Visible‐IR and Raman microspectroscopic investigation of three Itokawa particles collected by Hayabusa: Mineralogy and degree of space weathering based on nondestructive analyses

Hayabusa‐returned samples offer a unique perspective for understanding the link between asteroids and cosmomaterials available in the laboratory, and provide insights on the early stages of surface space weathering. This study characterizes the mineralogy and the extent of space weathering of the three Itokawa particles RA‐QD02‐0163, RA‐QD02‐0174, and RA‐QD02‐0213 provided by JAXA to our consortium. We report here a series of results based on nondestructive analyses through visible‐near‐infrared reflectance and Raman spectroscopy. Results were obtained on the raw particles, both in their original containers and deposited on diamond windows. Identification of the minerals, characterization of their elemental compositions, and measurements of their relative abundances were led through Raman spectroscopy in punctual and automatic mode. Reflectance spectra in the visible and near‐IR wavelengths constrain the mineralogy of the grains and allow direct comparison with the surface of Itokawa. The spectra reflect the extent of space weathering experienced by the three particles. Particle RA‐QD02‐0163 consists of a heterogeneous mixture of minerals: olivine (Fo₇₆) dominates an assemblage with both Ca‐rich (En₅₀, Wo₅₀) and Ca‐poor (En₈₅) pyroxenes. The elemental compositions of the silicates are consistent with those previously reported for distinct Hayabusa particles. Particles RA‐QD‐0174 and RA‐QD02‐0213 are solely composed of olivine, whose chemical composition is similar to that observed in RA‐QD02‐0163. It has been previously shown that the S‐type asteroid 25143 Itokawa is a breccia of poorly equilibrated LL4 and highly equilibrated LL5 and LL6 materials. The three particles studied here can be related to the least metamorphosed lithology (LL4) based on the high forsterite content of the olivine. Neither carbonaceous matter nor hydrated minerals were detected through Raman on the three allocated particles. The NIR‐VIS reflectance (incidence = 45°, light collection at e = 0°) spectra of the three particles, in particular the 1 μm band, are consistent with the presence of both olivine and pyroxene detected via Raman. The spectra of particles RA‐QD02‐0163 and RA‐QD02‐0213 are also fully compatible with the ground‐based observations of asteroid (25143) Itokawa in terms of both spectral features and slope. By contrast, particle RA‐QD02‐0174 has a similar 1 μm band depth but higher (redder) spectral slope than the surface of Itokawa. This probably reveals a variable extent of space weathering among the regolith particles. RA‐QD02‐0174 may contain a higher amount of nanophase metallic iron and nanophase FeS. Such phases are products by space weathering induced by solar wind, previously detected on other Itokawa particles.     

Correlations and zoning patterns of phosphorus and chromium in olivine from H chondrites and the LL chondrite Semarkona

Phosphorus zoning is observed in olivines in high‐FeO (type IIA) chondrules in H chondrites over the entire range of petrologic grades: H3.1–H6. Features in P concentrations such as oscillatory and sector zoning, and high P cores are present in olivines that are otherwise unzoned in the divalent cations. Aluminum concentrations are low and not significantly associated with P zoning in chondrule olivines. In highly unequilibrated H chondrites, phosphorus zoning is generally positively correlated with Cr. Atomic Cr:P in olivine is roughly 1:1 (3:1 for one zone in one olivine in RC 075), consistent with Cr³⁺ charge‐balancing P⁵⁺ substituting for Si⁴⁺. Normal igneous zonation involving the dominant chrome species Cr²⁺ was observed only in the LL3.0 chondrite Semarkona. In more equilibrated chondrites (H3.5–H3.8), Cr spatially correlated with P is occasionally observed but it is diffuse relative to the P zones. In H4–H6 chondrites, P‐correlated Cr is absent. One signature of higher metamorphic grades (≥H3.8) is the presence of near matrix olivines that are devoid of P oscillatory zoning. The restriction to relatively high metamorphic grade and to grains near the chondrule–matrix interface suggests that this is a response to metasomatic processes. We also observed P‐enriched halos near the chondrule–matrix interface in H3.3–H3.8 chondrites, likely reflecting the loss of P and Ca from mesostasis and precipitation of Ca phosphate near the chondrule surface. These halos are absent in equilibrated chondrites due to coarsening of the phosphate and in unequilibrated chondrites due to low degrees of metasomatism. Olivines in type IA chondrules show none of the P‐zoning ubiquitous in type IIA chondrules or terrestrial igneous olivines, likely reflecting sequestration of P in reduced form within metallic alloys and sulfides during melting of type IA chondrules.     

