon have substantially increased opportunities for the simultaneous dating of individual crystals using the U/Pb geochronometer and the (U-Th)/He thermochronometer. Here we review current and emerging methodologies that permit such “multichronometry”, particularly as applied to detrital minerals in modern sediments and ancient sedimentary (or metasedimentary) rocks.

 Applications at Arizona State University (Horne et al., 2016, Geochimica et Cosmochimica Acta)involve using an ArF excimer (193 nm) laser microprobe to analyze polished, previously characterized crystals (≥ 70 μm) in grain mounts. We ablate small pits for helium isotopic abundance measurements using a quadrupole or magnetic sector mass spectrometer. After measurement of these pits using an interferometric microscope, the same laser is then used, with an expanded beam, to ablate larger volumes of the crystals that surround the first ablation pits, and that material is analyzed for U, Th, and Pb by inductively coupled quadrupole mass spectrometry. After measuring the volumes of the second ablation pits, the combined U-Th-Pb-He datasets can be used to calculate both U/Pb and (U-Th)/He dates for each analyzed crystal. From the perspective of (U-Th)/He chronometry, this procedure eliminates the need for alpha ejection or injection corrections, and minimizes the impact of U+Th zonation and inclusions. A major advantage is that sample throughput is increased dramatically, increasing the number of crystals that can be analyzed practically for any given detrital sample.

Working with polished grain mounts also enables better chemical and structural characterization of dated crystals prior to isotopic analysis. For example, we routinely use Raman spectroscopy to map radiation damage (using the full width at half maximum of the ν₃ (SiO₄) peak as a proxy; Nasdala et al., 2005, Chemical Geology). This enables better interpretation of the age significance of the (U-Th)/He dates since He diffusivity in zircon appears to be correlated with radiation damage, and provides a third chronometer for the same crystal (Pidgeon, 2014, Chemical Geology).