Douglas L. Phinney

The temperature of formation of carbonate in martian meteorite ALH84001: constraints from cation diffusion

We have measured the rates of chemical diffusion of Mg in calcite and Ca in magnesite and used these new data to constrain the formation temperature and thermal history of carbonates in the Martian meteorite ALH84001. Our data have been collected at lower temperatures than in previous studies and provide improved constraints on carbonate formation during relatively low-temperature processes (#400°C). Mea- sured log D0 values for chemical diffusion of Mg in calcite and Ca in magnesite are 216.0 6 1.1 and 27.8 6 4.3 m 2 /s and the activation energies (EA) are 76 6 16 and 214 6 60 kJ/mol, respectively. Measured diffusion rates of Mg in calcite at temperatures between 400 and 550°C are substantially faster than expected from extrapolation of existing higher-temperature data, suggesting that different mechanisms may govern diffusion of Mg at temperatures above and below ;550°C. We have used these data to constrain thermal histories which will allow the ;1 mm variations in Ca-Mg composition in ALH84001 carbonates to survive homogenization by diffusion. Our results are generally consistent with models for formation of carbonates in ALH84001 at low temperatures. For initial cooling rates of between ;10 21 and 10 3 °/Ma our results demonstrate that carbonates formed at temperatures no higher than 400°C and most probably less than 200°C. This range of cooling rates is similar to those observed within the terrestrial crust, and suggests that formation of the carbonates by igneous, metamorphic or hydrothermal (or other) processes in the Martian crust most plausibly occurred at temperatures below 200 to 400°C. Models that suggest ALH84001 carbonates formed during a Martian impact event are also constrained by our data. The thermal histories of terrestrial impact structures suggest that cooling rates sufficiently rapid to allow preser- vation of the observed carbonate zoning at formation temperatures in excess of 600°C (.;10 7 °C/Ma) occur only within the uppermost, melt-rich portions of an impact structure. Material deeper within the impact structure (where cooling is dominated by uplifted crustal material and where much of the formation of hydrothermal minerals is concentrated) cools much slower, typically at rates of ;10 2 to 10 3 °/Ma. Carbonates formed within this region would also only preserve ;1 mm compositional zoning at formation temperatures of less than ;200 to 400°C. Copyright

Experimental study of radium partitioning between anorthite and melt at 1 atm

Abstract We present the first experimental radium mineral/melt partitioning data, specifically between anorthite and a CMAS melt at atmospheric pressure. 226Ra disequilibria are an important chronometer of recent magmatic activity. Ion microprobe measurement of coexisting anorthite and glass phases produces a molar DRa = 0.040 ± 0.006 and DRa/DBa = 0.24 ± 0.05 at 1400 °C. Our results indicate that lattice strain partitioning models fit the divalent (Ca, Sr, Ba, Ra) partition coefficient data of this study well, supporting previous work on crustal melting and magma chamber dynamics that has relied on such models to approximate radium partitioning behavior in the absence of experimentally determined values

Formation of spinel-,hibonite-rich inclusions found in CM2 carbonaceous chondrites

Abstract We report petrography, mineral chemistry, bulk chemistry, and bulk isotopic compositions of a suite of 40 spinel-rich inclusions from the Murchison (CM2) carbonaceous chondrite. Seven types of inclusions have been identified based on mineral assemblage: spinel-hibonite-perovskite; spinelperovskite- pyroxene; spinel-perovskite-melilite; spinel-hibonite-perovskite-melilite; spinel-hibonite; spinel-pyroxene; and spinel-melilite-anorthite. Hibonite-bearing inclusions have Ti-poor spinel compared to the hibonite-free ones, and spinel-hibonite-perovskite inclusions have the highest average bulk TiO2 contents (7.8 wt%). The bulk CaO/Al2O3 ratios of the inclusions range from 0.005 to 0.21, well below the solar value of 0.79. Hibonite-, spinel-rich inclusions consist of phases that are not predicted by condensation calculations to coexist; in the equilibrium sequence, hibonite is followed by melilite, which is followed by spinel. Therefore, hibonite-melilite or melilite-spinel inclusions should be dominant instead. One explanation for the .missing melilite. is that it condensed as expected, but was lost due to evaporation of Mg and Ca during heating and melting of spherule precursors. If this theory were correct, melilite-poor spherules would have isotopically heavy Mg and Ca, assuming Rayleigh fractionation accompanied evaporation. Except for one inclusion with FMg = 4.3 ± 2.6./amu and another with isotopically light Ca (FCa = -3.4 ± 2.0./amu), however, all the inclusions we analyzed have normal isotopic compositions within their 2σ uncertainties. Thus, we found no evidence for significant mass-dependent fractionation. Conditions necessary for non-Rayleigh evaporation are unlikely if not unrealistic, and our preferred explanation for the general lack of melilite among hibonite-, spinel-bearing inclusions is kinetic inhibition of melilite condensation relative to spinel. Because of similarities between the crystal structures of hibonite and spinel, it should be easier for spinel than for melilite to form from hibonite.

Nuclear forensics in law enforcement applications

Over the past several years, the Livermore Forensic Science Center has conducted analyses of nuclear-related samples in conjunction with domestic and international criminal investigations. Law enforcement officials have sought conventional and nuclear-forensic analyses of questioned specimens that have typically consisted of miscellaneous metal species or actinide salts. The investigated activities have included nuclear smuggling and the proliferation of alleged fissionable materials, nonradioactive hoaxes such as “Red Mercury,” and the interdiction of illegal laboratories engaged in methamphetamine synthesis.

Forensic Analyses of Suspect Illicit Nuclear Material

A small metal sample, alleged to be a substance that could substitute for highly enriched uranium in a nuclear weapon, was subjected to qualitative and quantitative forensic analyses using methods of materials science, radioisotopic chemistry, inorganic chemistry, and organic chemistry. The specimen was determined to be moderately pure Sc, likely derived from a uranium refining operation. Although no fissionable species or weaponization signatures were detected, the sample did exhibit some unusual properties. These anomalies included lanthanide fractionation, with concentrations of Dy, Ho, and Er elevated by factors greater than 100 over normal levels, and the presence of long, odd-chain fatty acids.

Advanced Techniques in Physical Forensic Science

The techniques and tools of modern physics have long been profitably applied to forensic science. Since the 1900s, forensic scientists have analyzed gunshot residues, examined fired munitions, determined the refractive indices of glass—to name a few of their diverse activities.