Shocked plagioclase in the lunar meteorites Yamato-793169 and Asuka-881757: Implications for their shock and thermal histories

Yamato (Y)-793169 and Asuka (A)-881757 are unbrecciated lunar mare meteorites that contain shocked plagioclase as a main constituent phase. Plagioclase in Y-793169 is composed of fine-grained microlitic grains that preserve the overall optical orientation of the original grain. Small vesicles are also observed within plagioclase grains. Plagioclase shows chemical zoning from the An-rich core (An_) to the An-poor rim (An_). These observations suggest that plagioclase in Y-793169 recrystallized from a diaplectic glass by thermal annealing as the heating experiment of a diaplectic plagioclase suggests. The thermal annealing event might occur sometime later than 750Ma that has been derived from the K-Ar study of plagioclase. A-881757 plagioclase has completely transformed into an isotropic glass similar to "maskelynite" in Martian meteorites. Chemical zoning of plagioclase is well preserved, showing a systematic change of composition from the An-rich core (An_) to the slightly An-poor rim (An_). Plagioclase glass partly converts to crystalline (optically single crystal) plagioclase near the fusion crust due to reheating during the atmospheric entry. All of these observations suggest that plagioclase glass in A-881757 is probably a diaplectic glass. Unlike Y-793169,plagioclase glass in A-881757 has not been modified by later thermal events, which is consistent with the old ^Ar-^Ar age (3.8Ga) of A-881757 plagioclase glass. Thus, Y-793169 and A-881757 contained (A-881757 still contains) a diaplectic plagioclase glass that was shock-induced perhaps during the late heavy bombardment of the Moon as their old crystallization ages (3.8-3.9Ga) indicate

Mineralogy and petrology of Y 002712 shergottite

第6回極域科学シンポジウム[OA] 南極隕石11月17日（火）　国立国語研究所 2階 講

Mineralogical similarities and differences between the Los Angeles basaltic shergottite and the Asuka-881757 lunar mare meteorite

Los Angeles, a new basaltic shergottite, is a coarse-grained rock composed of pyroxene and plagioclase ("maskelynite") plus abundant late-crystallization phases. Pyroxenes are Fe-rich and show extensive chemical zoning from pigeonite and augite individual cores to Fe-rich pigeonite rims. The chemical zoning of plagioclase, though it is only slightly zoned, suggests that it grew outward from the most calcic part near the pyroxene walls. One of the most remarkable characteristics of Los Angeles is the presence of complex mixtures of hedenbergite+fayalite+silica, presumably breakdown products of pyroxferroite. Among the previously known basaltic shergottites, QUE94201 shows similar mineralogy to Los Angeles (the melt pockets in Zagami is also similar to Los Angeles). However, Los Angeles is clearly different from QUE94201 in several mineralogical respects, and they do not appear geochemically related. In spite of the origin from the different parent body, the Asuka-881757 lunar mare meteorite shows remarkable similarities to Los Angeles. Asuka-881757 is composed of large Fe-rich pyroxene and plagioclase grains with abundant hedenbergite+fayalite+silica mixtures. The pyroxene zoning and exsolution features of Asuka-881757 are similar to those of Los Angeles rather than QUE 94201. It is likely that Los Angeles and Asuka-881757 experienced similar crystallization and cooling histories although they came from different planetary bodies, Mars and the Moon, respectively

Yamato-793605: A new lherzolitic shergottite from the Japanese Antarctic meteorite collection

Y-793605 is a new martian meteorite from Antarctica that can be classified as a lherzolitic shergottite. Y-793605 mainly shows a poikilitic texture (large pyroxene oikocryst with enclosed olivine and chromite), but partly contains non-poikilitic areas (mainly maskelynite, olivine, and pigeonite). Olivine in the non-poikilitic area is more Fe-rich and shows a narrower compositional distribution than that in the poikilitic area. Low-Ca pyroxenes in the non-poikilitic area are also more Fe-rich (En_Fs_Wo_7∿En_Fs_Wo_) than those in the poikilitic area (En_Fs_Wo_3∿En_Fs_Wo_). Augites in the poikilitic area are usually present rimming the oikocrysts (En_Fs_Wo_ to En_Fs_Wo_). The crystallization sequence of minerals in Y-793605 is considered to have begun by initial crystallization of cumulus phases (olivine and chromite) from the parent magma. Then, low-Ca pyroxenes and later augite poikilitically enclosed cumulus phases, and became a large oikocryst. Due to accumulation of phases, small interstitial melts formed between the oikocryst boundaries, and plagioclase crystallized from the Ca-Fe-rich melt along with pigeonite. After minor augite crystallization in the non-poikilitic area, all phases experienced re-equilibration (e. g., homogenization of olivine). Y-793605 shows a close relationship to previously known lherzolitic shergottites ALH77005 and LEW88516. Especially, olivine composition of Y-793605 is nearly identical to that of LEW88516. Pyroxene and maskelynite compositions are almost the same among these three meteorites. Although it is hardly possible to consider that Y-793605 is paired with ALH77005 or LEW88516 in the sense that Y-793605 was in the same fall with them, it can be concluded that Y-793605 originated from the same igneous body or rock in Mars as ALH77005 and LEW88516

