Achondrite Groups Research Articles

Asteroids accretion, differentiation, and break-up in the Vesta source region: Evidence from cosmochemistry of mesosiderites

The cosmochemistry of meteorites provides unique clues on asteroids accretion, differentiation, collisional break-up and reassembly - processes of critical importance for understanding planet formation in the early solar system. Mesosiderites are a complex group of achondrites whose nearly 50:50 metal-silicate composition is interpreted in the literature as resulting from the mixing of core and crustal materials derived from differentiated asteroids. Because of their complex nature and contrasting geochemical, isotopic, and spectroscopic data, the formation mechanism of mesosiderites is still poorly understood and is open to a large variety of planetary differentiation scenarios and collisional histories. In this study, based on new petrographic and geochemical data of 16 mesosiderites, we investigate in detail the proposal that mesosiderites are related to the howardite-eucrite-diogenite (HED) meteorite group, whose parent body is widely considered to be asteroid 4 Vesta (∼500 km diameter), the target of the recent NASA’s Dawn mission. We present the first high precision oxygen isotope analyses on the matrix of a set of mesosiderite samples, coupled with new chemical and petrographic analyses of mesosiderites Um Hadid, Estherville, and Mount Padbury. Concordant Δ17O values between mesosiderites (–0.241 ± 0.015 (2σ)) and howardite-eucrite-diogenites (−0.241 ± 0.017‰ (2σ)) indicate that they derived from the same oxygen isotope reservoir, but petrological evidence, in particular the distinctly lower Fe/Mn ratios and the larger lithological diversity in mesosiderites, indicates that they formed within different parent bodies. This suggests that mesosiderites and howardite-eucrite-diogenites originated in distinct parent bodies that accreted in the 4 Vesta source region but experienced different geologic evolution in terms of crustal differentiation and impact history, which were more complex and catastrophic in the mesosiderite parent body.

Parentage Identification of Differentiated Achondritic Meteorites by Hand‐held Energy Dispersive X‐Ray Fluorescence Spectrometry

We evaluate the potential of a hand‐held energy dispersive XRF spectrometer for the preliminary classification of non‐chondritic differentiated meteorites. The studied achondrites include nine lunar meteorites, seventeen Martian meteorites, five angrites and eighteen meteorites from asteroid 4 Vesta. Analytical precision and accuracy was tested on thirty‐nine terrestrial igneous rock slabs with a wide range of composition. Replicate analyses, performed on the studied meteorites, show that Fe/Mn values together with Si and Ca/K ratio can be used in the discrimination of different achondrite groups. Fusion crust's Fe/Mn values of meteorites from Vesta and Mars are indistinguishable from those of the interior implying that even measurements on the fusion‐crusted external surface could be sufficient to pigeonhole non‐chondritic meteorites. Hand‐held energy dispersive XRF spectrometer is a non‐destructive but very effective technique for preliminary classification of achondrites in the field and in laboratory and for the identification of mislabelled meteorites in museum collections.

Redox variations in the inner solar system with new constraints from vanadium XANES in spinels

![][1] Many igneous rocks contain mineral assemblages that are not appropriate for application of common mineral equilibria or oxybarometers to estimate oxygen fugacity. Spinel-structured oxides, common minerals in many igneous rocks, typically contain sufficient V for XANES measurements, allowing use of the correlation between oxygen fugacity and V K pre-edge peak intensity. Here we report V pre-edge peak intensities for a wide range of spinels from source rocks ranging from terrestrial basalt to achondrites to oxidized chondrites. The XANES measurements are used to calculate oxygen fugacity from experimentally produced spinels of known ![Formula][2] . We obtain values, in order of increasing ![Formula][3] , from IW-3 for lodranites and acapulcoites, to diogenites, brachinites (near IW), ALH 84001, terrestrial basalt, hornblende-bearing R chondrite LAP 04840 (IW+1.6), and finally ranging up to IW+3.1 for CK chondrites (where the ![Formula][4] of a sample relative to the ![Formula][5] of the IW buffer at specific T ). To place the significance of these new measurements into context we then review the range of oxygen fugacities recorded in major achondrite groups, chondritic and primitive materials, and planetary materials. This range extends from IW-8 to IW+2. Several chondrite groups associated with aqueous alteration exhibit values that are slightly higher than this range, suggesting that water and oxidation may be linked. The range in planetary materials is even wider than that defined by meteorite groups. Earth and Mars exhibit values higher than IW+2, due to a critical role played by pressure. Pressure allows dissolution of volatiles into magmas, which can later cause oxidation or reduction during fractionation, cooling, and degassing. Fluid mobility, either in the sub-arc mantle and crust, or in regions of metasomatism, can generate values >IW+2, again suggesting an important link between water and oxidation. At the very least, Earth exhibits a higher range of oxidation than other planets and astromaterials due to the presence of an O-rich atmosphere, liquid water, and hydrated interior. New analytical techniques and sample suites will revolutionize our understanding of oxygen fugacity variation in the inner solar system, and the origin of our solar system in general. [1]: /embed/graphic-1.gif [2]: /embed/mml-math-1.gif [3]: /embed/mml-math-2.gif [4]: /embed/mml-math-3.gif [5]: /embed/mml-math-4.gif

