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.     

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.     

Mineral chemistry of MUSES‐C Regio inferred from analysis of dust particles collected from the first‐ and second‐touchdown sites on asteroid Itokawa

The mineralogy and mineral chemistry of Itokawa dust particles captured during the first and second touchdowns on the MUSES‐C Regio were characterized by synchrotron‐radiation X‐ray diffraction and field‐emission electron microprobe analysis. Olivine and low‐ and high‐Ca pyroxene, plagioclase, and merrillite compositions of the first‐touchdown particles are similar to those of the second‐touchdown particles. The two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that MUSES‐C Regio is covered with dust particles of uniform mineral chemistry of LL chondrites. Quantitative compositional properties of 48 dust particles, including both first‐ and second‐touchdown samples, indicate that dust particles of MUSES‐C Regio have experienced prolonged thermal metamorphism, but they are not fully equilibrated in terms of chemical composition. This suggests that MUSES‐C particles were heated in a single asteroid at different temperatures. During slow cooling from a peak temperature of approximately 800 °C, chemical compositions of plagioclase and K‐feldspar seem to have been modified: Ab and Or contents changed during cooling, but An did not. This compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized Itokawa parent asteroid. After cooling, some particles have been heavily impacted and heated, which resulted in heterogeneous distributions of Na and K within plagioclase crystals. Impact‐induced chemical modification of plagioclase was verified by a comparison to a shock vein in the Kilabo LL6 ordinary chondrite where Na‐K distributions of plagioclase have been disturbed.     

Combining IR and X‐ray microtomography data sets: Application to Itokawa particles and to Paris meteorite

In the near future, a new generation of sample return missions (Hayabusa2, OSIRIS‐REx, MMX, etc.) will collect samples from small solar system bodies. To maximize the scientific outcome of laboratory studies and minimize the loss of precious extraterrestrial samples, an analytical sequence from less destructive to more destructive techniques needs to be established. In this work, we present a combined X‐ray and IR microtomography applied to five Itokawa particles and one fragment of the primitive carbonaceous chondrite Paris. We show that this analytical approach is able to provide a 3‐D physical and chemical characterization of individual extraterrestrial particles, using the measurement of their 3‐D structure and porosity, and the detection of mineral and organic phases, and their spatial co‐localization in 3‐D. We propose these techniques as an efficient first step in a multitechnique analytical sequence on microscopic samples collected by space missions.     

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.     

Mineralogy of fine‐grained material in the Krymka (LL3.1) chondrite

Abstract— Two dark lithic fragments and matrix of the Krymka LL3.1 chondrite were mineralogically and chemically studied in detail. These objects are characterised by the following chemical and mineralogical characteristics, which distinguish them from the host chondrite Krymka: (1) bulk chemical analyses revealed low totals (systematically lower than 94 wt%) due to high porosity; (2) enrichment in FeO and depletion in S, MgO and SiO₂ due to a high abundance of Fe‐rich silicates and low sulfide abundance; (3) fine‐grained, almost chondrule‐free texture with predominance of a porous, cryptocrystalline groundmass and fine grains; (4) occurrence of a small amount of once‐molten material (microchondrules) enclosed in fine‐grained materials; (5) occurrence of accretionary features, especially unique accretionary spherules; (6) high abundance of small calcium‐ aluminium‐rich inclusions (CAIs) in one of the fine‐grained fragments. It is suggested that the abundance of CAIs in this fragment is one of the highest ever found in an ordinary chondrite. Accretionary, fine‐grained spherules within one of the fragments bear fundamental information about the initial stages of accretion as well as on the evolution of the clast, its incorporation, and history within the bulk rock of Krymka. The differences in porosity, bulk composition, and mineralogy of cores and rims of the fine‐grained spherulitic objects allow us to speculate on the following processes: (1) Low velocity accretion of tiny silicate grains onto the surface of coarse metal or silicate grains in a dusty region of the nebula is the beginning of the formation of accretionary, porous (fluffy) silicate spherules. (2) Within a dusty environment with decreasing silicate/(metal + sulfide) ratio the porous spherules collected abundant metal and sulfide particles together with silicate dust, which formed an accretionary rim. Variations of the silicate/(sulfide + metal) ratio in the dusty nebular environment result in the formation of multi‐layered rims on the surface of the silicate‐rich spherules. (3) Soft accretion and lithification of rimmed, fluffy spherules, fine‐grained, silicate‐rich dust, metal‐sulfide particles, CAIs, silicate‐rich microchondrules, and coarse silicate grains and fragments followed. (4) After low‐temperature processing of the primary, accretionary rock collisional fragmentation occurred, the fragments were subsequently coated by fine‐grained material, which was highly oxidized and depleted in sulfides. (5) In a final stage this accretionary “dusty” rock was incorporated as a fragment within the Krymka host.     

