Origin of ureilites inferred from a SIMS oxygen isotopic and trace element study of clasts in the Dar al Gani 319 polymict ureilite

Secondary ion mass spectrometer (SIMS) oxygen isotope analyses were performed on 24 clasts, representing 9 clast types, in the Dar al Gani (DaG) 319 polymict ureilite with precisions better than 1‰. Olivine-rich clasts with typical ureilitic textures and mineral compositions have oxygen isotopic compositions that are identical to those of the monomict ureilites and plot along the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Other igneous clasts, including plagioclase-bearing clasts, also plot along the CCAM line, indicating that they were derived from the ureilite parent body (UPB). Thus, we suggest that some of the plagioclase-bearing clasts in the polymict ureilites represent the “missing basaltic component” produced by partial melting on the UPB.Trace element concentrations (Mg, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr and Ba) in ureilitic plagioclase and glass from 13 clasts were obtained by using the SIMS high mass resolution method. The trace element contents of the plagioclase generally show monotonic variations with anorthite content (mol%) that are consistent with partial melting and fractional crystallization. Incompatible trace element concentrations (K, Ti, and Ba) are low and variable for plagioclase with An > 40, indicating that the parental magmas for some clasts were derived from a depleted source. We performed partial melt modeling for CI and CM precursor compositions and compared the results to the observed trace element (K, Ba, and Sr) abundances in the plagioclase. Our results indicate that (1) the UPB evolved from a alkali-rich carbonaceous chondritic precursor, (2) parent melts of porphyritic clasts could have formed by 5-20% equilibrium partial melting and subsequent fractional crystallization, and (3) parent melts of the incompatible trace element-depleted clasts could be derived from fractional melting, where low degree (<10%) partial melts were repeatedly extracted from their solid sources.Thus, both the oxygen isotopic and trace element compositions of the plagioclase bearing clasts in DaG-319 suggest that the UPB underwent localized low degree-partial melting events. The partial melts could have been repeatedly extracted from the precursor, resulting in the formation of the olivine-pigeonite monomict ureilites as the final residue.     

Ureilitic breccias: clues to the petrologic structure and impact disruption of the ureilite parent asteroid

The majority of the 143 ureilite meteorites are monomict (unbrecciated) ultramafic rocks, which represent the mantle (olivine+low-Ca pyroxene residues and less abundant cumulates) of a partially melted (25–30%), carbon-rich asteroid 125 km in radius. Accumulated petrologic and geochemical studies of these meteorites have led to a picture of a ureilite parent body (UPB) that was stratified in mg#, pyroxene abundance and pyroxene type, due to the pressure dependence of carbon redox control, and which preserved a pre-magmatic heterogeneity in Δ¹⁷O. The absence, however, of ureilitic crustal rocks (i.e. basalts) in the meteorite record, leads to significant gaps in our knowledge of the geologic history of the UPB.

Ureilitic breccias provide considerable information that cannot be obtained from the monomict samples, and help to fill in those gaps. Fourteen ureilites are polymict breccias (at least three of which contain solar wind gases) that formed in a regolith. They contain a variety of clast types representing indigenous ureilitic lithologies not known among the monomict samples, as well as several types of non-indigenous impactor materials. In addition, one ureilite (FRO 93008) is a dimict breccia, consisting of two ultramafic lithologies that could not have formed in close proximity on the UPB.

Several feldspathic lithologies representing melts complementary to the monomict ureilite residues or cumulates have been recognized in polymict ureilites. From these lithologies we infer that melt extraction on the UPB was a rapid, fractional process in which trace element and oxygen isotopic equilibrium was not achieved. The majority of melts that reached the surface erupted explosively (due to high contents of CO/CO₂) and were lost into space. Thus, it is likely that the UPB never had an extensive basaltic crust. Melts generated at the shallowest depths and late fractionates, in which carbon had largely been consumed by reduction, were the most likely to have been preserved. Our sample of the UPB is limited to depths equivalent to 100 bars pressure or less, but minor augite-bearing feldspathic lithologies and related cumulates may represent melts derived from deeper.

In addition, we infer that the UPB was catastrophically disrupted, while still hot, by an impacting projectile. Meter-sized ejecta from this impact reaccreted into one or more daughter bodies, on which the brecciated ureilites formed. Ureilite meteorites are derived from these offspring, rather than from the UPB. The remnant of the original UPB may consist largely of olivine plus augite, and thus not resemble the majority of ureilites.     