Boron abundances and isotopic ratios of olivine grains on Itokawa returned by the Hayabusa spacecraft

We report the B abundances and isotopic ratios of two olivine grains from the S‐type asteroid Itokawa sampled by the Hayabusa spacecraft. Olivine grains from the Dar al Gani (DaG) 989 LL6 chondrite were used as a reference. Since we analyzed polished thin sections in both cases, we expect the contribution from the solar wind B (rich in ¹⁰B) to be minimal because the solar wind was implanted only within very thin layers of the grain surface. The Itokawa and DaG 989 olivine grains have homogeneous B abundances (~400 ppb) and ¹¹B/¹⁰B ratios compatible with the terrestrial standard and bulk chondrites. The observed homogeneous B abundances and isotopic ratios of the Itokawa olivine grains are likely the result of thermal metamorphism which occurred in the parent asteroid of Itokawa, which had a similar composition as LL chondrites. The chondritic B isotopic ratios of the Itokawa samples suggest that they contain little cosmogenic B (from cosmic‐ray spallation reactions) rich in ¹⁰B. This observation is consistent with the short cosmic‐ray exposure ages of Itokawa samples inferred from the small concentrations of cosmogenic ²¹Ne. If other Itokawa samples have little cosmogenic B as well, the enrichment in ¹⁰B found previously on the surface of another Itokawa particle (as opposed to the bulk grain study here) may be attributed to implanted solar wind B.     

Geochemistry and origin of metal,olivine clasts,and matrix in the Dong Ujimqin Qi mesosiderite

Abstract— The Dong Ujimqin Qi mesosiderite is the first recorded fall of a stony‐iron meteorite in China. According to silicate textures and metal composition, this meteorite is classified as a member of subgroup IB. Instrumental neutron activation analyses (INAA) of metals show that the matrix metal has lower concentrations of Os, Ir, Re, and Pt, but higher concentrations of Ni and Au than the 7.5 cm metal nodule present in the meteorite. We attribute these compositional differences to fractional crystallization of molten metal. Studies of olivine clasts show that FeO contents are uniform in individual olivine crystals but are variable for different olivine clasts. Although concentrations of rare earth elements (REEs) change within olivine clasts, they all exhibit a vee‐shaped pattern relative to CI chondrites. The relatively high concentrations of REEs in olivine and the shape of REE patterns require a liquid high in REEs and especially in light REEs. As such a liquid was absent from the region where basaltic and gabbroic clasts formed, mesosiderite olivine must have formed in a part of the differentiated asteroid that is different from the location where other mesosiderite silicate clasts formed.     

Refractory forsterite in primitive meteorites: Condensates from the solar nebula?

Abstract— All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements such as Ca, Al, and Ti. Ten such grains (52 to 754 μg) with minor amounts of adhering matrix were separated from the Allende meteorite. After bulk chemical analysis by instrumental neutron activation analysis (INAA), some samples were analyzed with an electron microprobe and some with an ion microprobe. Matrix that accreted to the forsterite grains has a well‐defined unique composition, different from average Allende matrix in having higher Cr and lower Ni and Co contents, which implies limited mixing of Allende matrix. All samples have approximately chondritic relative abundances of refractory elements Ca, Al, Sc, and rare‐earth elements (REE), although some of these elements, such as Al, do not quantitatively reside in forsterite; whereas others (e.g., Ca) are intrinsic to forsterite. The chondritic refractory element ratios in bulk samples, the generally high abundance level of refractory elements, and the presence of Ca‐Al‐Ti‐rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine. The Ca‐Al‐Ti‐rich glasses may have acted as nuclei for forsterite condensation. Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chondrules: (a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. (b) The low FeO content of refractory forsterite of 0.2‐0.4 wt% imposes an upper limit of ～1 wt% of FeO on the parental melt, too low for ordinary and carbonaceous chondrule melts, (c) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivines in all classes of meteorites. The observed Mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition, (d) As shown earlier, refractory forsterites have O‐isotopic compositions different from chondrules (Weinbruch et al., 1993a). Refractory olivines in carbonaceous chondrites are found in matrix and in chondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 1992). As refractory forsterite cannot have formed by crystallization from chondrule melts, we conclude that refractory forsterite from chondrules are relic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of refractory forsterite by condensation from an oxidized nebular gas.     

