Mineralogical characterization of a unique material having heavy oxygen isotope anomaly in matrix of the primitive carbonaceous chondrite Acfer 094 (original) (raw)
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Meteoritics & Planetary Science, 2007
available online at Abstract-We have used a variety of complementary microanalytical techniques to constrain the mineralogy, trace-element distributions, and oxygen-isotopic compositions in a 50 × 50 μm area of Acfer 094 matrix. The results reveal the exceptional mineralogical and compositional heterogeneity of this material at the sub-μm level. We observe μm-scale and sub-μm grains with elemental associations suggesting feldspar, metal with widely varying Ni contents, and a Cr-Fe alloy (in addition to forsterite, pyroxene, sulfide, ferrihydrite, and amorphous groundmass previously described). A new class of μmscale CAI (μCAI) is also observed, which show sub-μm compositional zoning, and a range of oxygen isotopic compositions. Unlike the larger CAIs in Acfer 094, which are uniformly 16 O-enriched, two of the three μCAIs we analyzed are isotopically normal. We also observed a Li-rich hotspot that detailed analysis by ToF-SIMS suggests may be a LiCr-oxide grain. Within the resolution of the NanoSIMS, this grain has isotopically normal Li. Finally, in our 50 × 50 μm area, we positively identified a presolar grain that is the most 18 O-rich silicate found so far in meteorites. The grain may originate from an asymptotic giant branch (AGB) star, or more likely, a supernova. In line with previous TEM studies (Greshake 1997), we find no evidence for clastic material (e.g., fragmental chondrules) in the matrix of Acfer 094: although the matrix is volatile-depleted, this depletion does not appear to result from dilution of a primordial starting material with (depleted) chondrule fragments. Assuming that matrix experienced the depletion event, our data on the detailed mineralogy of Acfer 094 are currently equivocal in constraining the nature of that event. We observe carrier phases for several elements consistent with conditions approaching equilibrium condensation; however, the presence of an amorphous groundmass is suggestive of more rapid cooling.
Meteoritics & Planetary Science, 2005
available online at http://meteoritics.org 1043 Abstract-Oxygen isotopes have been measured by ion microprobe in individual minerals (spinel, Al-Ti-diopside, melilite, and anorthite) within four relatively unaltered, fine-grained, spinel-rich Ca-Alrich inclusions (CAIs) from the reduced CV chondrite Efremovka. Spinel is uniformly 16 O-rich (Δ 17 O ≤ −20‰) in all four CAIs; Al-Ti-diopside is similarly 16 O-rich in all but one CAI, where it has smaller 16 O excesses (−15‰ ≤ Δ 17 O ≤ −10‰). Anorthite and melilite vary widely in composition from 16 Orich to 16 O-poor (−22‰ ≤ Δ 17 O ≤ −5‰)
Earth and Planetary Science Letters, 2006
Chondrules and matrix of the Ningqiang carbonaceous chondrite contain numerous opaque assemblages (OAs). These OAs are mainly composed of Ni-rich metal, magnetite, sulfides, phosphates with minor amounts of silicates and trace amounts of Pt-metal grains. Their mineral assemblage is very similar to that of OAs found in Allende refractory inclusions. In this study, we performed in situ analyses of O-isotopic compositions of magnetite and whitlockite in OAs and of olivine adjacent to OAs from Ningqiang. O-isotopic compositions of magnetite vary by 16‰ in δ 18 O and fall on a regression line with a slope of 0.51 and a mean Δ 17 O of −2.9 ± 0.3 (1σ)‰ in the three-O-isotope diagram. Magnetite grains have consistently higher Δ 17 O values than their adjacent olivine grains. The mineral chemistry and O-isotopic compositions of OAs in Ningqiang chondrules and matrix are most consistent with the formation of magnetites and whitlockite by aqueous alteration of preexisting metal alloys within a planetary body. Formation of magnetites and phosphates in OAs thus do not constrain processes in the early solar nebula, but rather provide information on metamorphism in a planetary environment.
Geochimica et Cosmochimica Acta, 2005
With one exception, the low-FeO relict olivine grains within high-FeO porphyritic chondrules in the type 3.0 Acfer 094 carbonaceous chondrite have ⌬ 17 O (ϭ ␦ 17 O Ϫ 0.52 ϫ ␦ 18 O) values that are substantially more negative than those of the high-FeO olivine host materials. These results are similar to observations made earlier on chondrules in CO3.0 chondrites and are consistent with two independent models: (1) Nebular solids evolved from low-FeO, low-⌬ 17 O compositions towards high-FeO, more positive ⌬ 17 O compositions; and (2) the range of compositions resulted from the mixing of two independently formed components. The two models predict different trajectories on a ⌬ 17 O vs. log Fe/Mg (olivine) diagram, but our sample set has too few values at intermediate Fe/Mg ratios to yield a definitive answer. Published data showing that Acfer 094 has higher volatile contents than CO chondrites suggest a closer link to CM chondrites. This is consistent with the high modal matrix abundance in Acfer 094 (49 vol.%). Acfer 094 may be an unaltered CM chondrite or an exceptionally matrix-rich CO chondrite. Chondrules in Acfer 094 and in CO and CM carbonaceous chondrites appear to sample the same population. Textural differences between Acfer 094 and CM chondrites are largely attributable to the high degree of hydrothermal alteration that the CM chondrites experienced in an asteroidal setting.