Magnetic properties of the LL5 ordinary chondrite Chelyabinsk (fall of February 15, 2013)

Here we characterize the magnetic properties of the Chelyabinsk chondrite (LL5, S4, W0) and constrain the composition, concentration, grain size distribution, and mineral fabric of the meteorite's magnetic mineral assemblage. Data were collected from 10 to 1073 K and include measurements of low‐field magnetic susceptibility (χ₀), the anisotropy of χ₀, hysteresis loops, first‐order reversal curves, Mössbauer spectroscopy, and X‐ray microtomography. The REM and REM′ paleointensity protocols suggest that the only magnetizations recorded by the chondrite are components of the Earth's magnetic field acquired during entry into our planet's atmosphere. The Chelyabinsk chondrite consists of light and dark lithologies. Fragments of the light lithology show logχ₀ = 4.57 ± 0.09 (s.d.) (n = 135), while the dark lithology shows 4.65 ± 0.09 (n = 39) (where χ₀ is in 10^?9 m³ kg^?1). Thus, Chelyabinsk is three times more magnetic than the average LL5 fall, but is similar to a subgroup of metal‐rich LL5 chondrites (Paragould, Aldsworth, Bawku, Richmond) and L/LL5 chondrites (Glanerbrug, Knyahinya). The meteorite's room‐temperature magnetization is dominated by multidomain FeNi alloys taenite and kamacite (no tetrataenite is present). However, below approximately 75 K remanence is dominated by chromite. The metal contents of the light and dark lithologies are 3.7 and 4.1 wt%, respectively, and are based on values of saturation magnetization.     

Flattened chondrules in the LAP 04581 LL5 chondrite: Evidence for an oblique impact into LL3 material and subsequent collisional heating

Abstract– LaPaz Icefield (LAP) 04581 is a shock‐stage S2 LL5 chondrite that initially consisted of unrecrystallized LL3 material with a moderately abundant fine‐grained porous matrix (on the order of 15 vol%). A rare oblique impact created shearing stresses that produced a petrofabric in the rock, induced frictional melting of troilite (thereby forming a large troilite vein), and caused chondrule flattening. The latter process was facilitated by impact‐induced collapse of matrix pores. Chondrule flattening could not have occurred if the rock had been impacted after it had been metamorphosed to type 5 levels because the fine‐grained matrix would have previously recrystallized and developed low porosity. Ar‐Ar dating of LAP 04581 yields an age of 4175 Ma. This date is long after ²⁶Al had decayed away and most likely reflects the timing of a second impact event that shocked the rock to S4–S5 levels. The troilite vein became polycrystalline at this time and the whole rock was annealed to petrologic type 5, perhaps by being buried beneath hot ejecta of low thermal diffusivity. After annealing, the rock was weakly shocked to S2 levels. LAP 04581 serves as an example of impact‐induced heating being a viable mechanism for chondrite metamorphism.     

