Total nitrogen in meteorites

Total N has been measured in a number of meteorites by neutron activation analysis using the reaction N¹⁴(n, p)C¹⁴. From each meteorite a number of chips have been analysed to investigate the variation of N contents in a sample. Many meteorites are found to contain a heterogeneous distribution of N. Eighteen chondrites, mostly of the classes C3, H4, H5, L4, L5, L6 and LL6, and six achondrites are found to have average N contents of 10–45 ppm. These do not show any clear-cut dependence of N on petrological group. However, the inherent heterogeneity or the fact that from most meteorite classes only single falls were studied might be responsible for this lack of correlation. In Cold Bokkeveld (C2) N is high (420 ppm). Unlike C, N content of ureilites is low (26 ppm). Nitrogen is enriched in the non-magnetic as compared to the magnetic fractions in H-group chondrites. Analyses of sieved Bjurböle phases show no enrichment of N in finer matrix material, nor any depletion in chondrules. In two gas-rich meteorites, Kapoeta and Assam, there is no excess N in the dark phases. Nine iron meteorites and three mesosiderites were analysed. Twenty analyses of Canyon Diablo and seven of Odessa establish a very heterogeneous N distribution in these meteorites.     

The distribution of total nitrogen in iron meteorites

Total nitrogen abundances in 123 iron meteorites have been determined by inert carrier-gas fusion extraction-gas chromatography. The median value for the iron meteorites was found to be 18 ppm N. The N contents of Sulfide inclusions are greater, in nine cases out of ten, than the corresponding metallic phase. The N content of the iron meteorites is positively correlated with germanium content. The effects of terrestrial weathering and heat treatment by man are discussed in relation to the N contents measured for certain specimens. A correlation between N and cooling rates was found, with lower cooling rates associated with greater N abundances.     

Origin of fossil meteorites from Ordovician limestone in Sweden

Based on the analysis of data in [1, 2] on the concentrations of noble gases and the cosmic ray exposure age (CREA) of chromite grains in fossil meteorites, it was demonstrated in [3] that the distributions of gas concentrations and cosmic ray exposure ages can be explained under the assumption of the fall of a single meteorite in the form of a meteorite shower in southern Sweden less than 0.2 Ma after the catastrophic destruction of the parental body (asteroid) of L chondrites in space at approximately 470 Ma. This assumption differs from the conclusion in [1, 2, 4] about the long-lasting (for 1–2 Ma) delivery of L chondrites to the Earth, with the intensity of the flux of this material one to two orders of magnitude greater than now. The analysis of newly obtained data on samples from the Brunflo fossil meteorite [5] corroborates the hypothesis of a meteorite shower produced by the fall of a single meteorite. The possible reason for the detected correlations between the cosmic ray exposure ages of meteorites and the masses of the samples with the ²⁰Ne concentrations can be the occurrence of Ne of anomalous isotopic composition in the meteorites.     

Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites

Inclusions of troilite and metallic Fe,Ni 0.2–4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites. Cooling rates in the interval 1400?950°C calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10^?7 to 10⁴°C/sec. In 8 of these chondrites, which are breccias containing some normal slow-cooled metal grains, the inclusions solidified before they were incorporated into the breccias. Their cooling rates of 1–300 °C/sec indicate cooling by radiation, or by conduction in contact with cold silicate or hot silicate volumes only 6–40 mm in size. This is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body (by impact), and were not formed at depth from an internally derived melt. In Ramsdorf, Rose City and Shaw, which show extensive reheating to ? 1000°C, Fe-FeS textures in melted areas are coarser and indicate cooling rates of 10^?1 to 10^?4°C/sec during solidification. This metal may have solidified inside hot silicate volumes that were 10–300 cm in size. As Shaw and Rose City are breccias of unmelted and melted material, their melted metal did not necessarily cool through 1000°C within a few m of the surface. Shock-melted, fine-grained, irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of ? 10⁵°C/sec. Their kamacite and taenite compositions may result from annealing at ~250°C of metallic glass or exceedingly fine-grained quench products.     

