Primitive Achondrites Research Articles

Isotopic analysis of lead in sub-nanogram quantities by TIMS using a202Pb–205Pb spike

We present a method for isotopic analyses of Pb in sub-nanogram quantities by thermal ionization mass spectrometry with fractionation correction using a 202Pb–205Pb spike. The procedure is simple, fast, easy to automate, and requires only one analysis of Pb per sample to acquire both fractionation-corrected Pb isotopic ratios and Pb concentration. The procedure is evaluated by analysis of multiple loads of NIST Pb isotopic standards SRM-981 and SRM-982, and Pb separated from USGS standard rock BCR-1 and chondrite Bruderheim, with the amount of Pb per load between 0.05–3.0 × 10−9 g. The data have been corrected using both the double spike (DS) fractionation correction described here, as well as by conventional external normalization (EN) procedures, thus providing a direct comparison between the correction methods on exactly the same analytical runs. The enhancement in precision and reproducibility achieved by DS normalization varies with the quantity of Pb analysed. For amounts of Pb of less than 0.2 ng, the improvement in precision and reproducibility of 207Pb/206Pb ratios is small but becomes more substantial with increasing sample size: 3–10 times lower within-run errors, and 2–3 times better reproducibility are obtained for 0.5–3.0 ng loads. Even greater enhancement in precision and reproducibility is achieved for the 208Pb/206Pb ratios, at twice the relative mass difference between isotopes. The advantage of DS for the 204Pb/206Pb ratio is, however, much smaller. This indicates that factors other than fractionation control precision and accuracy of isotopic ratios measured at the ion beam levels below 2–3 × 10−13 A—the intensity typically observed for 204Pb in all samples smaller than 2–3 ng, and for all Pb isotopes in samples smaller than ca. 0.1 ng. The method described here is applicable to high-precision Pb–Pb and U–Pb dating of milligram quantities of meteorites and their components, and single, microgram-size grains of zircon and other U-bearing minerals. The technique described here had been used for dating early solar system materials (Ca–Al rich inclusions and primitive achondrites) containing 0.01–0.3 ng of Pb per fraction, and yielded Pb–Pb isochrons with the age error of 0.16–0.33 million years, which was previously unattainable for such small samples.

Meteorite Phosphates Show Constant 176 Lu Decay Rate Since 4557 Million Years Ago

The use of radioactive decay of 176Lu to 176Hf to study the evolution of the Earth requires a precise and accurate value for the 176Lu decay constant. Recent determinations of this decay constant by age comparison to the more precisely calibrated U-Pb isotopic system produced internally consistent but discrepant values between terrestrial minerals and meteorites. New highly radiogenic Lu-Hf data for phosphate minerals from Richardton (ordinary chondrite) and Acapulco (primitive achondrite) yield decay constant values of 1.864 x 10(-11) +/- 0.016 x 10(-11) and 1.832 x 10(-11) +/- 0.029 x 10(-11) year(-1), respectively, identical to the value determined from terrestrial minerals.