Mineralogy and olivine cooling rate of the Dhofar 019 shergottite

Dhofar 019 is a new basaltic shergottite found in Oman. It is mainly composed of pyroxenes, plagioclase glass, and olivine. Olivine grains show variable size distributions from large (∿1mm) non-euhedral zoned grains with magnesian cores (Fa_) to small euhedral homogeneous grains with variable compositions (Fa_). This may suggest that some of the Dhofar 019 olivines are xenocrysts and others are phenocrysts. The Dhofar 019 olivines are compositionally distinct from those in other shergottites. Pyroxenes are extensively zoned, and pigeonite cores are usually surrounded with augite rims, suggesting undercooling of magma to a similar degree as that of EETA 79001 (lithology A) and Zagami. There appears to be a compositional change in major and minor elements from the magnesian pyroxenes to ferroan pyroxenes for both pigeonite and augite. The magnesian parts have similar compositions to EETA 79001 (lithology A) pyroxenes, and the ferroan parts are similar to Zagami pyroxenes. Plagioclase zoning in Dhofar 019 more closely resembles that of Zagami. Spinel compositions in Dhofar 019 overlap those of EETA79001,and the Dhofar 019 chromite may be a xenocryst like EETA79001. The cooling rate of Dhofar 019 calculated from Fe-Mg zoning profiles of olivine is 0.05-0.1℃/hour. This corresponds to a burial depth of ∿5m from the martian surface, possibly in a lava flow. Thus, the groundmass crystallization of Dhofar 019 occurred near the martian surface, although it is likely that Dhofar 019 has experienced a complex history

Diffuse reflectance spectra for heated samples of an H5 chondrite: Importance of oxygen fugacity at heating

We obtained (biconical) diffuse reflectance spectra of the Nuevo Mercurio (H5) ordinary chondrite in the 200-2500 nm wavelength region. The samples were heated in the temperature range of 800-1200℃ at constant oxygen fugacities at one log unit below the iron-wustite (IW) buffer (IW-1) and two log units above the IW buffer (IW+2). The spectra of the samples heated to temperatures lower than 1050℃ at IW+2 show low spectral contrast and shorter wavelength positions of UV drop-off compared with the unheated <100 μm sample. On the basis of our heating experiments, this work suggests that oxygen fugacity affects the spectra of the heated samples especially at IW+2,implying that oxygen-fugacity control is important for heating experiment and that oxygen fugacity may play a role in the surface processes on asteroids

Comparison of Fe-Mg interdiffusion coefficients in olivine

We have compared Fe-Mg interdiffusion coefficients in olivine reported in several literatures by analyzing experimentally produced diffusion profiles. The chemical zoning profiles of olivine measured with an electron microprobe were compared with those calculated by numerically solving the diffusion equation by using different diffusion coefficients. For our experimental results, the Fe-Mg interdiffusion coefficient in olivine reported by D. J. Misener (Carnegie Inst. Washington Publ., 634,117,1974) with oxygen fugacity dependence gives the best fit to the observed profile. The Fe-concentration dependence of the Fe-Mg interdiffusion coefficient in olivine is important when the Fe content varies widely

Micro-Raman spectroscopic analysis of darkened olivine in Martian meteorites

第6回極域科学シンポジウム[OA] 南極隕石11月17日（火）　国立国語研究所 2階 講

Cooling rates of olivine xenocrysts in the EET79001 shergottite

The chemical zoning profiles of olivine xenocrysts in lithology A of the Elephant Moraine (EET) 79001 basaltic shergottite have been measured in order to investigate its cooling history. Olivines are zoned from Fa_ in the cores to Fa_ in the rims. The major and minor element compositions of olivine xenocryst cores in EET79001 correspond to those of homogeneous olivines in the lherzolitic shergottites, reconfirming the previous studies. We assume that the initial profile of olivine was uniform and it was modified by atomic diffusion due to contact with the Fe-rich surrounding melt when olivine was incorporated as a xenocryst. Comparing the observed Fe zoning profiles with the calculated ones, we have estimated the minimum cooling rates of the xenocrystic olivines by numerically solving the diffusion equation. The inferred minimum cooling rates (0.07-0.5℃) are faster than that for Zagami estimated from the width of pyroxene lamella, consistent with a hypothesis that EET79001 experienced significant undercooling unlike Shergotty and Zagami. We also performed the calculations by applying two different Fe-Mg interdiffusion coefficients to evaluate them. There is no significant difference in the curvature of the calculated profiles between the two diffusion coefficients, whereas one gives roughly fivefold higher cooling rates than the other