Cosmochemical and spectroscopic properties of Northwest Africa 7325—A consortium study

AbstractThis work is part of a project to build an infrared database in order to link IR data of planetary materials (and therefore possible Mercury material) with remote sensing observations of Mercury, which will probably be obtained by the MERTIS instrument on the forthcoming BepiColombo mission.The unique achondrite Northwest Africa (NWA) 7325, which has previously been suggested to represent the first sample from Mercury, was investigated by optical and electron microscopy, and infrared and Raman spectroscopy. In addition, the oxygen, strontium, xenon, and argon isotopes were measured and the abundance of selected trace elements determined. The meteorite is a cumulate rock with subchondritic abundances of HFSE and REE and elevated Sr contents, which underwent a second heating and partial remelting process. Oxygen isotope measurements show that NWA 7325 plots in the ureilite field, close to the ALM‐A trachyandesitic fragment found in the unique Almahata Sitta meteorite breccia. On the other hand, mineralogical investigations of the pyroxenes in NWA 7325 provide evidence for similarities to the lodranites and acapulcoites. Furthermore, the rock is weakly shocked and argon isotope data record ancient (~4.5 Ga) plateau ages that have not been reset. The sample records a cosmogenic exposure age of ~19 Ma. Systematics of Rb‐Sr indicate an extreme early volatile depletion of the precursor material, similar to many other achondrite groups. However, despite its compositional similarities to other meteorite groups, our results suggest that this meteorite is unique and unrelated to any other known achondrite group. An origin for NWA 7325 as a sample from the planet Mercury is not supported by the results of our investigation. In particular, the evidence from infrared spectroscopy indicates that a direct relationship between NWA 7325 and the planet Mercury can be ruled out: no acceptable spectral match between laboratory analyses and remote sensing observations from Mercury has been obtained. However, we demonstrate that infrared spectroscopy is a rapid and nondestructive method to characterize mineral phases and thus an excellent tool for planetary surface characterization in space missions.

147Sm‐143Nd and 176Lu‐176Hf systematics of eucrite and angrite meteorites

AbstractComparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present 147Sm‐143Nd and 176Lu‐176Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a 147Sm‐143Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous 147Sm‐143Nd and 176Lu‐176Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.

Near infrared spectroscopy of HED meteorites: Effects of viewing geometry and compositional variations

The howardite, eucrite and diogenite (HED) meteorites are genetically related and represent the most voluminous group of achondrites. They are the closest analog materials to Vesta and V-type asteroids. Many of these meteorites were the focus of intense petrologic and visible to near infrared spectral studies. As ground-based and orbital observations of basaltic asteroids have increased, an improved understanding of HEDs is needed. For this study, we investigated 24 HED samples, mainly new finds from Northwest Africa (NWA). Visible to near infrared (up to 2.5μm) spectral measurements under varying illumination and observation geometries were acquired for 4 samples. Phase reddening and bluing (i.e., increase and decrease in spectral slope) is observed for the visible slope as phase angle increase. Monotonic phase reddening can occur for the near infrared slope as phase angle increase. Non-systematic changes with phase angle are found for the Band Area Ratio parameter. At phase angles higher than ∼60°, the decrease of reflectance and decrease of pyroxene bands depth are undistinguishable from admixture of low albedo material to HED samples. To assess the precision of empirical equations relating spectral properties and composition, the pyroxenes, feldspar, and olivine chemistry of the samples was determined. Using previous calibrations, systematic overestimations of the ferrosilite (Fs) and wollastonite (Wo) contents are found, especially in the 15–40 Fs range. To overcome such discrepancies, a new set of empirical equations is proposed. For an application of the new calibration, we selected two compositional end-member areas on Vesta on the basis of their iron content. For the iron-poor terrain we found an average pyroxene composition of Fs30Wo5 and for the iron-rich terrain an average of Fs47Wo14.

Highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body

Abstract– Aubrites exhibit a wide range of highly siderophile element (HSE—Re, Os, Ir, Ru, Rh, Pt, Pd, Au) concentrations and 187Os/188Os compositions. Their HSE concentrations are one to three orders of magnitude less than chondrites, with the exception of the Shallowater and Mt. Egerton samples. While most aubrites show chondritic HSE abundance ratios, significant enrichments of Pd and Re relative to Os, Ir, and Ru are observed in 12 of 16 samples. Present‐day 187Os/188Os ratios range from subchondritic values of 0.1174 to superchondritic values of up to 0.2263. Half of the samples have 187Os/188Os ratios of 0.127 to 0.130, which is in the range of enstatite chondrites. Along with the brecciated nature of aubrites, the HSE and Re‐Os isotope systematics support a history of extensive postaccretion processing, including core formation, late addition of chondritic material and/or core material and potential breakup and reassembly. Highly siderophile element signatures for some aubrites are consistent with a mixing of HSE‐rich chondritic fragments with a HSE‐free aubrite matrix. The enrichments in incompatible HSE such as Pd and Re observed in some aubrites, reminiscent of terrestrial basalts, suggest an extensive magmatic and impact history, which is supported by both the 187Re‐187Os isotope system and silicate‐hosted isotope systems (Rb‐Sr, K‐Ar) yielding young formation ages of 1.3–3.9 Ga for a subset of samples. Compared with other differentiated achondrites derived from small planetary bodies, aubrites show a wide range in HSE concentrations and 187Os/188Os, most similar to angrites. While similarities exist between the diverse groups of achondrites formed early in solar system history, the aubrite parent body(ies) clearly underwent a distinct evolution, different from angrites, brachinites, ureilites, howardites, eucrites, and diogenites.

Iron isotope fractionation in planetary crusts

We present new high precision iron isotope data (δ56Fe vs. IRMM-014 in per mil) for four groups of achondrites: one lunar meteorite, 11 martian meteorites, 32 howardite–eucrite–diogenite meteorites (HEDs), and eight angrites. Angrite meteorites are the only planetary materials, other than Earth/Moon system, significantly enriched in the heavy isotopes of Fe compared to chondrites (by an average of +0.12‰ in δ56Fe). While the reason for such fractionation is not completely understood, it might be related to isotopic fractionation by volatilization during accretion or more likely magmatic differentiation in the angrite parent-body. We also report precise data on martian and HED meteorites, yielding an average δ56Fe of 0.00±0.01‰. Stannern-trend eucrites are isotopically heavier by +0.05‰ in δ56Fe than other eucrites. We show that this difference can be ascribed to the enrichment of heavy iron isotopes in ilmenite during igneous differentiation. Preferential dissolution of isotopically heavy ilmenite during remelting of eucritic crust could have generated the heavy iron isotope composition of Stannern-trend eucrites. This supports the view that Stannern-trend eucrites are derived from main-group eucrite source magma by assimilation of previously formed asteroidal crust.These new results show that iron isotopes are not only fractionated in terrestrial and lunar basalts, but also in two other differentiated planetary crusts. We suggest that igneous processes might be responsible for the iron isotope variations documented in planetary crusts.