Portales Valley: Petrology of a metallic-melt meteorite breccia

Abstract— Portales Valley (PV) is an unusual metal-veined meteorite that has been classified as an H6 chondrite. It has been regarded either as an annealed impact melt breccia, as a primitive achondrite, or as a meteorite with affinities to silicated iron meteorites. We studied the petrology of PV using a variety of geochemical-mineralogical techniques. Our results suggest that PV is the first well-documented metallic-melt meteorite breccia. Mineral-chemical and other data suggest that the protolith to PV was an H chondrite. The composition of FeNi metal in PV is somewhat fractionated compared to H chondrites and varies between coarse vein and silicate-rich portions. It is best modeled as having formed by partial melting at temperatures of ˜940–1150 °C, with incomplete separation of solid from liquid metal. Solid metal concentrated in the coarse vein areas and S-bearing liquid metal concentrated in the silicate-rich areas, possibly as a result of a surface energy effect. Both carbon and phosphorus must have been scavenged from large volumes and concentrated in metallic liquid. Graphite nodules formed by crystallization from this liquid, whereas phosphate formed by reaction between P-bearing metal and clinopyroxene components, depleting clinopyroxene throughout much of the meteorite and growing coarse phosphate at metal-silicate interfaces. Some phosphate probably crystallized from P-bearing liquids, but most probably formed by solid-state reaction at ˜975-725 °C. Phosphate-forming and FeO-reduction reactions were widespread in PV and entailed a change in the mineralogy of the stony portion on a large scale. Portales Valley experienced protracted annealing from supersolidus to subsolidus temperatures, probably by cooling at depth within its parent body, but the main differences between PV and H chondrites arose because maximum temperatures were higher in PV. A combination of a relatively weak shock event and elevated pre-shock temperatures probably produced the vein-and-breccia texture, with endogenic heating being the main heat source for melting, and with stress waves from an impact event being an essential trigger for mobilizing metal. Portales Valley is best classified as an H7 metallic-melt breccia of shock stage S1. The meteorite is transitional between more primitive (chondritic) and evolved (achondrite, iron) meteorite types and offers clues as to how differentiation could have occurred in some asteroidal bodies.

Differentiation of metal‐rich meteoritic parent bodies: I. Measurements of PGEs, Re, Mo, W, and Au in meteoritic Fe‐Ni metal

Abstract— We describe an analytical technique for measurements of Fe, Ni, Co, Mo, Ru, Rh, W, Re, Os, Ir, Pt, and Au in bulk samples of iron meteorites. The technique involves EPMA (Fe, Ni, Co) and LA‐ICP‐MS analyses of individual phases of iron meteorites, followed by calculation of bulk compositions based on the abundances of these phases. We report, for the first time, a consistent set of concentrations of Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in the iron meteorites Arispe, Bennett County, Grant, Cape of Good Hope, Cape York, Carbo, Chinga, Coahuila, Duchesne, Gibeon, Henbury, Mundrabilla, Negrillos, Odessa, Sikhote‐Alin, and Toluca and the Divnoe primitive achondrite. The comparison of our LA‐ICP‐MS data for a number of iron meteorites with high‐precision isotope dilution and INAA data demonstrates the good precision and accuracy of our technique.The narrow ranges of variations of Mo and Pd concentrations within individual groups of iron meteorites suggest that these elements can provide important insights into the evolution of parent bodies of iron meteorites. Under certain assumptions, the Mo concentrations can be used to estimate mass fractions of the metal‐sulfide cores in the parent bodies of iron meteorites. It appears that a range of Pd variations within a group of iron meteorites can serve as a useful indicator of S content in the core of its parent body.

Brachinites: Igneous rocks from a differentiated asteroid

Abstract— We have done petrologic studies of brachinites Allan Hills (ALH) 84025, Elephant Moraine (EET) 99402, and EET 99407; bulk geochemical studies of EET 99402 and EET 99407; Ar‐Ar studies of Brachina and EET 99402; and a Xe isotopic study of Brachina. Textural, mineral compositional, and bulk compositional evidence show that EET 99402 and EET 99407 are paired. ALH 84025, EET 99402, and EET 99407 have igneous textures. Petrofabric analyses of ALH 84025 and EET 99407 demonstrate the presence of lineations and probable foliations of olivine grains that support formation as igneous cumulates. Mineral minor element chemistry and bulk rock incompatible lithophile element contents of the brachinites are distinct from those of acapulcoitelodranite clan meteorites, a suite of high‐grade metamorphic rocks and anatectic residues. The differences demonstrate a higher blocking temperature of equilibration for the brachinites and that cumulus plagioclase is present in EET 99402, EET 99407, and probably ALH 84025, thus indicating an igneous origin. Brachinites are differentiated, ultramafic achondrites, and are not part of a suite of primitive achondrites. We infer that their parent asteroid is a differentiated body.Brachina has an excess of 129Xe correlated with reactor‐produced 128Xe, demonstrating that short‐lived 129I was present at the time of formation. This, plus literature data, attests to early formation of the brachinites, within a few Ma of the formation of chondrites. Ar‐Ar age data show that Brachina and EET 99407 were degassed about 4.13 Ga ago, possibly by a common impact event. EET 99402 and EET 99407 show petrographic evidence for shock, including possible conversion of plagioclase to maskelynite followed by devitrification. Brachina is unshocked, making a direct association between the Ar‐Ar age and textures ambiguous.