Geochimica et Cosmochimica Acta, 2008
The R chondrite meteorite LaPaz Icefield (LAP) 04840 is unique among metamorphosed, non-carbonaceous chondrites in containing abundant OH-bearing silicate minerals: 1313% ferri-magnesiohornblende and 130.4% phlogopite by volume. Other minerals include olivine (Fo 62 ), orthopyroxene (En 69 Fs 30 Wo 1 ), albite (An 8 Ab 90 Or 2 ), magnetite, pyrrhotite, pentlandite, and apatite. Ferromagnesian minerals are rich in Fe 3+ , as determined by Mö ssbauer spectrometry and electron microprobe chemical analyses. Fe 3+ /Fe tot values are olivine 65%, amphibole 80%, phlogopite 65%, and magnetite 42%. Mineral compositions are nearly constant across grains and the section, except for a small variability in amphibole compositions reflecting the edenite exchange couple ( A Na + IV Al M A h + Si). These mineral compositions, the absence of Fe-Ni metal, and the oxygen isotope data support its classification as an R (Rumuruti) chondrite. LAP 04840 is classified as petrologic grade 5, based on the chemical homogeneity of its minerals, and the presence of distinctly marked chondrules and chondrule fragments in a finegrained crystalline matrix. The mineral assemblage of LAP 04840 allows calculation of physical and chemical conditions at the peak of its metamorphism: T = 670 ± 60°C from a amphibole-plagioclase thermometer; P H 2 O between 250 and 500 bars as constrained by the assemblage phlogopite + orthopyroxene + olivine + feldspar and the absence of diopside; P CO 2 unconstrained; f O 2 at QFM + 0.5 log units; logðf HF =f H 2 O Þ % À5:8; logðf HCl =f H 2 O Þ % À3:3; and logðf HCl =f HF Þ % À2:6. The hydrogen in LAP 04840 is very heavy, an average dD value of +3660 ± 75& in the magnesiohornblende. Only a few known sources of hydrogen have such high dD and are suitable sources for LAP 04840: ordinary chondrite phyllosilicates (as in the Semarkona chondrite), and insoluble organic matter (IOM) in ordinary chondrites and CR chondrites. Hydrogen from the IOM could have been released by oxidation, and then reacted with an anhydrous R chondrite (at high temperature), but it is not clear whether this scenario is correct. Geochimica et Cosmochimica Acta 72 (2008) 5757-5780 5758 M.C. McCanta et al. / Geochimica et Cosmochimica Acta 72 (2008) 5757-5780
Earth and Planetary Science Letters, 2007
Mid-Ordovician fossil meteorites found in the Thorsberg quarry, southern Sweden, are believed to have been deposited during a period of enhanced meteorite flux following the fragmentation of the L chondrite parent body. During diagenesis, the fossil meteorites were largely replaced by a secondary mineral assemblage. However, primary chromite grains have been preserved. High-precision oxygen isotope analysis by laser-assisted fluorination has been undertaken in order to confirm the chemical group (H, L or LL) to which the fossil meteorites belong. To test our methodology, chromites extracted from recent ordinary chondrite falls (Holbrook L6, Appley Bridge LL6 and Kernouve H6) have been analyzed and these show that ordinary chondrites can be classified into their respective groups (H, L, or LL) using the oxygen isotopic composition of chromite alone. Results from the Golvsten 001 meteorite demonstrate that this sample is an equilibrated L chondrite. The uniform major and minor element composition of chromites throughout the southern Swedish fossil meteorite section means that it is highly probable that all are L chondrites. High-precision oxygen isotope analysis of relict chromites thus further strengthens the link between the fossil meteorites and the disruption of the L chondrite parent body. The evidence presented here demonstrates that relict chromite grains survive diagenesis and can be used to classify ancient meteoritic material. Analysis of such fossil grains may prove to be a powerful tool, not only in the case of the mid-Ordovician event, but also in examining changes in the relative distribution of meteorite groups throughout geological time. when this paper was in the final stages of preparation. Robert was an ever enthusiastic and inspirational colleague. All who met him were given a warm smile and the benefit of his impressive scientific knowledge and understanding. We would like to dedicate this paper to his memory. 0012-821X/$ -see front matter
Geochimica et Cosmochimica Acta, 2011
