Patrick H. Donohue

Experimentally determined subsolidus metal-olivine element partitioning with applications to pallasites

Pallasite meteorites, which consist primarily of olivine and metal, may be remnants of disrupted core-mantle boundaries of differentiated asteroids or planetesimals. The early thermal histories of pallasites are potentially recorded by minor- and trace-element zonation in olivine. However, constraining this history requires knowledge of element behavior under the conditions of pallasite formation, which is lacking for many of the main elements of interest (e.g., Co, Cr, Mn). In this study, we experimentally determined metal/olivine partition coefficients for Fe, Ni, Co, Cr, and Mn in a pallasite analogue at subsolidus temperatures. Metal/olivine partition coefficients (KM ) increase in the order KMn < KCr < 1 < KFe < KCo < KNi , with five orders of magnitude separating KMn from KNi . Transition metals also become more siderophile with increasing experimental temperature (900 to 1550°C). The experiments incidentally produced diffusion profiles in olivine for these elements; Our results suggest they diffuse through olivine at similar rates. Core compositions of pallasite olivines are consistent with high-temperature equilibration with FeNi-metal. Olivine zonation toward crystal rims varies significantly for the investigated transition metals. We suggest rim zonation results from partial re-equilibration during late stage crystallization of minor phases (e.g., chromite, phosphates). This re- equilibration occurred over short timescales relative to overall pallasite cooling, likely tied to initial cooling rates on the order of 100-300°C/Myr.

Textural and mineral chemical evidence for the cumulate origin and evolution of high-titanium basalt fragment 71597

Abstract Basalt fragment 71597 is the sole high-titanium mare basalt showing evidence for olivine accumulation during formation. The petrogenesis of this unique sample was investigated using quantitative textural analysis and major- and trace-element mineral geochemistry. Crystal size distribution analysis identified two size populations of olivine, which we separate into cumulate and matrix olivine. The spatial distribution of olivine also supports clustering of olivine crystals, likely during accumulation. Observed mineral chemistry was consistent with an origin through olivine accumulation, although where this occurred cannot be discerned (e.g., in ponded melts at the base of or in the lunar crust, or within a thick high-Ti basalt flow). Attempts to place 71597 within a geochemical group were inconclusive both using subtraction of cumulate olivine from bulk composition, and by modal recombination of major phases. However, equilibrium liquid compositions of augite and plagioclase are determined to be consistent with an origin by fractionation from the Type B2 chemical suite of Apollo 17 high-Ti basalts. This method of classification has potential for placing other Type U (“Unclassified”) basalts into chemical suites.

Vesicle Size Distribution as a Novel Nuclear Forensics Tool

The first nuclear bomb detonation on Earth involved a plutonium implosion-type device exploded at the Trinity test site (33°40′38.28″N, 106°28′31.44″W), White Sands Proving Grounds, near Alamogordo, New Mexico. Melting and subsequent quenching of the local arkosic sand produced glassy material, designated “Trinitite”. In cross section, Trinitite comprises a thin (1–2 mm), primarily glassy surface above a lower zone (1–2 cm) of mixed melt and mineral fragments from the precursor sand. Multiple hypotheses have been put forward to explain these well-documented but heterogeneous textures. This study reports the first quantitative textural analysis of vesicles in Trinitite to constrain their physical and thermal history. Vesicle morphology and size distributions confirm the upper, glassy surface records a distinct processing history from the lower region, that is useful in determining the original sample surface orientation. Specifically, the glassy layer has lower vesicle density, with larger sizes and more rounded population in cross-section. This vertical stratigraphy is attributed to a two-stage evolution of Trinitite glass from quench cooling of the upper layer followed by prolonged heating of the subsurface. Defining the physical regime of post-melting processes constrains the potential for surface mixing and vesicle formation in a post-detonation environment.