Old Sm–Nd ages for cumulate eucrites and redetermination of the solar system initial 146Sm/ 144Sm ratio

Short-lived 146Sm– 142Nd and long-lived 147Sm– 143Nd chronometers have been measured in three cumulate eucrites (Binda, Moore County and Moama). The two major mineral phases (plagioclase and pyroxene) present in these achondrites are characterized by a wide range of Sm/Nd ratios that allows well-resolved Sm–Nd isochrons. This group of meteorites thus is suitable to better constrain the initial 146Sm/ 144Sm ratio of the solar system. Binda and Moore County give concordant ages of 4544 ± 88 and 4542 ± 85 Ma, respectively, with initial 143Nd/ 144Nd ratios slightly higher, to within error, of chondritic. These ages are in agreement with most of the radiometric ages determined on basaltic eucrites. A best estimate for the solar system initial 146Sm/ 144Sm ratio is obtained using the five-point regression line determined for Binda. The 146Sm/ 144Sm ratio of 0.00728 ± 57 obtained for this sample translates to a 146Sm/ 144Sm ratio at 4568 Ga of 0.0085 considering the age of isotopic closure obtained from 147Sm– 143Nd systematics. When 146Sm– 142Nd data from the literature are examined in detail, four eucrites have concordant 147Sm– 143Nd and 146Sm– 142Nd systematics. Their weighted average 147Sm– 143Nd age is equal to 4546 ± 8 Ma. An initial 146Sm/ 144Sm ratio at 4568 Ma calculated from these samples is 0.0084 ± 0.0005. A similar ratio of 0.0085 ± 0.0007 is calculated if data from different groups of achondrites (angrite and mesosiderite) are included in the calculation. No difference in the 146Sm/ 144Sm ratios or initial 142Nd/ 144Nd ratios is observed among different groups of achondrites relative to ordinary chondrites. This work suggests that 146Sm was homogeneously distributed and that both Sm and Nd were isotopically uniform at the planetary scale in the solar system, at least in the region around where these planetary bodies formed.

Appendix: Meteorites - A Brief Tutorial

There are four broad categories of meteorites—chondrites, achondrites, irons and stony irons. These are subdivided into meteorite groups, the basic unit of meteorite classification. Although no formal guideline is in place for the minimum number of meteorites needed to define a group, common practice is that there should be five or more members in a group. A defined meteorite group is thought to be derived from a single asteroid. However, some groups are genetically related and are derived from a common parent asteroid. Chondrites are primitive stony meteorites; rocks whose compositions are little changed since their formation in the solar nebula. There are fourteen defined groups of chondrites, and they make up the vast majority of meteorites falling to Earth in the current epoch. Achondrites are stony meteorites of two broad types. Some are primitive materials like chondrites, but most are the products of igneous differentiation. There are ten defined groups of achondrites, of which seven are differentiated types. Irons are also the products of asteroidal differentiation, having crystallized from metallic melts separated from chondritic precursors. There are thirteen defined groups of iron meteorites. Stony-irons are also differentiated materials, and both the rocky and metallic phases were formed by igneous processes on asteroids. There are two defined groups of stony-iron meteorites. In addition to those that fit into groups, there are many meteorites that are unique, or for which there are less than five examples. These ungrouped meteorites make up a substantial fraction of meteorites recovered to date.

Petrogenesis of angrites

The recent discovery of two new angrites, Sahara 99555 and D'Orbigny, has revived interest in this small group of achondrites. We measured trace element abundances in the individual minerals of these two angrites and compared them with the three Antarctic angrites, LEW 86010, LEW 87051 and Asuka 881371. Trace element variations in four of these meteorites (LEW 87051, Asuka 881371, Sahara 99555 and D'Orbigny) indicate rapid crystallization under near closed system conditions, consistent with their mineralogical and textural features. All four appear to be closely related and crystallized from very similar magmas. Discrepancies between their bulk REE compositions and melts calculated to be in equilibrium with the major phases may be due in part to kinetic effects of rapid crystallization. Prior crystallization of olivine and/or plagioclase may also account for the elevated parent melt composition of clinopyroxene in some of the angrites such as Asuka 881371. LEW 86010 also crystallized from a melt and represents a liquid composition, but trace element trends in clinopyroxene and olivine differ from those of the other angrites. This meteorite seems to have crystallized from a different source magma.

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.

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.