New noble gas data of primitive and differentiated achondrites including Northwest Africa 011 and Tafassasset

Abstract— This work reports on the noble gas inventory of 3 new acapulcoites, 3 brachinites, 2 new eucrites from the Dar al Gani region in Libya, the unique achondrite Dar al Gani (DaG) 896 from the same locality, the new eucrite‐like achondrite Northwest Africa (NWA) 011, and the controversial sample Tafassasset.We determined cosmic ray exposure and gas retention ages, evaluated shielding conditions, and discuss the trapped noble gas component of the specimens. All exposure ages are within the known range of stony meteorites and partly confirm previously established age clusters. Shielding conditions vary, suggesting substantial shielding for all 3 brachinites and Tafassasset. We cannot exclude, however, that the Mg‐rich composition of brachinites simply simulates heavy shielding. Regarding the trapped component, we found Q‐like compositions only for the acapulcoite Thiel Mountains (TIL) 99002. The brachinite Elephant Moraine (EET) 99402 yields a high, subsolar 36Ar/132Xe ratio of ˜400 along with a slightly elevated 84Kr/132atio, indicating minor atmospheric contamination. All the other samples, particularly the eucrite DaG 983, are characterized by clearly elevated Ar/Kr/Xe ratios due to significant terrestrial alteration.Tafassasset exhibits noble gas parameters that are different from those of CR chondrites, including a relatively high cosmic ray exposure age, the absence of a solar component, low 132Xe concentrations, a low trapped 36Ar/132Xe ratio of ˜30, and a noticeable amount of radiogenic 129Xe. Similar attributes have been observed for some primitive achondrites. These attributes are also consistent with the metamorphic character of the sample. We, therefore, consider Tafassasset's noble gas record to be inconclusive as to its classification (primitive achondrite versus metamorphosed CR chondrite).

The trapped noble gas component in achondrites

Abstract—The trapped noble gases Ar, Kr and Xe in several achondrites were analysed. We chose separates of the lodranites Lodran and Graves Nunataks 95209 and bulk samples of the Tatahouine diogenite, Pasamonte eucrite, five aubrites and two angrites. Among these, Lodran, Tatahouine, Pasamonte and the aubrite Norton County have been reported to contain U‐Xe, a noble gas component assumed to be the most primitive component in the solar system. U‐Xe might have been incorporated into the early Earth. We found large concentrations of Xe in several separates of the Lodran lodranite, however, none of the measurements revealed U‐Xe composition. The Xe composition of all achondrites can straightforwardly be explained with mixtures of trapped common Xe‐Q, absorbed air and various amounts of fissiogenic and cosmogenic Xe. Reanalysis of literature data for Pasamonte, Angra dos Reis and some aubrites is consistent with Xe‐Q as the trapped endmember component and contributions of fissiogenic Xe. The presence of Xe‐Q in many primitive achondrites is in agreement with the formation of their parent bodies from originally chondritic precursor material. The Ar‐Xe elemental composition of Lodran and the aubrites indicate subsolar composition, which is commonly found in E chondrites. This result supports a model of formation of the aubrites from E‐chondritic precursor material.

The Meteoritical Bulletin, No. 86, 2002 July

Abstract— Meteoritical Bulletin No. 86 lists information for 1154 newly classified meteorites, comprising 661 from Antarctica, 218 from Africa, 207 from Asia (203 of which are from Oman), 62 from North America, 3 from South America, and 3 from Europe. Information is provided for 5 falls (El Idrissia, Undulung, Dashoguz, El Tigre, and Yafa). Noteworthy specimens include 7 martian meteorites (Dhofar 378, Grove Mountains 99027, Northwest Africa 856, 1068, and 1110, and Sayh al Uhaymir 060 and 090); 4 lunar meteorites (Dhofar 301, 302, 303, and 489); 9 new iron meteorites; a mesosiderite (Northwest Africa 1242); an ungrouped stony‐iron meteorite (Dar al Gani 962); and a wide variety of other interesting stony meteorites, including CH, CK, CM, CR, CV, R, enstatite, unequilibrated ordinary, and ungrouped chondrites, primitive achondrites, howardite‐eucrite‐diogenite (HED) achondrites, and ureilites.

