Mineralogy,petrology, and shock history of lunar meteorite Sayh al Uhaymir 300: A crystalline impact‐melt breccia

Abstract— Sayh al Uhaymir (SaU) 300 comprises a microcrystalline igneous matrix (grain size <10 μm), dominated by plagioclase, pyroxene, and olivine. Pyroxene geothermometry indicates that the matrix crystallized at ?1100 °C. The matrix encloses mineral and lithic clasts that record the effects of variable levels of shock. Mineral clasts include plagioclase, low‐ and high‐Ca pyroxene, pigeonite, and olivine. Minor amounts of ilmenite, FeNi metal, chromite, and a silica phase are also present. A variety of lithic clast types are observed, including glassy impact melts, impact‐melt breccias, and metamorphosed impact melts. One clast of granulitic breccia was also noted. A lunar origin for SaU 300 is supported by the composition of the plagioclase (average An₉₅), the high Cr content in olivine, the lack of hydrous phases, and the Fe/Mn ratio of mafic minerals. Both matrix and clasts have been locally overprinted by shock veins and melt pockets. SaU 300 has previously been described as an anorthositic regolith breccia with basaltic components and a granulitic matrix, but we here interpret it to be a polymict crystalline impact‐melt breccia with an olivine‐rich anorthositic norite bulk composition. The varying shock states of the mineral and lithic clasts suggest that they were shocked to between 5–28 GPa (shock stages S1–S2) by impact events in target rocks prior to their inclusion in the matrix. Formation of the igneous matrix requires a minimum shock pressure of 60 GPa (shock stage >S4). The association of maskelynite with melt pockets and shock veins indicates a subsequent, local 28–45 GPa (shock stage S2–S3) excursion, which was probably responsible for lofting the sample from the lunar surface. Subsequent fracturing is attributed to atmospheric entry and probable breakup of the parent meteor.     

Shock,post‐shock annealing,and post‐annealing shock in ureilites

Abstract— The thermal and shock histories of ureilites can be divided into four periods: 1) formation, 2) initial shock, 3) post‐shock annealing, and 4) post‐annealing shock. Period 1 occurred ?4.55 Ga ago when ureilites formed by melting chondritic material. Impact events during period 2 caused silicate darkening, undulose to mosaic extinction in olivines, and the formation of diamond, lonsdaleite, and chaoite from indigenous carbonaceous material. Alkali‐rich fine‐grained silicates may have been introduced by impact injection into ureilites during this period. About 57% of the ureilites were unchanged after period 2. During period 3 events, impact‐induced annealing caused previously mosaicized olivine grains to become aggregates of small unstrained crystals. Some ureilites experienced reduction as FeO at the edges of olivine grains reacted with C from the matrix. Annealing may also be responsible for coarsening of graphite in a few ureilites, forming euhedral‐appearing, idioblastic crystals. Orthopyroxene in Meteorite Hills (MET) 78008 may have formed from pigeonite by annealing during this period. The Rb‐Sr internal isochron age of ?4.0 Ga for MET 78008 probably dates the annealing event. At this late date, impacts are the only viable heat source. About 36% of ureilites experienced period 3 events, but remained unchanged afterwards. During period 4, ?7% of the ureilites were shocked again, as is evident in the polymict breccia, Elephant Moraine (EET) 83309. This rock contains annealed mosaicized olivine aggregates composed of small individual olivine crystals that exhibit undulose extinction. Ureilites may have formed by impact‐melting chondritic material on a primitive body with heterogeneous O isotopes. Plagioclase was preferentially lost from the system due to its low impedance to shock compression. Brief melting and rapid burial minimized the escape of planetary‐type noble gases from the ureilitic melts. Incomplete separation of metal from silicates during impact melting left ureilites with relatively high concentrations of trace siderophile elements.     

