bluuebunny/crossref_metadata_2025 huggingface dataset:
Melted micrometeorites from Antarctic ice with evidence for the separation of immiscible Fe‐Ni‐S liquids during entry heating
Authors: GENGE MATTHEW J. , GRADY MONICA M.
Published: February 2010

Abstract—We report the discovery of four large (>50 μm) cosmic spherules (CSs) and a single scoriaceous micrometeorite (SMM) that contain evidence for the separation of immiscible Fe‐Ni‐S liquids during atmospheric entry heating. The particles contain segregated Fe‐rich regions dominated by either Ni‐S‐bearing Fe‐oxides or iron sulphides and have textural relations that suggest these separated from the silicate portions of the particles as metallic liquids. The oxides, which may be hydrous, are thought to result from alteration of metal and sulphide. The compositions of the silicate portions of the CSs are equivalent to spherules without Fe‐rich regions, implying that metallic liquids are exsolved during the heating of most spherules, but completely separate. The single SMM has a very different composition from other scoriaceous particles, and the occurrence of an exsolved metallic liquid probably indicates extreme reduction during entry heating. The pyrolysis of carbonaceous materials is the most likely explanation for reduction and suggests that the precursor material of this particle was unusually C‐rich. This SMM might be, therefore, an appropriate candidate for a large melted anhydrous or smectite interplanetary dust particle (IDP). The exsolution of immiscible Fe‐Ni‐S liquids during entry heating will result in systematic changes in the compositions of the remaining silicate melt.
The orbit and exposure history of the Piplia Kalan eucrite
Authors: BHANDARI N. , MURTY S. V. S. , SUTHAR K. M. , SHUKLA A. D. , BALLABH G. M. , SISODIA M. S. , VAYA V. K.
Published: February 2010

Abstract— Cosmogenic radionuclides, particle tracks and rare gases have been measured in two fragments of the Piplia Kalan eucrite that fell in Rajasthan, India on 1996 June 20. The cosmic‐ray exposure age of the meteorite is calculated to be 23 Ma, which is similar to ages of some other eucrites. The track density in feldspars and pyroxenes varies between 0.2 × 106 to ∼4.5 × 106 cm−2. The mass ablation of the meteorite, based on the distribution of track density in near‐surface samples of the two fragments, is calculated to be ∼75%, which corresponds to an entry velocity of ∼17 km/s. The orbital parameters of the eucrite have been computed from the radiant of the meteor trail and the geocentric velocity. The best estimates are a = 2.47 AU, e = 0.62 and i = 7.54°, which is similar to the orbital elements of other meteorites, most of which have been inferred to originate within 2.6 AU of the Sun. The activity of the radionuclide 26Al agrees with the expected production rate; whereas the shortlived radionuclides 22Na, 54Mn, 46Sc etc. have levels that are consistent with the galactic cosmic‐ray fluxes that are expected during the solar minimum period before the time of fall. All the cosmogenic effects (i.e., radio‐ and stable‐ nuclides and particle tracks) are consistent with the meteoroid having had a simple, one‐stage exposure history in interplanetary space. Lower radio genic ages of U, Th‐He (0.7 Ga) and K‐Ar (3.6 Ga) indicate severe losses of 4He and 40Ar, as observed in most eucrites. A Pu‐Xe age, concordant with Angra dos Reis, shows that Piplia belongs to the “old” eucrite group.
The Aouelloul crater, Mauritania: On the problem of confirming the impact origin of a small crater
Authors: KOEBERL CHRISTIAN , REIMOLD WOLF UWE , SHIREY STEVEN B.
Published: February 2010

Abstract— The impact origin of small craters in sedimentary rocks is often difficult to confirm because of the lack of characteristic shock metamorphic features. A case in point is the 3.1 Ma Aouelloul crater (Mauritania), 390 m in diameter, which is exposed in an area of Ordovician Oujeft and Zli sandstone. We studied several fractured sandstone samples from the crater rim for the possible presence of shock metamorphic effects. In thin section, a large fraction of the quartz grains show abundant subplanar and planar fractures. Many of the fractures are healed and are evident only as fluid inclusion trails. A few grains showed sets of narrow and densely spaced fluid inclusions trails in one (rarely two) orientations per grain, which could be possible remnants of planar deformation features (PDFs), although such an interpretation is not unambiguous. In contrast, an impact origin of the crater is confirmed by Re‐Os isotope studies of the target sandstone and glass found around the crater rim, which show the presence of a distinct extraterrestrial component in the glass.
Carbonaceous micrometeorites from Antarctica
Authors: ENGRAND CÉCILE , MAURETTE MICHEL
Published: February 2010

Abstract— Over 100 000 large interplanetary dust particles in the 50–500 μm size range have been recovered in clean conditions from ∼600 tons of Antarctic melt ice water as both unmelted and partially melted/dehydrated micrometeorites and cosmic spherules. Flux measurements in both the Greenland and Antarctica ice sheets indicate that the micrometeorites deliver to the Earth's surface ∼2000× more extraterrestrial material than brought by meteorites. Mineralogical and chemical studies of Antarctic micrometeorites indicate that they are only related to the relatively rare CM and CR carbonaceous chondrite groups, being mostly chondritic carbonaceous objects composed of highly unequilibrated assemblages of anhydrous and hydrous minerals. However, there are also marked differences between these two families of solar system objects, including higher C/O ratios and a very marked depletion of chondrules in micrometeorite matter; hence, they are “chondrites‐without‐chondrules.” Thus, the parent meteoroids of micrometeorites represent a dominant and new population of solar system objects, probably formed in the outer solar system and delivered to the inner solar system by the most appropriate vehicles, comets. One of the major purposes of this paper is to discuss applications of micrometeorite studies that have been previously presented to exobiologists but deal with the synthesis of prebiotic molecules on the early Earth, and more recently, with the early history of the solar system.
Geochemistry and petrology of lunar meteorite Queen Alexandra Range 94281, a mixed mare and highland regolith breccia, with special emphasis on very‐low‐titanium mafic components
Authors: JOLLIFF BRADLEY L. , KOROTEV RANDY L. , ROCKOW KAYLYNN M.
Published: February 2010