Space weathering on Eros: Constraints from albedo and spectral measurements of Psyche crater

Abstract— We present combined multi‐spectral imager (MSI) (0.95 μm) and near‐infrared spectrometer (NIS) (0.8–2.4 μm) observations of Psyche crater on S‐type asteroid 433 Eros obtained by the Near‐Earth Asteroid Rendezvous (NEAR)—Shoemaker spacecraft. At 5.3 km in diameter, Psyche is one of the largest craters on Eros which exhibit distinctive brightness patterns consistent with downslope motion of dark regolith material overlying a substrate of brighter material. At spatial scales of 620 m/ spectrum, Psyche crater wall materials exhibit albedo contrasts of 32–40% at 0.946 μm. Associated spectral variations occur at a much lower level of 4–8% (±2–4%). We report results of scattering model and lunar analogy investigations into several possible causes for these albedo and spectral trends: grain size differences, olivine, pyroxene, and troilite variations, and optical surface maturation. We find that the albedo contrasts in Psyche crater are not consistent with a cause due solely to variations in grain size, olivine, pyroxene or lunar‐like optical maturation. A grain size change sufficient to explain the observed albedo contrasts would result in strong color variations that are not observed. Olivine and pyroxene variations would produce strong band‐correlated variations that are not observed. A simple lunar‐like optical maturation effect would produce strong reddening that is not observed. The contrasts and associated spectral variation trends are most consistent with a combination of enhanced troilite (a dark spectrally neutral component simulating optical effects of shock) and lunar‐like optical maturation. These results suggest that space weathering processes may affect the spectral properties of Eros materials, causing surface exposures to differ optically from subsurface bedrock. However, there are significant spectral differences between Eros' proposed analog meteorites (ordinary chondrites and/or primitive achondrites), and Eros' freshest exposures of subsurface bright materials. After accounting for all differences in the measurement units of our reflectance comparisons, we have found that the bright materials on Eros have reflectance values at 0.946 μm consistent with meteorites, but spectral continua that are much redder than meteorites from 1.5 to 2.4 μm. Most importantly, we calculate that average Eros surface materials are 30–40% darker than meteorites.

The composition of 433 Eros: A mineralogical—chemical synthesis

Abstract— The near‐Earth asteroid rendezvous (NEAR) mission carried x‐ray/gamma‐ray spectrometers and multi‐spectral imager/near‐infrared spectrometer instrument packages which gave complementary information on the chemistry and mineralogy, respectively, of the target asteroid 433 Eros. Synthesis of these two data sets provides information not available from either alone, including the abundance of non‐mafic silicates, metal and sulfide minerals. We have utilized four techniques to synthesize these data sets. Venn diagrams, which examine overlapping features in two data sets, suggest that the best match for 433 Eros is an ordinary chondrite, altered at the surface of the asteroid, or perhaps a primitive achondrite derived from material mineralogically similar to these chondrites. Normalized element distributions preclude FeO‐rich pyroxenes and suggest that the x‐ray and gamma‐ray data can be reconciled with a common silicate mineralogy by inclusion of varying amounts of metal. Normative mineralogy cannot be applied to these data sets owing to uncertainties in oxygen abundance and lack of any constraints on the abundance of sodium. Matrix inversion for simultaneous solution of mineral abundances yields reasonable results for the x‐ray‐derived bulk composition, but seems to confirm the inconsistency between mineral compositions and orthopyroxene/clinopyroxene ratios. A unique solution does not seem possible in synthesizing these multiple data sets. Future missions including a lander to fully characterize regolith distribution and sample return would resolve the types of problems faced in synthesizing the NEAR data.