The origin of iron silicides in ureilite meteorites

Ureilite meteorites contain iron silicide minerals including suessite (Fe,Ni)₃Si, hapkeite (Fe₂Si) and xifengite (Fe₅Si₃). Despite occurring mostly in brecciated varieties presumed to be derived from the regolith of the ureilite parent asteroid, suessite has also been confirmed in one lithology of a dimict ureilite (NWA 1241). In contrast, Si-bearing Fe-metals occur in both brecciated and unbrecciated ureilites, implying that they were formed throughout the ureilite parent asteroid. We examined major, minor and trace element data of Fe-metals in seven brecciated ureilites (DaG 319, DaG 999, DaG 1000, DaG 1023, DaG 1047, EET 83309, and EET 87720) in addition to the dimict ureilite NWA 1241.In this study we show that the silicides and Si-bearing metals in ureilites have similar siderophile trace element patterns; therefore, the precursors to the silicides were indigenous to the ureilite parent body. Si-free kamacite grains in brecciated ureilites show flatter, more chondritic siderophile element patterns. They may also be derived from the interior of the ureilite parent body, but some may be of exogenous origin (impactor debris), as are rare taenite grains.On Earth, iron silicides are often formed under high-temperature and strongly reducing conditions (e.g. blast furnaces, lightning strikes). On the Moon, hapkeite (Fe₂Si) and other silicides have been found in the regolith where they were formed by impact-induced space weathering. In the Stardust aerogel, iron silicides derived from comet Wild2 were also formed by an impact-related reduction process. Silicides in ureilite regolith breccias may have formed by similar processes but ureilites additionally contain abundant elemental carbon which probably acted as a reducing agent, thus larger and more abundant silicide grains were formed than in the lunar regolith or cometary material. The origin of suessite in NWA 1241 may be analogous to that of reduced lithologies in the terrestrial mantle, although a regolith origin may also be possible since this sample is shown here to be a dimict breccia.     

Evidence from polymict ureilite meteorites for a disrupted and re-accreted single ureilite parent asteroid gardened by several distinct impactors

Ureilites are ultramafic achondrites that exhibit heterogeneity in mg# and oxygen isotope ratios between different meteorites. Polymict ureilites represent near-surface material of the ureilite parent asteroid(s). Electron microprobe analyses of >500 olivine and pyroxene clasts in several polymict ureilites reveal a statistically identical range of compositions to that shown by unbrecciated ureilites, suggesting derivation from a single parent asteroid. Many ureilitic clasts have identical compositions to the anomalously high Mn/Mg olivines and pyroxenes from the Hughes 009 unbrecciated ureilite (here termed the “Hughes cluster”). Some polymict samples also contain lithic clasts derived from oxidized impactors. The presence of several common distinctive lithologies within polymict ureilites is additional evidence that ureilites were derived from a single parent asteroid.In situ oxygen three isotope analyses were made on individual ureilite minerals and lithic clasts, using a secondary ion mass spectrometer (SIMS) with precision typically better than 0.2-0.4‰ (2SD) for δ¹⁸O and δ¹⁷O. Oxygen isotope ratios of ureilitic clasts fall on a narrow trend along the CCAM line, covering the range for unbrecciated ureilites, and show a good anti-correlation with mineral mg#. SIMS analysis identifies one ferroan lithic clast as an R-chondrite, while a second ferroan clast is unlike any known meteorite. An exotic enstatite grain is derived from an enstatite chondrite or aubrite, and another pyroxene grain with Δ¹⁷O of −0.4 ± 0.2‰ is unrelated to any known meteorite type.Ureilitic olivine clasts with mg#s < 85 are much more common than those with mg# > 85 which include the melt-inclusion-bearing “Hughes cluster” ureilites. Thus melt was present in regions of the parent ureilite asteroid with a bulk mg# > 85 when the asteroid was disrupted by impact, giving rise to two types of ureilites: common ferroan ones that were residual after melting and less common magnesian ones that were still partially molten when disruption occurred. One or more daughter asteroids re-accreted from the remnants of the mantle of the proto-ureilite asteroid. Polymict ureilite meteorites represent regolith that subsequently formed on the surface of a daughter asteroid, including impact-derived material from at least six different meteoritic sources.     