Abstract— Queen Alexandra Range (QUE) 94281, a lunar meteorite recently discovered in Antarctica, is a glassy‐matrix, clast‐rich regolith breccia containing a mixture of mafic, volcanic‐glass and gabbroic constituents and a diverse set of highland constituents. In thin section, the clast assemblage is dominated by coarse mineral debris from a shallow intrusive or hypabyssal setting, or from deep within a thick mare flow. Abundant coarse‐grained pyroxene clasts have fine‐scale exsolution lamellae and compositions similar to pyroxenes of known lunar very‐low‐Ti (VLT) basalts and other lunar meteorites of basaltic composition. Pyroxene compositions follow Fe‐enrichment extending to hedenbergite, which is associated with fayalite and cristobalite, indicating slow cooling. We refer to the protolith of the crystalline VLT component as VLT gabbro. Fragments of pyroclastic glasses that have high Fe and low Ti concentrations, similar to the pyroclastic green glasses known from Apollo samples, are common. Lithic clasts include abundant subrounded, glassy to cryptocrystalline, aluminous (∼17–30 wt% Al2O3) KREEP‐poor melt breccias of highland origin and a variety of other feldspathic impactites. On the basis of composition of our subsamples, QUE 94281 consists of ∼54 wt% mafic or “mare” components and 46 wt% feldspathic or “highland” components.

The bulk composition of QUE 94281 is similar to that of Yamato (Y) 793274, but QUE 94281 has slightly greater concentrations of some siderophile elements and slightly lower concentrations of those elements contributed mainly by mafic constituents. Differences in siderophile element concentrations are consistent with longer surface exposure of QUE 94281. Minor differences in trace element variations of subsamples of the two meteorites suggest subtle differences in the composition of their highland constituents. Nonetheless, the overall similarity of compositions supports the possibility that they were ejected from the same source region on the Moon.

The crystalline VLT component of QUE 94281 differs from those known from Apollo 17 and Luna 24 VLT lithologies and from that of basaltic breccia Elephant Moraine (EET) 87521. The VLT‐gabbro component and the ferroan VLT volcanic glasses in QUE 94281 have compositions that may be petrogenetically related by derivation from a common picritic parent composition, represented by an ultramafic glass found in QUE 94281.
Secondary calcium‐iron‐rich minerals in the Bali‐like and Allende‐like oxidized CV3 chondrites and Allende dark inclusions
Authors: KROT ALEXANDER N. , PETAEV MICHAEL I. , ZOLENSKY MICHAEL E. , KEIL KLAUS , SCOTT EDWARD R. D. , NAKAMURA KEIKO
Published: February 2010

Abstract— We have characterized Ca‐Fe‐rich silicates (salite‐hedenbergite pyroxenes (Fs10–50Wo45–50), andradite (Ca3Fe2Si3O12), kirschsteinite (CaFeSiO4), and wollastonite (Ca3Si3O9)) in the type I chondrules and matrices in the Bali‐like and Allende‐like oxidized CV3 chondrites and Allende dark inclusions. In type I chondrules in the Bali‐like CV3 chondrites, metal is oxidized to magnetite; magnetite‐sulfide nodules are replaced by Ca‐Fe‐rich pyroxenes with minor andradite and pure fayalite. We infer that Ca‐Fe‐rich pyroxenes, andradite, fayalite, magnetite, and phyllosilicates (which occur in mesostases) formed at relatively low temperatures (<300 °C) in the presence of aqueous solutions. Thermodynamic analysis of phase relations in the Si‐Fe‐Ca‐O‐H system and large O isotopic fractionation of the coexisting magnetite and fayalite (∼20%) (Krot et al., 1998) are consistent with this interpretation.

In type I chondrules in the Allende‐like CV3 chondrites and dark inclusions, magnetite‐sulfide nodules are replaced by Ca‐Fe‐rich pyroxenes and ferrous olivine; low‐Ca pyroxene and forsterite phenocrysts are rimmed and veined by ferrous olivine. It appear that the Ca‐Fe‐rich pyroxenes predate formation of ferrous olivine; the latter postdates formation of talc and biopyriboles (Brearley, 1997). The Allende dark inclusions are crosscut by Ca‐Fe‐pyroxene‐andradite veins and surrounded by Ca‐rich rims that consist of Ca‐Fe‐rich pyroxenes, andradite, wollastonite, and kirschsteinite. Calcium‐rich veins and rims formed after aggregation and lithification of the dark inclusions. The rimmed dark inclusions show zoned depletion in Ca, which is due to a lower abundance of Ca‐Fe‐rich pyroxenes close to the rim. Calcium was probably leached from the inclusions and redeposited along their edges. We infer that the Allende‐like chondrites and dark inclusions experienced similar aqueous alteration to the Bali‐like chondrites and were metamorphosed subsequently, which resulted in loss of aqueous solutions and dehydration of phyllosilicates.