Disequilibrium partial melting experiments on the Leedey L6 chondrite: Textural controls on melting processes

Abstract— A series of experiments were designed to investigate the textural and compositional changes that take place during disequilibrium partial melting of chondritic material. Chips of the L6 chondrite, Leedey, were heated at 1200 °C and log ƒO2 = IW‐1 for durations of 1 h to 21 days. We observed a progression of kinetically‐controlled textural changes in melt and restite minerals and changes in the liquidus mineralogy in response to factors such as volatile loss. During the course of the experiments, both olivine and orthopyroxene recrystallized at different times. Rare relic chondrules could still be identified after 21 days. The silicate melts that form are very heterogeneous, in terms of both major and trace element chemistry, reflecting heterogeneity of the localized mineral assemblage, particularly with respect to phosphates and clinopyroxene. Metal‐sulfide melts formed in short‐duration runs are also heterogeneous. The experimental data are relevant to aspects of the genesis of primitive achondrites such as the acapulcoites. The observed textures are consistent with a model for acapulcoite petrogenesis in which silicate melting was limited to only a few volume percent of the chondritic source rock. The experiments are also relevant to the behavior of chondritic material that has been partially melted in an impact environment.

Oxygen isotopes and the early Solar System

Meteorites preserve a wide range of oxygen isotopic signatures from the time of the Solar System9s formation. Most of these rocks record complex histories, each phase of which has the potential for overwriting initial oxygen signatures. The unequilibrated ordinary chondrites reveal evidence of hydrothermal alteration through isotopic disequilibrium within chondrules and in secondary magnetites, which we can now constrain to temperatures of 140–180 ° C. The effects of this alteration are progressively obliterated by later thermal metamorphism. Further heating leads to melting (shown in achondritic meteorites), producing well–defined mass–fractionation lines using high–precision analyses. The oxygen from low–temperature minerals in carbonaceous chondrites reveals high levels of isotopic uniformity, suggesting that the aqueous alteration occurred under open–system conditions. The initial isotopic composition of the water from the ordinary chondrites is quite distinct from that in the carbonaceous chondrites, but both fall on a single line of slope 1.0, as do the initial anhydrous silicate compositions. This is taken to show that a process generating a mass–independent fractionation was responsible for most of the oxygen–isotopic variation seen in meteorites. Subsequent aqueous alteration of the meteorite parent bodies involving these components is then capable of producing the full observed variation.

The Meteoritical Bulletin, No. 85, 2001 September

Abstract— Meteoritical Bulletin No. 85 lists information for 1376 newly classified meteorites, comprising 658 from Antarctica, 409 from Africa, 265 from Asia (262 of which are from Oman), 31 from North America, 7 from South America, 3 from Australia, and 3 from Europe. Information is provided for 11 falls (Dergaon, Dunbogan, Gujba, Independence, Itqiy, Morávka, Oued el Hadjar, Sayama, Sologne, Valera, and Worden). Noteworthy non‐Antarctic specimens include 5 martian meteorites (Dar al Gani 876, Northwest Africa 480 and 817, and Sayh al Uhaymir 051 and 094); 6 lunar meteorites (Dhofar 081, 280, and 287, and Northwest Africa 479, 482, and 773); an ungrouped enstatite‐rich meteorite (Itqiy); a Bencubbin‐like meteorite (Gujba); 9 iron meteorites; and a wide variety of other interesting stony meteorites, including CH, CK, CM, CO, CR, CV, R, enstatite, and unequilibrated ordinary chondrites, primitive achondrites, HED achondrites, and ureilites.

