On the significance of diopside and oldhamite in enstatite chondrites and aubrites

Abstract— Enstatite is the primary silicate phase of equilibrated enstatite chondrites (EECs). The CaO contents of these enstatites lie close to or on the enstatite-diopside phase boundary, yet, curiously, diopside has always been absent from EEC assemblages. In contrast, aubrites contain abundant diopside even though they are thought to be derived from an E chondrite-like protolith. A phase equilibrium analysis of the Ca-Mg-Fe-Mn-Si-O-S system under reducing conditions solves this enigma and shows that diopside-bearing EECs should commonly be found. When S fugacity is sufficiently high (e.g., Fe-FeS buffer), low O fugacity limits the stability of diopside in favor of oldhamite. Under such conditions, the relative stability of diopside and oldhamite is described by the reaction: CaMgSi₂O₆ + MgS = CaS + Mg₂Si₂O₆ A large bulk compositional field exists where diopside and oldhamite are simultaneously stable. The existence of oldhamite does not preclude the stability of diopside. Phase diagram topology demonstrates that bulk compositions lying in the enstatite-oldhamite field and enstatite-oldhamite-alabandite field have enstatite CaO contents nearly identical to that of enstatite in equilibrium with diopside alone. This explains the high enstatite CaO contents of all EECs that do not contain diopside. This study also reports the discovery of the first EEC to contain metamorphic diopside, the Antarctic meteorite EET 90102. Elephant Moraine 90102 has a typical EL6 texture and contains the assemblage: enstatite, diopside, albite, kamacite, troilite, sinoite, and graphite. Trace quantities of alabandite, oldhamite and daubreelite are also present. Diopside is stable in EET 90102 because its bulk composition lies within either the enstatite-diopside-oldhamite-alabandite or diopside-alabandite-enstatite stability fields. In contrast, all other EECs analyzed to date have bulk compositions lying in the enstatite-oldhamite-alabandite stability field. The discovery of diopside in EET 90102 helps confirm the predictions of the phase equilibrium analysis. Elephant Moraine 90102 experienced a high-temperature metamorphic equilibration from which it was quenched. The enstatite-diopside, CaS in alabandite and Fe in alabandite, geothermometers yield temperatures of last equilibration of ～900 °C. The absence of daubreelite and schreibersite along with high troilite Cr contents and high kamacite P contents confirm a high-temperature metamorphic quench. The EET 90102 chondrite experienced a somewhat different cooling history and has a slightly different bulk composition than all other EECs studied to date; however, the close mineralogic, petrologic and textural similarities between EET 90102 and nominal EL6 chondrites signify that it should be classified as a diopside- and sinoite-bearing EL6 chondrite. Assuming that the aubrites formed from an E chondrite-like protolith, a source rock similar to that of a diopside-bearing EEC offers a clear advantage for aubrite formation. Melting of a diopside-saturated EEC protolith would not require conversion of CaS to achieve diopside-saturation upon cooling.     

Potassium isotopic compositions of enstatite meteorites

Enstatite chondrites and aubrites are meteorites that show the closest similarities to the Earth in many isotope systems that undergo mass‐independent and mass‐dependent isotopic fractionations. Due to the analytical challenges to obtain high‐precision K isotopic compositions in the past, potential differences in K isotopic compositions between enstatite meteorites and the Earth remained uncertain. We report the first high‐precision K isotopic compositions of eight enstatite chondrites and four aubrites and find that there is a significant variation of K isotopic compositions among enstatite meteorites (from ?2.34‰ to ?0.18‰). However, K isotopic compositions of nearly all enstatite meteorites scatter around the bulk silicate earth (BSE) value. The average K isotopic composition of the eight enstatite chondrites (?0.47 ± 0.57‰) is indistinguishable from the BSE value (?0.48 ± 0.03‰), thus further corroborating the isotopic similarity between Earth's building blocks and enstatite meteorite precursors. We found no correlation of K isotopic compositions with the chemical groups, petrological types, shock degrees, and terrestrial weathering conditions; however, the variation of K isotopes among enstatite meteorite can be attributed to the parent‐body processing. Our sample of the main‐group aubrite MIL 13004 is exceptional and has an extremely light K isotopic composition (δ⁴¹K = ?2.34 ± 0.12‰). We attribute this unique K isotopic feature to the presence of abundant djerfisherite inclusions in our sample because this K‐bearing sulfide mineral is predicted to be enriched in ³⁹K during equilibrium exchange with silicates.     