Northwest Africa 428: Impact‐induced annealing of an L6 chondrite breccia

Abstract— Northwest Africa (NWA) 428 is an L chondrite that was successively thermally metamorphosed to petrologic type‐6, shocked to stage S4–S5, brecciated, and annealed to approximately petrologic type‐4. Its thermal and shock history resembles that of the previously studied LL6 chondrite, Miller Range (MIL) 99301, which formed on a different asteroid. The petrologic type‐6 classification of NWA 428 is based on its highly recrystallized texture, coarse metal (150 ± 150 μm), troilite (100 ± 170 μm), and plagioclase (20–60 μm) grains, and relatively homogeneous olivine (Fa_{24.4 ± 0.6}), low‐Ca pyroxene (Fs_{20.5 ± 0.4}), and plagioclase (Ab_{84.2 ± 0.4}) compositions. The petrographic criteria that indicate shock stage S4–S5 include the presence of chromite veinlets, chromite‐plagioclase assemblages, numerous occurrences of metallic Cu, irregular troilite grains within metallic Fe‐Ni, polycrystalline troilite, duplex plessite, metal and troilite veins, large troilite nodules, and low‐Ca clinopyroxene with polysynthetic twins. If the rock had been shocked before thermal metamorphism, low‐Ca clinopyroxene produced by the shock event would have transformed into orthopyroxene. Post‐shock brecciation is indicated by the presence of recrystallized clasts and highly shocked clasts that form sharp boundaries with the host. Post‐shock annealing is indicated by the sharp optical extinction of the olivine grains; during annealing, the damaged olivine crystal lattices healed. If temperatures exceeded those approximating petrologic type‐4 (?600–700°C) during annealing, the low‐Ca clinopyroxene would have transformed into orthopyroxene. The other shock indicators, likewise, survived the mild annealing. An impact event is the most plausible source of post‐metamorphic, post‐shock annealing because any ²⁶Al that may have been present when the asteroid accreted would have decayed away by the time NWA 428 was annealed. The similar inferred histories of NWA 428 (L6) and MIL 99301 (LL6) indicate that impact heating affected more than 1 ordinary chondrite parent body.     

Shock and annealing in the amphibole‐ and mica‐bearing R chondrites

MIL 11207 (R6) and LAP 04840 (R6) contain hornblende and phlogopite; MIL 07440 (R6) contains accessory titan‐phlogopite and no hornblende. All three meteorites have been shocked: MIL 11207 contains extensive sulfide veins, pyroxene that formed from dehydrated hornblende, and an extensive network of plagioclase glass; MIL 07440 contains chromite‐plagioclase assemblages, chromite veinlets and blebs, pincer‐shaped plagioclase patches, but no sulfide veins; LAP 04840 contains olivine grains with chromite‐bleb‐laden cores and opaque‐free rims, rare grains of pyroxene that formed from dehydrated hornblende, and no sulfide veins. These meteorites appear to have been heated to maximum temperatures of approximately 700–900 °C under conditions of moderately high PH₂O (perhaps 250–500 bars). All three samples underwent postshock annealing. During this process, olivine crystal lattices healed (giving the rocks the appearance of shock‐stage S1), and diffusion of Fe and S from thin sulfide veins to coarse sulfide grains caused the veins to disappear in MIL 07440 and LAP 04840. This latter process apparently also occurred in most S1–S2 ordinary chondrites of high petrologic type. The pressure–temperature conditions responsible for forming the amphibole and mica in these rocks may have been present at depths of a few tens of kilometers (as suggested in the literature). A giant impact or a series of smaller impacts would then have been required to excavate the hornblende‐ and biotite‐bearing rocks and bring them closer to the surface. It was in that latter location where the samples were shocked, deposited in a hot ejecta blanket of low thermal diffusivity, and annealed.     

Impact melting of lunar meteorite Dhofar 458: Evidence from polycrystalline texture and decomposition of zircon