Oxygen isotopes in chondrule olivine and isolated olivine grains from the CO3 chondrite Allan Hills A77307

Abstract— We have measured O‐isotopic ratios in a variety of olivine grains in the CO3 chondrite Allan Hills (ALH) A77307 using secondary ion mass spectrometry in order to study the chondrule formation process and the origin of isolated olivine grains in unequilibrated chondrites. Oxygen‐isotopic ratios of olivines in this chondrite are variable from δ¹⁷O = ?15.5 to +4.5% and δ¹⁸O = ?11.5 to +3.9%, with Δ¹⁷O varying from ?10.4 to +3.5%. Forsteritic olivines, Fa_<1, are enriched in ¹⁶O relative to the bulk chondrite, whereas more FeO‐rich olivines are more depleted in ¹⁶O. Most ratios lie close to the carbonaceous chondrite anhydrous minerals (CCAM) line with negative values of Δ¹⁷O, although one grain of composition Fa₄ has a mean Δ¹⁷O of +1.6%. Marked O‐isotopic heterogeneity within one FeO‐rich chondrule is the result of incorporation of relic, ¹⁶O‐rich, Mg‐rich grains into a more ¹⁶O‐depleted host. Isolated olivine grains, including isolated forsterites, have similar O‐isotopic ratios to olivine in chondrules of corresponding chemical composition. This is consistent with derivation of isolated olivine from chondrules, as well as the possibility that isolated grains are chondrule precursors. The high ¹⁶O in forsteritic olivine is similar to that observed in forsterite in CV and CI chondrites and the ordinary chondrite Julesburg and suggests nebula‐wide processes for the origin of forsterite that appears to be a primitive nebular component.     

Analysis of ordinary chondrites using powder X‐ray diffraction: 2. Applications to ordinary chondrite parent‐body processes

Abstract– We evaluate the chemical and physical conditions of metamorphism in ordinary chondrite parent bodies using X‐ray diffraction (XRD)‐measured modal mineral abundances and geochemical analyses of 48 type 4–6 ordinary chondrites. Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD‐derived olivine and low‐Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3–0.4 wt% H₂O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies.     

The oxygen isotopic properties of olivines in the Semarkona ordinary chondrite

Abstract— In order to explore the origin of chondrules and the chondrites, the O isotopic compositions of nine olivine grains in seven chondrules from the primitive Semarkona LL3.0 chondrite have been determined by ion microprobe. The data plot in the same general region of the three-isotope plot as whole-chondrule samples from ordinary chondrites previously measured by other techniques. There are no significant differences between the O isotopic properties of olivine in the various chondrule groups in the present study, but there is a slight indication that the data plot at the ¹⁶O-rich end of the ordinary chondrite field. This might suggest that the mesostasis contains isotopically heavy O. The olivines in the present study have O isotopic compositions unlike the ¹⁶O-rich olivine grains from the Julesburg ordinary chondrite. Even though olivines in group A chondrules have several properties in common with them, the ¹⁶O-rich Julesburg olivines previously reported are not simply olivines from group A chondrules.     

NWA 10214—An LL3 chondrite breccia with an assortment of metamorphosed,shocked, and unique chondrite clasts