Origin of planetary cores: evidence from highly siderophile elements in martian meteorites

We present new bulk compositional data for 6 martian meteorites, including highly siderophile elements Ni, Re, Os, Ir and Au. These and literature data are utilized for comparison versus the siderophile systematics of igneous rocks from Earth, the Moon, and the HED asteroid. The siderophile composition of ALH84001 is clearly anomalous. Whether this reflects a more reducing environment on primordial Mars when this ancient rock first crystallized, or secondary alteration, is unclear. QUE94201 shows remarkable similarity with EET79001-B for siderophile as well as lithophile elements; both are extraordinarily depleted in the “noblest” siderophiles (Os and Ir), to roughly 0.00001 × CI chondrites. As in terrestrial igneous rocks, among martian rocks Ni, Os and Ir show strong correlations vs. MgO. In the case of MgO vs. Ni, the martian trend is displaced toward lower Ni by a large factor (5), but the Os and Ir trends are not significantly displaced from their terrestrial counterparts. For Mars, Re shows a rough correlation with MgO, indicating compatible behavior, in contrast to its mildly incompatible behavior on Earth. Among martian MgO-rich rocks, Au shows a weak anticorrelation vs. MgO, resembling the terrestrial distribution except for a displacement toward 2–3 times lower Au. The same elements (Ni, Re, Os, Ir and Au) show similar correlations with Cr substituted for MgO. Data for lunar and HED rocks generally show less clear-cut trends (relatively few MgO-rich samples are available). These trends are exploited to infer the compositions of the primitive Earth, Mars, Moon and HED mantles, by assuming that the trend intercepts the bulk MgO or Cr content of the primitive mantle at the approximate primitive mantle concentration of the siderophile element. Results for Earth show good agreement with earlier estimates. For Mars, the implied primitive mantle composition is remarkably similar to the Earth’s, except for 5 times lower Ni. The best constrained of the extremely siderophile elements, Os and Ir, are present in the martian mantle at 0.005 times CI, in comparison to 0.007 times CI in Earth’s mantle. This similarity constitutes a key constraint on the style of core-mantle differentiation in both Mars and Earth. Successful models should predict similarly high concentrations of noble siderophile elements in both the martian and terrestrial mantles (“high” compared to the lunar and HED mantles, and to models of simple partitioning at typical low-pressure magmatic temperatures), but only predict high Ni for the Earth’s mantle. Models that engender the noble siderophile excess in Earth’s mantle through a uniquely terrestrial process, such as a Moon-forming giant impact, have difficulty explaining the similarity of outcome (except for Ni) on Mars. The high Ni content of the terrestrial mantle is probably an effect traceable to Earth’s size. For the more highly siderophile elements like Os and Ir, the simplest model consistent with available constraints is the veneer hypothesis. Core-mantle differentiation was notably inefficient on the largest terrestrial planets, because during the final ∼ 1% of accretion these bodies acquired sufficient H₂O to oxidize most of the later-accreting Fe-metal, thus eliminating the carrier phase for segregation of siderophile elements into the core.     

The concentration and isotopic composition of hydrogen,carbon and nitrogen in carbonaceous meteorites

Concentrations and isotopic compositions were determined for H₂, N₂ and C extracted by stepwise pyrolysis from powdered meteorites, from residues of meteorites partially dissolved with aqueous HF, and from residues of meteorites reacted with HF-HCl solutions. The meteorites treated were the carbonaceous chondrites, Orgueil, Murray, Murchison, Renazzo and Cold Bokkeveld. Data determined for whole rock samples are in approximate agreement with previously published data. Acidification of the meteorites removed the inorganic sources of H₂, so that H₂ in the HF-HCl acid residues came primarily from insoluble organic matter, which makes up 70–80% fraction of the total carbon in carbonaceous meteorites. The δD in the organic matter differs markedly from previously determined values in organic matter in meteorites. The δD values of organic matter from acid residues of C1 and C2 carbonaceous chondrites range from +650 to + 1150%. The acid residues of the Renazzo meteorite, whose total H₂ has a δD of +930‰, gave a δD value of +2500‰. Oxidation of the HF-HCl residue with H₂O₂ solution removes the high δD and the low δ¹⁵N components. The δ¹³C values range between ?10 and ?21 and δ¹⁵N values range between +40 and ?11. The δ¹⁵N of Renazzo is unusual; its values range between +150 and ?190.There is good correlation between δD and the concentration of H₂ in the acid residues, but no correlation exists between δD, δ¹³C and δ¹⁵N in them. A simple model is proposed to explain the high δD values, and the relationships between δD values and the concentration of H₂. This model depends on the irradiation of gaseous molecules facilitating reaction between ionic molecules, and indicates that an increase in the rate of polymerization and accumulation of organic matter on grains would produce an increase in the deuterium concentration in organic matter.     