The Meteoritical Bulletin, No. 84, 2000 August

Abstract— Meteoritical Bulletin No. 84 lists information for 1341 newly classified meteorites, comprising 842 from Antarctica, 341 from Africa, 66 from Australia, 48 from Asia (including 42 from the Arabian peninsula), 38 from North America, 4 from Europe, and 2 from South America. Information is provided for 11 recent falls (Bilanga, Devri‐Khera, Djoumine, Guangmingshan, Kitchener, Kobe, Leighlinbridge, Sabrum, Songyuan, Tagish Lake, and Vissannapeta); 4 martian meteorites (Dar al Gani 670/735, Dhofar 019, Los Angeles, and Sayh al Uhaymir 005/008); 3 lunar meteorites (Northwest Africa 032, Dhofar 025 and 026); an ungrouped enstatite‐rich meteorite (Zaklodzie); 11 iron meteorites; and a wide variety of other interesting stony meteorites, including CK, CM, CO, CR, CV, R, enstatite, and unequilibrated ordinary chondrites, primitive achondrites, HED achondrites, ureilites, and aubrites.

The pyroxene pallasites, Vermillion and Yamato 8451: Not quite a couple

Abstract— Two pallasites, Vermillion and Yamato (Y)‐8451, have been studied to obtain petrologic, trace element, and O‐isotopic data. Both meteorites contain low‐Ca and high‐Ca pyroxenes (<2% by volume) and have been dubbed “pyroxene pallasites.” Pyroxene occurs as large individual grains, as inclusions in olivine and in other pyroxene, and as grains along the edges of olivine. Symplectic overgrowths, sometimes found in Main Group and Eagle Station pallasites, are not seen in the pyroxene pallasites. Olivine compositions are Fa10–12, similar to those of Main Group pallasites. Siderophile trace element data show that metal in the two meteorites have significantly differing compositions that are, for many elements, outside the range of the Main Group and Eagle Station pallasites. These compositions also differ from those of IAB and IIIAB iron meteorites. Rare earth element (REE) patterns in merrillite are similar to those seen in other pallasites, indicating formation by subsolidus reaction between metal and silicate, with the merrillite inheriting its pattern from the surrounding silicates. The O‐isotopic compositions of Vermillion and Y‐8451 are similar but differ from Main Group or Eagle Station pallasites, as well as other achondrite and primitive achondrite groups. Although Vermillion and Y‐8451 have similar mineralogy, pyroxene compositions, REE patterns, and O‐isotopic compositions, there is sufficient evidence to resist formally grouping these two meteorites. This evidence includes the texture of Vermillion, siderophile trace element data, and the presence of cohenite in Vermillion.

Fe/Mg–Fe/Mn relations of meteorites and primary heterogeneity of primitive achondrite parent bodies

We examine Fe–Mg–Mn relations of meteorites, as represented on plots of molar Fe/Mg ratio vs. molar Fe/Mn ratio, and use them to consider whether it is meaningful to classify achondrites as “primitive” or “evolved.” First, we demonstrate that Fe/Mg–Fe/Mn plots can be used to identify some of the major processes that have affected meteorite parent bodies, during both the nebular (condensation and accretion) and the planetary (igneous) stages of their evolution. Examination of compositions of chondrites on this diagram reveals distinct patterns that result from three nebular processes: (1) volatility-controlled Mn–Mg fractionation; (2) metal/silicate fractionation under highly reducing conditions; and (3) subsequent oxidation leading to variations in FeO/MgO ratio. Ordinary chondrites (OCs) show a characteristic pattern that records all three of these processes and suggests that the latter two are coupled. Examination of the compositions of achondrites on this diagram shows that all of those that have been classified as primitive (lodranites/acapulcoites, winonaites/IAB silicate inclusions, brachinites, and ureilites) are easily distinguished from those that have been classified as evolved (SNCs, HEDs, and lunar basalts). The Mn/Mg ratios of the former are chondritic, while those of the latter are superchondritic; hence, they plot in two graphically distinct regions of the Fe/Mg–Fe/Mn diagram. A simple melting calculation reveals that this distinction results principally because the former are residues and the latter are melts or cumulate products of melts, rather than because the latter are more evolved. Additionally, however, some of the groups classified as primitive achondrites show Fe/Mg–Fe/Mn patterns similar to that of OCs—constant (but distinct) MnO/MgO ratios over large ranges of FeO/MgO ratio. We argue that these trends reflect primary heterogeneity, resulting from the same nebular processes as those that affected OCs, and therefore preserve evidence that more of the materials that accreted to form planetesimals experienced these processes than is apparent from known chondrites. This evidence is preserved principally because these meteorites are residues. In contrast, groups commonly classified as evolved achondrites show Fe/Mg–Fe/Mn patterns that reflect only igneous processing, and any evidence for primary heterogeneity of their parent bodies has been obliterated. Again, this is principally because these meteorites represent melts, rather than beause they are more evolved. Usage of the terms primitive and evolved to describe achondrites should be clearly defined.