Mineralogical composition of (25143) Itokawa 1998 SF36 from visible and near‐infrared reflectance spectroscopy: Evidence for partial melting

Abstract— –In March 2001, asteroid (25143) Itokawa, the target of the Japanese Hayabusa spacecraft mission, was in a favorable viewing geometry for ground‐based telescopic study. Visible/near‐infrared (VNIR) spectra (～～0.48 to 0.9 μm) obtained on March 24, 26, and 27 UT, and near‐infrared (NIR) spectra (～～0.75 to 2.5 μm) obtained on March 10, 11, 12, 23, and 24 UT collectively show absorption features centered near 1.0 and 2.0 μm, which are indicative of olivine and pyroxene. Analyses of these absorption features indicate an abundance ratio of olivine to pyroxene of approximately 75:25 ± 5, respectively, with no significant variation in the relative abundance of these minerals across its surface on a regional scale. The band center positions indicate that the mean pyroxene chemistry is ～～Wo_{14 ± 5}Fs_{43 ± 5}. There appear to be at least two pyroxene components: primarily a low‐Ca orthopyroxene accompanied by a spectrally significant (～～15–20%) high Fe‐rich pigeonite phase. The mean pyroxene composition is significantly more Fe‐rich than the Fs_14–26 range found in ordinary chondrites. These pyroxene compositions are suggestive of phases crystallized from partial melts. This would indicate that the parent body of (25143) Itokawa reached temperatures sufficient to initiate partial melting (～～1050 to 1250 °C), but that it did not attain the degree of melting required for significant melt mobilization and efficient segregation of the basaltic melt component from the unmelted residual olivine portion. Itokawa's spectral band parameters place it near the S(III)/S(IV) boundary, but within the S(III) taxonomic field. In meteoritic nomenclature, Itokawa would be most analogous to an olivine‐rich primitive achondrite. Alternatively, if the high Fs value is not related to partial melting, then Itokawa could also represent a rare atypical LL chondrite, or a previously unsampled oxidized Fe‐rich chondritic‐like assemblage.     

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.     

The Richfield LL3 chondrite

Abstract— Richfield is a moderately shocked (shock stage S4) LL3.7 genomict breccia find consisting mainly of light-colored recrystallized clasts and dark clasts exhibiting significant silicate darkening; a few impact-melt-rock clasts and LL5 chondrite clasts also occur. The cosmic-ray exposure age of 14.5 Ma is indistinguishable from the main exposure peak for LL chondrites (15 Ma). Although the exposure ages indicate little He loss, the gas-retention ages indicate high gas losses that must have occurred prior to or during ejection from the LL parent body.     

Orbit and origin of the LL7 chondrite Dishchii'bikoh (Arizona)

The trajectory and orbit of the LL7 ordinary chondrite Dishchii'bikoh are derived from low‐light video observations of a fireball first detected at 10:56:26 UTC on June 2, 2016. Results show a relatively steep ~21° inclined orbit and a short 1.13 AU semimajor axis. Following entry in Earth's atmosphere, the meteor luminosity oscillated corresponding to a meteoroid spin rate of 2.28 ± 0.02 rotations per second. A large fragment broke off at 44 km altitude. Further down, mass was lost to dust during flares at altitudes of 34, 29, and 25 km. Surviving meteorites were detected by Doppler weather radar and several small 0.9–29 g meteorites were recovered under the radar reflection footprint. Based on cosmogenic radionuclides and ground‐based radiometric observations, the Dishchii'bikoh meteoroid was 80 ± 20 cm in diameter assuming the density was 3.5 g/cm³. The meteoroid's collisional history confirms that the unusual petrologic class of LL7 does not require a different parent body than three previously observed LL chondrite falls. Dishchii'bikoh was ejected 11 Ma ago from parent body material that has a 4471 ± 6 Ma U‐Pb age, the same as that of Chelyabinsk (4452 ± 21 Ma). The distribution of the four known pre‐impact LL chondrite orbits is best matched by dynamical modeling if the source of LL chondrites is in the inner asteroid belt in a low inclined orbit, with the highly inclined Dishchii'bikoh being the result of interactions with Earth before impacting.     