Petrogenesis of anomalous Queen Alexandra Range enstatite meteorites and their relation to enstatite chondrites,primitive enstatite achondrites,and aubrites

Queen Alexandra Range (QUE) meteorite 94204 is an anomalous enstatite meteorite whose petrogenesis has been ascribed to either partial melting or impact melting. We studied the meteorite pairs QUE 94204, 97289/97348, 99059/99122/99157/99158/99387, and Yamato (Y)‐793225; these were previously suggested to represent a new grouplet. We present new data for mineral abundances, mineral chemistries, and siderophile trace element compositions (of Fe,Ni metal) in these meteorites. We find that the texture and composition of Y‐793225 are related to EL6, and that this meteorite is unrelated to the QUEs. The mineralogy and siderophile element compositions of the QUEs are consistent with petrogenesis from an enstatite chondrite precursor. We caution that potential re‐equilibration during melting and recrystallization of enstatite chondrite melt‐rocks make it unreliable to use mineral chemistries to assign a specific parent body affinity (i.e., EH or EL). The QUEs have similar mineral chemistries among themselves, while slight variations in texture and modal abundances exist between them. They are dominated by inclusion‐bearing millimeter‐sized enstatite (average En_99.1–99.5) with interstitial spaces filled predominantly by oligoclase feldspar (sometimes zoned), kamacite (Si approximately 2.4 wt%), troilite (≤2.4 wt% Ti), and cristobalite. Siderophile elements that partition compatibly between solid metal and liquid metal are not enriched like in partial melt residues Itqiy and Northwest Africa (NWA) 2526. We find that the modal compositions of the QUEs are broadly unfractionated with respect to enstatite chondrites. We conclude that a petrogenesis by impact melting, not partial melting, is most consistent with our observations.     

The oxygen‐isotopic record in enstatite meteorites

Abstract— Oxygen‐isotopic compositions were determined for a suite of enstatite chondrites and aubrites. In agreement with previous work (Clayton et al., 1984), most samples have O‐isotopic compositions close to the terrestrial fractionation line (TFL), and there appear to be no significant differences in O‐isotopic compositions between individual EH and EL chondrites and aubrites. Five enstatite meteorites have O‐isotopic compositions that are significantly different from the other samples and >0.2% away from the TFL. Two of these have petrographic evidence of brecciation and interaction between other meteorite types; for the other three, similar scenarios are suggested. There appears to be a systematic increase in δ¹⁸O from enstatite chondrites (both EH and EL) of petrologic type 3 to those of type 6. There is also good evidence that the EH meteorites do not fall along a mass fractionation line but along a line slope 0.66. At the present time, detailed understanding of the origin of these O‐isotopic systematics remain elusive but clearly point to a complex accretion history, parent‐body evolution, or both.     

Understanding the textures and origin of shock melt pockets in Martian meteorites from petrographic studies,comparisons with terrestrial mantle xenoliths,and experimental studies

Abstract— We present a textural comparison of localized shock melt pockets in Martian meteorites and glass pockets in terrestrial, mantle‐derived peridotites. Specific textures such as the development of sieve texture on spinel and pyroxene, and melt migration and reaction with the host rock are identical between these two apparently disparate sample sets. Based on petrographic and compositional observations it is concluded that void collapse/variable shock impedance is able to account for the occurrence of pre‐terrestrial sulfate‐bearing secondary minerals in the melts, high gas emplacement efficiencies, and S, Al, Ca, and Na enrichments and Fe and Mg depletion of shock melt compositions compared to the host rock; previously used as arguments against such a formation mechanism. Recent experimental studies of xenoliths are also reviewed to show how these data further our understanding of texture development and can be used to shed light on the petrogenesis of shock melts in Martian meteorites.     