NWA 10214 is an LL3‐6 breccia containing ~8 vol% clasts including LL5, LL6, and shocked‐darkened LL fragments as well as matrix‐rich Clast 6 (a new kind of chondrite). This clast is a dark‐colored, subrounded, 6.1 × 7.0 mm inclusion, consisting of 60 vol% fine‐grained matrix, 32 vol% coarse silicate grains, and 8 vol% coarse opaque grains. The large chondrules and chondrule fragments are mainly Type IB; one small chondrule is Type IIA. Also present are one 450 × 600 μm spinel‐pyroxene‐olivine CAI and one 85 × 110 μm AOI. Clast 6 possesses a unique set of properties. (1) It resembles carbonaceous chondrites in having relatively abundant matrix, CAIs, and AOIs; the clast's matrix composition is close to that in CV3 Vigarano. (2) It resembles type‐3 OC in its olivine and low‐Ca pyroxene compositional distributions, and in the Fe/Mn ratio of ferroan olivine grains. Its mean chondrule size is within 1σ of that of H chondrites. The O‐isotopic compositions of the chondrules are in the ordinary‐ and R‐chondrite ranges. (3) It resembles type‐3 enstatite chondrites in the minor element concentrations in low‐Ca pyroxene grains and in having a high low‐Ca pyroxene/olivine ratio in chondrules. Clast 6 is a new variety of type‐3 OC, somewhat more reduced than H chondrites or chondritic clasts in the Netschaevo IIE iron; the clast formed in a nebular region where aerodynamic radial drift processes deposited a high abundance of matrix material and CAIs. A chunk of this chondrite was ejected from its parent asteroid and later impacted the LL body at low relative velocity.     

New measurement technique for characterizing small extraterrestrial materials by X‐ray diffraction using the Gandolfi attachment

Identification and characterization of small extraterrestrial samples, such as small Antarctic meteorites <~1 cm, require the development of convenient laboratory‐based nondestructive analytical techniques using X‐ray diffraction (XRD). We explore the characterization criteria using an X‐ray diffractometer with a Gandolfi attachment using sub‐mm small fragments and powder aggregates for various kinds of stony meteorites and develop a new analytical technique. We primarily focus on olivine and pyroxene because they are the most abundant and important minerals for stony meteorite classification. A new calibration is performed to estimate the FeO content of the olivine in unequilibrated ordinary chondrites, which is useful for determining the meteorite chemical group irrespective of powder aggregate diameter but dependent on fragment grain diameter. This is because X‐ray intensity absorption is more effective for grains than for powders. Clinoenstatite (Cen) and orthoenstatite (Oen) were distinguished using the presence or absence of the isolated Oen 511 index peak. The method is also applied to other stony meteorites including carbonaceous chondrites and achondrites. The XRD results are consistent with studies based on polished sections involving textural observations by scanning microscope and chemical compositions of the constituent minerals. The new measurement technique presented here is convenient because of its use in air by the laboratory‐based X‐ray diffractometer, which makes it useful for the initial analyses of restricted extraterrestrial sample characterization.     

Isolated olivines in the Yamato 82042 CM2 chondrite: The tracing of major condensation events in the solar nebula

Abstract— Yamato 82042 is an unusual CM2 chondrite consisting mainly of phyllosilicates, a few olivines and carbonates, very minor sulphides and trace metal. Olivine occurs: (1) as isolated grains dispersed in the phyllosilicate matrix, (2) as constituents of mineral aggregates or accretionary fragments associated with abundant phyllosilicates and minor sulphides, and (3) as objects which resemble barred olivine chondrules also associated with phyllosilicates. Olivine, from all occurrences, ranges in composition from 0.26 to 22.6 weight % FeO, but generally contains less than 1.25 wt.% FeO. Minor element contents, particularly Ca, Al, and Cr, are relatively high and are generally correlated, as reported for olivines in other carbonaceous chondrites. However, we report here uncorrected trends for the same minor elements which occur in distinct areas (volumes) within the same olivines. These compositional trends may be due to condensation of olivine from a vapor of non-solar composition and partial mobilization of Ca during later annealing. If this is the case, the data may be used to trace changes in the Ca/Al ratio of the parent medium during the formation of these olivines, provided that it is possible to distinguish the effects of any post-formation annealing which could have redistributed the minor elements. Some isolated olivines show distinctive minor element zoning which severely limits the possibility of any post-formation redistribution of these elements. Accordingly, these isolated olivines indeed retain evidence of early condensation processes in the solar nebula, though non-classic conditions are implied for their formation.     

