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METEORITICS &
PLANETARY
SCIENCE
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Amino acid analyses of Antarctic CM2 meteorites using liquid
chromatography–time of flight–mass spectrometry
Amino acid analyses of the Antarctic CM2 chondrites Allan Hills (ALH)
83100 and Lewis Cliff (LEW) 90500 using liquid chromatography–time of
flight–mass spectrometry (LC-ToF-MS) coupled with UV fluorescence
detection revealed that these carbonaceous meteorites contain a suite of
indigenous amino acids not present in Antarctic ice. Several amino acids
were detected in ALH 83100, including glycine, alanine, β-alanine,
γ-amino-n-butyric acid (γ-ABA), and α-aminoisobutyric acid (AIB) with
concentrations ranging from 250 to 340 parts per billion (ppb). In
contrast to ALH 83100, the CM2 meteorites LEW 90500 and Murchison had a
much higher total abundance of these amino acids (440–3200 ppb). In
addition, ALH 83100 was found to have lower abundances of the α-dialkyl
amino acids AIB and isovaline than LEW 90500 and Murchison. There Northwest Africa 1500:
Plagioclase-bearing monomict ureilite or ungrouped achondrite? Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (~100–500 µm) olivine (95–96%), augite (2–3%), and chromite (0.6–1.6%) in an equilibrated texture. Plagioclase (0.7–1.8%) occurs as poikilitic grains (up to ~3 mm) in vein-like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl-apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65–73, with a strong peak at Fo 68–69. Most grains are reversezoned, and also have ~10–30 µm reduction rims. In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo ≥75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite-bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augitebearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800–1000 °C, from two-pyroxene and olivine-chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history. Confirmation of a
meteoritic component in impact-melt rocks of the Chesapeake Bay impact
structure, Virginia, USA—Evidence from osmium isotopic and PGE systematics The osmium isotope ratios and platinum-group element (PGE) concentrations of impact-melt rocks in the Chesapeake Bay impact structure were determined. The impact-melt rocks come from the cored part of a lower-crater section of suevitic crystalline-clast breccia in an 823 m scientific test hole over the central uplift at Cape Charles, Virginia. The 187Os/188Os ratios of impact-melt rocks range from 0.151 to 0.518. The rhenium and platinum-group element (PGE) concentrations of these rocks are 30–270× higher than concentrations in basement gneiss, and together with the osmium isotopes indicate a substantial meteoritic component in some impact-melt rocks. Because the PGE abundances in the impact-melt rocks are dominated by the target materials, interelemental ratios of the impact-melt rocks are highly variable and nonchondritic. The chemical nature of the projectile for the Chesapeake Bay impact structure cannot be constrained at this time. Model mixing calculations between chondritic and crustal components suggest that most impact-melt rocks include a bulk meteoritic component of 0.01–0.1% by mass. Several impact-melt rocks with lowest initial 187Os/188Os ratios and the highest osmium concentrations could have been produced by additions of 0.1%–0.2% of a meteoritic component. In these samples, as much as 70% of the total Os may be of meteoritic origin. At the calculated proportions of a meteoritic component (0.01–0.1% by mass), no mixtures of the investigated target rocks and sediments can reproduce the observed PGE abundances of the impact-melt rocks, suggesting that other PGE enrichment processes operated along with the meteoritic contamination. Possible explanations are 1) participation of unsampled target materials with high PGE abundances in the impact-melt rocks, and 2) variable fractionations of PGE during syn- to post-impact events. Magmatic cristobalite and
