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Grain selection Grains are hand-picked with fine-tipped tweezers from mineral concentrates and selected on the basis of morphology, size, optical clarity, and, ideally, compositional uniformity. Euhedral crystals or grains with morphologies that can be geometrically approximated as one of several typical morphologies, such as hexagonal or orthorhombic prisms with pryamidal terminations, are favored, because these simplify alphaejection corrections. Highly irregular or rounded grains can also be dated if such corrections are not needed or can be applied to prolate spheroids. Especially in the case of apatite, larger grains are generally favored over smaller ones, because this provides higher analyte contents that often, but not always, lead to better analytical precision, and minimizes consequences of inaccurate alpha-ejection corrections. Selection of grains based on these criteria is usually accomplished with a high-powered (~120-160x) stereo-zoom microscope and/or SEM, often with the aid of digital photographic documentation and measurement. Finally, grains may be selected on the basis of compositional uniformity, particularly with respect to U and Th, to minimize complications related to alpha-ejection corrections, perturbations to He diffusion profiles, and potential intragrain He diffusivity variations arising from radiation damage (Farley et al., 2011). Grain selection on this basis may be accomplished using electron microbeam or CL imaging, or LA-ICP-MS, including on grain interiors (e.g., Hourigan et al., 2005). 4

He measurement Helium is extracted from aliquots using laser heating using long-wavelength lasers such as those with diode (~800-900 nm), fundamental Nd:YAG (1064 nm), or CO2 (10.6 um) sources. Nb foil tubes are used as "microfurnaces" to contain analyzed minerals. Crystal-bearing foil tubes (and also empty ones to check 4He blanks) are individually heated under a laser beam to about 900-1300 °C for several minutes in an ultrahigh vacuum chamber typically pumped to