Browsing Meteoritics & Planetary Science, Volume 42, Number 2 (2007) by Title
Now showing items 7-10 of 10
Olivine-dominated asteroids and meteorites: Distinguishing nebular and igneous historiesMelting models indicate that the composition and abundance of olivine systematically co-vary and are therefore excellent petrologic indicators. However, heliocentric distance, and thus surface temperature, has a significant effect on the spectra of olivine-rich asteroids. We show that composition and temperature complexly interact spectrally, and must be simultaneously taken into account in order to infer olivine composition accurately. We find that most (7/9) of the olivine-dominated asteroids are magnesian and thus likely sampled mantles differentiated from ordinary chondrite sources (e.g.,pallasites). However, two other olivine-rich asteroids (289 Nenetta and 246 Asporina) are found to be more ferroan. Melting models show that partial melting cannot produce olivine-rich residues that are more ferroan than the chondrite precursor from which they formed. Thus, even moderately ferroan olivine must have non-ordinary chondrite origins, and therefore likely originate from oxidized R chondrites or melts thereof, which reflect variations in nebular composition within the asteroid belt. This is consistent with the meteoritic record in which R chondrites and brachinites are rare relative to pallasites.
Petrology of the Miller Range 03346 nakhlite in comparison with the Yamato-000593 nakhliteWe petrologically examined the Miller Range (MIL) 03346 nakhlite. The main-phase modal abundances are 67.7 vol% augite, 0.8 vol% olivine, and 31.5 vol% mesostasis. Among all known nakhlites, MIL 03346's modal abundance of olivine is the smallest and of mesostasis is the largest. Augite occurs as cumulus phenocrysts having a homogeneous core composition (En36-38Fs24-22Wo40), which is identical with other nakhlites. They accompany thin ferroan rims divided into inner and outer rims with a compositional gap at the boundary between the two rims. Olivine grains have magnesian cores (Fa is greater than or equal to 55) and show normal zoning toward ferroan rims (Fa is less than or equal to 84). Mesostasis consists mostly of glass (26.0 vol%) with minor skeletal fayalites, skeletal titanomagnetites, acicular phosphate, massive cristobalite, and sulfides. We conclude that MIL 03346 is the most rapidly cooled nakhlite among all known nakhlites based on the petrography.We obtain the intercumulus melt composition for MIL 03346 from the mass balance calculation using the modal abundances and discuss the crystallization sequence of MIL 03346 in comparison with that of Yamato (Y-) 000593. Although magnesian olivines of Y-000593 are phenocrystic, magnesian olivine grains of MIL 03346 seem to have texturally crystallized from the intercumulus melt. After the MIL 03346 magma intruded upward to the Martian surficial zone, the magnesian olivine crystallized, and then the ferroan inner rim formed on phenocrystic core augite. The outer rim of phenocrystic augites formed after the crystallization of skeletal fayalites and skeletal titanomagnetites, resulting in a compositional gap between the inner and outer rims. Finally, glassy mesostasis formed from the residual melt. This crystallization sequence of MIL 03346 is different from those of other nakhlites, including Y-000593.
Spectroscopy of synthetic Mg-Fe pyroxenes I: Spin-allowed and spin-forbidden crystal field bands in the visible and near-infraredUnderstanding the fundamental crystal chemical controls on visible and near-infrared reflectance spectra of pyroxenes is critical to quantitatively assessing the mineral chemistry of pyroxenes viewed by remote sensing. This study focuses on the analysis ofspectroscopic measurements of a comprehensive set of synthetic Mg-Fe pyroxenes from the visible through the near-infrared (0.3-2.6 micrometers) to address the constraints of crystal structure and Fe^2+ content on spin-forbidden and spin-allowed crystal field absorptions in Ca-freeorthopyroxenes. The chemistry and oxidation state of the synthetic pyroxenes are characterized. Coordinated Mössbauer spectroscopy is used to determine site occupancy of Fe^2+ in the M1 and M2 crystallographic sites. Properties of visible and near-infrared absorption bands of the synthetic pyroxenes are quantified using the modified Gaussian model. The 1 and 2 m spin-allowed crystal field absorption bands move regularly with increasing iron content, defining a much tighter trend than observed previously. A spin-allowed crystal field absorption band at 1.2 micrometers is explicitly verified, even at low total iron contents, indicating that some portion of Fe^2+ resides in the M1 site. The 1.2 micrometers band intensifies and shifts to longer wavelengths with increasing iron content. At visible wavelengths, spin-forbidden crystal field absorptions are observed in all iron-bearing samples. The most prominent absorption near 506 nm, attributed to iron in the M2 site, shifts to slightly longer wavelengths with iron content. The purity and extent of this pyroxene series allows visible wavelength absorption bands to be directly assigned to specific transitions of Fe^2+ in the M1 and M2 sites.
Stardust—An artificial, low-velocity "meteor" fall and recovery: 15 January 2006On January 15, 2006, Stardust, a man-made space capsule, plummeted to Earth for a soft landing after spending seven years in space. Since the expected initial speed of the body was about 12.9 km/s, a four-element ground-based infrasound array was deployed to Wendover, Nevada, USA, to measure the hypersonic booms from the re-entry. At a distance of ~33 km from the nominal trajectory, we easily recorded the weak acoustic arrivals and their continued rumbling after the main hypersonic boom arrival. In this paper, we report on subsequent analyses of these data, including an assessment of the expected entry characteristics (dynamics, energetics, ablation and panchromatic luminosity, etc.) on the basis of a bolide/meteor/fireball entry model that was specifically adapted for modeling a re-entering man-made object.Throughout the infrasonic data analyses, we compared our results for Stardust to those previously obtained for Genesis. From the associated entry parameters, we were also able to compute the kinetic energy density conservation properties for the propagating line source blast wave and compared the inviscid theoretical predictions against observed ground-based infrasound amplitude and wave period data as a function of range. Finally, we made a top-down bottom-up assessment of the line source wave normals propagating downward into the complex temperature/sound speed and horizontal wind speed environment during January 15, 2006. This assessment proved to be generally consistent with the signal processing analysis and with the observed time delay between the known Stardust entry and the time of observations of infrasound signals, and so forth.