Fluorine in meteorites

Microanalysis using a resonant nuclear reaction was used to measure F concentrations in USGS standard rocks and 21 meteorites. The F appears to be a moderately depleted element, but there were significant variations within each sample. Measurements on separated metal phases suggest that about 20% of meteoritic F is in the metal or in a phase closely associated with it. Simultaneous measurements of F, Mg, Na, Al and Si in the non-magnetic fractions of meteorites suggest plagioclase as a F containing phase.     

Origin of light hydrocarbon gases from the Matsukawa geothermal area in Japan

Light hydrocarbon and isotope compositions of methane were analyzed in well steam samples from the Matsukawa vapour-dominated type geothermal system. Alkanes (C₁-C₄) and alkene (C₂) were detected in all samples. Light hydrocarbon contents of CO₂-type steam are slightly higher than those of CO₂-H₂S-type steam. The isotope composition of methane and the relationship between methane/ethane ratio and δ¹³C-value of methane suggest that these light hydrocarbon gases are mixtures of thermogenic and abiogenic components. The abiogenic hydrocarbon may be attributed to magmatic hydrocarbon gases equilibrated with carbon dioxide at f_o2 defined by the fayalite-magnetite-quartz buffer (FMQ).     

Titanium isotopic anomalies in meteorites

High-precision analyses of Ti are reported for samples from a variety of meteorite classes. The expanded data base for Allende inclusions still shows Ti isotopic anomalies in every inclusion. All the coarse-grained inclusions give quite similar patterns, but fine-grained inclusions show more variable, and sometimes larger, anomalies. One inclusion, 3675A, was analyzed because others identified it as a possible “FUN” inclusion due to its mass-fractionated Mg. This designation is supported by the significantly more complex Ti isotopic pattern for 3675A compared to all our other Allende inclusions. Available data fail to suggest that any particular Allende mineral phase, including a chromite-carbon fraction from an acid residue, is especially rich in anomalous Ti. We also find anomalous Ti in a bulk sample of a Cl chondrite and in matrix separates from C2 chondrites. The excesses of ⁵⁰Ti are smaller than for Allende inclusions, and subtle differences in Ti isotopic patterns tentatively suggest that parent materials for C1-C2 matrix and Allende inclusions are not directly related. Analyses of chondrules from unequilibrated ordinary chondrites did not yield clear evidence for anomalous Ti, but some “larger than usual” deficits at $\text{50}\text{46}$ give encouragement for future work in this direction. Comparing the magnitude of isotopic shifts at ⁵⁰Ti and ¹⁶O for all these meteorite samples indicates that they are not correlated, but the data do not preclude a correlation between concentrations of “exotic” ⁵⁰Ti and ¹⁶O atoms.Whether or not Allende “FUN” inclusions are considered, at least 4 distinct isotopic components of Ti are required to account for the observed isotopic variations. The Ti data cannot be plausibly explained in terms of an early solar-system particle irradiation; instead, neutron-rich hydrostatic burning within a star is probably responsible for the dominant ⁵⁰Ti anomalies, while s-process mechanisms are viable sources for some of the more subtle Ti variations. We suggest that the Ti anomalies are linked to a diversity of nucleosynthetic sources and the highly refractory behavior of Ti. Therefore, some form of “chemical memory” from the ISM, rather than “late stage supernova injection”, is most likely responsible for the preservation of observed isotopic heterogeneities.     

Isotopic composition of silicon in meteorites

Samples of bulk meteorites show only mass-dependent fractionation of silicon isotopes. No isotopic anomalies were found. The variation of the ratios ²⁹Si/²⁸Si and ³⁰Si/²⁸Si over the meteorite classes is small; 1%. per mass unit difference. The average Si isotopic composition for each class of meteorites is identical, within analytical uncertainties. This is quite unlike O, whose anomalous isotopic abundances in bulk samples differentiate among the classes of meteorites. The overlapping abundance ranges of Si isotopes among many classes of meteorites suggest closed-system behavior for this element prior to meteorite accretion and allow calculation of an average solar system Si isotope composition.     

