Northwest Africa 11522: A New Paired Stone of Martian Polymict Regolith Breccia Northwest Africa 7034

No abstract available

The Nakhlites Sample Multiple Igneous Units: Evidence From 40Ar/39Ar Chronology And Geochemistry

No abstract available

NWA 7034 Martian Breccia: 40Ar/39Ar Ages of 1.2 to 1.4 Ga

No abstract available

40Ar/39Ar of Meteorites at SUERC

No abstract available

NWA 7034 Martian Breccia: 40Ar/39Ar Ages of 1.2 to 1.4 Ga

No abstract available

Significance of the Cosmogenic Argon Correction in Deciphering the 40Ar/39Ar Ages of the Nakhlite (Martian) Meteorites

All meteorites contain variable amounts of cosmogenic 38Ar and 36Ar produced during extraterrestrial exposure, and in order to calculate reliable 40Ar/39Ar ages this cosmogenic Ar must be removed from the total Ar budget. The amount of cosmogenic Ar has usually been calculated from the step-wise 38Ar/36Ar, minimum 36Ar/37Ar, or average 38Arcosmogenic/37Ar from the irradiated meteorite fragment. However, if Cl is present in the meteorite, then these values will be disturbed by Ar produced during laboratory neutron irradiation of Cl. Chlorine is likely to be a particular issue for the Nakhlite group of Martian meteorites, which can contain over 1000 ppm Cl [1]. An alternative method for the cosmogenic Ar correction uses the meteorites exposure age as calculated from an un-irradiated fragment and step-wise production rates based on the measured Ca/K [2]. This calculation is independent of the Cl concentration. We applied this correction method to seven Nakhlites, analyzed in duplicate or triplicate. Selected samples were analyzed at both Lawrence Livermore National Laboratory and SUERC to ensure inter-laboratory reproducibility. We find that the cosmogenic argon correction of [2] has a significant influence on the ages calculated for individual steps, particularly for those at lower temperatures (i.e., differences of several tens of million years for some steps). The lower-temperature steps are moreinfluenced by the alternate cosmogenic correction method of [2], as these analyses yielded higher concentrations of Cl-derived 38Ar. As a result, the Nakhlite data corrected using [2] yields step-heating spectra that are flat or nearly so across >70% of the release spectra (in contrast to downward-stepping spectra often reported for Nakhlite samples), allowing for the calculation of precise emplacement ages for these meteorites

40Ar/39Ar of Meteorites at SUERC

No abstract available

Neon diffusion kinetics in olivine, pyroxene and feldspar: retentivity of cosmogenic and nucleogenic neon

We performed stepwise degassing experiments by heating single crystals of neutron- or proton-irradiated olivine, pyroxene and feldspar to study diffusion kinetics of neon. This is important in evaluating the utility of these minerals for cosmogenic ²¹Ne measurements and, potentially, for Ne thermochronometry. Degassing patterns are only partially explained by simple Arrhenius relationships; most samples do not exhibit a precisely-determined activation energy in an individual diffusion domain. Regardless, we find clear differences in diffusion kinetics among these minerals. Based on sub-selected data, our estimates for neon diffusion kinetics (activation energy Ea and pre-exponential factor Do, assuming the analyzed fragments approximate the diffusion domain) in each mineral are as follows: for the feldspars, Ea ranges from !65 to 115 kJ/mol and <i>D</i>_o from 3.9 x 10^-3 to 7.1 x 10² cm²s^-1; for the pyroxenes, <i>E</i>_a ranges from !292 to 480 kJ/mol and Do from 1.6 x 10² to 2.9 x 10¹¹ cm²s^-1; for the olivines, <i>E</i>^a ranges from ~360 to 370 kJ/mol and <i>D</i>_o from 1.5 x 10⁶ to 5.0 x 10⁶ cm²s^-1. Differences in these parameters are broadly consistent with the expected effect of structural differences between feldspar, and olivine and pyroxene. These results indicate that cosmogenic 21Ne will be quantitatively retained within olivine and pyroxene at Earth surface temperatures over geological timescales. The diffusion kinetics for feldspars, on the other hand, predicts that ²¹Ne retention at Earth surface temperatures will vary significantly with domain size, crystal microtexture, surface temperature, and exposure duration. Quantitative retention is expected only in favorable conditions. This conclusion is reinforced by additional measurements of cosmogenic ²¹Ne in coexisting quartz and feldspar from naturally irradiated surface samples; sanidine from a variety of rhyolitic ignimbrites exhibits quantitative retention, whereas alkali–feldspar from several granites does not