Reclassification of four aubrites as enstatite chondrite impact melts: Potential geochemical analogs for Mercury

We present petrologic and isotopic data on Northwest Africa (NWA) 4799, NWA 7809, NWA 7214, and NWA 11071 meteorites, which were previously classified as aubrites. These four meteorites contain between 31 and 56 vol% of equigranular, nearly endmember enstatite, Fe,Ni metal, plagioclase, terrestrial alteration products, and sulfides, such as troilite, niningerite, daubréelite, oldhamite, and caswellsilverite. The equigranular texture of the enstatite and the presence of the metal surrounding enstatite indicate that these rocks were not formed through igneous processes like the aubrites, but rather by impact processes. In addition, the presence of pre‐terrestrially weathered metal (7.1–14 vol%), undifferentiated modal abundances compared to enstatite chondrites, presence of graphite, absence of diopside and forsterite, low Ti in troilite, and high Si in Fe,Ni metals suggest that these rocks formed through impact melting on chondritic and not aubritic parent bodies. Formation of these meteorites on a parent body with similar properties to the EHa enstatite chondrite parent body is suggested by their mineralogy. These parent bodies have undergone impact events from at least 4.5 Ga (NWA 11071) until at least 4.2 Ga (NWA 4799) according to ³⁹Ar‐⁴⁰Ar ages, indicating that this region of the solar system was heavily bombarded early in its history. By comparing NWA enstatite chondrite impact melts to Mercury, we infer that they represent imperfect petrological analogs to this planet given their high metal abundances, but they could represent important geochemical analogs for the behavior and geochemical affinities of elements on Mercury. Furthermore, the enstatite chondrite impact melts represent an important petrological analog for understanding high‐temperature processes and impact processes on Mercury, due to their similar mineralogies, Fe‐metal‐rich and FeO‐poor silicate abundances, and low oxygen fugacity.     

39Ar‐40Ar chronology of R chondrites

Abstract— This study presents the first determinations of ³⁹Ar‐⁴⁰Ar ages of R chondrites for the purpose of understanding the thermal history of the R chondrite parent body. The ³⁹Ar‐⁴⁰Ar ages were determined on whole‐rock samples of four R chondrites: Carlisle Lakes, Rumuruti, Acfer 217, and Pecora Escarpment #91002 (PCA 91002). All samples are breccias except for Carlisle Lakes. The age spectra are complicated by recoil and diffusive loss to various extents. The peak ³⁹Ar‐⁴⁰Ar ages of the four chondrites are 4.35, ?4.47 ± 0.02, 4.30 ± 0.07 Ga, and 4.37 Ga, respectively. These ages are similar to Ar‐Ar ages of relatively unshocked ordinary chondrites (4.52–4.38 Ga) and are older than Ar‐Ar ages of most shocked ordinary chondrites («4.2 Ga). Because the meteorites with the oldest (Rumuruti, ?4.47 Ga) and the youngest (Acfer 217, ?4.30 Ga) ages are both breccias, these ages probably do not record slow cooling within an undisrupted asteroidal parent body. Instead, the process of breccia formation may have differentially reset the ages of the constituent material, or the differences in their age spectra may arise from mixtures of material that had different ages. Two end‐member type situations may be envisioned to explain the age range observed in the R chondrites. The first is if the impact(s) that reset the ages of Acfer 217 and Rumuruti was very early. In this case, the ?170 Ma maximum age difference between these meteorites may have been produced by much deeper burial of Acfer 217 than Rumuruti within an impact‐induced thick regolith layer, or within a rubble pile type parent body following parent body re‐assembly. The second, preferred scenario is if the impact that reset the age of Acfer 217 was much later than that which reset Rumuruti, then Acfer 217 may have cooled more rapidly within a much thinner regolith layer. In either scenario, the oldest age obtained here, from Rumuruti, provides evidence for relatively early (?4.47 Ga) impact events and breccia formation on the R chondrite parent body.     