Sylvite and halite on particles recovered from 25143 Itokawa: A preliminary report

We observed cross sectional ultra‐thin sections near the surface of 12 particles recovered from the S‐type asteroid Itokawa by the Hayabusa spacecraft in 2010, using spherical aberration–corrected STEM and conventional TEM. Although their mineralogy is almost identical to the equilibrated LL chondrites and therefore basically anhydrous, micrometer‐to‐submicrometer‐sized sylvite was identified on the surface of Itokawa particle RA‐QD02‐0034. Separately, micrometer‐sized halite was also identified on the surface of Itokawa particle RA‐QD02‐0129. Detailed inspection of the sample processing procedures at the JAXA's Planetary Materials Sample Curation Facility and textural observation of the sylvite and halite indicate that they were clearly present on two Itokawa particles before they were removed from Clean Chamber #2 at JAXA. However, there is no direct evidence for their extraterrestrial origin at present. If the sylvite and halite are extraterrestrial, their presence suggests that they may be more abundant on the surface of S‐type asteroids than previously thought.     

Terrestrial weathering of ordinary chondrites in nature and continuing during laboratory storage and processing: Review and implications for Hayabusa sample integrity

Abstract– The Hayabusa mission recently returned the first samples from an ordinary chondrite (OC) parent body. Olivine, low‐Ca pyroxene, and kamacite compositions fall within the known ranges of minerals from LL4 to LL6 chondrites. Hayabusa samples are being processed and stored in a pure N₂ atmosphere. However, during recovery, prior to receiving, and during preliminary examination, some Hayabusa samples were briefly exposed to terrestrial atmosphere. Some of the minerals already identified in the Hayabusa samples (olivine, sulfides) are known to be among the most vulnerable to weathering reactions in moist, oxidizing terrestrial environments. Oxidation of Fe in metal, sulfides, and ferrous silicates is ubiquitous in naturally weathered OC finds, in samples of falls subjected to even a few decades of weathering before recovery, and in OC falls recovered and curated promptly after recovery. All prerecovery oxidation, hydrolysis, hydration, and product‐forming phenomena documented to affect OC finds have been documented to continue in OC samples in curatorial and laboratory settings, producing mineralogical and textural effects at scales easily discernable by electron microscopy, on timescales of decades. Hayabusa samples will be exposed to similar terrestrial conditions at times throughout sample processing, allocation, and examination. Maximizing the science yield from these important samples requires thorough understanding of how LL chondrite minerals like those in the Hayabusa samples react with terrestrial moisture and oxidants in support of proper planning for maintaining Hayabusa sample integrity after allocation, and for proper anticipation of the effects of inevitable exposure to Earth’s atmosphere during storage and examination in terrestrial analytical laboratories.     

Effect of a synchrotron X‐ray microtomography imaging experiment on the amino acid content of a CM chondrite

X‐ray microcomputed tomography and synchrotron X‐ray microcomputed tomography (μCT) are becoming popular tools for the reconnaissance imaging of chondrites. However, there are occasional concerns that the use of μCT may be detrimental to organic components of a chondrite. Soluble organic compounds represent ~2–10% of the total solvent extractable carbon in CI and CM carbonaceous chondrites and amino acids are among the most abundant compounds in the soluble organic fraction. We irradiated two samples of the Murchison CM2 carbonaceous chondrite under conditions slightly harsher (increased beam exposure time) than those typically used for x‐ray μCT imaging experiments to determine if detectable changes in the amino acid abundance and distribution relative to a nonexposed control sample occurred. After subjecting two meteorite portions to ionizing radiation dosages of 1.1 kiloGray (kGy) and 1.2 kGy with 48.6 and 46.6 keV monochromatic X‐rays, respectively, we analyzed the amino acid content of each sample. Within analytical errors, we found no differences in the amino acid abundances or enantiomeric ratios when comparing the control samples (nonexposed Murchison) and the irradiated samples. We show with calculations that any sample heating due to x‐ray exposure is negligible. We conclude that a monochromatic synchrotron X‐ray μCT experiment at beamline 13‐BM‐D of the Advanced Photon Source, which imparts ~1 kGy doses, has no detectable effect on the amino acid content of a carbonaceous chondrite. These results are important for the initial reconnaissance of returned samples from the OSIRIS‐REx and Hayabusa 2 asteroid sample return missions.     