Ar‐Ar ages and thermal histories of enstatite meteorites

Abstract– Compared with ordinary chondrites, there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies. In this study, we report ³⁹Ar‐⁴⁰Ar dating results for five EL chondrites: Khairpur, Pillistfer, Hvittis, Blithfield, and Forrest; five EH chondrites: Parsa, Saint Marks, Indarch, Bethune, and Reckling Peak 80259; three igneous‐textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies: Zaklodzie, Queen Alexandra Range 97348, and Queen Alexandra Range 97289; and three aubrites, Norton County, Bishopville, and Cumberland Falls Several Ar‐Ar age spectra show unusual ³⁹Ar recoil effects, possibly the result of some of the K residing in unusual sulfide minerals, such as djerfisherite and rodderite, and other age spectra show ⁴⁰Ar diffusion loss. Few additional Ar‐Ar ages for enstatite meteorites are available in the literature. When all available Ar‐Ar data on enstatite meteorites are considered, preferred ages of nine chondrites and one aubrite show a range of 4.50–4.54 Ga, whereas five other meteorites show only lower age limits over 4.35–4.46 Ga. Ar‐Ar ages of several enstatite chondrites are as old or older as the oldest Ar‐Ar ages of ordinary chondrites, which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies, or were metamorphosed to lower temperatures compared to other chondrite types. Many enstatite meteorites are brecciated and/or shocked, and some of the younger Ar‐Ar ages may record these impact events. Although impact heating of ordinary chondrites within the last 1 Ga is relatively common for ordinary chondrites, only Bethune gives any significant evidence for such a young event.     

Alkali elements in the Earth's core: Evidence from enstatite meteorites

Abstract— The abundances of alkali elements in the Earth's core are predicted by assuming that accretion of the Earth started from material similar in composition to enstatite chondrites and that enstatite achondrites (aubrites) provide a natural laboratory to study core-mantle differentiation under extremely reducing conditions. If core formation on the aubrite parent body is comparable with core formation on the early Earth, it is found that 2600 (±1000) ppm Na, 550 (±260) ppm K, 3.4 (±2.1) ppm Rb, and 0.31 (±0.24) ppm Cs can reside in the Earth's core. The alkali-element abundances are consistent with those predicted by independent estimates based on nebula condensation calculations and heat flow data.     

Search for isotopic anomalies in oldhamite (CaS) from unequilibrated (E3) enstatite chondrites

Abstract— The Ca isotopic compositions of 32 oldhamite (CaS) grains from the Qingzhen (EH3), MAC88136 (EL3), and Indarch (EH4) enstatite chondrites were determined by ion microprobe mass spectrometry. Also measured were the S isotopic compositions of eight oldhamite, two niningerite (MgS), and seven troilite (FeS) grains. The S isotopic compositions of all minerals are normal, but oldhamite grains of the first two meteorites exhibit apparent small ⁴⁸Ca excesses and deficits that are correlated with isotopic mass fractionation as determined from the ⁴⁰Ca-⁴⁴Ca pair. The interpretation of these results is complicated by the fact that none of the established mass fractionation laws can account for the data in the Norton County oldhamite standard. The method of analysis is carefully scrutinized for experimental artifacts. Neither interferences nor any known mass fractionation effect can satisfactorily explain the observed small deviations from normal isotopic composition. If these are truly isotopic anomalies, they are much smaller than those observed in hibonite. The nucleosynthetic origin of Ca isotopes is discussed.     