Abstract– Dhofar 458 is a lunar meteorite consisting mainly of olivine‐plagioclase intergrowths, pyroxene‐plagioclase intergrowths, and plagioclase fragments. Pyroxene‐plagioclase globules are also common. In this study, we report the discovery of a polycrystalline zircon in this lunar meteorite. The polycrystalline zircon contains small vesicles and rounded baddeleyite grains at its margin. The polycrystalline and porous texture of the zircon indicates high‐pressure shock‐induced melting and degassing. Baddeleyite grains are derived from decomposition of zircon under high postshock temperature. The shock features in zircon indicates that the shock pressure in Dhofar 458 was greater than approximately 60 GPa and the postshock temperature greater than approximately 1700 °C. The polycrystalline and degassing texture and decomposition zircon also strongly indicates that Dhofar 458 is a clast‐rich impact melt rock. During this shock event, most components were melted and grains of mafic minerals are interstitial to lath‐like plagioclase grains. Large fragments of olivine and chromite also formed polycrystalline texture at margins and chemically reequilibrated with surrounding melts. We suggest that pyroxene‐plagioclase globules could be remains of melted target clasts, whereas vesicles may form during shock‐induced degassing of the rock. The U‐Pb isotopic data plot on a well‐defined discordant line, yielding the age of the zircon of 3434 ± 15 Ma (2σ). This age is interpreted as the time of the impact event that melted Dhofar 458 and caused decomposition and recrystallization of this zircon in Dhofar 458, which reset this zircon’s U‐Pb age.     

Redistribution of elements in the heavily shocked Yanzhuang chondrite

Abstract— Compositions of metal, sulfide, olivine, pyroxene, and plagioclase/plagioclase glass were studied for the melted and unmelted parts of the heavily shocked H6(S6) chondrite‐Yanzhuang. We found that the partitioning of some trace elements significantly changed between the 2 parts; compared with the corresponding minerals in the unmelted part, Ga is enriched in the metal, Co, Cr, and Zn are enriched in the sulfide, Cr is enriched in olivine and pyroxene, and Ti is enriched in the plagioclase glass of the melt pocket. These detailed studies of the mineral phases put constraints on 3 important parameters (temperature, pressure, and duration) associated with the post‐shock melting process. The coexistence of melted and unmelted olivine in the melt pocket of Yanzhuang implies a peak temperature after shock that approaches the melting point of olivine. The lack of Ni in the olivine crystallized from a melt suggests crystallization of olivine at pressures below 10 kbar. The resetting of Ga partitioning between metal and silicate in the melt pocket indicates that the interval from the peak temperature after shock to the crystallization of metal‐sulfide and plagioclase glass in the melted part of Yanzhuang is longer than 500 sec.     

Shock and annealing in aubrites: Implications for parent‐body history

Several aubrites (e.g., LAP 03719, Bishopville, Khor Temiki, ALH 83015) contain orthopyroxene grains that exhibit more‐pronounced shock effects than associated olivine grains. The orthopyroxene grains in these samples have clinoenstatite lamellae on (100) and exhibit weak mosaic extinction, characteristic of shock stage S4; the olivine grains exhibit either sharp optical extinction, characteristic of shock stage S1 (as in LAP 03719), or undulose extinction (shock stage S2), as in Bishopville and ALH 83015. The Khor Temiki regolith breccia contains S1 and S2 olivine grains. Because literature data show that diffusion is much slower in orthopyroxene than in olivine, it seems likely that aubrites experienced postshock, impact‐induced annealing. After differentiation, the aubrite parent asteroid suffered major collisions that caused extensive brecciation of near‐surface materials and damaged orthopyroxene and olivine crystal lattices. As a result of these impact events, some aubrites were shocked and buried within warm ejecta blankets or beneath fallback debris under the crater floor. Entombed olivine crystal lattices healed (and became unstrained, reaching shock stage S1), but orthopyroxene lattices retained their S4‐level shock‐damaged features. Aubrites with S4 orthopyroxene and S2 olivine were probably very weakly shocked again after olivine was annealed to S1.     

Comment on: “New” lunar meteorites: Impact melt and regolith breccias and large‐scale heterogeneities of the upper lunar crust,by P. H. Warren,F. Ulff‐Møller,and G. W. Kallemeyn