The Galim LL/EH polymict breccia: Evidence for impact-induced exchange between reduced and oxidized meteoritic material

Abstract— Galim is a polymict breccia consisting of a heavily shocked (shock stage S6) LL6 chondrite, Galim (a), and an impact-melted EH chondrite, Galim (b). Relict chondrules in Galim (b) served as nucleation sites for euhedral enstatite grains crystallizing from the impact melt. Many of the reduced phases typical of EH chondrites (e.g., Si-bearing metallic Fe-Ni; Ti-bearing troilite) are absent. Galim (b) was probably shock-melted while in contact with a more oxidized source, namely, Galim (a); during this event, Si was oxidized from the metal and Ti was oxidized from troilite. Galim (a) contains shock veins and recrystallized, unzoned olivine. The absence of evidence for reduction in Galim (a) may indicate that the amount of LL material greatly exceeded that of EH material; shock metamorphism may have taken place on the LL parent body. Shock-induced redox reactions such as those inferred for the Galim breccia appear to be restricted mainly to asteroids because the low-end tail of their relative-velocity distribution permits mixing of intact disparate materials (including accretion of projectiles of different oxidation states), whereas the peak of the distribution leads to high equilibration shock pressures (allowing impact-induced exchange between previously accreted, disequilibrated materials). Galim probably formed by a two-stage process: (1) accretion to the LL parent body of an intact EH projectile at low relative velocities, and (2) shock metamorphism of the assemblage by the subsequent impact of another projectile at significantly higher relative velocities.     

The Melnikovo LL6 chondrite: A new find from Ukraine

Abstract Melnikovo is a relatively unweathered 545.6-g LL6 chondrite that was found in 1983. Only a few poorly defined chondrules are discernable in the examined sections; two of these are enriched in chromite. The meteorite contains olivine (Fa_27,8), low-Ca pyroxene (Fs_24,4), plagioclase, rare clinopyroxene, chlorapatite, merrillite and opaque minerals, which have a modal abundance (in wt%) of troilite (3.9%), kamacite (0.4%), taenite plus tetrataenite (0.7%), chromite (0.8%), and trace amounts of ilmenite and Mn-ilmenite. The meteorite appears unbrecciated on a centimeter scale.     

Mesoscale and microscale shock effects in the LL6 S4 chondrites Saint‐Séverin and Elbert: A tale of two breccias

Saint‐Séverin and Elbert, two LL6 chondrite breccias, were systematically studied to evaluate multiple deformation effects on spatial scales ranging from thin section (mesoscale) to micron‐submicron (microscale) using optical microscopy, electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The different techniques provide consistent results but have complementary strengths, together providing a powerful approach to unravel even complex impact histories. Both meteorites have an S4 conventional shock stage, but interclast areas are more deformed, and clasts are more deformed in Elbert than in Saint‐Séverin. TEM and EBSD data provide compelling evidence that Saint‐Séverin experienced significant shock deformation while already hot, and cooled rapidly afterward, as a result of a major, possibly disruptive impact on the LL chondrite parent body ~4.4 Ga ago. In contrast, Elbert was shocked from a cold initial state but was heated significantly during shock, and cooled in a localized hot impact deposit on the LL asteroid. Both meteorites probably were shocked at least twice; data for Saint‐Séverin are best reconciled with a three‐impact model.     

High‐pressure phases in shock‐induced melt of the unique highly shocked LL6 chondrite Northwest Africa 757

Northwest Africa 757 is unique in the LL chondrite group because of its abundant shock‐induced melt and high‐pressure minerals. Olivine fragments entrained in the melt transform partially and completely into ringwoodite. Plagioclase and Ca‐phosphate transform to maskelynite, lingunite, and tuite. Two distinct shock‐melt crystallization assemblages were studied by FIB‐TEM analysis. The first melt assemblage, which includes majoritic garnet, ringwoodite plus magnetite‐magnesiowüstite, crystallized at pressures of 20–25 GPa. The other melt assemblage, which consists of clinopyroxene and wadsleyite, solidified at ~15 GPa, suggesting a second veining event under lower pressure conditions. These shock features are similar to those in S6 L chondrites and indicate that NWA 757 experienced an intense impact event, comparable to the impact event that disrupted the L chondrite parent body at 470 Ma.     

Shock stage distribution of 2280 ordinary chondrites—Can bulk chondrites with a shock stage of S6 exist as individual rocks?