We conclude that Ca‐Fe‐rich silicates in the oxidized CV3 chondrites and Allende dark inclusions are secondary and resulted from aqueous fluid‐rock interactions during progressive metamorphism of a heterogeneous mixture of hydrous (ices?) and anhydrous materials; the latter were possibly mineralogically similar to the reduced CV3 chondrites.
Exposure of Allan Hills 84001 and other achondrites on the Antarctic ice
Authors: KräHENBÜHL U. , NOLL K. , DÖBELI M. , GRAMBOLE D. , HERRMANN F. , TOBLER L.
Published: February 2010

Abstract— The enrichment of F on Antarctic meteorites is the result of their exposure to the atmosphere, and its measurement allows a subdivision of the terrestrial age into a duration of exposure on the ice and the time a meteorite was enclosed by the ice. In many cases, the periods of surface exposure are only small fractions of the terrestrial ages of meteorites collected in Antarctica. The enrichment of F on the surfaces of Antarctic achondrites was investigated by means of nuclear reaction analysis (NRA): scanning proton beams with an energy of 2.7 and 3.4 MeV were used to induce the reactions 19F(p,αγ)16O and 19F(p, p'γ)19F, respectively. Gamma signals proportional to the F content were measured. The following Antarctic achondrites were investigated: Martian meteorite ALH 84001; diogenite ALHA77256; the eucrites ALHA81011 and ALHA78132; and in addition, the H5 chondrite ALHA79025. For ALH 84001, our data indicate a period of exposure on the ice of <500 years. Thus, this specimen was enclosed in the ice >95% of its terrestrial age of 13 000 years.
Recognition of minor constituents in reflectance spectra of Allan Hills 84001 chips and the importance for remote sensing on Mars
Authors: BISHOP JANICE L. , MUSTARD JOHN F. , PIETERS CARLÉ M. , HIROI TAKAHIRO
Published: February 2010

Abstract— Reflectance spectra of splits 92 and 271 from the Martian meteorite Allan Hills (ALH) 84001 are presented and analyzed in this paper. Although the visible and infrared spectra of both chips show that the dominant mineralogy is low‐Ca pyroxene, the focus here is on identification of the minor constituents. Infrared spectra measured at multiple spots along the surface of chips 92 and 271 show subtle spectroscopic variations due to changes in the low‐Ca pyroxene texture and composition and to the presence of secondary minerals. Absorption bands observed near 0.93 and 1.95 μm are characteristic of low‐Ca pyroxene. Strong mid‐infrared reststrahlen bands are observed near 9 and 19.5 μm in all surface spectra, and additional bands near 7, 10.5, 11.4, 17.8, 20.5 and 23 μm are variable depending on the low‐Ca pyroxene texture and the presence of secondary minerals. Selected spectra exhibit carbonate features near 4, 6.4–7.1 and 11.3 μm. Detailed analysis of these carbonate features indicates the presence of Mg‐Fe carbonate, which is consistent with petrographic studies. Many of these spectra with strong carbonate features exhibit a magnetite feature near 17.9 μm and a shoulder near 20.5 μm that cannot be uniquely ascribed to one mineral. Spectroscopic identification of the minor carbonate and magnetite minerals in this probable piece of Mars indicates that detection of small amounts of these minerals of possible biological significance will be possible using infrared hyperspectral analyses of the Martian surface. Also of importance for remote sensing on Mars is the result that Mg, Fe and Mg‐Fe carbonates in a low‐Ca pyroxene matrix should be distinguishable from one another in the spectral region measured by the thermal emmitance spectrometer (TES).
Martian “microfossils” in lunar meteorites?
Authors: SEARS DEREK W. G. , KRAL TIMOTHY A.
Published: February 2010

Abstract— One of the five lines of evidence used by McKay et al. (1996) for relic life in the Martian meteorite Allan Hills (ALH) 84001 was the presence of objects thought to be microfossils. These ovoid and elongated forms are similar to structures found in terrestrial rocks and described as “nanobacteria” (Folk, 1993; McBride et al, 1994). Using the same procedures and apparatus as McKay et al. (1996), we have found structures on internal fracture surfaces of lunar meteorites that cannot be distinguished from the objects described on similar surfaces in ALH 84001. The lunar surface is currently a sterile environment and probably always has been. However, the lunar and Martian meteorites share a common terrestrial history, which includes many thousands of years of exposure to Antarctic weathering. Although we do not know the origin of these ovoid and elongated forms, we suggest that their presence on lunar meteorites indicates that the objects described by McKay et al. (1996) are not of Martian biological origin.
Presolar silicon carbide grains and their parent stars
Authors: LODDERS K. , FEGLEY B.
Published: February 2010

Abstract— Carbon stars are an important source of presolar TiC, SiC, and graphite grains found in meteorites. The elemental abundances in the stellar sources of the SiC grains are inferred by using condensation calculations. These elemental abundances, together with C isotopic compositions, are used to identify possible groups of carbon stars that may have contributed SiC grains to the presolar dust cloud. The most likely parent stars of meteoritic SiC mainstream grains are N‐type carbon stars and evolved subgiant CH stars. Both have s‐process element abundances higher than solar and 10 < 12C/13C < 100 ratios. The J stars and giant CH stars, with solar and greater than solar abundances of s‐process elements, respectively, are good candidate parents for the ‘A’ and ‘B’ SiC grains with low 12C/13C ratios. A special subgroup of CH giant stars with very large 12C/13C ratios could have parented the ‘Y’ SiC grains with 12C/13C ratios > 100. The carbon star population (e.g., N, R, J, CH groups) needed to provide the observed SiC grains is compared to the current population of carbon stars. This comparison suggests that low‐metallicity CH stars may have been more abundant in the past (>4.5 Ga ago) than at present. This suggestion is also supported by condensation‐chemistry modeling of the trace element patterns in the SiC grains that shows that subsolar Fe abundances may be required in the stellar sources for many SiC grains. The results of this study suggest that presolar SiC grains in meteorites can provide information about carbon stars during galactic evolution.
Progressive alteration in CV3 chondrites: More evidence for asteroidal alteration
Authors: KROT ALEXANDER N. , PETAEV MICHAEL I. , SCOTT EDWARD R. D. , CHOI BYEON‐GAK , ZOLENSKY MICHAEL E. , KEIL KLAUS
Published: February 2010