Impact ages of meteorites: A synthesis

Abstract— Isotopic ages of meteorites that indicate chronometer resetting due to impact heating are summarized. Most of the ages were obtained by the ³⁹Ar-⁴⁰Ar technique, but several Rb-Sr, Pb-Pb, and Sm-Nd ages also suggest some degree of impact resetting. Considerations of experimental data on element diffusion in silicates suggest that various isotopic chronometers ought to differ in their ease of resetting during shock heating in the order K-Ar (easiest), Rb-Sr, Pb-Pb, and Sm-Nd, which is approximately the order observed in meteorites. Partial rather than total chronometer resetting by impacts appears to be the norm; consequently, interpretation of the event age is not always straightforward. Essentially all ³⁹Ar-⁴⁰Ar ages of eucrites and howardites indicate partial to total resetting in the relatively narrow time interval of 3.4–4.1 Ga ago (1 Ga = 10⁹ years). Several disturbed Rb-Sr ages appear consistent with this age distribution. This grouping of ages and the brecciated nature of many eucrites and all howardites argues for a large-scale impact bombardment of the HED parent body during the same time period that the Moon received its cataclysmic bombardment. Other meteorite parent bodies such as those of mesosiderites, some chondrites, and IIE irons also may have experienced this bombardment. These data suggest that the early bombardment was not lunar specific but involved much of the inner Solar System, and may have been caused by breakup of a larger planetismal. Although a few chondrites show evidence of age resetting ～3.5–3.9 Ga ago, most impact ages of chondrites tend to fall below 1.3 Ga in age. A minimum of ～4 impact events, including events at 0.3, 0.5, 1.2, and possibly 0.9 Ga appear to be required to explain the younger ages of H, L, and LL chondrites, although additional events are possible. Most L chondrites show evidence of shock, and the majority of ³⁹Ar-⁴⁰Ar ages of L chondrites fall near 0.5 Ga. The L chondrite parent body apparently experienced a major impact at this time, which may have disrupted it. The observations (1) that lunar highland rocks experienced major impact resetting of various isotopic chronometers ～3.7–4.1 Ga ago; (2) that the HED parent body experienced widespread impact resetting of the K-Ar chronometer but only modest disturbance of other isotopic systems, during a similar time period; (3) that ordinary chondrite parent bodies show much more recent and less extensive impact resetting; and (4) that impacts, which initiated cosmic-ray exposure of most stone meteorites almost never reset isotopic chronometers, may all be a consequence of relative parent body size. Greater degrees of isotopic chronometer resetting occur in larger and warmer impact ejecta deposits that cool slowly. The relatively greater size of bodies like the Moon and Vesta (assumed to be the parent asteroid of HED meteorites) both permit such favorable ejecta deposits to occur more easily compared to smaller parent bodies (generally assumed for chondrites) and also protect parent objects from collisional disruption. Thus, impacts on larger bodies would tend to more easily reset chronometers, consistent with the observed relative ease of resetting of Moon (easiest), HED, chondrites and of K-Ar (easiest), Rb-Sr, other chronometers. In contrast, the more recent impact ages of chondrites are postulated to represent collisional disruption of smaller parent objects whose fragments are more readily removed from the meteorite source reservoirs. Impacts that initiate cosmic-ray exposure are mostly small in scale and produce little heating.     