Testing an integrated chronology: I‐Xe analysis of enstatite meteorites and a eucrite

Abstract— We have determined initial ¹²⁹I/¹²⁷I ratios for mineral concentrates of four enstatite meteorites and a eucrite. In the case of the enstatite meteorites the inferred ages are associated with the pyroxene‐rich separates giving pyroxene closure ages relative to the Shallowater standard of Indarch (EH4, 0.04 ± 0.67 Ma), Khairpur (EL6, ?4.22 ± 0.67 Ma), Khor Temiki (aubrite, ?0.06 Ma), and Itqiy (enstatite achondrite, ?2.6 ± 2.6 Ma), negative ages indicate closure after Shallowater. No separate from the cumulate eucrite Asuka (A?) 881394 yielded a consistent ratio, though excess ¹²⁹Xe was observed in a feldspar separate, suggesting disturbance by thermal metamorphism within 25 Ma of closure in Shallowater. Iodine‐129 ages are mapped to the absolute Pb‐Pb time scale using the calibration proposed by Gilmour et al. (2006) who place the closure age of Shallowater at 4563.3 ± 0.4 Ma. Comparison of the combined ¹²⁹I‐Pb data with associated ⁵³Mn ages, for objects that have been dated by both systems, indicates that all three chronometers evolved concordantly in the early solar system. The enstatite chondrites are offset from the linear array described by asteroid‐belt objects when ⁵³Mn ages are plotted against combined ¹²⁹I‐Pb data, supporting the suggestion that ⁵³Mn was radially heterogeneous in the early solar system.     

Experimental rare-earth-element partitioning in oldhamite: Implications for the igneous origin of aubritic oldhamite

Abstract— Aubritic oldhamite (CaS) has been the subject of intense study recently because it is the major rare-earth-element (REE) carrier in aubrites, has a variety of REE patterns comparable to those in unequilibrated enstatite chondrites and has an extraordinarily high melting point as a pure substance (2525 °C). These latter two facts have caused some authors to assert that much of the aubritic oldhamite is an unmelted nebular relict, rather than of igneous origin. We have conducted REE partitioning experiments between oldhamite and silicate melt using an aubritic bulk composition at 1200 °C and 1300 °C and subsolidus annealing experiments. All experiments produced crystalline oldhamite, with a range of compositions, glass and Fe metal, as well as enstatite, SiO₂, diopside and troilite in some charges. Rare-earth-element partitioning is strongly dependent on oldhamite composition and temperature. Subsolidus annealing results in larger partition coefficients for some oldhamite grains, particularly those in contact with troilite. All experimental oldhamite/silicate melt partition coefficients are <20 and the vast majority are <5, which is similar to those reported in the literature and is two orders of magnitude less than those inferred for natural aubritic oldhamite. These partition coefficients preclude a simple igneous model, since REE abundances in aubritic oldhamite are greater than would be predicted on the basis of the experimental partition coefficients. Our experimental partition coefficients are consistent with a relict nebular origin for aubritic oldhamite, although experimental evidence that suggests melting of oldhamite at temperatures lower than that reached on the aubrite parent body are clearly inconsistent with the nebular model. Our experiments are consistent also with a complex igneous history. Oldhamite REE patterns may reflect a complex process of partial melting, melt removal, fractional crystallization and subsolidus annealing and exsolution. These mechanisms (primarily fractional crystallization and subsolidus annealing) can produce a wide range of REE patterns in aubritic oldhamite, as well as elevated (100–1000 × CI) REE abundances observed in aubritic oldhamite.     

