Micro raman spectroscopy of diamonds in the Canyon Diablo iron meteorite: Implication for shock origin

Raman spectra were measured for diamonds in an inclusion of the Canyon Diablo iron meteorite. The wavenumber position of a Raman peak was found in the range between 1333 and 1318cm^, and the full width at half maximum (FWHM) of the Raman peak ranges from 25 to 117cm^. These results are similar to those of Raman spectra of diamonds synthesized by shock-induced high pressure and are consistent with the consensus that diamond in Canyon Diablo formed by impact shock. No diamond grains showing a relatively narrow FWHM (7cm^) were found in the inclusion

Infrared diffuse reflectance spectra of several thermallymetamorphosed carbonaceous chondrites

Infrared diffuse reflectance spectra (2.53-25μm) were measured for several Antarctic carbonaceous chondrites. Thermally metamorphosed carbonaceous chondrites with CI-CM affinities (Belgica (B)-7904,Yamato (Y)-793321,Y-82162,and Y-86720), which were recently found in the Antarctic collection, show weaker absorption bands near 3μm due to hydrous minerals, compared with the Murchison (CM) carbonaceous chondrite. These results show that hydrous minerals are dehydrated as a result of thermal metamorphism, consistent with the results of chemical, mineralogical, and petrologic studies. B-7904,Y-793321,and Y-86720 show weaker absorption bands near 6.9μm due to carbonates than those of Murchison. For Y-82162,the 1450cm^ (6.9μm) band is relatively strong unlike the other thermally metamorphosed carbonaceous chondrites despite the weak 3μm band of all the thermally metamorphosed ones measured. The shape of the spectral curves near 3μm of the thermally metamorphosed carbonaceous chondrites resembles those of Antarctic ordinary chondrites affected by terrestrial weathering, implying that major absorption features near 3μm of the thermally metamorphosed carbonaceous chondrites may be due to secondary hydrous minerals produced by terrestrial weathering

A model for analysis of the spectral reflectance of mineral mixtures

We proposed a model to determine the spectral reflectance of mineral mixtures as a function of relative abundance of mineral components on the basis of both diffuse spectral reflectance and diffuse spectral transmittance of the components. In order to test the model, we measured spectral reflectances of sized olivine and pyroxene samples separated from Nuevo Mercurio ordinary chondrite and examined variations of spectral reflictances of the olivine-pyroxene mixtures as a function of relative mineral abundance. Spectral reflectances calculated for olivine-pyroxene mixtures based on our model agree well with measured spectra of the mixtures

Lherzolitic Martian meteorites Allan Hills 77005, Lewis Cliff 88516 and Yamato-793605: Major and minor element zoning in pyroxene and plagioclase glass

Three lherzolitic Martian meteorites (ALH77005,LEW88516 and Y-793605) show very similar petrography and mineralogy. They consist of the poikilitic and non-poikilitic (interstitial) areas that are heterogeneous on a cm-scale. A detailed electron microprobe analysis of pyroxene and plagioclase glass from these meteorites gives characteristic distributions of major and minor elements. It is striking that pyroxene and plagioclase glass in all three meteorites have nearly identical zoning patterns of these elements, while olivine shows different chemistry. This is because olivine chemistry was largely controlled by a late-stage re-equilibration which did not significantly modify the major and minor element distributions in pyroxene and plagioclase glass. However, a close look at minor element zoning in plagioclase glass exhibits slightly different zoning patterns among these three meteorites, corresponding to different degrees of re-equilibration (degrees of re-equilibration : ALH77005>LEW88516>Y-793605). Nevertheless, it is evident that they experienced a very similar igneous crystallization history and they are likely to have originated from the same igneous unit on Mars

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

Infrared diffuse reflectance spectra of weathered and unweathered CR2 chondrites: Comparison with asteroids

The presence and intensity of absorption band near 3μm in the reflectance spectra are due to the presence and abundance of hydrates and/or hydroxylions. Previously, we have shown that Renazzo (CR2 chondrite) is an excellent spectral match to 2 Pallas (B-type asteroid) in the wavelength region near 3μm. In this paper, we have measured infrared diffuse reflectance spectra of eight Antarctic CR2 chondrites and have compared reflectance spectra of CR2 chondrites with those of eight asteroids which belong to the C, G, B and F taxonomic classes. The spectral absorption near 3μm of Antarctic CR2 chondrites are deepened and rounded compared to that of Renazzo (fell 1824). This difference implies the presence of corrosion products probably due to terrestrial weathering of the Antarctic CR2 chondrites. This is the reason why no asteroids has been found that has an Antarctic CR2 meteorite counterpart. In addition to the close similarity between Pallas and Renazzo, the overall spectral profile of 511 Davida also exhibit similarity to the Renazzo spectrum

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

Weathering of some Antarctic meteorites: Infrared spectroscopy

We studied the degree of weathering of some Antarctic meteorites using the integrated intensity of absorption bands near 3μm to enlarge the data base of the degree of weathering. There is no clear correlation between the integrated intensity and the degree of weathering on the A-B-C scale. A rough correlation can be seen between the integrated intensity of absorption bands near 3μm and the total amount of H_2O in a meteorite analyzed by a standard wet chemical analysis method. Some Yamato meteorites measured show weaker integrated intensities than the Allan Hills meteorites measured. Our method to determine the degree of weathering may be useful for choosing less weathered meteorites from among the Antarctic meteorite collection