40Ar‐39Ar age of Northwest Africa 091: More evidence for a link between L chondrites and fossil meteorites

Abstract— Most ⁴⁰Ar‐³⁹Ar ages of L chondrites record an event at approximately 500 Ma, indicating a large collisional impact at that time. However, there is a spread in ages from 400 to 600 Ma in these meteorites that is greater than the analytical uncertainty. Identification of, and correction for, trapped Ar in a few L chondrites has given an age of 470 ± 6 Ma. This age coincides with Ordivician fossil meteorites that fell to Earth at 467 ± 2 Ma. As these fossil meteorites were originally L chondrites, the apparent conclusion is that a large impact sent a flood of L chondrite material to Earth, while material that remained on the L chondrite parent body was strongly heated and reset. We have reduced ⁴⁰Ar‐³⁹Ar data for Northwest Africa 091 using various techniques that appear in the literature, including identification and subtraction of trapped Ar. These techniques give a range of ages from 455 to 520 Ma, and show the importance of making accurate corrections. By using the most straightforward technique to identify and remove a trapped Ar component (which is neither terrestrial nor primordial), an ⁴⁰Ar‐³⁹Ar age of 475 ± 6 Ma is found for Northwest Africa 091, showing a temporal link to fossil meteorites. In addition, high temperature releases of Northwest Africa 091 contain evidence for a second trapped component, and subtraction of this component indicates a possible second collisional impact at approximately 800 Ma. This earlier age coincides with ⁴⁰Ar‐³⁹Ar ages of some H and L chondrites, and lunar samples.     

Preservation of ancient impact ages on the R chondrite parent body: 40Ar/39Ar age of hornblende‐bearing R chondrite LAP 04840

The hornblende‐ and biotite‐bearing R chondrite LAP 04840 is a rare kind of meteorite possibly containing outer solar system water stored during metamorphism or postshock annealing deep within an asteroid. Because little is known regarding its age and origin, we determined ⁴⁰Ar/³⁹Ar ages on hornblende‐rich separates of the meteorite, and obtained plateau ages of 4340(±40) to 4380(±30) Ma. These well‐defined plateau ages, coupled with evidence for postshock annealing, indicate this meteorite records an ancient shock event and subsequent annealing. The age of 4340–4380 Ma (or 4.34–4.38 Ga) for this and other previously dated R chondrites is much older than most impact events recorded by ordinary chondrites and points to an ancient event or events that predated the late heavy bombardment that is recorded in so many meteorites and lunar samples.     

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.     

Cosmogenic and trapped noble gases in individual chondrules: Clues to chondrule formation

Abstract— We studied the elemental and isotopic abundances of noble gases (He, Ne, Ar in most cases, and Kr, Xe also in some cases) in individual chondrules separated from six ordinary, two enstatite, and two carbonaceous chondrites. Most chondrules show detectable amounts of trapped ²⁰Ne and ³⁶Ar, and the ratio (³⁶Ar/²⁰Ne)_t (from ordinary and carbonaceous chondrites) suggests that HL and Q are the two major trapped components. A different trend between (³⁶Ar/²⁰Ne)_t and trapped ³⁶Ar is observed for chondrules in enstatite chondrites indicating a different environment and/or mechanism for their formation compared to chondrules in ordinary and carbonaceous chondrites. We found that a chondrule from Dhajala chondrite (DH‐11) shows the presence of solar‐type noble gases, as suggested by the (³⁶Ar/²⁰Ne)_t ratio, Ne‐isotopic composition, and excess of ⁴He. Cosmic‐ray exposure (CRE) ages of most chondrules are similar to their host chondrites. A few chondrules show higher CRE age compared to their host, suggesting that some chondrules and/or precursors of chondrules have received cosmic ray irradiation before accreting to their parent body. Among these chondrules, DH‐11 (with solar trapped gases) and a chondrule from Murray chondrite (MRY‐1) also have lower values of (²¹Ne/²²Ne)_c, indicative of SCR contribution. However, such evidences are sporadic and indicate that chondrule formation event may have erased such excess irradiation records by solar wind and SCR in most chondrules. These results support the nebular environment for chondrule formation.     