Rare-earth abundances in chondritic meteorites

Fifteen chondrites, including eight carbonaceous chondrites, have been analyzed for rare earth element (REE) abundances by isotope dilution. These analyses complement and extend earlier isotope dilution REE determinations in chondrites, performed in other laboratories, so that coverage of major chondrite classes is now complete. An examination of this body of precise and comparable REE data from individual chondrites reveals that only a small proportion of the analyses have flat, unfractionated REE patterns within experimental error. A statistical procedure is used to derive revised chondritic abundances of REE by selection of unfractionated patterns. A number of the remaining analyses show Eu anomalies and fractionated patterns consistent with magmatic fractionation as encountered in the products of planetary differentiation. However, many patterns exhibit features not readily explicable by known magmatic processes; in particular, positive Ce anomalies are often encountered. Abundance anomalies can be quantitatively determined by the use of a least-squares curve fitting procedure. The wide variety of anomalous patterns and the uncertainties in model parameters preclude detailed modeling of the origin of anomalies, but it is probable that at least some arise from fractional condensation in the solar nebula, as has been demonstrated for Allende inclusions. Elemental abundance anomalies are found in all major chondrite classes. If these anomalies are ignored, the range and nature of variation within chondrite classes are consistent with a parent body model, in which solid-liquid or solid-solid equilibria induce variations from an unfractionated bulk composition. Absolute abundances in the H, L and LL parent bodies are almost twice those of the E parent body.The persistence of anomalies in chondritic materials relatively removed from direct condensational processes implies that anomalous components are resistant to equilibration or were introduced at a late stage of chondrite formation. Large scale segregation of gas and condensate is also implied, and raises the possibility of bulk variations in REE abundances between planetary bodies.     

Isotopic fractionation of nitrogen and carbon in Paleoarchean cherts from Pilbara craton, Western Australia: Origin of N-depleted nitrogen

Nitrogen and carbon isotopic compositions, together with mineralogy and trace element geochemistry, were studied in a few kerogen-rich Paleoarchean cherts, a barite and a dolomitic stromatolite belonging to the eastern (Dixon Island Formation) and western (Dresser and Strelley Pool Chert Formations; North Pole Dome and Marble Bar) terranes of Pilbara Craton, Western Australia. The aim of the study was to search for ¹⁵N-depleted isotopic signatures, often found in kerogens of this period, and explain the origin of these anomalies. Trace elements suggest silica precipitation by hydrothermal fluids as the main process of chert formation with a contamination from volcanoclastic detritus. This is supported by the occurrence of hydrothermal-derived minerals in the studied samples indicating precipitation temperatures up to 350 °C. Only a dolomitic stromatolite from Strelley Pool shows a superchondritic Y/Ho ratio of 72 and a positive Eu/Eu^* anomaly of 1.8, characteristic of chemical precipitates from the Archean seawater. The bulk δ¹³C vs. δ¹⁵N values measured in the cherts show a roughly positive co-variation, except for one sample from the North Pole (PI-85-00). The progressive enrichment in ¹⁵N and ¹³C from a pristine source having δ¹³C ? −36‰ and δ¹⁵N ? −4‰ is correlated with a progressive depletion in N content and to variations in Ba/La and Co/As ratios. These trends have been interpreted as a progressive hydrothermal alteration of the cherts by metamorphic fluids. Isotopic exchange at 350 °C between NH₄⁺_(rock) and N_2(fluid) may explain the isotopic and elemental composition of N in the studied cherts. However, we need to assume isotopic exchange at 350 °C between carbonate C and graphite to explain the large ¹³C enrichment recorded. Only sample PI-85-00 shows a large N loss (90%) with a positive δ¹⁵N value (+11‰), while C (up to 120 ppm and δ¹³C −38‰) seems to be unaffected. This pattern has been interpreted as the result of devolatilization and alteration (oxidation) of graphite by low-temperature fluids. The ¹⁵N-¹³C-depleted pristine source has δ ¹⁵N values from −7‰ to −4‰ and ⁴⁰Ar/³⁶Ar ratios from 30,000 to 60,000, compatible with an inorganic mantle N source, although the elemental abundance ratios N/C and ⁴⁰Ar/C are not exactly the same with the mantle source. The component alternatively could be explained by elemental fractionation from metabolic activity of chemolithoautotrophs and methanogens at the proximity to the hydrothermal vents. However, ambiguities between mantle vs organic sources of N subsist and need further experimental work to be fully elucidated.     