Lewis Cliff 87223, an anomalous enstatite chondrite and the diversity of enstatite chondrites

We studied a thin section of Lewis Cliff (LEW) 87223, an unusual EL3-related, enstatite chondrite (EC) that has primary and secondary features not observed in other ECs. We studied its metal-rich nodules, possible shock features, and chondrules, eight of which are Al-rich chondrules (ARCs). LEW 87223 has petrologic and compositional features similar to EL3s. Enstatite is the dominant mineral; chondrule boundaries are well defined; Si content of metal (0.5–0.6 wt%) is consistent with typical EL3; it has Cr-bearing troilite, oldhamite, and alabandite; and its O-isotopic composition is similar to other ECs. However, metal abundance in LEW 87223 (~13 vol%) is slightly higher than in other EL3s and its metal nodules are texturally and mineralogically different from other ECs. Both high and low Ni metals are present, and its alabandite has higher Fe (27.8 wt% Fe) than in other EL3s. Silicates appear darkened in plane polarized light, largely due to reduction of Fe from silicate. A remarkable feature of LEW 87223 is the high abundance of ARCs, which contain Ca-rich plagioclase and varying amounts of Na-rich plagioclase along chondrule edges and as veins. This suggests Na metasomatism and the possibility of hydrothermal fluids, potentially related to an impact event. LEW 87223 expands the range of known EC material. It shows that ECs are more diverse and record a wider range of parent body processes than previously known. LEW 87223 is an anomalous EL3, potentially the first member of a new EC group should similar samples be discovered.     

Comets, meteorites and atmospheres

The relatively low value of Xe/Kr in the atmospheres of Earth and Mars seems to rule out meteorites as the major carriers of noble gases to the inner planets. Laboratory experiments on the trapping of gases in ice forming at low temperatures suggest that comets may be a better choice. It is then possible to develop a model for the origin of inner planet atmospheres based on volatiles delivered by comets added to volatiles originally trapped in planetary rocks. The model will be tested by results from the Galileo Entry Probe.     

Comets, meteorites and atmospheres

The relatively low value of Xe/Kr in the atmospheres of Earth and Mars seems to rule out meteorites as the major carriers of noble gases to the inner planets. Laboratory experiments on the trapping of gases in ice forming at low temperatures suggest that comets may be a better choice. It is then possible to develop a model for the origin of inner planet atmospheres based on volatiles delivered by comets added to volatiles originally trapped in planetary rocks. The model will be tested by results from the Galileo Entry Probe.     

The evolution of enstatite and chondrules in unequilibrated enstatite chondrites: Evidence from iron-rich pyroxene

Abstract— FeO-rich (Fs^6–34) pyroxene lacking cathodoluminescence (CL), hereafter black pyroxene, is a major constituent of some of the chondrules and fragments in unequilibrated (type 3) enstatite chondrites (UECs). It contains structurally oriented zones of Cr-, Mn-, V-rich, FeO-poor enstatite with red CL, associated with mm-sized blebs of low-Ni, Fe-metal and, in some cases, silica. These occurrences represent clear evidence of pyroxene reduction. The black pyroxene is nearly always rimmed by minor element (Cr, Mn, V)-poor enstatite having a blue CL. More commonly, red and blue enstatites, unassociated with black pyroxene, occur as larger grains in chondrules and fragments, and these constitute the major silicate phases in UECs. The REE abundance patterns of the black pyroxene are LREE-depleted. The blue enstatite rims, however, have a near-flat to LREE-enriched pattern, ～0.5–4x chondritic. The petrologic and trace element data indicate that the black pyroxene is from an earlier generation of chondrules that formed in a nebular region that was more oxidizing than that of the enstatite chondrites. Following solidification, these chondrules experienced a more reducing nebular environment and underwent reduction. Some, perhaps most, of the red enstatite that is common throughout the UECs may be the product of solid-state reduction of black pyroxene. The blue enstatite rims grew onto the surfaces of the black pyroxene and red enstatite as a result of condensation from a nebular gas. The evolutionary history of some of the enstatite and chondrules in enstatite chondrites can be expressed in a four-stage model that includes: Stage 1. Formation of chondrules in an oxidizing nebular environment Stage 2. Solid-state reduction of the more oxidized chondrules and fragments to red enstatite in a more reducing nebular environment Stage 3. Formation of blue enstatite rims on the black pyroxene as well as on the red enstatite. Stage 4. Reprocessing, by various degrees of melting, of many of the earlier-formed materials.     

Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil‐1 borehole,Chicxulub impact structure,Mexico: Implications for target composition

Abstract— We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil‐1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end‐member requires a mafic component in order to explain the low SiO₂ content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned—a high‐Ba, low‐Ta group and a high‐Fe, high‐Zn, and high‐Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil‐1 drill core likely register a multistage deformation history that spans the period from pre‐ to post‐impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347–1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event.     

Density,porosity, and magnetic susceptibility of achondritic meteorites

As part of a large‐scale survey of meteorite bulk and grain densities, porosities, and magnetic susceptibilities, we measured these properties for 174 stones from 106 achondritic meteorites. These include four lunar meteorites, 15 stones from 10 shergottites, nakhlites, and chassignites (SNCs), 96 stones from 56 howardites, eucrites, and diogenites (HEDs), 17 stones from nine aubrites, two angrites, and 16 stones from 10 ureilites, four stones of three acapulcoites, as well as four stones of three lodranites, and 15 stones from eight primitive achondrites. Those meteorites derived from basalts and crustal material of differentiated parent bodies have lower densities and magnetic susceptibilities, on an average, than the more primitive achondrites, which have a higher percentage metal. A notable exception is the one chassignite in the study (Chassigny), which has a high grain density of 3.73 ± 0.04 g cm^?3. Ureilites have magnetic susceptibilities consistent with primitive achondrites, but lower grain densities. Porosities do not vary considerably between most of the groups, with most stones 5–14% porous, although on an average, ureilites and brachinites have lower porosities, with most stones less than 7% porous. For primitive achondrites, the higher metal content causes finds to exhibit weathering effects similar to what is observed in ordinary chondrites, with a reduction in grain density, magnetic susceptibility, and porosity as compared with unweathered falls. For lunites, SNCs, and HEDs, no such effect is observed. We also observe that grain density and magnetic susceptibility used in conjunction distinguish shergottites, nakhlites, and chassignites from each other. Shergottites and nakhlites have low grain densities (averaging 3.31 and 3.41 g cm^?3, respectively) whereas Chassigny is 3.7 g cm^?3. In magnetic susceptibility, shergottities and chassignites are similar (averaging 2.85 and 2.98 in log units of 10^?9 m³ kg^?1, respectively) with nakhlites averaging higher at 3.42.     

Geochemical zoning and magnetic mineralogy at Fe,Ni‐alloy–troilite interfaces of three iron meteorites from Morasko,Coahuila II,and Mundrabilla

We combined high‐resolution and space‐resolved elemental distribution with investigations of magnetic minerals across Fe,Ni‐alloy and troilite interfaces for two nonmagmatic (Morasko and Mundrabilla) IAB group iron meteorites and an octahedrite found in 1993 in Coahuila/Mexico (Coahuila II) preliminarily classified on Ir and Au content as IIAB group. The aim of this study was to elucidate the crystallization and thermal history using gradients of the siderophile elements Ni, Co, Ge, and Ga and the chalcophile elements Cr, Cu, and Se with a focus on magnetic minerals. The Morasko and Coahuila II meteorite show a several mm‐thick carbon‐ and phosphorous‐rich transition zone between Fe,Ni‐alloy and troilite, which is characterized by magnetic cohenite and nonmagnetic or magnetic schreibersite. At Morasko, these phases have a characteristic trace element composition with Mo enriched in cohenite. In both Morasko and Coahuila II, Ni is enriched in schreibersite. The minerals have crystallized from immiscible melts, either by fractional crystallization and C‐ and P‐enrichment in the melt, or by partial melting at temperatures slightly above the eutectic point. During crystallization of Mundrabilla, the field of immiscibility was not reached. Independent of meteorite group and cooling history, the magnetic mineralogy (daubreelite, cohenite and/or schreibersite, magnetite) is very similar to the troilite (and transition zone) for all three investigated iron meteorites. If these minerals can be separated from the metal, they might provide important information about the early solar system magnetic field. Magnetite is interpreted as a partial melting or a terrestrial weathering product of the Fe,Ni‐alloy under oxidizing conditions.     