Abstract We described lunar meteorite Dhofar 026 (Cohen et al. 2004) and interpreted this rock as a strongly shocked granulitic breccia (or fragmental breccia consisting almost entirely of granulitic‐breccia clasts) that was partially melted by post‐shock heating. Warren et al. (2005) objected to many aspects of our interpretation: they were uncertain whether or not the bulk rock had been shocked; they disputed our identification of the precursor as granulitic breccia; and they suggested that mafic, igneous‐textured globules within the breccia, which we proposed were melted by post‐shock heating, are clasts with relict textures. The major evidence for shock of the bulk rock is the fact that the plagioclase in the lithologic domains that make up 80–90% of the rock is devitrified maskelynite. The major evidence for a granulitic‐breccia precursor is the texture of the olivine‐plagioclase domain that constitutes 40—45% of the rock; Warren et al. apparently overlooked or ignored this lithology. Textures of the mafic, igneous‐textured globules, and especially of the vesicles they contain, demonstrate that these bodies were melted and crystallized in situ. Warren et al. suggested that the rock might have originally been a regolith breccia, but the textural homogeneity of the rock and the absence of solar wind—derived noble gases preclude a regolith‐breccia precursor. Warren et al. classified the rock as an impact‐melt breccia, but they did not identify any fraction that was impact melt.     

Silicate darkening in the Kobe CK chondrite: Evidence for shock metamorphism at high temperature

Abstract— The Kobe CK4 chondrite, like most metamorphosed CK chondrites, exhibits pronounced silicate darkening of matrix and chondrule mesostases. Our petrographic and scanning electron microscopic study reveals that the matrix of Kobe consists mostly of intermixtures of two types of fine‐grained olivine. One forms subhedral to anhedral normal crystals. The other fills interstices of the subhedral to anhedral olivine crystals, exhibiting a complex network of veinlets. The latter type of olivine contains high densities of small spherical vesicles (<0.05‐3 μm in diameter) and grains (<0.05‐5 μm) of magnetite and pentlandite as well as round to anhedral grains (1–10 μm) of plagioclase, low‐Ca pyroxene, diopside and chlorapatite. The vesicular olivine is particularly abundant in regions of matrix that exhibit a relatively high degree of darkening and commonly fills chondrule mesostases. The vesicular olivine is clearly the principal cause of the silicate darkening in Kobe. The internal texture of the vesicular olivine closely resembles those of local melts produced from the matrices of experimentally and naturally shocked carbonaceous chondrites. The occurrence and texture of the vesicular olivine suggest that it resulted from recrystallization of partially melted matrix olivine by shock. Kobe exhibits light shock effects in olivine that are consistent with shock stage S2 that is too low to explain the occurrence of olivine melting. We suggest that the vesicular olivine in Kobe was produced by shock metamorphism at a relatively mild shock pressure (<25 GPa) and a high temperature (>600 °C). Thus, it is probable that the shock effects in olivine, manifest as fracturing and deformation, were relatively minor, but heating was strong enough to cause partial melting of matrix olivine.     

Impact melting of the largest known enstatite meteorite: Al Haggounia 001, a fossil EL chondrite

Al Haggounia 001 and paired specimens (including Northwest Africa [NWA] 2828 and 7401) are part of a vesicular, incompletely melted, EL chondrite impact melt rock with a mass of ~3 metric tons. The meteorite exhibits numerous shock effects including (1) development of undulose to weak mosaic extinction in low‐Ca pyroxene; (2) dispersion of metal‐sulfide blebs within silicates causing “darkening”; (3) incomplete impact melting wherein some relict chondrules survived; (4) vaporization of troilite, resulting in S₂ bubbles that infused the melt; (5) formation of immiscible silicate and metal‐sulfide melts; (6) shock‐induced transportation of the metal‐sulfide melt to distances >10 cm; (7) partial resorption of relict chondrules and coarse silicate grains by the surrounding silicate melt; (8) crystallization of enstatite in the matrix and as overgrowths on relict silicate grains and relict chondrules; (9) crystallization of plagioclase from the melt; and (10) quenching of the vesicular silicate melt. The vesicular samples lost almost all of their metal during the shock event and were less susceptible to terrestrial weathering; in contrast, the samples in which the metal melt accumulated became severely weathered. Literature data indicate the meteorite fell ~23,000 yr ago; numerous secondary phases formed during weathering. Both impact melting and weathering altered the meteorite's bulk chemical composition: e.g., impact melting and loss of a metal‐sulfide melt from NWA 2828 is responsible for bulk depletions in common siderophile elements and in Mn (from alabandite); weathering of oldhamite caused depletions in many rare earth elements; the growth of secondary phases caused enrichments in alkalis, Ga, As, Se, and Au.     

