Effects of terrestrial weathering on the matrix mineralogy of Colony CO3 chondrite

Colony is the least metamorphosed CO chondrite (CO3.0). However, it is badly weathered. In order to show both primary and secondary features of the matrix and discuss the effects of terrestrial weathering on the matrix mineralogy of Colony, we performed combined SEM, EPMA, CL, and TEM studies on this meteorite. EPMA data of the matrix of Colony show that matrix composition was changed by terrestrial weathering. The matrix composition can be represented by three hypothetical components : an Fe-rich component (Fe hydroxide-rich weathering products), a component with olivine-like composition, and an Fe-poor, Si-rich component. TEM observation reveals that the matrix contains abundant Fe-rich poorly crystalline interstitial materials, abundant ferroan olivine (Fa_-Fa_), less abundant magnesian low-Ca pyroxene (En_), magnetite, and minor amounts of spinel group minerals, troilite, and rare Ca-rich pyroxene. Among these phases, anhydrous minerals are primary minerals. The Fe-rich component estimated from EPMA data is composed of abundant Fe-enriched, nearly amorphous material, minute phyllosilicates (mainly saponite, serpentine, and minor amounts of chlorite and montomorillonite), and goethite. The Fe-poor, Si-rich component estimated from EPMA data is also composed of basically the same but seems to contain more phyllosilicates than the Fe-rich one. Matrix olivine crystals often contain planar defects parallel to (001). Such planar defects were probably formed in the initial stage of iddingsite formation by terrestrial weathering

Thermoluminescence studies of ordinary chondrites in the Japanese Antarctic meteorite collection, III: Asuka and Yamato type 3

We measured thermoluminescence (TL) properties of 37 Asuka and 13 Yamato type 3 ordinary chondrites from Japanese Antarctic meteorite collection for determining subtypes and pairing. Most of the meteorites are of petrologic type 3.6-3.9; however, we found three primitive ordinary chondrites (A-9043,A-87319 and Y-793384) of petrologic type ≦3.1. We found 22 TL potential pairing groups in 26 Asuka H3 chondrites comprising a chain of pairing groups, which implies an H3 chondrite shower near the Asuka area

Thermoluminescence study of ordinary chondrites by TL spatial distribution readout system

The thermoluminescence (TL) image reading technique by the TL spatial distribution readout system is improved 1) to obtain a quantitative glow curve in any part of the TL image, 2) to get fine structure of a TL image and 3) to heat a sample to a higher temperature. This technique is applied to measure the natural and artificial TL glow curves of chondrules in ordinary chondrites, ALH-77294 (H5) and ALH-77216 (L3.8). The fluctuation in the natural LT/HT (region) ratios (LT(region); photons counted in a low temperature region, HT (region); in a high temperature region) of the equilibrated chondrite ALH-77294 is small though that in the unequilibrated chondrite ALH-77216 is large. The equivalent doses of ALH-77294 and ALH-77216 can be estimated from the correlation between natural LT (region) and artificial LT (region) to be about 240krad and 16krad respectively, and are consistent with isotopic ages

Thermoluminescence studies of ordinary chondrites in the Japanese Antarctic meteorite collection, IV: Asuka ordinary chondrites

We measured TL properties of ninety Asuka (A) ordinary chondrites (LL: 16, L: 27, H: 47) from Japanese Antarctic meteorite collection. Most of the chondrites had TL sensitivities over 0.1, corresponding to petrologic subtype 3.5-3.9. Eight chondrites, A-881244 (L3), A-881607 (LL3), A-881328 (LL3), A-881408 (LL3), A-881397 (LL3), A-881522 (L3), A-881357 (LL3 or L3), and A-881199 (LL3) were revealed to be primitive ordinary chondrites under petrologic subtype 3.4, and therefore they are particularly significant in understanding the nature of primitive material in the solar system. Twenty-one chondrites with low TL sensitivities below 0.1 (Dhajala=1) had suffered remarkable shock and/or terrestrial weathering. Samples were mainly from three different dirt bands south of the Sr Rondane Mountains at D1, D2, and D3 sites. LT/HT distribution at D1 site suggested that chondrites at D1 site had shorter terrestrial ages than those at D2 and D3. A-880709, A-88710, A-88774, A-881324 which had extremely low LT/HT ratio under 0.1, and A-881484 and A-881546 which might be heated over 400°C were presumed to have small perihelia. We found 26 TL potential paired fragments, and 9 groups. A group of H3 at D1 site comprises a chain of paired fragments. A H3 chondrite might shower near the Asuka

Thermoluminescence of Japanese Antarctic ordinary chondrite collection

Thermoluminescence (TL) data for Japanese Antarctic chondrites obtained by laboratories in Arkansas and Okayama were compared and found to be in good agreement. Data for three large Antarctic chondrites were used to develop new TL pairing criteria which were found to be less restrictive than previously used. These new criteria were applied to ten equilibrated and twenty-eight unequilibrated Japanese ordinary chondrites. The petrographic subtype of the forty-three unequilibrated ordinary chondrites were determined from their TL sensitivity and nine were found to have petrographic types under 3.3 and therefore are particularly primitive samples of solar system material