Abstract— The oxidized CV3 chondrites can be divided into two major subgroups or lithologies, Bali‐like (CV3oxB) and Allende‐like (CV3oxA), in which chondrules, calcium‐aluminum‐rich inclusions (CAIs) and matrices show characteristic alteration features (Weisberg et al, 1997; Krot et al, 1997d; Kimura and Ikeda, 1997). The CV3oxB lithology is present in Bali, Kaba, parts of the Mokoia breccia and, possibly, in Grosnaja and Allan Hills (ALH) 85006. It is characterized by the presence of the secondary low‐Ca phyllosilicates (saponite and sodium phlogopite), magnetite, Ni‐rich sulfides, fayalite (Fa>90), Ca‐Fe‐rich pyroxenes (Fs10–50Wo45–50) and andradite. Phyllosilicates replace primary Ca‐rich minerals in chondrules and CAIs, which suggests mobilization of Ca during aqueous alteration. Magnetite nodules are replaced to various degrees by fayalite, Ca‐Fe‐rich pyroxenes and minor andradite. Fayalite veins crosscut fine‐grained rims around chondrules and extend into the matrix. Thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures (<300 °C) in the presence of aqueous solutions. Oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large Δ18Ofayalite‐magnetite fractionation (∼20%). We infer that phyllosilicates, magnetite, fayalite, Ca‐Fe‐rich pyroxenes and andradite formed at relatively low temperatures (<300 °C) by fluid‐rock interaction in an asteroidal environment.

Secondary fayalite and phyllosilicates are virtually absent in chondrules and CAIs in the CV3oxA lithology, which is present in Allende and its dark inclusions, Axtell, ALHA81258, ALH 84028, Lewis Cliff (LEW) 86006, and parts of the Mokoia and Vigarano breccias. Instead secondary nepheline, sodalite, and fayalitic olivine are common. Fayalitic olivine in chondrules replaces low‐Ca pyroxenes and rims and veins forsterite grains; it also forms coarse lath‐shaped grains in matrix. Secondary Ca‐Fe‐rich pyroxenes are abundant. We infer that the CV3oxA lithology experienced alteration at higher temperatures than the CV3oxB lithology. The presence of the reduced and CV3oxA lithologies in the Vigarano breccia and CV3oxA and CV3oXB lithologies in the Mokoia breccia indicates that all CV3 chondrites came from one heterogeneously altered asteroid. The metamorphosed clasts in Mokoia (Krot and Hutcheon, 1997) may be rare samples of the hotter interior of the CV asteroid. We conclude that the alteration features observed in the oxidized CV3 chondrites resulted from the fluid‐rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced CV3 chondrites.
Hydrous and anhydrous alterations of chondrules in Kaba and Mokoia CV chondrites
Authors: KIMURA M. , IKEDA Y.
Published: February 2010

Abstract— Chondrules in the Bali‐like CV chondrite Kaba and the Allende‐like portion of the Mokoia breccia have been studied to explore the relationship between hydrous alteration to form phyllosilicates and anhydrous alteration resulting in secondary olivine zonation, replacement of enstatite by ferroan olivine and formation of feldspathoids (nepheline and sodalite). All Kaba chondrules experienced extensive hydrous alteration; whereas, anhydrous alteration was minor and resulted only in the olivine zonation. On the other hand, all of the Mokoia chondrules experienced both extensive anhydrous and hydrous alteration. Bronzite rims formed between relic enstatite grains and phyllosilicates in both Kaba and Mokoia during the hydrous alteration. Petrographic observations indicate that phyllosilicates in Mokoia postdate formation of the secondary ferroan olivine and feldspathoids. We conclude that anhydrous alteration in Kaba and Mokoia predated hydrous alteration and took place before accretion of chondrules into the CV parent asteroid.
Implications of iodine‐xenon studies for the timing and location of secondary alteration
Authors: SWINDLE TIMOTHY D.
Published: February 2010

Abstract— Iodine‐xenon ages (based on 15.7 Ma 129I) of meteoritic samples are highly susceptible to secondary alteration processes, so they have the potential to determine both the timing, and in some cases the location, of those secondary processes. Iodine‐xenon studies can determine the location in two cases. First, if the length of time required is greater than the lifetime of the nebula, then the process must have occurred on a parent body. Ages from sodalite in Allende, dark inclusions in Efremovka (CV3), and some samples from CM chondrites all suggest durations of several million years, in some cases marginally longer than the predicted duration of the nebula. Second, in some cases the evolution of the 129Xe/132Xe ratio can be used to determine the I/Xe elemental ratio of the reservoir in which the evolution occurred. For chondrules from the unequilibrated ordinary chondrites Chainpur and Tieschitz, the isotopic evolution is quantitatively consistent with evolution in a chondritic (parent body), rather than nebular, reservoir.
Transportation of gaseous elements and isotopes in a thermally evolving chondritic planetesimal
Authors: HASHIZUME KO , SUGIURA NAOJI
Published: February 2010