Fe, Mg, Mn-bearing phosphates in the GRA 95209 meteorite; occurrences and mineral chemistry

Two large regions of Fe, Mg, Mn-bearing phosphates, together with metal and various silicate phases, have been observed in the GRA 95209 meteorite (a primitive achondrite belonging to the acapulcoite-lodranite group). Numerous grains of an orthophosphate intermediate in composition between farringtonite and graftonite/sarcopside are found in association with Fe- and Mn-rich chladniite. The chladniite occurs as rims around the Ca-free phosphate (termed Mg-graftonite here) and along boundaries between Fe, Ni metal and silicate grains. Relict metal grains are included within both phosphates. Textures, as well as mineral compositions, suggest formation of the Mg-graftonite by reaction of Fe, Ni metal with surrounding silicates (olivine and orthopyroxene). Chladniite probably formed both through replacement of Mg-graftonite and directly by reaction of metal with silicates, including plagioclase. Apatite appears to have played only a minor role in the formation of these phosphates.

Are ureilites residues from partial melting of chondritic material? The answer from MAGPOX

Abstract— Calculations performed using MAGPOX show that no bulk compositions having chondritic Ca/Al ratio and within the range of chondritic Si/Mg ratios can produce the olivine‐pigeonite ureilites (which constitute 65% of those for which modal abundances are known) as residues of single‐stage equilibrium partial melting. Calcium/aluminum ratios of 2–3.5 × CI are required. In addition, all the ureilites could not have formed from a single composition at various degrees of reduction, because they show no correlation between pigeonite/olivine ratio and mg ratio. Materials with various Al/Mg ratios, ranging from subchondritic to superchondritic, are required. If these materials are primitive (i.e., created by nebular processes rather than planetary igneous processes), they are unknown in the meteorite record. Excess accretion (relative to chondrites) of 5–10 mol% of a high‐temperature condensate component, which was itself almost completely depleted in corundum due to early fractionation, could create the necessary compositions. The plausibility that such processes occurred on a parent‐body sized scale is difficult to assess. In contrast, lodranites can be produced as residues of ∼3–30% equilibrium partial melting of an average ordinary chondritic composition at the appropriate level of reduction. Although many features of ureilites suggest that they are relatively primitive residues produced by low degrees of melting of chondritic materials, and thus resemble lodranites and other groups of primitive achondrites, their predominantly pigeonite + olivine mineralogy remains difficult to explain within this simple scenario.

A petrologic and isotopic study of winonaites: evidence for early partial melting, brecciation, and metamorphism