High‐pressure polymorphs in Yamato‐790729 L6 chondrite and their significance for collisional conditions

Shock pressure recorded in Yamato (Y)‐790729, classified as L6 type ordinary chondrite, was evaluated based on high‐pressure polymorph assemblages and cathodoluminescence (CL) spectra of maskelynite. The host‐rock of Y‐790729 consists mainly of olivine, low‐Ca pyroxene, plagioclase, metallic Fe‐Ni, and iron‐sulfide with minor amounts of phosphate and chromite. A shock‐melt vein was observed in the hostrock. Ringwoodite, majorite, akimotoite, lingunite, tuite, and xieite occurred in and around the shock‐melt vein. The shock pressure in the shock‐melt vein is about 14–23 GPa based on the phase equilibrium diagrams of high‐pressure polymorphs. Some plagioclase portions in the host‐rock occurred as maskelynite. Sixteen different CL spectra of maskelynite portions were deconvolved using three assigned emission components (centered at 2.95, 3.26, and 3.88 eV). The intensity of emission component at 2.95 eV was selected as a calibrated barometer to estimate shock pressure, and the results indicate pressures of about 11–19 GPa. The difference in pressure between the shock‐melt vein and host‐rock might suggest heterogeneous shock conditions. Assuming an average shock pressure of 18 GPa, the impact velocity of the parent‐body of Y‐790729 is calculated to be ~1.90 km s^?1. The parent‐body would be at least ~10 km in size based on the incoherent formation mechanism of ringwoodite in Y‐790729.     

Estimating shock pressures based on high‐pressure minerals in shock‐induced melt veins of L chondrites

Abstract— Here we report the transmission electron microscopy (TEM) observations of the mineral assemblages and textures in shock‐induced melt veins from seven L chondrites of shock stages ranging from S3 to S6. The mineral assemblages combined with phase equilibrium data are used to constrain the crystallization pressures, which can be used to constrain shock pressure in some cases. Thick melt veins in the Tenham L6 chondrite contain majorite and magnesiowüstite in the center, and ringwoodite, akimotoite, vitrified silicate‐perovskite, and majorite in the edge of the vein, indicating crystallization pressure of ?25 GPa. However, very thin melt veins (5–30 μm wide) in Tenham contain glass, olivine, clinopyroxene, and ringwoodite, suggesting crystallization during transient low‐pressure excursions as the shock pressure equilibrated to a continuum level. Melt veins of Umbarger include ringwoodite, akimotoite, and clinopyroxene in the vein matrix, and Fe₂SiO₄‐spinel and stishovite in SiO₂‐FeO‐rich melt, indicating a crystallization pressure of ?18 GPa. The silicate melt veins in Roy contain majorite plus ringwoodite, indicating pressure of ?20 GPa. Melt veins of Ramsdorf and Nakhon Pathon contain olivine and clinoenstatite, indicating pressure of less than 15 GPa. Melt veins of Kunashak and La Lande include albite and olivine, indicating crystallization at less than 2.5 GPa. Based upon the assemblages observed, crystallization of shock veins can occur before, during, or after pressure release. When the assemblage consists of high‐pressure minerals and that assemblage is constant across a larger melt vein or pocket, the crystallization pressure represents the equilibrium shock pressure.     

Petrogenesis of lunar meteorite Northwest Africa 2977: Constraints from in situ microprobe results