Light noble gases in 12 meteorites from the Omani desert,Australia, Mauritania,Canada, and Sweden

We measured the concentrations and isotopic compositions of He, Ne, and Ar in 14 fragments from 12 different meteorites: three carbonaceous chondrites, six L chondrites (three most likely paired), one H chondrite, one R chondrite, and one ungrouped chondrite. The data obtained for the CV3 chondrites Ramlat as Sahmah (RaS) 221 and RaS 251 support the hypothesis of exposure age peaks for CV chondrites at approximately 9 Ma and 27 Ma. The exposure age for Shi?r 033 (CR chondrite) of 7.3 Ma is also indicative of a possible CR chondrite exposure age peak. The three L chondrites Jiddat al Harasis (JaH) 091, JaH 230, and JaH 296, which are most likely paired, fall together with Hallingeberg into the L chondrite exposure age peak of approximately 15 Ma. The two L chondrites Shelburne and Lake Torrens fall into the peaks at approximately 40 Ma and 5 Ma, respectively. The ages for Bassikounou (H chondrite) and RaS 201 (R chondrite) are approximately 3.5 Ma and 5.8 Ma, respectively. Six of the studied meteorites show clear evidence for ³He diffusive losses, the deficits range from approximately 17% for one Lake Torrens aliquot to approximately 45% for RaS 211. The three carbonaceous chondrites RaS 221, RaS 251, and Shi?r 033 all have excess ⁴He, either of planetary or solar origin. However, very high ⁴He/²⁰Ne ratios occur at relatively low ²⁰Ne/²²Ne ratios, which is unexpected and needs further study. The measured ⁴⁰Ar ages fit well into established systematics. They are between 2.5 and 4.5 Ga for the carbonaceous chondrites, older than 3.6 Ga for the L and H chondrites, and about 2.4 Ga for the R chondrite as well as for the ungrouped chondrite. Interestingly, none of our studied L chondrites has been degassed in the 470 Ma break‐up event. Using the amount of trapped ³⁶Ar as a proxy for noble gas contamination due to terrestrial weathering we are able to demonstrate that the samples studied here are not or only very slightly affected by terrestrial weathering (at least in terms of their noble gas budget).     

Shocked meteorites: Argon-40-argon-39 evidence for multiple impacts

Abstract— To contribute to the understanding of the impact history of asteroids, we performed a high-resolution ⁴⁰Ar-³⁹Ar study of ten moderately to highly shocked chondrites, which we selected according to the shock classification given by Stöffler et al. (1991). Two recent shocked chondrite falls and two highly shocked eucrites completed our sample suite. When possible, we separated impact melt from host rock for separate analysis. In total, we studied 28 samples from 14 meteorites. In some cases, atmospheric Ar that we associate with terrestrial weathering was identified and corrected for. The ages we obtained range between ～100 Ma and ～4.1 Ga and are clearly distinct from primordial ages that correspond to solar system formation. We reproduced the previously reported cluster of L-chondrite ages, ～500 Ma. The most prominent result of our study is that, in the case of chondrites, melts generally are older than host rocks or melt-embedded unmolten rocks. To solve this apparent paradox, we propose that the melt-forming event, which was the most severe shock episode in the history of these meteorites, has not been the only occasion affecting their K-Ar systems. At least one later impact metamorphism must have occured. The response of the K-Ar clock to this second event was more severe in the host rock than in the previously (in the first event) generated melt veins and pockets because of different Ar retention rates. Hence, impact metamorphism on meteorite parent bodies indeed was a multistage process extending in time over billions of years.     