Comparisons between Antarctic and non-Antarctic meteorites based on carbon isotope geochemistry

The whole-rock C isotopic composition of a suite of ordinary chondrites indicates that there is a difference between Antarctic and non-Antarctic meteorites. However, this apparent distinction is not due to an inherent difference in the meteorite source populations, but is rather a result of the presence of Antarctic weathering products, mainly bicarbonates, which affect the overall δ 13 C of these meteorites. When these weathering products are removed, either by acid-washing or combustion to 500°C, the perceived division also disappears, leading to the conclusion that ordinary chondrites (both Antarctic and non-Antarctic) have similar δ 13 C values. A similar effect is noted in data from three separate groups of achondrites: the ureilites, HEDs, and SNCs. However, here the weathering products do not influence total C data. In the case of the ureilites, this is readily understood, since the indigenous C content is high. The HEDs and SNCs, on the other hand, with low indigenous C abundance, contain relatively lower concentrations of weathering products compared to ordinary chondrites, which can be removed by combustion to 200°C. This suggests a different complement of weathering products in the achondritic and chondritic meteorites, an effect which may be related to differences in chemistry or porosity. The almost ubiquitous occurrence of weathering products with distinctive C isotopic composition highlights the importance of removing all terrestrial contaminants from meteorites with low indigenous C budgets before attempting any interpretation concerning their C abundance or isotopic composition. The C chemistry of CI, CM, and CR chondrites is also subject to problematic interpretation. Whilst the indigenous C abundance is too high to be compromised by the addition of terrestrial weathering products, the inhomogeneous distribution of an array of components with widely varying isotopic signatures leads to a random distribution of δ 13 C values. Analyses of individual components (insoluble macromolecular material, carbonates, and silicon carbide) reveal that there are no gross differences in C chemistry between Antarctic and non-Antarctic chondrites. The few distinctions that do exist might readily be explained by weathering or isotopic exchange in Antarctic. Whilst not forming a separate population, Antarctic carbonaceous chondrites may extend the range of properties of non-Antarctic chondrites beyond the currently known limits.

Formation of ureilites by nebular processes

Oxygen isotopic data are presented for 23 ureilites, including 14 Antarctic meteorites. They show a much greater range in isotopic composition than those previously studied. Most noteworthy is the fact that, unlike other achondrite groups, the ureilites do not fall on a mass-dependent isotopic fractionation trend, but rather fall along an extrapolation of the 16O-mixing line defined by components of Allende and other C3 chondrites. Correlations between isotopic and chemical data imply that the isotopic and chemical variations are due primarily to processes in the solar nebula and not to igneous processes within a parent body. The ureilite parent body accreted with isotopic and chemical heterogeneities which were not much modined by subsequent parent-body processes. The precursor materials were related in some way to the C3 carbonaceous chondrites.

Mineralogy of Yamato 791073 with reference to crystal fractionation of the Howardite parent body

Shock textures and mineral chemistry indicate that Yamato 791073 is a type of meteorite intermediate between diogenite and cumulate eucrite and is similar to the Yamato 75032‐type achondrites. The presence of vesicles in shock melt glass and swirly glasses of plagioclase composition as in the suevite in the matrix indicates that it is the most intensely shocked sample among this group of achondrites. Pyroxene compositions in Y791073 fall in a limited range from Mg/(Mg + Fe) = 0.7 to 0.6 and are compatible with the continuum of rock types produced presumably by differentiation on the howardite parent body. The chemical variation of pyroxene is wider than that of the monomict Y75032 group. In addition to Fe‐rich diogenite‐like pyroxene (Ca2Mg66Fe31) common in Y75032, Y791073 contains clasts of pyroxene texturally like those in Moore County. The plagioclase glass with An89 may originally have been associated with this pyroxene as a cumulate eucrite. Such clasts are uncommon in the Y75032‐type achondrites, Y791073 also contains a very differentiated Si‐ and K‐rich clast. Y791073 shows a more polymict nature than other Y75032‐type achondrites, but the lithic variation more limited than normal howardites suggests that it sampled two or three layers of the layered crust or pluton produced by the crystal fractionation.