Abstract– Northwest Africa (NWA) 2977 is an olivine‐gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts. We studied this rock with a series of in situ analytical methods. NWA 2977 consists mainly of olivine and pyroxene with minor plagioclase. It shows evidence of intense shock metamorphism, locally as high as shock‐stage S6. Olivine adjacent to a melt vein has been partially transformed into ringwoodite and Al,Ti‐rich chromite grains have partially transformed into their high‐pressure polymorph (possibly CaTi₂O₄‐structure). Olivine in NWA 2977 contains two types of lithic inclusions. One type is present as Si,Al‐rich melt inclusions that are composed of glass and, in most cases, dendritic pyroxene. The other type is mafic and composed of relatively coarse‐grained augite with accessory chromite, RE‐merrillite, and baddeleyite. Two Si,Al‐rich melt inclusions are heavy rare earth elements (REE) enriched, whereas the mafic inclusion has high REE concentrations and a KREEP‐like pattern. The mafic inclusion could be a relict fragment captured during the ascent of the parent magma of NWA 2977, whereas the Si,Al‐rich inclusions may represent the original NWA 2977 melt. The calculated whole‐rock composition has a KREEP‐like REE pattern, suggesting that NWA 2977 has an affinity to KREEP rocks. Baddeleyite has recorded a young crystallization age of 3123 ± 7 Ma (2σ), which is consistent with results from previous whole‐rock and mineral Sm‐Nd and Rb‐Sr studies. The petrography, mineralogy, trace element geochemistry, and young crystallization age of NWA 2977 support the possibility of pairing between NWA 2977 and the olivine‐gabbro portion of NWA 773.     

Eutectic metal + troilite + Fe‐Mn‐Na phosphate + Al‐free chromite assemblage in shock‐produced chondritic melt of the Yanzhuang chondrite

An assemblage with FeNi metal, troilite, Fe‐Mn‐Na phosphate, and Al‐free chromite was identified in the metal‐troilite eutectic nodules in the shock‐produced chondritic melt of the Yanzhuang H6 meteorite. Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have the composition of Na‐bearing graftonite (Fe_,Mn,Na)₃(PO₄)₂, whereas most others correspond to Mn‐bearing galileiite Na(Fe,Mn)₄(PO₄)₃ and a possible new phosphate phase of Na₂(Fe,Mn)₁₇(PO₄)₁₂ composition. The Yanzhuang meteorite was shocked to a peak pressure of 50 GPa and a peak temperature of approximately 2000 °C. All minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus. The volatile elements P and Na released from whitlockite and plagioclase along with elements Cr and Mn released from chromite are concentrated into the shock‐produced Fe‐Ni‐S‐O melt at high temperatures. During cooling, microcrystalline olivine and pyroxene first crystallized from the chondritic melt, metal‐troilite eutectic intergrowths, and silicate melt glass finally solidified at about 950–1000 °C. On the other hand, P, Mn, and Na in the Fe‐Ni‐S‐O melt combined with Fe and crystallized as Fe‐Mn‐Na phosphates within troilite, while Cr combined with Fe and crystallized as Al‐free chromite also within troilite.     

Northwest Africa 4797: A strongly shocked ultramafic poikilitic shergottite related to compositionally intermediate Martian meteorites

Abstract– Northwest Africa (NWA) 4797 is an ultramafic Martian meteorite composed of olivine (40.3 vol%), pigeonite (22.2%), augite (11.9%), plagioclase (9.1%), vesicles (1.6%), and a shock vein (10.3%). Minor phases include chromite (3.4%), merrillite (0.8%), and magmatic inclusions (0.4%). Olivine and pyroxene compositions range from Fo_66–72,En_58–74Fs_19–28Wo_6–15, and En_46–60Fs_14–22Wo_34–40, respectively. The rock is texturally similar to “lherzolitic” shergottites. The oxygen fugacity was QFM?2.9 near the liquidus, increasing to QFM?1.7 as crystallization proceeded. Shock effects in olivine and pyroxene include strong mosaicism, grain boundary melting, local recrystallization, and pervasive fracturing. Shock heating has completely melted and vesiculated igneous plagioclase, which upon cooling has quench‐crystallized plagioclase microlites in glass. A mm‐size shock melt vein transects the rock, containing phosphoran olivine (Fo_69–79), pyroxene (En_44–51Fs_14–18Wo_30–42), and chromite in a groundmass of alkali‐rich glass containing iron sulfide spheres. Trace element analysis reveals that (1) REE in plagioclase and the shock melt vein mimics the whole rock pattern; and (2) the reconstructed NWA 4797 whole rock is slightly enriched in LREE relative to other intermediate ultramafic shergottites, attributable to local mobilization of melt by shock. The shock melt vein represents bulk melting of NWA 4797 injected during pressure release. Calculated oxygen fugacity for NWA 4797 indicates that oxygen fugacity is decoupled from incompatible element concentrations. This is attributed to subsolidus re‐equilibration. We propose an alternative nomenclature for “lherzolitic” shergottites that removes genetic connotations. NWA 4797 is classified as an ultramafic poikilitic shergottite with intermediate trace element characteristics.     