Chlorine abundances in meteorites

Abstract— We used the nuclear reaction ³⁷Cl (n,γ) ³⁸Ar, achieved during neutron irradiation for dating meteorites by the ³⁹Ar‐⁴⁰Ar technique, to calculate the elemental Cl concentration of 132 samples of 94 different meteorites (mostly finds) representing several different classes. determined k and ca concentrations are also reported. Total [Cl] varies considerably, both among meteorites of the same class and among different meteorite classes. The range in [Cl] is approximately 15–177 ppm for ordinary chondrites; approximately 24–650 ppm for enstatite chondrites; approximately 4–177 ppm for eucrites; approximately 7–128 ppm for mesosiderites; approximately 35–268 ppm for acapulcoites and lodranites; and approximately 12–507 ppm for winonaites and iron silicates. As expected, most differentiated meteorites have lower [Cl] compared to chondrites and iron silicates. Analyses of 11 interior samples (～0.1 g each) of a large L6 chondrite varied over 68–129 ppm, which is a measure of the homogeneity of Cl distribution. By evaluating Ar release during stepwise sample degassing, we separated the Cl into low‐temperature and high‐temperature components, the former of which may consist of terrestrial contamination. Most samples show low‐temperature Cl concentrations of <40 ppm, but for several samples terrestrial Cl contamination constitutes significant fractions of the total Cl. Among most differentiated meteorites, finds show considerably greater low‐temperature [Cl] compared to falls.     

40Ar‐39Ar age determinations of lunar basalt meteorites Asuka 881757, Yamato 793169, Miller Range 05035, La Paz Icefield 02205, Northwest Africa 479, and basaltic breccia Elephant Moraine 96008

Abstract— ⁴⁰Ar‐³⁹Ar data are presented for the unbrecciated lunar basaltic meteorites Asuka (A‐) 881757, Yamato (Y‐) 793169, Miller Range (MIL) 05035, LaPaz Icefield (LAP) 02205, Northwest Africa (NWA) 479 (paired with NWA 032), and basaltic fragmental breccia Elephant Moraine (EET) 96008. Stepped heating ⁴⁰Ar‐³⁹Ar analyses of several bulk fragments of related meteorites A‐881757, Y‐793169 and MIL 05035 give crystallization ages of 3.763 ± 0.046 Ga, 3.811 ± 0.098 Ga and 3.845 ± 0.014 Ga, which are comparable with previous age determinations by Sm‐Nd, U‐Pb Th‐Pb, Pb‐Pb, and Rb‐Sr methods. These three meteorites differ in the degree of secondary ⁴⁰Ar loss with Y‐793169 showing relatively high Ar loss probably during an impact event ?200 Ma ago, lower Ar loss in MIL 05035 and no loss in A‐881757. Bulk and impact melt glass‐bearing samples of LAP 02205 gave similar ages (2.985 ± 0.016 Ga and 2.874 ± 0.056 Ga) and are consistent with ages previously determined using other isotope pairs. The basaltic portion of EET 96008 gives an age of 2.650 ± 0.086 Ga which is considered to be the crystallization age of the basalt in this meteorite. The Ar release for fragmental basaltic breccia EET 96008 shows evidence of an impact event at 631 ± 20 Ma. The crystallization age of 2.721 ± 0.040 Ga determined for NWA 479 is indistinguishable from the weighted mean age obtained from three samples of NWA 032 supporting the proposal that these meteorites are paired. The similarity of ⁴⁰Ar‐³⁹Ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the K‐Ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime.     

Geochemistry and 40Ar‐39Ar geochronology of impact‐melt clasts in feldspathic lunar meteorites: Implications for lunar bombardment history

Abstract— We studied 42 impact‐melt clasts from lunar feldspathic regolith breccias MacAlpine Hills (MAC) 88105, Queen Alexandra Range (QUE) 93069, Dar al Gani (DaG) 262, and DaG 400 for texture, chemical composition, and/or chronology. Although the textures are similar to the impactmelt clasts identified in mafic Apollo and Luna samples, the meteorite clasts are chemically distinct from them, having lower Fe, Ti, K, and P, thus representing previously unsampled impacts. The ⁴⁰Ar‐³⁹Ar ages on 31 of the impact melts, the first ages on impact‐melt samples from outside the region of the Apollo and Luna sampling sites, range from ～4 to ～2.5 Ga. We interpret these samples to have been created in at least six, and possibly nine or more, different impact events. One inferred impact event may be consistent with the Apollo impact‐melt rock age cluster at 3.9 Ga, but the meteorite impact‐melt clasts with this age are different in chemistry from the Apollo samples, suggesting that the mechanism responsible for the 3.9 Ga peak in lunar impact‐melt clast ages is a lunar‐wide phenomenon. No meteorite impact melts have ages more than 1s? older than 4.0 Ga. This observation is consistent with, but does not require, a lunar cataclysm.     