We have conducted detailed petrologic, chemical, and isotopic studies of winonaites to ascertain the genesis of this group of meteorites. Winonaites have reduced mineral compositions and mineralogy, and oxygen isotopic compositions distinct from primitive achondrite groups other than silicate inclusions in IAB and IIICD irons. However, winonaites differ from IAB and IIICD irons in that they lack the metallic matrices of the latter and consist mostly of silicates. On the basis of these criteria, Winona, Mount Morris (Wisconsin), Tierra Blanca, Pontlyfni, Y-74025, Y-75300, Y-8005, and QUE 94535 are winonaites and Y-75305 and Y-75261 may be winonaites. Winonaites are fine- to medium-grained, mostly equigranular rocks. Pontlyfni and Mount Morris (Wisconsin) contain what appear to be relict chondrules. Several winonaites contain mm-sized areas that differ substantially in grain size and/or silicate mineralogy from the surrounding matrix. Fe,Ni-FeS veins are common in many winonaites. Mineral compositions are intermediate between E and H chondrites, and reduced sulfides are observed in low-FeO winonaites. Bulk major element compositions are roughly chondritic, although REE elements are fractionated. The 39Ar- 40Ar ages of three winonaites range from ≥4.40 Ga (Mount Morris, Wisconsin) to 4.54 Ga (Pontlyfni). Cosmic-ray exposure ages are ∼20–80 Ma. Trapped noble gases in these winonaites resemble those in enstatite chondrites. We suggest that the winonaites formed from a chondritic precursor material unlike that of known chondrites in mineral and oxygen isotopic compositions, and this material may have been heterogeneous in composition. Extensive heating caused metamorphism and partial melting of both Fe,Ni-FeS and silicate material. Impact brecciation during cooling mixed lithologies with different thermal histories, and subsequent metamorphism produced recrystallization, grain growth, and reduction of mafic silicates. The 39Ar- 40Ar ages indicate that cooling may have been more rapid than observed in IAB irons, although later resetting may have occurred.

Mineralogy of meteorite groups

Abstract— Approximately 275 mineral species have been identified in meteorites, reflecting diverse redox environments, and, in some cases, unusual nebular formation conditions. Anhydrous ordinary, carbonaceous and R chondrites contain major olivine, pyroxene and plagioclase; major opaque phases include metallic Fe‐Ni, troilite and chromite. Primitive achondrites are mineralogically similar. The highly reduced enstatite chondrites and achondrites contain major enstatite, plagioclase, free silica and kamacite as well as nitrides, a silicide and Ca‐, Mg‐, Mn‐, Na‐, Cr‐, K‐ and Ti‐rich sulfides. Aqueously altered carbonaceous chondrites contain major amounts of hydrous phyllosilicates, complex organic compounds, magnetite, various sulfates and sulfides, and carbonates.In addition to kamacite and taenite, iron meteorites contain carbides, elemental C, nitrides, phosphates, phosphides, chromite and sulfides. Silicate inclusions in IAB/IIICD and IIE iron meteorites consist of mafic silicates, plagioclase and various sulfides, oxides and phosphates. Eucrites, howardites and diogenites have basaltic to orthopyroxenitic compositions and consist of major pyroxene and calcic plagioclase and several accessory oxides. Ureilites are made up mainly of calcic, chromian olivine and low‐Ca clinopyroxene embedded in a carbonaceous matrix; accessory phases include the C polymorphs graphite, diamond, lonsdaleite and chaoite as well as metallic Fe‐Ni, troilite and halides. Angrites are achondrites rich in fassaitic pyroxene (i.e., Al‐Ti diopside); minor olivine with included magnesian kirschsteinite is also present. Martian meteorites comprise basalts, lherzolites, a dunite and an orthopyroxenite. Major phases include various pyroxenes and olivine; minor to accessory phases include various sulfides, magnetite, chromite and Ca‐phosphates. Lunar meteorites comprise mare basalts with major augite and calcic plagioclase and anorthositic breccias with major calcic plagioclase.Several meteoritic phases were formed by shock metamorphism. Martensite (α2‐Fe,Ni) has a distorted body‐centered‐cubic structure and formed by a shear transformation from taenite during shock reheating and rapid cooling. The C polymorphs diamond, lonsdaleite and chaoite formed by shock from graphite. Suessite formed in the North Haig ureilite by reduction of Fe and Si (possibly from olivine) via reaction with carbonaceous matrix material. Ringwoodite, the spinel form of (Mg,Fe)2SiO4, and majorite, a polymorph of (Mg,Fe)SiO3 with the garnet structure, formed inside shock veins in highly shocked ordinary chondrites.Secondary minerals in meteorite finds that formed during terrestrial weathering include oxides and hydroxides formed directly from metallic Fe‐Ni by oxidation, phosphates formed by the alteration of schreibersite, and sulfates formed by alteration of troilite.