Abstract— The behavior of H, C, N and their isotopes in a thermally evolving planetesimal was evaluated by numerical simulation. Transportation of heat and gas molecules, and the chemical equilibrium involving these elements, were simulated. Our modeled planetesimals initially contain homogeneous amounts of radioactive heat source (26Al); and H, C, and N in forms of organic materials, graphite, and in some models, water ice. Vaporized gas molecules were transported from the interior of the planetesimal to its surface, although their transportation efficiencies were quite different among the three elements, primarily due to differences in their affinities to metallic Fe. Significant portions of these elements were redistributed into metallic Fe when the planetesimal was heated at 600 °C and above. Nitrogen showed the most prominent siderophile characteristics, resulting in fairly large concentrations of N trapped in metallic Fe, which is consistent with observations by Hashizume and Sugiura (1997). Efficiency of C transportation crucially depended on O fugacity. To realize effective C transportation, it was necessary to assume an oxidizing condition (log fO2 > log fO2,(FiF) + 1) in the initially accreted material. Water vapor, generated at the interior of the planetesimal and transported to its near surface, formed a water‐rich layer under certain conditions, providing an environment sufficient for aqueous alteration of chondritic materials to occur. Variations in isotopic ratios of N in taenite observed among equilibrated ordinary chondrites can be explained by our gas transportation model. It is required, however, that carriers of isotopically anomalous N, perhaps presolar grains, were initially localized on a large spatial scale within a single planetesimal, which possibly suggests incorporation of preaccretionary objects as large as 0.1 × of the final mass of the ordinary chondrite parent body.
An impact‐melt origin for lithology A of martian meteorite Elephant Moraine A79001
Authors: MITTLEFEHLDT David W. , LINDSTROM David J. , LINDSTROM Marilyn M. , MARTINEZ René R.
Published: February 2010

Abstract— Mixing models using major and trace elements show that the bulk composition of lithology A (xenocryst‐bearing magnesian basalt) of Elephant Moraine A79001 (EETA79001) can be reasonably approximated as a simple mixture of ∼44% EETA79001 lithology B (ferroan basalt) and ∼56% of Allan Hills A77005 (ALHA7705) light lithology (incompatible element‐poor lherzolite). Micro‐instrumental neutron activation analysis (INAA) data on xenocryst‐free groundmass samples of lithology A show that about 20–25% of the melt phase could be dissolved lherzolite. The bulk and groundmass samples of lithology A have excesses in Au, which indicates either meteoritic contamination or addition by some unknown martian geochemical process.

Previous workers have suggested that lithology A was formed by either assimilation of cumulates (like ALHA77005), by a basalt (like lithology B), or by mixing of basaltic and lherzolitic magmas. The former scenario is energetically improbable and unlikely to explain the normal Fe/Mg zonation in lithology A groundmass pyroxenes, whereas the latter scenario is unlikely to satisfy the constraints of the mixing model indicating the ultramafic component is poor in incompatible elements. We suggest rather that EETA79001 lithology A is an impact melt composed dominantly of basalt like lithology B and lherzolitic cumulates like the trace‐element‐poor fraction of ALHA77005 or Y‐793605. This model can satisfy the energetic, petrologic, and geochemical constraints imposed by the samples. If EETA79001 lithology A is an impact melt, this would have considerable consequences for current models of martian petrologic evolution. It would call into question the generally accepted age of magmatism of martian basalts and preclude the use of lithology A groundmass as a primary martian basalt composition in experimental studies. Regardless, the latter is required because lithology A groundmass is a hybrid composition.
Geochemical signals of the late Jurassic, marine Mjølnir impact
Authors: DYPVIK Henning , ATTREP Moses
Published: February 2010

Abstract— Of the only seven submarine impact craters that have been found globally, the Mjølnir crater is one of the best preserved and retains crater and ejecta. Geochemical studies (organic pyrolysis using the Rock Eval technique and XRF analysis for major, minor, and trace elements) of the Institute for Petroleum Research (IKU) core 7430/10‐U‐01 that was taken from a drillhole located ∼30 km north‐northeast of the crater rim show gradual establishment of anoxic sea floor conditions through the late Jurassic. These poorly ventilated water conditions were overturned due to the Mjølnir impact event. Waves and currents transported impact glass (which is now partly weathered to smectite) into the depositional area where the drillhole is located. The succeeding crater collapse transported impact material (e.g., shocked quartz and Ir) from the crater rim and deeper levels to the core site. Normal marine depositional conditions were established a short time after the crater collapsed.
The composition and thickness of the crust of Mars estimated from rare earth elements and neodymium‐isotopic compositions of Martian meteorites
Authors: NORMAN Marc D.
Published: February 2010

Abstract— Isotopic and trace element compositions of Martian meteorites show that early differentiation of Mars produced complementary crustal and mantle reservoirs that were sampled by later magmatic events. This paper describes a mass balance model that estimates the rare earth element (REE) content and thickness of the crust of Mars from the compositions of shergottites. The diverse REE and Nd isotopic compositions of shergottites are most easily explained by variable addition of light rare earth element (LREE)–enriched crust to basaltic magmas derived from LREE‐depleted mantle source regions. Antarctic shergottites EET 79001, ALH 77005, LEW 88516, and QUE 94201 all have strongly LREE‐depleted patterns and positive initial 143Nd isotopic compositions, which is consistent with the generation of these magmas from depleted mantle sources and little or no interaction with enriched crust. In contrast, Shergotty and Zagami have negative initial 143Nd isotopic compositions and less pronounced depletions of the LREE, which have been explained by incorporation of enriched crustal components into mantle‐derived magmas (Jones, 1989; Longhi, 1991; Borg et al., 1997).