quartz in the NWA 856 Martian meteorite Silica-rich late-stage crystallization pockets in the Martian meteorite Northwest Africa (NWA) 856 were investigated by transmission electron microscopy (TEM). The pockets occur as wedges between maskelynite laths or between maskelynite and pyroxene. They consist of elongated grains of cristobalite and quartz embedded in a silica-rich glass. Interstitial to the amorphous phase and silica minerals, a number of small accessory minerals have been identified, typical for late-stage crystallization products. They are ilmenite, tranquillityite, fayalite, troilite, baddeleyite, apatite, and chloroapatite. Cristobalite and quartz are shocked, as revealed by the occurrence of numerous amorphous lamellae. This assemblage suggests metastable dendritic crystallization under hydrous conditions. Cristobalite crystallization was probably facilitated by the presence of impurities such as Na or H2O. Our observations show that silica minerals can be formed under magmatic conditions on Mars. Survival of organic phases
in porous IDPs during atmospheric entry: A pulse-heating study In this study, we have performed pulse-heating experiments at different temperatures for three organic molecules (a polycyclic aromatic hydrocarbon [PAH], a ketone, and an amino acid) absorbed into microporous aluminum oxide (Al2O3) in order to imitate the heating of the organic molecules in interplanetary dust particles (IDPs) and micrometeorites (MMs) during atmospheric entry and to investigate their survival. We have shown that modest amounts (a few percent) of these organic molecules survive pulse-heating at temperatures in the 700 to 900 °C range. This suggests that the porosity in IDPs and MMs, combined with a sublimable phase (organic material, water), produces an ablative cooling effect, which permits the survival of organic molecules that would otherwise be lost either by thermal degradation or evaporation during atmospheric entry. Formation of the binary
near-Earth object 1996 FG3: Can binary NEOs be the source of
short-CRE meteorites? 1996 FG3 is a binary near-Earth object (NEO) that was likely formed during a tidal disruption event. Our results indicate that the formation of this binary object was unlikely to have occurred when the progenitor had a encounter velocity with the Earth significantly smaller than its current value (10.7 km/s); The formation of the binary object on an orbit similar to the present one is possible, and the survival of the satellite constrains this to have happened less than 1.6 Ma ago. However, the binary object could also have been formed when the progenitor's encounter velocity with Earth was >12 km/s, and in this case we cannot constrain its formation age. Our results indicate that tidal disruptions occurring among NEOs with low velocity encounters with Earth are unlikely to produce long-lasting NEO binaries. Thus, tidal disruption may not be able to completely re-supply the observed population. This would imply that a significant fraction of the observed NEO binaries evolved out of the main asteroid belt. Overall, our results suggest to us that the CM2 meteorites having cosmic ray exposure (CRE) ages of ~200,000 yr were likely liberated by the tidal disruption of a primitive NEO with a relative velocity with the Earth significantly smaller than that of 1996 FG3. We propose a list of such objects, although as far as we know, none of the candidates is a binary for the reasons described above. Cosmic-ray exposure
age and
heliocentric distance of the parent bodies of enstatite chondrites ALH
85119 and MAC 88136 We measured concentrations and
isotopic ratios of noble gases in enstatite (E) chondrites Allan Hills (ALH)
85119 and MacAlpine Hills (MAC) 88136. These two meteorites contain solar
and cosmogenic noble gases. Based on the solar and cosmogenic noble gas
compositions, we calculated heliocentric distances, parent body exposure
ages, and space exposure ages of the two meteorites. The parent body
exposure ages are longer than 6.7 Ma for ALH 85119 and longer than 8.7 Ma
for MAC 88136. The space exposure ages are shorter than 2.2 Ma for ALH
85119 and shorter than 3.9 Ma for MAC 88136. The estimated heliocentric
distances are more than 1.1 AU for ALH 85119 and 1.3 AU for MAC 88136.