Thermoluminescence characteristics and chemical compositions of mesostases in ordinary chondrites

Induced thermoluminescence (TL) images of ordinary chondrites, ALH-77214 (L3.4-3.5), Y-74191 (L3.6), ALH-77216 (L3.8) and ALH-78043 (L6), were measured by the TL spatial distribution readout system combined with a microscope and TL characteristics [peak temperature and peak width] of mesostases were analyzed. Their chemical compositions were also analyzed by an electron probe X-ray microanalyzer. We found that; (1) The mesostasis was responsible for much of the TL in the ordinary chondrites, (2) A mesostasis of normative anorthite compositions showed low peak temperature (∿90℃) and narrow width (∿65℃), while a mesostasis of normative albite compositions showed high peak temperature (∿125℃) and wide width (∿100℃), (3) A main phosphor in a low petrologic grade chondrite 3.5 was a high albite mesostasis, (4) Some chondrules in the same fragments of the type 3 chondrites showed no or weak TL emission and these mesostases had high normative albite. These facts suggest that in type 3 ordinary chondrites; (1) The post-accretional metamorphism cannot account for the coexistence of high albite mesostases with TL emission and no emission and a high anorthite mesostasis with TL emission, (2) Low petrologic grade chondrites 3.5 have a large population of slowly cooled chondrules

Thermoluminescence studies of ordinary chondrites in the Japanese Antarctic meteorite collection, II: New measurements for thirty type 3 ordinary chondrites

We have measured the thermoluminescence (TL) properties of thirty type 3 ordinary chondrites from the Japanese Antarctic meteorite collection. This brings to 73 the total number of Japanese type 3 ordinary chondrites examined in this way by the Arkansas-Okayama collaboration. Fifteen pairing groups were found using TL and geographical criteria. Most of the new meteorites are of petrologic types 3.6-3.9,but fourteen are of petrologic type ≤3.4. Six of the 73 meteorites (Yamato (Y)-790448,Y-793596,Y-793565,Y-791324,Y-791558,Y-74660) have petrographic types 3.5) tend to have higher induced TL peak temperature and peak width than those with low TL sensitivity, in confirmation of earlier work and consistent with peak temperature and width as well as TL sensitivity, being independent parameters of thermal history. Samples not obeying these trends (Y-75029,Y-86706,Y-793567 and Y-790787), are either heavily weathered or experienced atypical thermal histories

Shock and thermal annealing history of the ALH 77005 Martian meteorite: a micro-Raman spectroscopical investigation

Abstract We studied optical microscopic and micro-Raman spectroscopic signatures of shocked olivine from the ALH 77005 Martian meteorite sample. The purpose of this study is to document pressure and temperature-related effects in olivine over the entire sample, which can aid in understanding structural changes due to shock metamorphism and the post-shock thermal annealing processes of lherzolitic Martian meteorites. According to the optical microscope observations, three areas may be discernible in olivine of the ALH 77005 in the vicinity of the melt pocket. The first area is the thermally undisturbed part of a grain, which contains a high density of shock-induced planar microdeformations such as Planar Deformation Features (PDFs) and Planar Fractures (PFs). Compared to the first area, the second area shows less shock-induced microstructures, while the third area is a strongly recrystallized region, but not formed from a melt. A common Raman spectral feature of these olivines is a regular doublet peak centered at 823 and 852 cm−1; additionally, two new peaks at 535 and 755 cm−1 appear in the weakly annealed transition zones

Non-luminescent nature of the planar deformation features in shocked quartz from the Ries impact structure, Germany: A new interpretation

Quartz grains from the Ries impact structure containing shock-induced microstructures were investigated using Scanning Electron Microscopy in cathodoluminescence (SEM-CL), secondary electron (SEM-SE) and back-scattered electron (SEM-BSE) modes as well as Mott–Seitz analysis. The purpose of this study is to evaluate the mechanism by which CL detects Planar Deformation Features (PDFs) in quartz, which is one of the most important indicators of shock metamorphism in rock-forming minerals. PDFs are micron-scale features not easily identified using optical microscopy or scanning electron microscopy. The CL spectrum of PDFs in quartz that has suffered relatively high shock pressure shows no or a relatively weak emission band at around 385 nm, whereas an emission band with a maximum near 650 nm is observed independent of shock pressure. Thus, the ~385 nm intensity in shocked quartz demonstrates a tendency to decrease with increasing shock metamorphic stage, whereas the 650 nm band remains fairly constant. The result indicates that the emission band at 385 nm is related to the deformed structure of quartz as PDFs