Impact ejection,spallation, and the origin of meteorites

Recent discoveries suggest that some meteorites have originated from major planets or satellites. Although it has been suggested that a large primary impact event might eject rock fragments as secondaries, it was previously supposed that material ejected at several kilometers per second would be highly shocked or perhaps melted. It is shown that a small amount of material (0.01 to 0.05 projectile mass) may be ejected at high velocity shock pressures. The approach utilizes observations of stress-wave propagation from large underground explosions to predict stresses and particle velocities in the near-surface environment. The largest fragments ejected at any velocity are spalls that originate from the target planet's surface. The spall size is proportional to the radius of the primary impactor and the target tensile strength and inversely proportional to ejection velocity. The shock level in the spalls is low, typically half of the dynamic crushing strength of the rock. The model also predicts the aspect ratio of the spalled fragments, the angle of ejection, and the sizes and shock level of other fragments originating deeper in the target. Comparison with data from laboratory experiments, the Ries Crater, and secondary crater sizes shows generally good agreement, although the observed fragment size at ejection velocities greater than 1 km/sec is considerably smaller than the simple version of the theory predicts. The theory indicates that although significant masses of solid material could be ejected from the Moon or Mars by large meteorite impacts, the fragments ejected from ca. 30-km-diameter craters are at most a few tens of meters in diameter if the most optimistic assumptions are made. The maximum fragment diameter is more likely to be about a meter. This theory, however, applies rigorously only up to ejection velocities of ca 1 km/sec. Further numerical extensions are necessary before film conclusions can be drawn, especially for Martian ejecta.     

The natural thermoluminescence of meteorites VI: Carbon-14, thermoluminescence and the terrestrial ages of meteorites

Abstract Research on meteorite finds, especially those from the Antarctic and from desert regions in Australia, Africa, and America, has become increasingly important, notably in studies of possible changes in the nature of the meteorite flux in the past. One important piece of information needed in the study of such meteorites is their terrestrial age which can be determined using a variety of methods, including ¹⁴C, ³⁶Cl, and ⁸¹Kr. Natural thermoluminescence (TL) levels in meteorites can also be used as an indicator of terrestrial age. In this paper, we compare ¹⁴C-determined terrestrial ages with natural TL levels in finds from the Prairie States (central United States), a group of finds from Roosevelt County (New Mexico, USA), and a group from the Sahara Desert. We find that, in general, the natural TL data are compatible with the ¹⁴C-derived terrestrial ages using a 20 °C TL decay curve for the Prairie States and Roosevelt County and a 30 °C decay curve for the Saharan meteorites. We also present TL data for a group of meteorites from the Sahara desert which has not been studied using cosmogenic radionuclides. Within these data there are distinct terrestrial age clusters which probably reflect changes in meteorite preservation efficiency over ～ 15, 000 years in the region.     

Occurrence and implications of silicon nitride in enstatite chondrites

Abstract— Silicon nitride, Si₃N₄, has previously been observed to be a common constituent of acid residues of Qingzhen (EH3) and Indarch (EH4). Ion probe analysis of the Si, N and C isotopic compositions of individual Si₃N₄ grains from Qingzhen and Indarch acid residues suggest most, if not all, grains are Solar System in origin. A few grains have isotopically anomalous C but this is probably due to small presolar SiC grains adhering to them. In situ observations of the Si₃N₄ in Qingzhen show that it is only present within, and probably exsolved from, host phases which contain elemental Si in solid solution. Thermodynamic calculations suggest that the Si₃N₄ probably formed during metamorphism and not in the nebula. Thermodynamic calculations also show that sinoite (Si₂N₂O) and not Si₃N₄ should be the stable phase during metamorphism. It appears that kinetic factors must have inhibited the formation of sinoite in Qingzhen and Indarch.