FeO‐rich silicates in the Sahara 97159 (EH3) enstatite chondrite: Mineralogy,oxygen isotopic compositions,and origin

Abstract— We report the mineralogy and oxygen isotopic compositions of FeO‐rich silicates in the Sahara 97159 EH3 chondrite. This component is referred to as FeO‐rich because it contains substantially more FeO than the characteristic FeO‐poor silicates in the highly reduced enstatite meteorites. These FeO‐rich silicates are mostly low‐Ca pyroxene (Fs5–35) and their compositions suggest an origin under more oxidizing conditions, like those for the ordinary chondrites. However, the mafic silicates in ordinary and carbonaceous chondrites are dominantly olivine, and the FeO‐rich silicates in the E chondrites are less commonly olivine. The oxygen isotopic compositions of the FeO‐rich silicates are indistinguishable from those of FeO‐poor silicates in Sahara 97159. These observations suggest that both the FeO‐rich silicates and the FeO‐poor silicates in EH chondrites formed from the same oxygen reservoir where redox conditions varied widely.     

Mineral compositions in Antarctic and Greenland micrometeorites

Abstract— The mineral compositions of 250 micrometeorites have been studied and olivines and low-calcium pyroxenes with crystals larger than 5 μm have been analyzed. While magnesium-rich grains dominate, the Fa content of olivine may reach 50% and the Fs content of pyroxene may reach 26%. The Ca and Mn of the olivine show no consistent trends with increasing Fe, but Cr shows a negative correlation. For low-Ca pyroxene, Al and Cr contents are generally higher than in pyroxenes of equilibrated chondrites but similar to those of highly unequilibrated chondrites. Calcium-bearing pyroxene, feldspar and chromite are rare in the micrometeorites which were selected because of their high Mg, Si, Fe and their low Ca and Al content. All these minerals are found as coarse-grained particles often with adhering iron-rich scoria or as clasts in fine-grained or scoriaceous micrometeorites. Apart from a few particles which could be the debris of ordinary chondrites, most micrometeorites probably come from a common source similar, but not identical to carbonaceous chondrites, as shown by their lower Ni and S content and their different oxygen isotopic composition assuming two measurements performed on olivine grains prove to be typical.     

Heavily metamorphosed clasts from the CV chondrite breccias Mokoia and Yamato‐86009

Abstract– Metamorphosed clasts in the CV carbonaceous chondrite breccias Mokoia and Yamato‐86009 (Y‐86009) are coarse‐grained, granular, polymineralic rocks composed of Ca‐bearing (up to 0.6 wt% CaO) ferroan olivine (Fa_34–39), ferroan Al‐diopside (Fs_9–13Wo_47–50, approximately 2–7 wt% Al₂O₃), plagioclase (An_37–84Ab_63–17), Cr‐spinel (Cr/(Cr + Al) = 0.19–0.45, Fe/(Fe + Mg) = 0.60–0.79), nepheline, pyrrhotite, pentlandite, Ca‐phosphate, and rare grains of Ni‐rich taenite; low‐Ca pyroxene is absent. Most clasts have triple junctions between silicate grains, indicative of prolonged thermal annealing. Based on the olivine‐spinel and pyroxene thermometry, the estimated metamorphic temperature recorded by the clasts is approximately 1100 K. Few clasts experienced thermal metamorphism to a lower degree and preserved chondrule‐like textures. The Mokoia and Y‐86009 clasts are mineralogically unique and different from metamorphosed chondrites of known groups (H, L, LL, R, EH, EL, CO, CK) and primitive achondrites (acapulcoites, brachinites, lodranites). On a three‐isotope oxygen diagram, compositions of olivine in the clasts plot along carbonaceous chondrite anhydrous mineral line and the Allende mass‐fractionation line, and overlap with those of the CV chondrule olivines; the Δ¹⁷O values of the clasts range from about ?4.3‰ to ?3.0‰. We suggest that the clasts represent fragments of the CV‐like material that experienced metasomatic alteration, high‐temperature metamorphism, and possibly melting in the interior of the CV parent asteroid. The lack of low‐Ca pyroxene in the clasts could be due to its replacement by ferroan olivine during iron‐alkali metasomatic alteration or by high‐Ca ferroan pyroxene during melting under oxidizing conditions.     