Cosmic‐ray exposure age and heliocentric distance of the parent bodies of enstatite chondrites ALH 85119 and MAC 88136

Abstract— We measured concentrations and isotopic ratios of noble gases in enstatite (E) chondrites Allan Hills (ALH) 85119 and MacAlpine Hills (MAC) 88136. These two meteorites contain solar and cosmogenic noble gases. Based on the solar and cosmogenic noble gas compositions, we calculated heliocentric distances, parent body exposure ages, and space exposure ages of the two meteorites. The parent body exposure ages are longer than 6.7 Ma for ALH 85119 and longer than 8.7 Ma for MAC 88136. The space exposure ages are shorter than 2.2 Ma for ALH 85119 and shorter than 3.9 Ma for MAC 88136. The estimated heliocentric distances are more than 1.1 AU for ALH 85119 and 1.3 AU for MAC 88136. Derived heliocentric distances indicate the locations of parent bodies in the past when constituents of the meteorites were exposed to the Sun. From the mineralogy and chemistry of E chondrites, it is believed that E chondrites formed in regions within 1.4 AU from the Sun. The heliocentric distances of the two E chondrite parent bodies are not different from the formation regions of E chondrites. This may imply that heliocentric distances of E chondrites have been relatively constant from their formation stage to the stage of exposure to the solar wind.     

Comparison of cosmic‐ray exposure ages and trapped noble gases in chondrule and matrix samples of ordinary,enstatite, and carbonaceous chondrites

Abstract— We performed a comprehensive study of the He, Ne, and Ar isotopic abundances and of the chemical composition of bulk material and components of the H chondrites Dhajala, Bath, Cullison, Grove Mountains 98004, Nadiabondi, Ogi, and Zag, of the L chondrites Grassland, Northwest Africa 055, Pavlograd, and Ladder Creek, of the E chondrite Indarch, and of the C chondrites Hammadah al Hamra 288, Acfer 059, and Allende. We discuss a procedure and necessary assumptions for the partitioning of measured data into cosmogenic, radiogenic, implanted, and indigenous noble gas components. For stone meteorites, we derive a cosmogenic ratio ²⁰Ne/²²Ne of 0.80 ± 0.03 and a trapped solar ⁴He/³He ratio of 3310 ± 130 using our own and literature data. Chondrules and matrix from nine meteorites were analyzed. Data from Dhajala chondrules suggest that some of these may have experienced precompaction irradiation by cosmic rays. The other chondrules and matrix samples yield consistent cosmic‐ray exposure (CRE) ages within experimental errors. Some CRE ages of some of the investigated meteorites fall into clusters typically observed for the respective meteorite groups. Only Bath's CRE age falls on the 7 Ma double‐peak of H chondrites, while Ogi's fits the 22 Ma peak. The studied chondrules contain trapped ²⁰Ne and ³⁶Ar concentrations in the range of 10^?6–10^?9 cm³ STP/g. In most chondrules, trapped Ar is of type Q (ordinary chondritic Ar), which suggests that this component is indigenous to the chondrule precursor material. The history of the Cullison chondrite is special in several respects: large fractions of both CR‐produced ³He and of radiogenic ⁴He were lost during or after parent body breakup, in the latter case possibly by solar heating at small perihelion distances. Furthermore, one of the matrix samples contains constituents with a regolith history on the parent body before compaction. It also contains trapped Ne with a ²⁰Ne/²²Ne ratio of 15.5 ± 0.5, apparently fractionated solar Ne.     