The mass balance model presented here derives the REE composition of the crustal component in Shergotty by assuming it represents a mixture between a mantle‐derived magma similar in composition to EET 79001A and a LREE‐enriched crustal component. The amount of crust in Shergotty is constrained by mixing relations based on Nd‐isotopic compositions, which allows the REE pattern of the crustal component to be calculated by mass balance. The effectiveness of this model is demonstrated by the successful recovery of important characteristics of the Earth's continental crust from terrestrial Columbia River basalts.

Self‐consistent results for Nd‐isotopic compositions and REE abundances are obtained if Shergotty contains ∼10–30% of LREE‐enriched crust with >10 ppm Nd. This crustal component would have moderately enriched LREE (Sm/Nd = 0.25–0.27; 147Sm/144Nd = 0.15–0.17; La/Yb = 2.7–3.8), relatively unfractionated heavy rare earth elements (HREE), and no Eu anomaly. Crust with these characteristics can be produced from a primitive lherzolitic Martian mantle by modest amounts (2–8%) of partial melting, and it would have a globally averaged thickness of <45 km, which is consistent with geophysical estimates. Mars may serve as a laboratory to investigate planetary differentiation by extraction of a primary basaltic crust.
Terrestrial ages of ordinary chondrites from the Lewis Cliff stranding area, East Antarctica
Authors: WELTEN K. C. , LINDNER L. , ALDERLIESTEN C. , van der BORG K.
Published: February 2010

Abstract— We determined terrestrial ages of ordinary chondrites from the Lewis Cliff stranding area, East Antarctica, on the basis of the concentrations of cosmogenic 10Be (t½; = 1.51 Ma), 26Al (t½; = 0.705 Ma), and 36Cl (t½; = 0.301 Ma). After an initial 26Al γ‐ray survey of 91 meteorites suggested that many have terrestrial ages >0.1 Ma, we selected 62 meteorites for 10Be and 26Al measurements by accelerator mass spectrometry (AMS) and measured 36Cl in twelve of those. Low terrestrial ages (<0.1 Ma) were found for ∼60% of the meteorites, whereas all others have ages between 0.1 and 0.5 Ma, except for one exceptional age of >2 Ma (Welten et al., 1997). Our major conclusions are: (1) The Lewis Cliff H‐chondrites show similar ages to those from the Allan Hills icefields, but the L‐chondrites are about a factor of 2 younger than those from Allan Hills, which indicates that Lewis Cliff is a younger stranding area. (2) The terrestrial age distributions at different parts of the Lewis Cliff stranding area generally agree with simple meteorite concentration models, although differences in weathering rate may also play a role. (3) We confirm that meteorites with natural thermoluminescence (TL) levels >80 krad are associated with low terrestrial ages (Benoit et al., 1992) but conclude that natural TL levels <80 krad can not be used to calculate the terrestrial age of a meteorite. Natural TL levels do seem useful to estimate relative terrestrial ages of large groups of meteorites and to determine differences in the surface exposure age of paired meteorite fragments. (4) Of the 62 meteorites measured with AMS, 31 were assigned to 11 different pairing groups, mainly on the basis of their cosmogenic nuclide record. The meteorites are estimated to represent between 42 and 52 distinct falls.
Samarium‐neodymium and rubidium‐strontium systematics of nakhlite Governador Valadares
Authors: SHIH C‐Y. , NYQUIST L. E. , WIESMANN H.
Published: February 2010

Abstract— The Sm‐Nd systematics of whole‐rock and mineral separate samples from nakhlite Governador Valadares define a good 147Sm‐143Nd mineral isochron age of 1.37 ± 0.02 Ga. This age is in excellent agreement with the 39Ar‐40Ar and Rb‐Sr ages obtained previously for this meteorite. However, the Rb‐Sr isotopic data for our sample show that the isotopic system is disturbed. The lack of isotopic equilibrium is probably caused by the weathering of the sample as indicated by the presence of secondary alteration phases. The whole‐rock and acid‐washed mineral data yield a Rb‐Sr age of 1.20 ± 0.05 Ga, which probably represents a lower limit to the crystallization age of the rock. The petrographic evidence indicates that this meteorite is a clinopyroxene cumulate that probably crystallized in a subsurface sill (McSween, 1994). Thus, the Sm‐Nd isotopic age probably represents the age of such a magmatic event. The initial ε143Nd value determined for the rock at 1.37 Ga is +17 ± 1, indicating that the parent magma of the rock came from a light‐rare‐earth‐element‐depleted source of 147Sm/144Nd = ∼0.237 based on a simple two‐stage evolution model. Results of the same model calculation for the initial 87Sr/86Sr ratio of the rock suggest that its source material was depleted in 87Rb/86Sr by ∼50% relative to the estimated martian value at 1.37 Ga. Both the high Sm/Nd and low Rb/Sr values support a clinopyroxene‐rich cumulate source for the genesis of the nakhlite Governador Valadares. Furthermore, our Sm‐Nd age and ε143Nd data and the previously published ε142Nd datum for the rock (Harper et al., 1995) are consistent with early differentiation of the parent planet, formation of cumulate sources ∼4.56 Ga ago, and late melting of the sources and formation of the rock ∼1.37 Ga ago. The good agreement of isotopic ages and petrographic features among Governador Valadares, Nakhla, and Lafayette strongly suggests that all three nakhlites have undergone similar evolutionary histories. The nakhlite age data suggest that isotopic heterogeneity in the martian mantle sources existed up to ∼1.37 Ga ago and early mantle structures probably have not been disturbed for a significant portion of martian history.
Mineralogy, petrography, and thermometry of the H5 chondrite Carcote, Chile
Authors: Kleinschrot Dorothée , Okrusch Martin
Published: February 2010