Within 5 million years after formation of calcium aluminium rich inclusions (CAI), high temperature anhydrous phases were transformed to hydrous phyllosilicates, mostly serpentines, which dominate the matrices of the most primitive carbonaceous chondrites. CMs are the largest group of meteorites to provide samples of this material. To understand the nature of the availability, and role of H 2 O in the early solar system -as well as the settings of aqueous alteration -defining CM petrogenesis is critical. By Position Sensitive Detector X-ray Diffraction (PSD-XRD), we determine the modal abundance of crystalline phases present in volumes >1% for a suite of CMs -extending Part 1 of this work that dealt only with CM2 falls ) to now include CM2 and CM1 finds. CM2 samples contain 13-31% Fe,Mg silicates (olivine + pyroxene) and from 67% to 82% total phyllosilicate (mean 75% ± 1.3 2r). CM1 samples contain 6-10% olivine + pyroxene and 86-88% total phyllosilicate. Magnetite (0.6-5.2%), sulphide (0.6-3.9%), calcite (0-1.9%) and gypsum (0-0.8%) are minor phases across all samples. Since phyllosilicate forms from hydration of anhydrous Fe,Mg silicates (olivine + pyroxene), the ratio of total phyllosilicate to total anhydrous Fe,Mg silicate defines the degree of hydration and the following sequence results (in order of increasing hydration): QUE 97990 < Y 791198 < Murchison < Murray < Mighei < ALHA 81002 < Nogoya 6 Cold Bokkeveld 6 Essebi < QUE 93005 < ALH 83100 < MET 01070 < SCO 06043. High activities of Al (mostly from reactive mesostasis) and Si help to explain the composition and structure of CM serpentines that are distinct from terrestrial standards. Our data allows inference as to CM mineralogy at the point of accretion and challenges the conceptual validity of progressive alteration sequences. Modal mineralogy also provides new insights into CM petrogenesis and hints at a component of aqueous alteration occurring in the nebula, in addition to on the CM parent body(ies).
Geochimica Et Cosmochimica Acta, 2010
The oxygen-isotope compositions (obtained by laser fluorination) of hand-picked separates of isolated forsterite, isolated olivine and chondrules from the Tagish Lake carbonaceous chondrite describe a line (δ 17O = 0.95 * δ 18O - 3.24; R2 = 0.99) similar to the trend known for chondrules from other carbonaceous chondrites. The isolated forsterite grains (Fo 99.6-99.8; δ 18O = -7.2‰ to -5.5‰; δ 17O = -9.6‰ to -8.2‰) are more 16O-rich than the isolated olivine grains (Fo 39.6-86.8; δ 18O = 3.1‰ to 5.1‰; δ 17O = -0.3‰ to 2.2‰), and have chemical and isotopic characteristics typical of refractory forsterite. Chondrules contain olivine (Fo 97.2-99.8) with oxygen-isotope compositions (δ 18O = -5.2‰ to 5.9‰; δ 17O = -8.1‰ to 1.2‰) that overlap those of isolated forsterite and isolated olivine. An inverse relationship exists between the Δ 17O values and Fo contents of Tagish Lake isolated forsterite and chondrules; the chondrules likely underwent greater exchange with 16O-poor nebular gases than the forsterite. The oxygen-isotope compositions of the isolated olivine grains describe a trend with a steeper slope (1.1 ± 0.1, R2 = 0.94) than the carbonaceous chondrite anhydrous mineral line (CCAM slope = 0.95). The isolated olivine may have crystallized from an evolving melt that exchanged with 16O-poor gases of somewhat different composition than those which affected the chondrules and isolated forsterite. The primordial components of the Tagish Lake meteorite formed under conditions similar to other carbonaceous chondrite meteorite groups, especially CMs. Its alteration history has its closest affinities to CI carbonaceous chondrites.
Meteoritics & Planetary Science, 2008
available online at Abstract-We present noble gas analyses of sediment-dispersed extraterrestrial chromite grains recovered from ~470 Myr old sediments from two quarries (Hällekis and Thorsberg) and of relict chromites in a coeval fossil meteorite from the Gullhögen quarry, all located in southern Sweden. Both the sediment-dispersed grains and the meteorite Gullhögen 001 were generated in the Lchondrite parent body breakup about 470 Myr ago, which was also the event responsible for the abundant fossil meteorites previously found in the Thorsberg quarry. Trapped solar noble gases in the sediment-dispersed chromite grains have partly been retained during ~470 Myr of terrestrial residence and despite harsh chemical treatment in the laboratory. This shows that chromite is highly retentive for solar noble gases. The solar noble gases imply that a sizeable fraction of the sedimentdispersed chromite grains are micrometeorites or fragments thereof rather than remnants of larger meteorites. The grains in the oldest sediment beds were rapidly delivered to Earth likely by direct injection into an orbital resonance in the inner asteroid belt, whereas grains in younger sediments arrived by orbital decay due to Poynting-Robertson (P-R) drag. The fossil meteorite Gullhögen 001 has a low cosmic-ray exposure age of ~0.9 Myr, based on new He and Ne production rates in chromite determined experimentally. This age is comparable to the ages of the fossil meteorites from Thorsberg, providing additional evidence for very rapid transfer times of material after the L-chondrite parent body breakup.