Accessory silicate mineral assemblages in the Bilanga diogenite: A petrographic study

Abstract— The petrographic relationships in diogenites between orthopyroxene and minor phases such as chromite, troilite, diopside, plagioclase, and silica are often obscured by the intense brecciation that characterizes these meteorites. Although brecciated, Bilanga preserves numerous clasts displaying primary textural relations between orthopyroxene and these minor phases that are large enough to analyze by electron microprobe. In this study, we focus on the distribution, composition, and origin of the minor phases in Bilanga to provide new insights into the crystallization and metamorphic history of these rocks. The samples examined consist mainly of orthopyroxene grains plus five types of assemblages containing diopside + a Fe‐rich phase (chromite, troilite, and/or Fe‐Ni metal) ± plagioclase ± silica. We interpret type 1 assemblages as being remnants of intercumulus melt trapped in the interstices between orthopyroxene grains after crystal settling in a magma chamber. Type 2 assemblages appear to have formed by heterogeneous exsolution during thermal metamorphism. Type 3 assemblages are believed to be remnants of other assemblages that have been shocked, melted, and rapidly recrystallized by impact events. Type 4 assemblages consist of veins that also appear to have formed from trapped intercumulus melt. Regions of silica‐rich mesostasis (type 5) appear to be larger patches of more evolved intercumulus melt that have been significantly affected by late‐stage impact melting. Finally, large clasts containing plagioclase ± diopside are interpreted to be exotic fragments of a different but possibly related rock type incorporated in the Bilanga breccia.     

Localized shock melting in lherzolitic shergottite Northwest Africa 1950: Comparison with Allan Hills 77005

Abstract— The lherzolitic Martian meteorite Northwest Africa (NWA) 1950 consists of two distinct zones: 1) low‐Ca pyroxene poikilically enclosing cumulate olivine (Fo_70–75) and chromite, and 2) areas interstitial to the oikocrysts comprised of maskelynite, low‐ and high‐Ca pyroxene, cumulate olivine (Fo_68–71) and chromite. Shock metamorphic effects, most likely associated with ejection from the Martian subsurface by large‐scale impact, include mechanical deformation of host rock olivine and pyroxene, transformation of plagioclase to maskelynite, and localized melting (pockets and veins). These shock effects indicate that NWA 1950 experienced an equilibration shock pressure of 35–45 GPa. Large (millimeter‐size) melt pockets have crystallized magnesian olivine (Fo_78–87) and chromite, embedded in an Fe‐rich, Al‐poor basaltic to picro‐basaltic glass. Within the melt pockets strong thermal gradients (minimum 1 °C/μm) existed at the onset of crystallization, giving rise to a heterogeneous distribution of nucleation sites, resulting in gradational textures of olivine and chromite. Dendritic and skeletal olivine, crystallized in the melt pocket center, has a nucleation density (1.0 × 10³ crystals/mm²) that is two orders of magnitude lower than olivine euhedra near the melt margin (1.6 × 10⁵ crystals/mm²). Based on petrography and minor element abundances, melt pocket formation occurred by in situ melting of host rock constituents by shock, as opposed to melt injected into the lherzolitic target. Despite a common origin, NWA 1950 is shocked to a lesser extent compared to Allan Hills (ALH) 77005 (45–55 GPa). Assuming ejection in a single shock event by spallation, this places NWA 1950 near to ALH 77005, but at a shallower depth within the Martian subsurface. Extensive shock melt networks, the interconnectivity between melt pockets, and the ubiquitous presence of highly vesiculated plagioclase glass in ALH 77005 suggests that this meteorite may be transitional between discreet shock melting and bulk rock melting.     