Abstract— The Carcote meteorite, detected in 1888 in the northern Chilean Andes, is a brecciated, weakly shocked H5 chondrite. It contains a few barred olivine chondrules and, even more rarely, fan‐shaped or granular orthopyroxene chondrules. The chondrules are situated in a fine‐grained matrix that consists predominantly of olivine and orthopyroxene with accessory clinopyroxene, troilite, chromite, merrillite, and plagioclase. The metal phase is mainly kamacite with subordinate taenite and traces of native Cu. In its bulk rock composition, Carcote compares well with other H5 chondrites so far analysed, except for a distinctly higher C content.

Microprobe analyses revealed the following mineral compositions: olivine (Fa16.5–20), orthopyroxene (Fs14–17.5), diopsidic clinopyroxene (FS6–7), plagioclase (An15–20). Troilite is stoichimetric FeS with traces of Ni and Cr; chromite has Cr/(Cr + Al) of 0.86, Fe2+/(Fe2+ + Mg) of 0.80‐0.88 and contains considerable amounts of Ti, Mn, and Zn. Merrillite is close to the theoretical formula Ca18(Mg, Fe)2Na2(PO4)14, although with a Na deficiency not compensated for by excess Ca; the Mg/(Mg + Fe2+) ratio of the Carcote merrilite is 0.93‐0.95. Kamacite and taenite have Ni contents of 5.6–7.2 and 17.1–23.4 wt%, respectively. Native Cu contains about 3.1–3.3 wt% Fe and 1.6 wt% Ni.

Application of different geothermometers to the Carcote H5 chondrite yielded apparently inconsistent results. The highest temperature range of 850–950 °C (at 1 bar) is derived from the Ca‐in‐opx thermometer. From the cpx‐opx solvus geothermometers and the two‐pyroxene Fe‐Mg exchange geothermometer, a lower temperature range of 750–840 °C is estimated, whereas lower and more variable temperatures of 630–770 °C are obtained from the Ca‐in‐olivine geothermometer. Recent calibrations of the olivine‐spinel geothermometer yielded a still lower temperature range of 570–670 °C, which fits well to the temperature information derived from the Ni distribution between kamacite and taenite. Judging from crystal chemical considerations, we assume that these different temperatures reflect the closure of different exchange equilibria during cooling of the meteorite parent body.
A study of the minor element concentrations of spinels from two type B calcium‐aluminum‐rich inclusions: An investigation into potential formation conditions of calcium‐aluminum‐rich inclusions
Authors: CONNOLLY Harold C. , BURNETT D. S.
Published: February 2010

Abstract— We have conducted an electron microprobe study of minor element distributions among spinels from two type B1 calcium‐aluminum‐rich inclusions (CAIs): Allende TS‐23 and Leoville 3537–2. We show that by maintaining the petrologic context (edge, middle, and center of the inclusion plus their host silicate phase), four populations of spinels are resolvable based on their minor element contents. One population resides within the edge area (mainly mantle melilite) and is characterized by the highest V contents. Unlike Leoville 3537–2, many edge grains from Allende TS‐23 also have high‐Fe contents (up to 4.0 wt%) and low‐Cr values. Based on their V and Ti concentrations (which is positively correlated), middle and center grains define a trend that is divided into three populations: spinels enclosed by melilite, fassaite, and anorthite. The overall range in Ti concentration based on fractional crystallization should be much less than a factor of 2; however, the observed range is considerably larger. The minor element contents of these grains are interpreted as recording alteration, primary fractional crystallization, and a complex igneous history that may involve remelting and recrystallization. From our data, Allende TS‐23 has experienced more alteration than Leoville 3537–2, which is consistent with previous petrologic studies of silicates within these objects; yet both objects have likely been remelted (at least one additional melting event, possibly two, postdating the initial formation of these CAIs). By invoking a remelting history, the large range ir Ti concentrations and the different populations of spinels can be explained. Although our data suggest that more than one generation of spinels exist within these objects, we are unable to establish any population of relic spinel grains that predate the initial melting event.
Evidence for the insignificance of ordinary chondritic material in the asteroid belt
Authors: MEIBOM Anders , CLARK Beth E.
Published: February 2010

Abstract—We review the meteoritical and astronomical literature to answer the question: What is the evidence for the importance of ordinary chondritic material to the composition of the asteroid belt?

From the meteoritical literature, we find that currently (1) our meteorite collections sample at least 135 different asteroids; (2) out of 25+ chondritic meteorite parent bodies, 3 are (by definition) ordinary chondritic; (3) out of 14 chondritic grouplets and unique chondrites, 11 are affiliated with a carbonaceous group/clan of chondrites; (4) out of 24 differentiated groups of meteorites, only the HE iron meteorites clearly formed from ordinary chondritic precursor material; (5) out of 12 differentiated grouplets and unique differentiated meteorites, 8 seem to have had carbonaceous chondritic precursors; (6) a high frequency of carbonaceous clasts in ordinary chondritic breccias suggests that ordinary chondrites have been embedded in a swarm of carbonaceous material. The rare occurrence (only one example) of ordinary chondritic clasts in carbonaceous chondritic breccias indicates that ordinary chondritic material has not been widespread in the asteroid belt; (7) cosmic spherules, micrometeorites, and stratospheric interplanetary dust particles—believed to represent a less biased sampling of asteroidal material—show that only a very small fraction (less than ∼1%) of asteroidal dust has an ordinary chondritic composition. From the astronomical literature, we find that currently (8) spectroscopic surveys of the main asteroid belt are finding more and more nonordinary chondritic primitive material in the inner main belt; (9) the increase in spectroscopic data has increased the inferred mineralogical diversity of main belt asteroids; and (10) no ordinary chondritic asteroids have been directly observed in the main belt.

