APPLICATION OF X-RAY MICRO-COMPUTED TOMOGRAPHY IN (U-TH)/HE THERMOCHRONOLOGY

A practical issue to be addressed during the determination of a (U-Th)/He age is the need to attribute a correction factor (F_He or F_T) to account for the potential He loss due to ejection of alpha-particles during the decay of U and Th parent isotopes located in the outer 20 µm of mineral grains. The accurate determination of an F_He is affected by the quality of the grain and the need for a subjective assessment of crystal morphology relative to an ideal crystal shape. Other issues that can confound researchers include the presence of latent mineral inclusions or fracture planes that can provide potential pathways for helium loss. These issues are most profound when dealing with detrital grains where crystal shape and transparency have been affected by physical weathering processes. We investigated X-ray Micro-Computed Tomography (MicroCT) analysis as a method to objectively and quantitatively address these issues. Using traditional microscopy methods, four experienced U-Th/He researchers independently examined the same apatite and zircon grains, and reported F_He values to within 4% and 9% respectively. The same grains were then tomographically imaged and measured using a MicroCT scanner to obtain X-Y-Z dimension, surface area (S) and volume (V) data. Machine-based F_He values were up to 31% different than microscopy-based F_He values, with the largest discrepancies occurring in small grains with deviations from ideal hexagonal/tetragonal dipyramidal grain morphology. The real advantage of MicroCT analysis however is the ability to computationally extract a 20 µm outer layer from the grain model via a 3D-erosion procedure. A MicroCT F_He is equivalent to (eroded (S/V)/original (S/V)) and objectively accounts for the daughterless parent component for each grain, independent of morphology. In addition, the MicroCT method can be successfully used at an early stage of laboratory analysis to: (i) reveal internal fractures, mineral and fluid inclusions to 1 µm resolution via density and elemental contrasts, (ii) provide accurate volume measurements for the calculation of U and Th concentrations in single grains, and (iii) translate grain measurements into equivalent spherical dimensions for use in forward modeling applications that use time-temperature data to constrain the cooling histories of crustal rocks.