The origin of SNC meteorites: An alternative to Mars

Shergottites, Nakhlites, and Chassignites (SNC) are a small group of achondrites with crystallization ages of approximately 1.3 AE. Although it has recently been postulated the these meteorites came from Mars, the dynamical difficulties of ejecting large meteorites from a major planet have caused us to examine the alternative possibility that they crystallized from an impact melt formed on a large asteroid. The kinetic energy necessary to produce a crater of a given size is estimated; it is postulated that 25% of this energy is partitioned into heat, and the heat is distributed in this model in a pattern suggested by the impact melt distribution in Brent Crater and the radioactivity distribution in Cactus nucelear explosion crater. The time evolution of the temperature by heat conduction for several locations around the crater is computed. Crystallization times for the more deeply buried impact melts are form 5 × 10 4 years for 60-km-diameter craters and increase for larger craters. These times are long enough for the observed cumulate textures to develop. Once solidified, these rocks may be ejected from the asteroid by subsequent cratering events. Since asteroidal escape velocities are low, ejection may be accomplished by shock pressures too low to produce petrologically detectable shock features. The SNC meteorites could thus have originated in the asteroid belt, their young crystallization ages being due to melting induced by impacts occurring on asteroids long after condensation from the solar nebula. This scenario avoids the dynamical difficulties of a major planet origin, but raises questions of how the SNC's acquired their chemical and REE characteristics. To date, there seems to be no internally consistent model for the origin of these strange meteorites. The impact melt hypothesis is offered as a rational alternative to a Martian origin. Neither hypothesis explains all the problems.

Petrology and origin of the shergottite meteorites

Shergottites contain cumulus pigeonite and augite, probably without cumulus plagioclase and crystallized under relatively oxidizing conditions. Shergotty and Zagami may differ in the relative proportions of cumulus pyroxenes and crystallized intercumulus liquid, but the compositions of pyroxenes and liquid are similar in both meteorites. Absence of olivine in melting experiments suggests that the shergottites crystallized from fractionated derivatives of primary liquids. Low-Ca pyroxene and augite apparently began to crystallize from these primary liquids prior to plagioclase. Shergottites can be readily distinguished from other achondrite groups by their mineralogies, crystallization sequences and inferred source region compositions. However, the source regions of the shergottites may be related to those of other achondrite types by addition or loss of volatile components. The bulk composition of the Earth's upper mantle overlaps that of permissible shergottite source regions. Shergottites and terrestrial basalts display similarities in oxidation state and concentrations of trace and minor elements with a wide range of cosmochemical and geochemical affinities. Accretion of similar materials to produce the terrestrial upper mantle and the shergottite parent body or accretion of the Earth's upper mantle from planetesimals similar to the shergottite parent body may account for many of their similarities. Models of the origin of the Earth's upper mantle which attribute its oxidation state, its siderophile element abundances and its volatile element abundances to uniquely terrestrial processes or conditions, or to factors unique to the origin and differentiation of large bodies, are unattractive in light of the similarities between shergottites and terrestrial basalts.

THE MOAMA EUCRITE — A PYROXENE‐PLAGIOCLASE ADCUMULATE

A non‐brecciated, relatively coarsely crystalline eucrite was found in 1940 near Moama, southern New South Wales, Australia. The texture and mineralogy (plagioclase 49.1 volume %, pyroxene 48.9%, tridymite 0.72%, chromite 1.0%, troilite 0.24% and metallic iron 0.14%) are characteristic of an adcumulate formed in a terrestrial layered igneous intrusion. Its composition and texture most closely resemble the eucrite Serra de Magé. Moama and Serra de Magé are the best approximations to the pyroxeneplagioclase cumulates which McCarthy et al. (1973) propose have separated from a primary magma to produce the eucrite group of achondrites. Primary (coarse) and secondary (fine) augitic exsolution lamellae in the host hypersthene grains indicate a cooling history for the Moama eucrite similar to that of mafic rocks in slowly cooled, large, layered terrestrial intrusions such as Bushveld.

Iron-magnesium-aluminum relationships in achondrites

The atomic proportions of total iron, magnesium and aluminum of 41 analyses of achondrites taken from the literature were plotted on a ternary variation diagram. The four main achondrite groups, eucrites, howardites, diogenites and aubrites, are readily distinguished without knowledge of the oxidation states of iron. Aluminum closely follows calcium in almost all cases. The extensive fractionation of an aubrite magma, as suggested by Moore (1962), cannot be substantiated but the achondrite Shallowater may represent a link between the diogenites and the aubrites.