Derived heliocentric distances indicate the locations of parent bodies in
the past when constituents of the meteorites were exposed to the Sun. From
the mineralogy and chemistry of E chondrites, it is believed that E
chondrites formed in regions within 1.4 AU from the Sun. The heliocentric
distances of the two E chondrite parent bodies are not different from the
formation Stellar nucleosynthetic
contribution of extinct short-lived nuclei in the early solar system and
the associated isotopic effects A wide range of stellar nucleosynthetic sources has been analyzed to derive their contributions of short-lived and stable nuclei to the presolar cloud. This detailed study is required to infer the most plausible source(s) of short-lived nuclei through a critical comparison among the various stellar sources that include AGB stars, novae, supernovae II, Ia, and Wolf-Rayet stars that evolved to supernovae Ib/c. In order to produce the canonical value of 26Al/27Al in the early solar system, almost all stellar sources except low-mass AGB stars imply large isotopic anomalies in Ca-Al-rich inclusions (CAIs). This is contrary to the observed isotopic compositions of CAIs. The discrepancy could impose stringent constraints on the formation and thermal evolution of CAIs from different chondrites. Among the various stellar scenarios, the injection of short-lived nuclei into the previously formed solar protoplanetary disc by a massive star of an ad hoc chosen high-injection mass cut is a possible scenario. There is a possibility of the contribution of short-lived nuclides by a 1.5–3 Mu AGB star as it implies the smallest shift in stable isotopes. A low-mass AGB star of relatively low metallicity would be even a better source of short-lived nuclei. However, this scenario would require independent gravitational collapse of the presolar cloud coupled with ambipolar diffusion of magnetic flux. Alternatively, numerous scenarios can be postulated that involve distant (≥10 pc) massive stars can contribute 60Fe to the presolar cloud and can trigger its gravitational collapse. These scenarios would require production of 26Al and 41Ca by irradiation in the early solar system. Significant production of 26Al and 60Fe can be explained if massive, rotating Wolf-Rayet stars that evolved to supernovae Ib/c were involved. Ureilite petrogenesis:
A
limited role for smelting during anatexis and catastrophic disruption A popular model for ureilites assumes that during anatexis in an asteroidal mantle, pressure-buffered equilibrium smelting (partial reduction coincident with partial melting) engendered their conspicuous mafic-silicate-core mg diversity (75–96 mol%). Several mass-balance problems arise from this hypothesis. Smelting inevitably consumes a large proportion of any plausible initial carbon while generating significant proportions of Fe metal and copious proportions of CO gas. The most serious problem concerns the yield of CO gas. If equilibrium smelting produced the ureilites’ entire 21 mol% range in olivine-core mg, the proportion of gas within the asteroidal mantle (assuming plausibly low pressure <~80 bar) should have reached ≥85 vol%. Based on the remarkably stepwise cooling history inferred from ureilite texture and mineralogy, a runaway, CO-leaky process that can loosely be termed smelting appears to have occurred, probably triggered by a major impact. The runaway scenario appears likely because, by Le Châtelier’s principle, CO leakage would tend to accelerate the smelting process. Also, the copious volumes of gas produced by smelting would have led to explosive, mass-leaky eruptions into the vacuum surrounding the asteroid. Loss of mass would mean diminution of interior pressure, which would induce further smelting, leading to further loss of mass (basalt), and so on. Such a disruptive runaway process may have engendered the ureilites’ distinctive reduced olivine rims. But the only smelting, according to this scenario, was a short-lived disequilibrium process that reduced only the olivine rims, not the cores; and the ureilites were cooling, not melting, during the abortive “smelting” episode. An anomalous eucrite,
Dhofar 007, and a possible genetic relationship with mesosiderites We studied the texture, mineralogy, and bulk chemical composition of Dhofar 007, a basaltic achondrite. Dhofar 007 is a polymict breccia that is mostly composed of coarse-grained granular (CG) clasts with a minor amount of xenolithic components, such as a fragment of Mg-rich pyroxene. The coarse-grained, relict gabbroic texture, mineral chemistry, and bulk chemical data of the coarse-grained clast indicate that the CG clasts were originally a cumulate rock crystallized in a crust of the parent body. However, in contrast to monomict eucrites, the siderophile elements are highly enriched and could have been introduced by impact events. Dhofar 007 appears to have experienced a two-stage postcrystallization thermal history: rapid cooling at high temperatures and slow cooling at lower temperatures. The presence of pigeonite with closely spaced, fine augite lamellae suggests that this rock was cooled rapidly from higher temperatures (>0.5 °C/yr at ~1000 °C) than typical cumulate eucrites. However, the presence of the cloudy zone in taenite and the Ni profile across the kamacite-taenite boundaries indicates that the cooling rate was very slow at lower temperatures (~1–10 °C/Myr at <600–700 °C). The slow cooling rate is comparable to those in mesosiderites and pallasites. The two-stage thermal history and the relative abundance of siderophile elements similar to those for metallic portions in mesosiderites suggest that Dhofar 007 is a large inclusion of mesosiderite. However, we cannot rule out a possibility that Dhofar 007 is an anomalous eucrite. |
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This website is maintained by Agnieszka P. Baier. Website credits. Last updated: 10/27/06.
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