• Northwest Africa 011: A "eucritic" basalt from a non-eucrite parent body

      Floss, C.; Taylor, L. A.; Promprated, P.; Rumble, D. (The Meteoritical Society, 2005-01-01)
      We have carried out a detailed petrographic, mineralogical, and trace element study of Northwest Africa (NWA) 011. This meteorite bears many similarities to the eucrites it was initially identified with, although oxygen isotopic compositions rule out a genetic relationship. Like many eucrites, NWA 011 crystallized from a source with approximately chondritic proportions of REE, although a slightly LREE-enriched bulk composition with a small positive Eu anomaly, as well as highly fractionated Fe/Mg ratios and depleted Sc abundances (Korotchantseva et al. 2003), suggest that the NWA 011 source experienced some pyroxene and/or olivine fractionation. Thermal metamorphism resulted in homogenization of REE abundances within grains, but NWA 011 did not experience the intergrain REE redistribution seen in some highly metamorphosed eucrites. Despite a similarity in oxygen isotopic compositions, NWA 011 does not represent a basaltic partial melt from the acapulcoite/lodranite parent body. The material from which NWA 011 originated may have been like some CH or CB chondrites, members of the CR chondrite clan, which are all related through oxygen isotopic compositions. The NWA 011 parent body is probably of asteroidal origin, possibly the basaltic asteroid 1459 Magnya.
    • Erratum

      The Meteoritical Society, 2005-01-01
    • "New" lunar meteorites: Implications for composition of the global lunar surface, lunar crust, and the bulk Moon

      Warren, Paul H. (The Meteoritical Society, 2005-01-01)
      New data for lunar meteorites and a synthesis of literature data have significant implications for the interpretation of global Th data and for the Moons bulk composition. As presently calibrated (Prettyman et al. 2002), the Lunar Prospector gamma-ray data imply that the average global surface Th = 1.58 micrograms/g. However, that calibration yields implausibly high concentrations for the three most Th-poor documented sampling sites, it extrapolates to a nonzero Lunar Prospector Th, ~0.7 micrograms/g, at zero sample Th, and it results in a misfit toward too-high Th when compared with the global regolith Th spectrum as constrained using mainly lunaite regolith breccias. Another problem is manifested by Th versus K systematics. Ground truth data plot consistently to the high-Th/K side of the Prospector data trend, offset by a factor of 1.2. A new calibration is proposed that represents a compromise between the Th levels indicated by ground truth constraints and the Prettyman et al. (2002) calibration. Conservatively assuming that the Th versus K issue is mostly a K problem, the average global surface Th is estimated to be ~1.35 micrograms/g. The Moons remarkable global asymmetry in KREEP abundance is even more pronounced than previously supposed. The surface Th concentration ratio between the hemisphere antipodal to the Procellarum basin and the hemisphere centered on Procellarum is reduced to 0.24 in the new calibration. This extreme disparity is most simply interpreted as a consequence of Procellarums origin at a time when the Moon still contained at least a thin residual layer of a global magma ocean. Allowing for diminution of Th with depth, the extrapolated bulk crustal Th is ~0.73 micrograms/g. Further extrapolation to bulk Moon Th yields ~0.07 micrograms/g, which is nearly identical to the consensus estimate for Earths primitive mantle. Assuming chondritic proportionality among refractory lithophile elements implies Al2O3 of approximately 3.8 wt%. The Moons bulk mantle mg ratio is only weakly constrained by seismic and mare-basaltic data. KREEPand mare-free lunaite regolith samples, other thoroughly polymict lunar meteorites, and a few KREEP-free Apollo highland samples manifest a remarkable anticorrelation on a plot of Al2O3 versus mg. This trend implies that an important component of the Moon is highly magnesian. The bulk Moon is inferred to have an Earth-like oxide mg ratio of ~87-88 mol%. The close resemblance between the bulk Moon and Earths primitive mantle extends to moderately volatile elements, most clearly Mn. Unless major proportions of Cr and V are sequestered into deep mantle spinel, remarkably Earth-like depletions (versus chondrites) are also inferred for bulk Moon Cr and V.
    • A unique type B inclusion from Allende with evidence for multiple stages of melting