Interstellar organic matter in meteorites

Hydrogen which is highly enriched in deuterium is present in organic matter in a variety of meteorites including non-carbonaceous chondrites. The concentrations of this hydrogen are quite large. For example Renazzo contains 140 μmoles/g of the 10,000‰ δD hydrogen. The $\text{D}\text{H}$ ratios of hydrogen in the organic matter vary from 8 × 10^?5 to 170 × 10^?5 (δD ranges from ? 500‰ to 10,000‰) as compared to 16 × 10^?5 for terrestrial hydrogen and 2 × 10^?5 for cosmic hydrogen. The majority of the unequilibrated primitive meteorites contain hydrogen whose $\text{D}\text{H}$ ratios are greater than 30 × 10^?5. If the $\text{D}\text{H}$ ratios in these compounds were due to enrichment relative to cosmic hydrogen by isotope exchange reactions, it would require that these reactions take place below 150 K. In addition the organic compounds having $\text{D}\text{H}$ ratios above 50 × 10^?5 would require temperatures of formation of < 120 K. These types of deuterium enrichments must take place by ion-molecule reactions in interstellar clouds where both ionization and low temperatures exist. Astronomically observed $\text{D}\text{H}$ ratios in organic compounds in interstellar clouds are typically 180 × 10^?5 and range between about 40 × 10^?5 and 5000 × 10^?5. The $\text{D}\text{H}$ values we have determined are the lower limits for the organic compounds derived from interstellar molecules because all processes subsequent to their formation, including terrestrial contamination, decrease their $\text{D}\text{H}$ ratios.In contrast, the $\text{D}\text{H}$ ratios of hydrogen associated with hydrated silicates are relatively uniform for the meteorites we have analyzed with an average value of 14 × 10^?5; very similar to the terrestrial value. These phyllosilicates values suggest equilibration of H₂O with H₂ in the solar nebula at temperatures of about 200 K and higher.The ${}^{13}\text{C}{}^{12}\text{C}$ ratios of organic matter, irrespective its $\text{D}\text{H}$ ratio, lie well within those observed for the earth. If organic matter originated in the interstellar medium, our data would indicate that the ${}^{13}\text{C}{}^{12}\text{C}$ ratio of interstellar carbon five billion years ago was similar to the present terrestrial value.Our findings suggest that other interstellar material, representing various inputs from various stars, in addition to the organic matter is preserved and is present in the meteorites which contain the high $\text{D}\text{H}$ ratios. We feel that some elements existing in trace quantities which possess isotopic anomalies in the meteorites may very well be such materials.     

Hydrocarbon components in carbonaceous meteorites

Currently, the presence of free n-alkanes and isoprenoid alkanes in carbonaceous meteorites is usually explained either by microbial contamination during the period between the meteorite fall and collection or by contamination from the environment of analytical laboratories and museums. The goal of this research was to repeat analysis of hydrocarbon components in meteorites and to investigate possible meteorite contamination routes discussed in the literature. Experimental analysis of free organic constituents in five carbonaceous meteorites by infrared spectroscopy (IR) and gas chromatographic (GC) methods confirmed the presence of extractable aliphatic components, n-alkanes in the C₁₅H₃₂-C₂₇H₅₆ range and isoprenoid alkanes (phytane, pristane, and norpristane), in some of these meteorites. The contents of these compounds vary depending on the source. Insoluble organic components of two meteorites (meteorite kerogens) were isolated, and their composition was analyzed by IR and cracking/GC methods. Comparison with the data on several terrestrial contamination sources proposed in the literature shows that the presence of free saturated hydrocarbons in meteorites and the composition of the meteorite kerogen could not be explained either by microbial contamination or by contamination from the laboratory environment. The types of the hydrocarbons in meteorites resemble those typical of ancient terrestrial deposits of organic-rich sediments, except for the absence of lighter hydrocarbons, which apparently slowly evaporated in space, and multi-ring naphthenic compounds of the biologic origin, steranes, terpanes, etc. The prevailing current explanation for the presence of free linear saturated hydrocarbons in carbonaceous meteorites, apart from contamination, is the abiotic route from hydrogen and carbon monoxide. However, the data on the structure of meteorite kerogens require a search for different routes that initially produce complex polymeric structures containing n-alkyl and isoprenoid chains which are attached, via polar links (esters, salts, etc.), to a cross-linked polymer matrix. Later, the polymer slowly decomposes with the liberation of free aliphatic hydrocarbons.     

