Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p11c1600m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P11C-1600
Other
[1040] Geochemistry / Radiogenic Isotope Geochemistry, [1140] Geochronology / Thermochronology, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Thermal histories of Martian meteorites provide important clues regarding the surface conditions of Mars, ejection dynamics of meteorites from Mars, and their delivery process to Earth. To investigate the peak shock temperatures of Zagami and ALHA77005 Martian meteorites during their ejection from Mars, we applied (U-Th)/He thermochronometry to multiple phosphate aggregates. A total of 248 phosphate aggregates (merrillite crystals with other attached phases) were identified using Scanning Electron Microscopy (SEM), and then grouped together in samples of five to twenty aggregates in order to measure U, Th, Sm and He abundances. The resulting (U-Th)/He ages are widely distributed in the range of 19.8 Ma - 202.4 Ma for Zagami. The ages from large aggregates (150-250 μm: 147 ± 35 Ma) are generally older than the ages from the smaller groups (75-250 μm, 51 ± 31 Ma). The textural relationships of the phosphates with neighboring phases in the aggregates were investigated using SEM and BSE (Back-Scattered Electron) imaging to understand the age distribution. Among the most contrasting features is that the larger aggregates contain thicker (>20 μm) layers of attached phases, whereas the smaller aggregates exhibit much thinner (<20 μm) corresponding layers. These observations imply that the large aggregates, with thick exotic layers, experienced least alpha-recoil loss, thus generating reliable (U-Th)/He ages. In contrast, the smaller aggregates' external layers lack sufficient thickness to shield recoiled alphas, and subsequently yielding apparently younger (U-Th)/He ages. For ALHA77005, the (U-Th)/He ages are relatively well constrained in the range of 5.9 Ma - 17.9 Ma (11.4 ± 5.8 Ma) with one outlier of 78.2 Ma. The limited amount of sample for ALHA77005 preempted us from testing any size effects on the ages. The peak shock temperatures of these two meteorites were estimated using a simple volume diffusion model with an assumption that all He loss occurred during the ejection-related shock events. For this modeling, it is required to constrain input parameters, with the most important being fHe (He fractional loss) and r (diffusion domain radius). The fHe values of 0.17 and 0.94 were calculated from the most reliable (U-Th)/He ages of Zagami (147 Ma) and ALHA77005 (11 Ma), respectively. The diffusion domain radii were determined to be 0.05-6 μm for Zagami and 5-15 μm for ALHA77005 through detailed image analyses of 56 phosphate grains found in two thin sections. These parameters combined with the He diffusion properties of merrillite yield model peak shock temperatures of 213°C - 407°C for Zagami and 520°C - 615°C for ALHA77005. The peak temperatures estimated for Zagami are comparable to post-shock temperatures of 220 ± 50°C [1] when converted to absolute temperatures (>150 ± 50°C) with Martian surface temperatures of -70°C or higher. However, the new estimates are higher than the post-shock temperatures of 70 ± 5°C estimated by [2]. For ALHA77005, the estimated shock temperatures fall between the two post-shock temperatures of 450-600°C [1] and 800 ± 200°C [2] when converted using the average Martian surface temperature of -70°C.
Farah Alejandro
Min Kyoung
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