      Simon, S. B.; Grossman, L.; Davis, A. M. (The Meteoritical Society, 2005-01-01)
      A large (7 mm in diameter) Allende type B inclusion has a typical bulk composition and a unique structure: a fassaite-rich mantle enclosing a melilite-rich core. The core and mantle have sharply contrasting textures. In the mantle, coarse (~1 mm across), subhedral fassaite crystals enclose radially oriented melilite laths about 500 micrometers long that occur at the inclusion rim. The core consists of blocky melilite grains 20-50 micrometers across and poikilitically enclosed in anhedral fassaite grains that are optically continuous over ~1 mm. Another unique feature of this inclusion is that melilite laths also extend from the core into the mantle. Fassaite in both the core and mantle is very rich in fine-grained (1-10 micrometer) spinel. The rim laths are normally zoned (Åk 30-70) inward from the rim of the inclusion with reverse zoning over the last ~200 micrometers to crystallize. A very wide range of melilite compositions is found in the core of the inclusion, where gehlenitic grains (Åk 5-12) occur. These grains are enclosed in strongly zoned (Åk 15-70) overgrowths. The gehlenitic cores and innermost parts of the overgrowths are Na2O-free, but the outer parts of the overgrowths are not. In the laths at the rim, Na2O decreases inward from the rim, then increases. Fassaite in the core has the same range of Ti contents as that in the mantle: 29 wt% TiO2 + Ti2O3. Two melting events are required to account for the features of this inclusion. In the first event, the precursor assemblage is heated to ~1400 degrees C and melts except for gehlenitic (Åk 5-12) melilite and some spinel. These grains become concentrated in the core. During cooling, Na2O-free melilite nucleates at the rim of the inclusion and on the relict grains in the core. After open system secondary alteration, the inclusion is heated again, but only to ~1260 degrees C. Melilite more gehlenitic than Åk40 does not melt. During cooling, Na2O-bearing melilite crystallizes as small, blocky grains and laths in the core and as overgrowths on relict grains in the core and at the rim. Eventually melilite co-crystallizes with fassaite, leading to the reverse zoning observed in the laths. The coexistence in this inclusion of Na-free and Na-bearing melilite, plus a positive correlation between Na2O and ååkermanite contents in melilite in an inclusion with a bulk Mg isotopic composition that is mass-fractionated in favor of the heavy isotopes, are both consistent with at least two melting events. Several other recently described coarse-grained inclusions also have features consistent with a sequence of early, high-temperature melting, secondary alteration, and remelting at a lower temperature, suggesting that remelting of refractory inclusions was a common occurrence in the solar nebula.
    • The non-igneous genesis of angrites: Support from trace element distribution between phases in D'Orbigny

      Varela, M. E.; Kurat, G.; Zinner, E.; Hoppe, P.; Ntaflos, T.; Nazarov, M. A. (The Meteoritical Society, 2005-01-01)
      D'Orbigny is an exceptional angrite. Chemically, it resembles other angrites such as Asuka-881371, Sahara 99555, Lewis Cliff (LEW) 87051, and LEW 86010, but its structure and texture are peculiar. It has a compact and porous lithology, abundant glasses, augite-bearing druses, and chemical and mineralogical properties that are highly unusual for igneous rocks. Our previous studies led us to a new view on angrites: they can possibly be considered as CAIs that grew to larger sizes than the ones we know from carbonaceous chondrites. Thus, angrites may bear a record of rare and special conditions in some part of the early solar nebula. Here we report trace element contents of D'Orbigny phases. Trace element data were obtained from both the porous and the compact part of this meteorite. We have confronted our results with the popular igneous genetic model. According to this model, if all phases of D'Orbigny crystallized from the same system, as an igneous origin implies, a record of this genesis should be expressed in the distribution of trace elements among early and late phases. Our results show that the trace element distribution of the two contemporaneous phases olivine and plagioclase, which form the backbone of the rock, seem to require liquids of different composition. Abundances of highly incompatible elements in all olivines, including the megacrysts, indicate disequilibrium with the bulk rock and suggest liquids very rich in these elements (>10,000 x CI), which is much richer than any fractional crystallization could possibly produce. In addition, trace element contents of late phases are incompatible with formation from the bulk systems residual melt. These results add additional severe constraints to the many conflicts that existed previously between an igneous model for the origin of angrites and the mineralogical and chemical observations. This new trace element content data, reported here, corroborate our previous results based on the shape, structure, mineralogy, chemical, and isotopic data of the whole meteorite, as well as on a petrographic and chemical composition study of all types of glasses and give strength to a new genetic model that postulates that D'Orbigny (and possibly all angrites) could have formed in the solar nebula under changing redox conditions, more akin to chondritic constituents (e.g., CAIs) than to planetary differentiated rock.
    • Revisiting spectral parameters of silicate-bearing meteorites