Trapped extraterrestrial argon in meteorites

The article considers different points of view on the genesis of extraterrestrial trapped argon with a composition different from primordial/solar in meteorites and lunar rocks. An alternative hypothesis of the origin of this component is discussed.     

Index of Australian meteorites

A new compilation of Australian meteorite discoveries suggests that many meteorites remain to be discovered by field geologists and others, in outback areas of Australia.     

Silicate mineralogy of martian meteorites

Basalts and basaltic cumulates from Mars (delivered to Earth as meteorites) carry a record of the history of that planet - from accretion to initial differentiation and subsequent volcanism, up to recent times. We provide new microprobe data for plagioclase, olivine, and pyroxene from 19 of the martian meteorites that are representative of the six types of martian rocks. We also provide a comprehensive WDS map dataset for each sample studied, collected at a common magnification for easy comparison of composition and texture. The silicate data shows that plagioclase from each of the rock types shares similar trends in Ca-Na-K, and that K₂O/Na₂O wt% of plagioclase multiplied by the Al content of the bulk rock can be used to determine whether a rock is “enriched” or “depleted” in nature. Olivine data show that meteorite Y 980459 is a primitive melt from the martian mantle as its olivine crystals are in equilibrium with its bulk rock composition; all other olivine-bearing Shergottites have been affected by fractional crystallization. Pyroxene quadrilateral compositions can be used to isolate the type of melt from which the grains crystallized, and minor element concentrations in pyroxene can lend insight into parent melt compositions.In a comparative planetary mineralogy context, plagioclase from Mars is richer in Na than terrestrial and lunar plagioclase. The two most important factors contributing to this are the low activity of Al in martian melts and the resulting delayed nucleation of plagioclase in the crystallizing rock. Olivine from martian rocks shows distinct trends in Ni-Co and Cr systematics compared with olivine from Earth and Moon. The trends are due to several factors including oxygen fugacity, melt compositions and melt structures, properties which show variability among the planets. Finally, Fe-Mn ratios in both olivine and pyroxene can be used as a fingerprint of planetary parentage, where minerals show distinct planetary trends that may have been set at the time of planetary accretion.Although the silicate mineralogical data alone cannot support one specific model of martian magmatism over another, the data does support the basic igneous reservoirs proposed for Mars, and may also be used to constrain some aspects of specific petrogenetic models. Examples include enriched and depleted reservoirs that can be identified by plagioclase K, Na and Al composition, multivalent element partitioning in olivine and pyroxene (V, Cr) elucidates oxygen fugacity conditions of the reservoirs, and minor element concentrations (i.e., Cr in pyx) show that proposed fractional crystallization models linking Y 980459 to QUE 94201 will not work.     