The nature and origin of ureilites

Ureilites are carbon-bearing olivine-pigeonite achondrites which constitute a unique achondrite type. We performed a comprehensive mineralogical and petrological study on eight ureilites (Kenna, Novo Urei, Goalpara, Haverö, Dingo Pup Donga, Dyalpur, North Haig, and ALHA77257) the results of which were used to construct a hypothesis for the origin of ureilites. This hypothesis suggests that ureilites are primarily olivine-pigeonite cumulates which crystallized from a silicate liquid that also contained suspended solid carbon phases, mainly graphite. This carbon caused reduction of the melt and influenced ureilite mineral compositions. Carbonaceous material was trapped by settling cumulus mafic silicates along with other intercumulus material to form the carbon-rich ‘veins’ common to ureilites. Petrofabric analyses show that mafic silicates are oriented in lineated and foliated patterns characteristic of cumulate rocks, specifically adcumulates. Strain state of silicates suggests that ureilites were deformed subsequent to lithification by mild tectonic stress as well as by moderate to severe shock. The latter event caused the formation of micron-sized diamonds and lonsdaleite from graphite and carbon-induced reduction of silicate grain margins during temporal shock-heating.     

Asteroid (4) Vesta: I. The howardite-eucrite-diogenite (HED) clan of meteorites

The howardite, eucrite and diogenite (HED) clan of meteorites are ultramafic and mafic igneous rocks and impact-engendered fragmental debris derived from a thoroughly differentiated asteroid. Earth-based telescopic observation and data returned from vestan orbit by the Dawn spacecraft make a compelling case that the asteroid (4) Vesta is the parent asteroid of HEDs, although this is not universally accepted. Diogenites are petrologically diverse and include dunitic, harzburgitic and noritic lithologic types in addition to the traditional orthopyroxenites. Diogenites form the lower crust of Vesta. Cumulate eucrites are gabbroic rocks formed by accumulation of pigeonite and plagioclase from a mafic magma at depth within the crust, while basaltic eucrites are melt compositions that likely represent shallow-level dikes and sills, and flows. Some basaltic eucrites are richer in incompatible trace elements compared to most eucrites, and these may represent mixed melts contaminated by partial melts of the mafic crust. Differentiation occurred within a few Myr of formation of the earliest solids in the Solar System. Evidence from oxygen isotope compositions and siderophile element contents favor a model of extensive melting of Vesta forming a global magma ocean that rapidly (period of a few Myr) segregated and crystallized to yield a metallic core, olivine-rich mantle, orthopyroxene-rich lower crust and basaltic upper crust. The igneous lithologies were subjected to post-crystallization thermal processing, and most eucrites show textural and mineral-compositional evidence for metamorphism. The cause of this common metamorphism is unclear, but may have resulted from rapid burial of early basalts by later flows caused by high effusion rates on Vesta. The observed surface of Vesta is covered by fragmental debris resulting from impacts, and most HEDs are brecciated. Many eucrites and diogenites are monomict breccias indicating a lack of mixing. However, many HEDs are polymict breccias. Howardites are the most thoroughly mixed polymict breccias, yet only some of them contain evidence for residence in the true regolith. Based on the numbers of meteorites, compositions of howardites, and models of magma ocean solidification, cumulate eucrites and their residual ferroan mafic melts are minor components of the vestan crust.     

Highly siderophile elements in ureilites

The abundances of the highly siderophile elements (HSE) Ru, Pd, Re, Os, Ir, and Pt were determined by isotope dilution mass spectrometry for 22 ureilite bulk rock samples, including monomict, augite-bearing, and polymict lithologies. This report adds significantly to the quantity of available Pt and Pd abundances in ureilites, as these elements were rarely determined in previous neutron activation studies. The CI-normalized HSE abundance patterns of all ureilites analyzed here except ALHA 81101 show marked depletions in the more volatile Pd, with CI chondrite-normalized Pd/Os ratios (excluding ALHA 81101) averaging 0.19 ± 0.23 (2σ). This value is too low to be directly derived from any known chondrite group. Instead, the HSE bulk rock abundances and HSE interelement ratios in ureilites can be understood as physical mixtures of two end member compositions. One component, best represented by sample ALHA 78019, is characterized by superchondritic abundances of refractory HSE (RHSE—Ru, Re, Os, Ir, and Pt), but subchondritic Pd/RHSE, and is consistent with residual metal after extraction of a S-bearing metallic partial melt from carbonaceous chondrite-like precursor materials. The other component, best represented by sample ALHA 81101, is RHSE-poor and has HSE abundances in chondritic proportions. The genesis of the second component is unclear. It could represent regions within the ureilite parent body (UPB), in which metallic phases were completely molten and partially drained, or it might represent chondritic contamination that was added during disruption and brecciation of the UPB. Removal of carbon-rich melts does not seem to play an important role in ureilite petrogenesis. Removal of such melts would quickly deplete the ureilite precursors in Re/Os and As/Au, which is inconsistent with measured osmium isotope abundances, and also with literature As/Au data for the ureilites. Removal of ²⁶Al during silicate melting may have acted as a switch that turned off further metal extraction from ureilite source regions.     