      Duffard, R.; Lazzaro, D.; De León, Len, J. (The Meteoritical Society, 2005-01-01)
      Visible and near-infrared reflectance spectra of a sample of silicate-bearing meteorites have been used to evaluate the spectral parameters space defined in the pioneering work of Gaffey et al. (1993). The studied sample consisted of 91 ordinary chondrites, 47 basaltic achondrites, and 21 different laboratory mixtures obtained from the RELAB database. Our results indicate that the spectral parameter space, in particular the BAR versus band I center, is not suitable enough to identify the mineralogy of meteorites and asteroids. The grain size of the sample also appears as a very sensitive parameter and can play an important role in locating an object in the spectral parameter space. Finally, the application of our study to the question of a genetic link between V-type asteroids and HED meteorites shows that these bodies plot in distinct regions in the BAR versus band I center space. This result further confirms that those spectral parameters cannot uniquely define the mineralogy of a sample.
    • Evidence of space weathering in regolith breccias I: Lunar regolith breccias

      Noble, S. K.; Keller, L. P.; Pieters, C. M. (The Meteoritical Society, 2005-01-01)
      We have analyzed a suite of lunar regolith breccias in order to assess how well space weathering products can be preserved through the lithification process and therefore whether or not it is appropriate to search for space weathering products in asteroidal regolith breccia meteorites. It was found that space weathering products, vapor/sputter deposited nanophase-iron-bearing rims in particular, are easily identified in even heavily shocked/compacted lunar regolith breccias. Such rims, if created on asteroids, should thus be preserved in asteroidal regolith breccia meteorites. Two additional rim types, glass rims and vesicular rims, identified in regolith breccias, are also described. These rims are common in lunar regolith breccias but rare to absent in lunar soils, which suggests that they are created in the breccia-forming process itself. While not "space weathering products" in the strictest sense, these additional rims give us insight into the regolith breccia formation process. The presence or absence of glass and/or vesicular rims in asteroidal regolith breccias will likewise tell us about environmental conditions on the surface of the asteroid body on which the breccia was created.
    • Melt inclusions in augite of the Nakhla martian meteorite: Evidence for basaltic parental melt

      Stockstill, K. R.; McSween, H. Y.; Bodnar, R. J. (The Meteoritical Society, 2005-01-01)
      Nakhla contains crystallized melt inclusions that were trapped in augite and olivine when these phases originally formed on Mars. Our study involved rehomogenization (slow-heating and fast-heating) experiments on multiphase melt inclusions in Nakhla augite. We studied melt inclusions trapped in augite because this phase re-equilibrated with the external melt to a lesser extent than olivine and results could be directly compared with previous Nakhla melt inclusion studies. Following heating and homogenization of encapsulated melt inclusions, single mineral grains were mounted and polished to expose inclusions. Major element chemistry was determined by electron microprobe. The most primitive melt inclusion analyzed in Nakhla NA03 is basaltic and closely matches previously reported nakhlite parent melt compositions. MELTS equilibrium and fractional crystallization models calculated for NA03 and previous Nakhla parent melt estimates at QFM and QFM-1 produced phase assemblages and compositions that can be compared to Nakhla. Of these models, equilibrium crystallization of NA03 at QFM-1 produced the best match to mineral phases and compositions in Nakhla. In all models, olivine and augite co-crystallize, consistent with the hypothesis that olivine is not xenocrystic but has undergone subsolidus re-equilibration. In addition, measured melt inclusion compositions plot along the MELTS-calculated liquid line of descent and may represent pockets of melt trapped at various stages during crystallization. We attempt to resolve discrepancies between previous estimates of the Nakhla parental melt composition and to reinterpret the results of a previous study of rehomogenized melt inclusions in Nakhla. Melt inclusions demonstrate that Nakhla is an igneous rock whose parent melt composition and crystallization history reflect planetary igneous processes.
    • Primordial noble gases in a graphite-metal inclusion from the Canyon Diablo IAB iron meteorite and their implications