Shock‐induced melting,recrystallization, and exsolution in plagioclase from the Martian lherzolitic shergottite GRV 99027

Abstract— Plagioclase in the Martian lherzolitic shergottite Grove Mountains (GRV) 99027 was shocked, melted, and recrystallized. The recrystallized plagioclase contains lamellae of pyroxene, olivine, and minor ilmenite (<1 μm wide). Both the pyroxene and the olivine inclusions enclosed in plagioclase and grains neighboring the plagioclase were partially melted into plagioclase melt pools. The formation of these lamellar inclusions in plagioclase is attributed to exsolution from recrystallizing melt. Distinct from other Martian meteorites, GRV 99027 contains no maskelynite but does contain recrystallized plagioclase. This shows that the meteorite experienced a slower cooling than maskelynite‐bearing meteorites. We suggest that the parent rock of GRV 99027 could have been embedded in hot rocks, which facilitated a more protracted cooling history.     

Mineral and chemical composition of the Jezersko meteorite—A new chondrite from Slovenia

The Jezersko meteorite is a newly confirmed stony meteorite found in 1992 in the Karavanke mountains, Slovenia. The meteorite is moderately weathered (W2), indicating short terrestrial residence time. Chondrules in partially recrystallized matrix are clearly discernible but often fragmented and have mean diameter of 0.73 mm. The meteorite consists of homogeneous olivine (Fa_19.4) and low‐Ca pyroxenes (Fs_16.7Wo_1.2), of which 34% are monoclinic, and minor plagioclase (Ab₈₃An₁₁Or₆) and Ca‐pyroxene (Fs₆Wo_45.8). Troilite, kamacite, zoned taenite, tetrataenite, chromite, and metallic copper comprise about 16.5 vol% of the meteorite. Phosphates are represented by merrillite and minor chlorapatite. Undulatory extinction in some olivine grains and other shock indicators suggests weak shock metamorphism between stages S2 and S3. The bulk chemical composition generally corresponds to the mean H chondrite composition. Low siderophile element contents indicate the oxidized character of the Jezersko parent body. The temperatures recorded by two‐pyroxene, olivine‐chromite, and olivine‐orthopyroxene geothermometers are 854 °C, 737–787 °C, and 750 °C, respectively. Mg concentration profiles across orthopyroxenes and clinopyroxenes indicate relatively fast cooling at temperatures above 700 °C. A low cooling rate of 10 °C Myr^?1 was obtained from metallographic data. Considering physical, chemical, and mineralogical properties, meteorite Jezersko was classified as an H4 S2(3) ordinary chondrite.     

Spectral properties and mineral compositions of acapulcoite–lodranite clan meteorites: Establishing S‐type asteroid–meteorite connections

Except for asteroid sample return missions, measurements of the spectral properties of both meteorites and asteroids offer the best possibility of linking meteorite groups with their parent asteroid(s). Visible plus near‐infrared spectra reveal distinguishing absorption features controlled mainly by the Fe²⁺ contents and modal abundances of olivine and pyroxene. Meteorite samples provide relationships between spectra and mineralogy. These relationships are useful for estimating the olivine and pyroxene mineralogy of stony (S‐type) asteroid surfaces. Using a suite of 10 samples of the acapulcoite–lodranite clan (ALC), we have developed new correlations between spectral parameters and mafic mineral compositions for partially melted asteroids. A well‐defined relationship exists between Band II center and ferrosilite (Fs) content of orthopyroxene. Furthermore, because Fs in orthopyroxene and fayalite (Fa) content in olivine are well correlated in these meteorites, the derived Fs content can be used to estimate Fa of the coexisting olivine. We derive new equations for determining the mafic silicate compositions of partially melted S‐type asteroid parent bodies. Stony meteorite spectra have previously been used to delineate meteorite analog spectral zones in Band I versus band area ratio (BAR) parameter space for the establishment of asteroid–meteorite connections with S‐type asteroids. However, the spectral parameters of the partially melted ALC overlap with those of ordinary (H) chondrites in this parameter space. We find that Band I versus Band II center parameter space reveals a clear distinction between the ALC and the H chondrites. This work allows the distinction of S‐type asteroids as nebular (ordinary chondrites) or geologically processed (primitive achondrites).