Three‐dimensional microstructure of samples recovered from asteroid 25143 Itokawa: Comparison with LL5 and LL6 chondrite particles

In this study, the three‐dimensional (3‐D) microstructure of 48 Itokawa regolith particles was examined by synchrotron microtomography at SPring‐8 during the preliminary examination of Hayabusa samples. Moreover, the 3‐D microstructure of particles collected from two LL6 chondrites (Ensisheim and Kilabo meteorites) and an LL5 chondrite (Tuxtuac meteorite) was investigated by the same method for comparison. The modal abundances of minerals, especially olivine, bulk density, porosity, and grain size are similar in all samples, including voids and cracks. These results show that the Itokawa particles, which are surface materials from the S‐type asteroid Itokawa, are consistent with the LL chondrite materials in terms of not only elemental and isotopic composition of the minerals but also 3‐D microstructure. However, we could not determine whether the Itokawa particles are purely LL5, LL6, or a mixture of the two. No difference between the particles collected from Rooms A and B of the sample chamber, corresponding to the sampling sequence of the spacecraft's second and first touchdowns, respectively, was detected because of the statistically small amount of particles from Room B.     

Deciphering the nebular and asteroidal record of silicates and organic material in the matrix of the reduced CV3 chondrite Vigarano

We have conducted scanning electron microscope (SEM) and transmission electron microscope (TEM) studies of a variety of occurrences of matrix in the reduced CV3 chondrite breccia Vigarano. Matrix, which occurs as clastic interchondrule material and finer‐grained rims, is dominated by morphologically variable olivines that host submicron, hercynitic spinel, and carbonaceous inclusions. Clastic matrix and fine‐grained rims show significant differences in their olivine morphologies, abundance, and composition of olivine inclusions, and characteristics of the carbonaceous matter. We suggest that these differences are the result of different degrees of alteration of clastic matrix and rims and are not due to variability in their precursor materials. Textural and compositional characteristics of olivine in the matrix are consistent with formation by growth, possibly from an amorphous precursor material during asteroidal metamorphism, in the presence of limited quantities of aqueous fluids. Spinel inclusions in olivine may be nebular condensates that acted as seeds for nucleation of olivine or may have formed during metamorphism and were subsequently overgrown by olivine. Carbonaceous material occurs as nanometer‐sized inclusions within olivine in both fine‐grained rims and clastic matrix, but is most abundant as 100–200 nm grains, interstitial to matrix olivines. Most carbonaceous material is amorphous, but poorly graphitized carbon (PGC) also occurs as a minor component in both olivine inclusions and interstitial C. The widespread occurrence of fine‐grained amorphous carbon grains in the interstitial regions between olivine grains may preserve the distribution and grain size of nebular organic material. No clear textural relationships exist between carbonaceous grains and the other mineralogical components of Vigarano matrix that could help constrain the origin of the organic grains (i.e., evidence for Fischer‐Tropsch‐type reactions). Finally, there are considerable differences between matrix olivines in Vigarano in comparison with those in oxidized CV3 chondrites. In particular, the mineralogy and morphology of the matrix olivines and the nature, composition, and distribution of inclusions in the olivine grains are distinct. Based on these differences, we conclude that matrix in the oxidized CV3 chondrites could not have formed by thermal processing of Vigarano‐like material.     

Thermochemical stability of low‐iron,manganese‐enriched olivine in astrophysical environments

Abstract– Low‐iron, manganese‐enriched (LIME) olivine grains are found in cometary samples returned by the Stardust mission from comet 81P/Wild 2. Similar grains are found in primitive meteoritic clasts and unequilibrated meteorite matrix. LIME olivine is thermodynamically stable in a vapor of solar composition at high temperature at total pressures of a millibar to a microbar, but enrichment of solar composition vapor in a dust of chondritic composition causes the FeO/MnO ratio of olivine to increase. The compositions of LIME olivines in primitive materials indicate oxygen fugacities close to those of a very reducing vapor of solar composition. The compositional zoning of LIME olivines in amoeboid olivine aggregates is consistent with equilibration with nebular vapor in the stability field of olivine, without re‐equilibration at lower temperatures. A similar history is likely for LIME olivines found in comet samples and in interplanetary dust particles. LIME olivine is not likely to persist in nebular conditions in which silicate liquids are stable.