Thermal evolution and physics of melt extraction on the ureilite parent body

We develop a physical model of the thermal history of the ureilite parent body (UPB) that numerically tracks the history of its heating, hydration, dehydration, partial melting and smelting as a function of its formation time and the initial values of its composition, formation temperature and water ice content. Petrologic and chemical data from the main group (non-polymict) ureilite meteorites, which sample the interior of the UPB between depths corresponding to pressures in the range 3-10 MPa, are used to constrain the model. We find that to achieve the ∼30% melting inferred for ureilites from all sampled depths, the UPB must have had a radius between ∼80 and ∼130 km and must have accreted about 0.55 Ma after CAI formation. Melting began in the body at ∼1 Ma after CAI, and the time at which 30% melting was reached varied with depth in the asteroid but was always between ∼4.5 and ∼5.8 Ma after CAI. The total rate at which melt was produced in the UPB varied from more than 100 m³ s⁻¹ in the very early stages of melting at ∼1 Ma after CAI to ∼5 m³ s⁻¹ between 2 and 3 Ma after CAI, decreasing to extremely small values as the end of melting was approached beyond ∼5 Ma. Although the initial period of high melt production occupied only a short time around 1 Ma after CAI, it corresponded to ∼half (16%) of total silicate melting, and all strictly basaltic (i.e. plagioclase-saturated) melts must have been produced during this period.A very efficient melt transport network, consisting of a hierarchy of veins and larger pathways (dikes), developed quickly at the start of melting, ensuring rapid (timescales of months) transport of any single parcel of melt to shallow levels, thus ensuring that chemical interaction between melts and the rocks through which they subsequently passed was negligible. Volatile (mainly carbon monoxide) production due to smelting began at the start of silicate melting in the shallowest parts of the UPB and at later times at greater depths. Except at the very start and very end of melting, the volatile content of the melts produced was always high - generally between 15 and 35 mass % - and most of the melt produced was erupted at the surface of the UPB with speeds well in excess of the escape velocity and was lost into space. However, we show that 30% melting at the 3 MPa pressure level was only possible if ∼15% of the total melt produced in the asteroid was retained as a small number (∼5) of very extensive, sill-like intrusions centered at a depth of ∼7 km below the surface, near the base of the ∼8 km thick outer crust of the asteroid that was maintained at temperatures below the basalt solidus by conductive heat loss to the surface. The horizontal extents of these sills occupied about 75% of the surface area of the UPB, and the sills acted as buffers between the steady supply of melt from depth and the intermittent explosive eruption of the melt into space. We infer that samples from these intrusions are preserved as the rare feldspathic (loosely basaltic) clasts in polymict ureilites, and show that the cooling histories of the sills are consistent with these clasts reaching isotopic closure at ∼5 Ma after CAI, as given by ²⁶Al-²⁶Mg, ⁵³Mn-⁵³Cr and Pb-Pb age dates.     

Experimental constraints on ureilite petrogenesis

This experimental study explores the petrogenesis of ureilites by a partial melting/smelting process. Experiments have been performed over temperature (1150-1280 °C), pressure (5-12.5 MPa), and low oxygen fugacity (graphite-CO gas) conditions appropriate for a hypothetical ureilite parent body ∼200 km in size. Experimental and modeling results indicate that a partial melting/smelting model of ureilite petrogenesis can explain many of the unique characteristics displayed by this meteorite group. Compositional information preserved in the pigeonite-olivine ureilites was used to estimate the composition of melts in equilibrium with the ureilites. The results of 20 experiments saturated with olivine, pyroxene, metal, and liquid with appropriate ureilite compositions are used to calibrate the phase coefficients and pressure-temperature dependence of the smelting reaction. The calibrated coefficients are used to model the behavior of a hypothetical residue that is experiencing fractional smelting. The residue is initially olivine-rich and smelting progressively depletes the olivine content and enriches the pyroxene and metal contents of the residues. The modeled residue composition at 1260 °C best reproduces the trend of ureilite bulk compositions. The model results also indicate that as a ureilite residue undergoes isothermal decompression smelting over a range of temperatures, Ca/Al values and Cr₂O₃ contents are enriched at lower temperatures (below ∼1240 °C) and tend to decrease at higher temperatures. Therefore, fractional smelting can account for the high Ca/Al and Cr₂O₃ wt% values observed in ureilites. We propose that ureilites were generated from an olivine-rich, cpx-bearing residue. Smelting began when the residue was partially melted and contained liquid, olivine, and carbon. These residues experienced varying degrees of fractional smelting to produce the compositional variability observed within the pigeonite-bearing ureilites. Variations in mineral composition, modal proportions, and isotopic signatures are best described by heterogeneous accretion of the ureilite parent body followed by minimal and variable degrees of igneous processing.     