Fission-track ages of four meteorites

A method for selective annealing of cosmic-ray tracks has been developed, permitting determination of fission-track ages in the presence of a large background of cosmic-ray tracks. The mesosiderite Bondoc contains 41 fission tracks/cm², of which about 75% are due to neutron-induced fission of U²³⁵ during cosmic-ray exposure. Its net fission-track age is 140 ± 40 Myr, nearly identical to its cosmic-ray exposure age of 150 Myr. The mesosiderite Mincy has a fission-track age of 1500 ± 400 Myr.Nakhla (nakhlite) contains an excess of apparent fission tracks, which may be either genuine fission tracks from Pu²⁴⁴ or etch pits mimicking fission tracks in length, thermal stability, random orientation, and other characteristics. On the assumption that they are fission tracks, the Pu²⁴⁴/U²³⁸ ratio at the onset of track retention in Nakhla was (3.1 ± 1.3) × 10^?3, nearly an order of magnitude lower than the initial solar system ratio. This may reflect a chemical fractionation of Pu and U, or a late impact or magmatic event. Different minerals of the Washougal howardite have different Pu²⁴⁴/U²³⁸ ratios, from (24 ± 7) × 10^?3 to (2.3 ± 0.7) × 10^?3. This may imply a succession of impacts over a period of time. Additionally, Pu and U may have been chemically fractionated from each other in this meteorite.Shocked meteorites show no consistent pattern in the retentivity of fission tracks and of fissiogenic or radiogenic noble gases. Some meteorites, e.g. Bondoc, Serra de Magé, and Mincy, retain gases more completely than tracks; others, e.g. Nakhla and Allende, retain them less completely.Uranium was determined in feldspar and/or pyroxene from 19 Ca-rich achondrites and mesosiderites. For most, only upper limits of 0.01–0.03 ppb were obtained. Apparently the uranium in these meteorites resides almost exclusively in minor phases, as in terrestrial and lunar rocks.     

Impact features of enstatite-rich meteorites

Enstatite-rich meteorites include EH and EL chondrites, rare ungrouped enstatite chondrites, aubrites, a few metal-rich meteorites (possibly derived from the mantle of the aubrite parent body), various impact-melt breccias and impact-melt rocks, and a few samples that may be partial-melt residues ultimately derived from enstatite chondrites. Members of these sets of rocks exhibit a wide range of impact features including mineral-lattice deformation, whole-rock brecciation, petrofabrics, opaque veins, rare high-pressure phases, silicate darkening, silicate-rich melt veins and melt pockets, shock-produced diamonds, euhedral enstatite grains, nucleation of enstatite on relict grains and chondrules, low MnO in enstatite, high Mn in troilite and oldhamite, grains of keilite, abundant silica, euhedral graphite, euhedral sinoite, F-rich amphibole and mica, and impact-melt globules and spherules. No single meteorite possesses all of these features, although many possess several. Impacts can also cause bulk REE fractionations due to melting and loss of oldhamite (CaS) – the main REE carrier in enstatite meteorites. The Shallowater aubrite can be modeled as an impact-melt rock derived from a large cratering event on a porous enstatite chondritic asteroid; it may have been shock melted at depth, slowly cooled and then excavated and quenched. Mount Egerton may share a broadly similar shock and thermal history; it could be from the same parent body as Shallowater. Many aubrites contain large pyroxene grains that exhibit weak mosaic extinction, consistent with shock-stage S4; in contrast, small olivine grains in some of these same aubrites have sharp or undulose extinction, consistent with shock stage S1 to S2. Because elemental diffusion is much faster in olivine than pyroxene, it seems likely that these aubrites experienced mild post-shock annealing, perhaps due to relatively shallow burial after an energetic impact event. There are correlations among EH and EL chondrites between petrologic type and the degree of shock, consistent with the hypothesis that collisional heating is mainly responsible for enstatite-chondrite thermal metamorphism. Nevertheless, the apparent shock stages of EL6 and EH6 chondrites tend to be lower than EL3-5 and EH3-5 chondrites, suggesting that the type-6 enstatite chondrites (many of which possess impact-produced features) were shocked and annealed. The relatively young Ar–Ar ages of enstatite chondrites record heating events that occurred long after any ²⁶Al that may have been present initially had decayed away. Impacts remain the only plausible heat source at these late dates. Some enstatite meteorites accreted to other celestial bodies: Hadley Rille (EH) was partly melted when it struck the Moon; Galim (b), also an EH chondrite, was shocked and partly oxidized when it accreted to the LL parent asteroid. EH, EL and aubrite-like clasts also occur in the polymict breccias Kaidun (a carbonaceous chondrite) and Almahata Sitta (an anomalous ureilite). The EH and EL clasts in Kaidun appear unshocked; some clasts in Almahata Sitta may have been extensively shocked on their parent bodies prior to being incorporated into the Almahata Sitta host.