      Matsuda, J.; Namba, M.; Maruoka, T.; Matsumoto, T.; Kurat, G. (The Meteoritical Society, 2005-01-01)
      We have carried out noble gas measurements on graphite from a large graphite-metal inclusion in Canyon Diablo. The Ne data of the low-temperature fractions lie on the mixing line between air and the spallogenic component, but those of high temperatures seem to lie on the mixing line between Ne-HL and the spallogenic component. The Ar isotope data indicate the presence of Q in addition to air, spallogenic component and Ar-HL. As the elemental concentration of Ne in Q is low, we could not detect the Ne-Q from the Ne data. On the other hand, we could not observe Xe-HL in our Xe data. As the Xe concentration and the Xe/Ne ratio in Q is much higher than that in the HL component, it is likely that only the contribution of Q is observed in the Xe data. Xenon isotopic data can be explained as a mixture of Q, air, and "El Taco Xe." The Canyon Diablo graphite contains both HL and Q, very much like carbonaceous chondrites, retaining the signatures of various primordial noble gas components. This indicates that the graphite was formed in a primitive nebular environment and was not heated to high, igneous temperatures. Furthermore, a large excess of 129Xe was observed, which indicates that the graphite was formed at a very early stage of the solar system when 129I was still present. The HL/Q ratios in the graphite in Canyon Diablo are lower than those in carbonaceous chondrites, indicating that some thermal metamorphism occurred on the former. We estimated the temperature of the thermal metamorphism to about 500-600 degrees C from the difference of thermal retentivities of HL and Q. It is also noted that El Taco Xe is commonly observed in many IAB iron meteorites, but its presence in carbonaceous chondrites has not yet been established.
    • The gentle separation of presolar SiC grains from meteorites

      Tizard, J.; Lyon, I.; Henkel, T. (The Meteoritical Society, 2005-01-01)
      This paper describes the development of a new, effective, and non-destructive method of SiC isolation from meteorites by freeze-thaw disaggregation, size, and density separation. This new method is important because there is evidence that current methods, which use strong acids and chemical treatments to dissolve silicates and separate out the interstellar grains, may alter the surfaces of the grains chemically and isotopically. Furthermore, any non-refractory coating present on the grains would be destroyed. Using our new separation method, SiC grains were enriched from ~6 ppm abundance in Murchison whole rock to 0.67% abundance in the 0.4-1.4 micrometer size range and 0.27% abundance in the 1.4-17 micrometer size range. Individual SiC grains were easily identified using electron probe microanalysis (EPMA) mapping of grains distributed thinly on gold foil; a small aliquot from these fractions has so far yielded >150 SiC grains for isotopic analysis. The method separates out SiC grains efficiently, is applicable to very small or rare samples, and avoids the harsh acid treatments that may alter possible amorphous or non-refractory coats on the grains. The procedure also preserves the remainder of the original sample and it is hoped that it may be extended to other micron-sized presolar grains found in meteorites such as corundum, graphite, and silicon nitride.
    • Mineralogy and petrology of the angrite Northwest Africa 1296

      Jambon, A.; Barrat, J. A.; Boudouma, O.; Fonteilles, M.; Badia, D.; Göpel, C.; Bohn, M. (The Meteoritical Society, 2005-01-01)
      We report on a new angrite, Northwest Africa (NWA) 1296, a fine-grained rock with a magmatic texture of rapid cooling. Dendritic olivine (Fo50) crystallized first in association with anorthite microcrysts (An98100) forming composite chains separated from one another by intergrown Al-Fe diopside-hedenbergite pyroxenes. In addition, some olivines with lower Mg# and increased CaO (up to 12%) are found between the chains as equant microphenocrysts. Pyroxenes and olivines are both normally zoned from Mg# = 0.52 to less than 0.01 in the rims. Ca-rich olivines are surrounded by, intergrown with, or replaced by subcalcic kirschsteinite. They appear after plagioclase crystallization stopped, at the end of the crystallization sequence. Minor phases are pyrrhotite, Fapatite, and titanomagnetite. Pyroxene is the last silicate phase to grow, interstitial to idiomorphic olivine-kirschsteinite. Numerous small vesicles and some channels are filled with microcristalline carbonate. The mode (vol%) is about 28% olivine, 3% kirschsteinite, 32% anorthite, 34% pyroxene, and 3% of the minor phases--close to that reported previously for D'Orbigny and Sahara (SAH) 99555. The bulk chemical composition of NWA 1296 is similar to D'Orbigny and SAH 99555; NWA 1296 differs by its texture and mineralogy, which are interpreted as resulting from rapid crystallization--an evidence of impact melting. Angrites cannot be produced by partial melting of a CV source because segregation of a "planetary" core is necessary to explain the low FeO/MgO ratio of magnesian olivines. Neither the odd Ca/Al ratio nor the very low SiO2 content can be explained by conventional partial melting scenarios. We suggest that carbonate is the key to angrite genesis. This is supported by the striking similarities with terrestrial melilitites (low SiO2, superchondritic Ca/Al ratio, presence of carbonate). The lack of alkalies could be the result of either loss after impact melting or absence of alkalies in the source.