Heat capacities of ordinary chondrite falls below 300 K

Low‐temperature specific heat capacities of meteorites provide valuable data for understanding the composition and evolution of meteorites and modeling the thermal behavior of their source asteroids. By liquid nitrogen immersion, we measured average low‐temperature heat capacities for 60 ordinary chondrite falls from the Vatican collection. We further characterized the temperature dependence of ordinary chondrite by direct measurement of C_p(T) over the range 5–320 K for five OC falls, coupled by composition‐based models for 94 ordinary chondrites. We find that the heat capacity as a function of temperature for typical ordinary chondrites can be closely approximated by a third‐order polynomial in temperature. Furthermore, those polynomial coefficients can be estimated from the single‐value average heat capacity measurement. These measurements have important implications for the orbital and spin evolution of S‐ and Q‐type asteroids via the various Yarkovsky effects and the thermal evolution of meteorite parent bodies.     

The Ar‐Ar age and petrology of Miller Range 05029: Evidence for a large impact in the very early solar system

Abstract– Miller Range (MIL) 05029 is a slowly cooled melt rock with metal/sulfide depletion and an Ar‐Ar age of 4517 ± 11 Ma. Oxygen isotopes and mineral composition indicate that it is an L chondrite impact melt, and a well‐equilibrated igneous rock texture with a lack of clasts favors a melt pool over a melt dike as its probable depositional setting. A metallographic cooling rate of approximately 14 °C Ma^?1 indicates that the impact occurred at least approximately 20 Ma before the Ar‐Ar closure age of 4517 Ma, possibly even shortly after accretion of its parent body. A metal grain with a Widmanstätten‐like pattern further substantiates slow cooling. The formation age of MIL 05029 is at least as old as the Ar‐Ar age of unshocked L and H chondrites, indicating that endogenous metamorphism on the parent asteroid was still ongoing at the time of impact. Its metallographic cooling rate of approximately 14 °C Ma^?1 is similar to that typical for L6 chondrites, suggesting a collisional event on the L chondrite asteroid that produced impact melt at a minimum depth of 5–12 km. The inferred minimum crater diameter of 25–60 km may have shattered the 100–200 km diameter L chondrite asteroid. Therefore, MIL 05029 could record the timing and petrogenetic setting for the observed lack of correlation of cooling rates with metamorphic grades in many L chondrites.     

40Ar‐39Ar ages of H‐chondrite impact melt breccias

Abstract— ⁴⁰Ar‐³⁹Ar analyses of a total of 26 samples from eight shock‐darkened impact melt breccias of H‐chondrite affinity (Gao‐Guenie, LAP 02240, LAP 03922, LAP 031125, LAP 031173, LAP 031308, NWA 2058, and Ourique) are reported. These appear to record impacts ranging in time from 303 ± 56 Ma (Gao‐Guenie) to 4360 ± 120 Ma (Ourique) ago. Three record impacts 300–400 Ma ago, while two others record impacts 3900–4000 Ma ago. Combining these with other impact ages from H chondrites in the literature, it appears that H chondrites record impacts in the first 100 Ma of solar system history, during the era of the “lunar cataclysm” and shortly thereafter (3500–4000 Ma ago), one or more impacts ?300 Ma ago, and perhaps an impact ?500 Ma ago (near the time of the L chondrite parent body disruption). Records of impacts on the H chondrite parent body are rare or absent between the era of planetary accretion and the “lunar cataclysm” (4400‐4050 Ma), during the long stretch between heavy bombardment and recent breakup events (3500‐1000 Ma), or at the time of final breakup into meteorite‐sized bodies (<50 Ma).     