The brecciation and shock classification of 2280 ordinary chondrites of the meteorite thin section collection at the Institut für Planetologie (Münster) has been determined. The shock degree of S3 is the most abundant shock stage for the H and LL chondrites (44% and 41%, respectively), while the L chondrites are on average more heavily shocked having more than 40% of rocks of shock stage S4. Among the H and LL chondrites, 40–50% are “unshocked” or “very weakly shocked.” Considering the petrologic types, in general, the shock degree is increasing with petrologic type. This is the case for all meteorite groups. The main criteria to define a rock as an S6 chondrite are the solid‐state recrystallization and staining of olivine and the melting of plagioclase often accompanied by the formation of high‐pressure phases like ringwoodite. These characteristics are typically restricted to local regions of a bulk chondrite in or near melt zones. In the past, the identification of high‐pressure minerals (e.g., ringwoodite) was often taken as an automatic and practical criterion for a S6 classification during chondrite bulk rock studies. The shock stage classification of many significantly shocked chondrites (>S3) revealed that most ringwoodite‐bearing rocks still contain more than 25% plagioclase (74%). Thus, these bulk chondrites do not even fulfill the S5 criterion (e.g., 75% of plagioclase has to be transformed into maskelynite) and have to be classified as S4. Studying chondrites on typically large thin sections (several cm²) and/or using samples from different areas of the meteorites, bulk chondrites of shock stage S6 should be extremely rare. In this respect, the paper will discuss the probability of the existence of bulk rocks of S6.     

Young asteroid mixing revealed in ordinary chondrites: The case of NWA 5764, a polymict LL breccia with L clasts

Polymict chondritic breccias—rocks composed of fragments originating from different chondritic parent bodies—are of particular interest because they give insights into the mixing of asteroids in the main asteroid belt (occurrence, encounter velocity, transfer time). We describe Northwest Africa (NWA) 5764, a brecciated LL6 chondrite that contains a >16 cm³ L4 clast. The L clast was incorporated in the breccia through a nondestructive, low‐velocity impact. Identical cosmic‐ray exposure ages of the L clast and the LL host (36.6 ± 5.8 Myr), suggest a short transfer time of the L meteoroid to the LL parent body of 0.1 ± 8.1 Myr, if that meteoroid was no larger than a few meters. NWA 5764 (together with St. Mesmin, Dimmitt, and Glanerbrug) shows that effective mixing is possible between ordinary chondrite parent bodies. In NWA 5764 this mixing occurred after the peak of thermal metamorphism on the LL parent body, i.e., at least several tens of Myr after the formation of the solar system. The U,Th‐He ages of the L clast and LL host, identical at about 2.9 Ga, might date the final assembly of the breccia, indicating relatively young mixing in the main asteroid belt as previously evidenced in St. Mesmin.     

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.     

Shock and thermal history of Northwest Africa 4859, an annealed impact‐melt breccia of LL chondrite parentage containing unusual igneous features and pentlandite

Abstract– Northwest Africa 4859 (NWA 4859) is a meteorite of LL chondrite parentage that shows unusual igneous features and contains widely distributed pentlandite. The most obvious unusual feature is a high proportion of large (≤3 cm diameter) igneous‐textured enclaves (LITEs), interpreted as shock melts that were intruded into an LL chondrite host. One such LITE appears to have been produced by whole rock melting of LL chondrite, initial rapid partial crystallization, and subsequent slow cooling of the residual melt in the host to produce a differentiated object. Other unusual features include mm‐sized “overgrowth objects,” fine‐grained plagioclase‐rich bands, and coarse troilite (≤7 mm across) grains. All these features are interpreted as having crystallized from melts produced by a single transient shock event, followed by slow cooling. A subsequent shock event of moderate (S3) intensity produced veining and transformed some of the pyroxene into the clinoenstatite polytype. Pentlandite (together with associated troilite) in NWA 4859 probably formed by the breakdown of a monosulfide precursor phase at low temperature (≤230 °C) following the second shock event. NWA 4859 is interpreted to be an unusual impact‐melt breccia that contains shock melt which crystallized in different forms at depth within the parent body.