These lines of evidence strongly suggest a scenario in which ordinary chondritic asteroids were never abundant in the main belt. The S‐type asteroids may currently be primarily differentiated, but the precursor material is more likely to have been carbonaceous chondritic, not ordinary chondritic. Historically, carbonaceous material could have dominated the entire main belt. This could explain the presence in the inner main belt of asteroids linked to the primitive carbonaceous chondrites, and the absence of asteroids linked to the ordinary chondrites. The implications of this scenario for the asteroid heating mechanism(s) are briefly discussed.
A transmission electron microscope study of Chassigny: Evidence for strong shock metamorphism
Authors: LANGENHORST Falko , GRESHAKE Ansgar
Published: February 2010

Abstract— Chassigny is a shock‐metamorphosed dunite of probable Martian origin. In order to determine its degree of shock metamorphism and to define the starting conditions prior to its ejection from Mars, the shock signature of Chassigny has been carefully examined by optical and electronoptical techniques. Dominant shock effects are the conversion of feldspars to diaplectic glass (maskelynite), the clino‐/orthoenstatite inversion, strong mosaicism of olivine, and the activation of numerous planar fractures and c dislocations in olivine. High‐resolution transmission electron microscopy (TEM) reveals additionally the coexistance of planar fractures with, so far, unknown discontinuous fractures in olivine. These findings point to a shock pressure of 35 GPa. Chassigny has thus experienced a high and similar degree of shock metamorphism as the shergottites. The results of this study suggest that Chassigny was at a shallow target position, close to the point of impact, when it was ejected.
The solar oxygen‐isotopic composition: Predictions and implications for solar nebula processes
Authors: WIENS R. C. , HUSS G. R. , BURNETT D. S.
Published: February 2010

Abstract— The outer layers of the Sun are thought to preserve the average isotopic and chemical composition of the solar system. The solar O‐isotopic composition is essentially unmeasured, though models based on variations in meteoritic materials yield several predictions. These predictions are reviewed and possible variations on these predictions are explored. In particular, the two‐component mixing model of Clayton and Mayeda (1984) (slightly revised here) predicts solar compositions to lie along an extension of the calcium‐aluminum‐rich inclusion (CAI) 16O line between (δ18O, δ17O) = (16.4, 11.4)%0 and (12.3, 7.5)%0. Consideration of data from ordinary chondrites suggests that the range of predicted solar composition should extend to slightly lower δ18O values. The predicted solar composition is critically sensitive to the solid/gas ratio in the meteorite‐forming region, which is often considered to be significantly enriched over solar composition. A factor of two solid/gas enrichment raises the predicted solar (δ18O, δ17O) values along an extension of the CAI 16O line to (33, 28)%0. The model is also sensitive to the nebular O gas phase. If conversion of most of the gaseous O from CO to H2O occurred at relatively low temperatures and was incomplete at the time of CM aqueous alteration, the predicted nebular gas composition (and hence the solar composition) would be isotopically heavier along a slope 1/2 line. The likelihood of having a single solid nebular O component is discussed. A distribution of initial solid compositions along the CAI 16O line (rather than simply as an end‐member) would not significantly change the predictions above in at least one scenario. Even considering these variations within the mixing model, the predicted range of solar compositions is distinct from that expected if the meteoritic variations are due to non‐mass‐dependent fractionation. Thus, a measurement of the solar O composition to a precision of several permil would clearly distinguish between these theories and should clarify a number of other important issues regarding solar system formation.
The granulitic impactite suite: Impact melts and metamorphic breccias of the early lunar crust
Authors: ÇUSHING Janet A. , TAYLOR G. Jeffrey , NORMAN Marc D. , KEIL Klaus
Published: February 2010

Abstract— An important and poorly understood group of rocks found in the ancient lunar highlands is called “feldspathic granulitic impactites.” Rocks of the granulite suite occur at most of the Apollo highlands sites as hand samples, rake samples, clasts in breccias, and soil fragments. Most lunar granulites contain 70–80% modal plagioclase, but they can range from anorthosite to troctolite and norite. Previous studies have led to different interpretations for the thermal history of these rocks, including formation as igneous plutons, long‐duration metamorphism at high temperatures, and short‐duration metamorphism at low temperatures. This paper reports on a study of 24 polished thin sections of lunar granulites from the Apollo 15, 16, and 17 missions. We identify three different textural types of granulitic breccias: poikilitic, granoblastic, and poikilitic‐granoblastic breccias. These breccias have similar equilibration temperatures (1100 ± 50 °C), as well as common compositions. Crystal size distributions in two granoblastic breccias reveal that Ostwald ripening took place during metamorphism. Solid‐state grain growth and diffusion calculations indicate relatively rapid cooling during metamorphism (0.5 to 50 °C/year), and thermal modeling shows that they cooled at relatively shallow depths (<200 m). In contrast, we conclude that the poikilitic rocks formed by impact melting, whereas the poikilitic‐granoblastic rocks were metamorphosed and may have partially melted. These results indicate formation of lunar granulites in relatively small craters (30–90 km in diameter), physically associated with the impact‐melt breccia pile, and possibly from fine‐grained fragmental precursor lithologies.