40Ar-39Ar Ages of Types 3 and 4, L and H Chondrites from Antarctica

Abstract— This is a report on ⁴⁰Ar-³⁹Ar studies of 7 low petrographic type L and H chondrites from Antarctica. From petrographic similarities it has been argued that the L3 chondrites ALHA77015, ?77167, ?77249, and ?77260 are pieces from a common fall (McKinley et al., 1981). Our results now confirm this supposition: The four meteorites have identical characteristic Ar-degassing patterns, very similar K, Ca, Cl, and ³⁶Ar_trapped contents, and similar ⁴⁰Ar-³⁹Ar ages of <4 Ga which are rather unusual for ordinary chondrites and might be due to shock. The undulating age patterns could be due to weathering or to ³⁹Ar recoil. The L4 chondrite ALHA77230 shows no age plateau and only a lower limit for the time of a severe degassing, 4.0 Ga, can be given. ALHA77226 and RKPA78002, two H4 chondrites, exhibit reasonably well defined age plateaus at about 4.3 and 4.4 Ga. Two individual chondrules from RKPA78002 have the same age as the whole rock sample.     

QUE 94204: A primitive enstatite achondrite produced by the partial melting of an E chondrite‐like protolith

Abstract– Queen Alexandra Range (QUE) 94204, an enstatite achondrite, is a coarse‐grained, highly recrystallized, chondrule‐free and unbrecciated rock dominated (about 70 vol%) by anhedral, equigranular crystals of orthoenstatite of nearly endmember composition (Fs_0.1–0.4, Wo_0.3–0.4) with interstitial plagioclase, kamacite, and troilite. Abundance of approximately 120° triple junctions and the close association of metal–sulfide and plagioclase‐rich melts indicate that QUE 94204 has undergone limited partial melting with inefficient melt extraction. Mineral chemistry indicates a high degree of thermal metamorphism. Kamacite in QUE 94204 contains between 2.09 and 2.55 wt% Si, similar to highly metamorphosed EL chondrites. Plagioclase has between 4.31 and 6.66 wt% CaO, higher than other E chondrites but closer in composition to plagioclase from metamorphosed EL chondrites. QUE 94204 troilite contains up to 2.55 wt% Ti, consistent with extensive thermal metamorphism of an E chondrite‐like precursor. Results presented in this study indicate that QUE 94204 is the result of low degree, (about 5–20 vol%, probably toward the lower end of this range) partial melting of an E chondrite protolith. Textural and chemical evidence suggests that during the metamorphism of QUE 94204, melts formed first at the Fe,Ni‐FeS cotectic near approximately 900 °C, followed by plagioclase‐pyroxene silicate partial melts near approximately 1100 °C. Neither the Fe,Ni‐FeS nor the plagioclase‐pyroxene melts were efficiently segregated or extracted. QUE 94204 belongs to a grouplet of similar “primitive enstatite achondrites” that are analogous to the acapulcoites‐lodranites, but that have resulted from the partial melting of an E chondrite‐like protolith.     

A new metal‐rich chondrite grouplet

Abstract— A new grouplet of primitive, metal‐rich chondrites, here called the CB (C, carbonaceous; B, bencubbinite) chondrites, has been recognized. It includes Bencubbin, Weatherford, Hammadah al Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627. Their mineral compositions, as well as their oxygen and nitrogen isotopic compositions, indicate that they are closely related to the CR and CH chondrites, all of which are members of the more inclusive CR clan. CB chondrites have much greater metal/silicate ratios than any other chondrite group, widely increasing the range of metal/silicate fractionation recorded in solar nebular processes. They also have the greatest moderately volatile lithophile element depletions of any chondritic materials. Metal has compositional trends and zoning patterns that suggest a primitive condensation origin, in contrast with metal from other chondrite groups. CB chondrites, as well as other CR clan chondrites, have much heavier nitrogen (higher ¹⁵N/¹⁴N) than that in other chondrite groups. The primitive characteristics of the CB chondrites suggest that they contain one of the best records of early nebular processes. Another chondrite, Grosvenor Mountains 95551, is petrographically similar to the CB chondrites, but its mineral and oxygen and nitrogen isotope compositions indicate that it formed from a different nebular reservoir.