A mantle or mafic crustal source for Proterozoic anorthosites?

Calculations of fractional crystallization (FC) and assimilation fractional crystallization (AFC) at 11 kb for a variety of primitive magmatic compositions and a mafic assimilant demonstrate that none of them has a bulk composition suitable to be parental to massif anorthosites. Mafic compositions thought to be parental to massif anorthosites have Mg′ values of 0.6 to 0.4 and form coherent arrays with moderately steep slopes on plots of TiO₂, K₂O, and P₂O₅ versus Mg′. The calculated liquid lines of descent (LLD) of basaltic magmas undergoing FC or AFC processes pass through the arrays of anorthosite parent magma compositions with much shallower slopes than the natural arrays, which indicates that the arrays of natural parental magmas were produced by a process other than FC/AFC. Also, by the time most crystallizing basaltic magmas with or without assimilation reach plagioclase saturation, their residual liquids have Mg′ values that are too low to be parental to anorthosites. MORB-like olivine tholeiites and high-aluminum olivine tholeiites (HAOT) from convergent plate margins do reach plagioclase saturation while sufficiently magnesian, but their Wo (Wollastonite) contents are too high such that they reach plagioclase saturation coexisting only with augite and do not reach orthopyroxene saturation (if at all) until Mg′ is too low. Calculations show it is not possible to produce a high-Al melt from typical mantle peridotites that has sufficient TiO₂ to make andesine-type anorthosite.

Calculation of partial melting for an average mafic crustal composition at 11 kbar provides a much closer match to the array of natural parental compositions in terms of minor element concentrations and proportions of mineral components. However, accounting for the entire array requires a more magnesian source composition. Such compositions exist in several crustal xenolith localities. Similar results were obtained using the bulk composition of the Stillwater Complex, which is used as a model mafic source (here the premise is that overdense crustal intrusions might sink back into the mantle). As with the terrain composition, this particular layered intrusion composition is not sufficiently magnesian, however, the fit improves when mixtures of early and late stage portions of the complex (i.e., the denser portions) were run as potential source regions.     

The geochemistry and provenance of Apollo 16 mafic glasses

The regolith of the Apollo 16 lunar landing site is composed mainly of feldspathic lithologies but mafic lithologies are also present. A large proportion of the mafic material occurs as glass. We determined the major element composition of 280 mafic glasses (>10 wt% FeO) from six different Apollo 16 soil samples. A small proportion (5%) of the glasses are of volcanic origin with picritic compositions. Most, however, are of impact origin. Approximately half of the mafic impact glasses are of basaltic composition and half are of noritic composition with high concentrations of incompatible elements. A small fraction have compositions consistent with impact mixtures of mare material and material of the feldspathic highlands. On the basis of major-element chemistry, we identified six mafic glass groups: VLT picritic glass, low-Ti basaltic glass, high-Ti basaltic glass, high-Al basaltic glass, KREEPy glass, and basaltic-andesite glass. These glass groups encompass 60% of the total mafic glasses studied. Trace-element analyses by secondary ion mass spectroscopy for representative examples of each glass group (31 total analyses) support the major-element classifications and groupings. The lack of basaltic glass in Apollo 16 ancient regolith breccias, which provide snapshots of the Apollo 16 soil just after the infall of Imbrium ejecta, leads us to infer that most (if not all) of the basaltic glass was emplaced as ejecta from small- or moderate-sized impacts into the maria surrounding the Apollo 16 site after the Imbrium impact. The high-Ti basaltic glasses likely represent a new type of basalt from Mare Tranquillitatis, whereas the low-Ti and high-Al basaltic glasses possibly represent the composition of the basalts in Mare Nectaris. Both the low-Ti and high-Al basaltic glasses are enriched in light-REEs, which hints at the presence of a KREEP-bearing source region beneath Mare Nectaris. The basaltic andesite glasses have compositions that are siliceous, ferroan, alkali-rich, and moderately titaniferous; they are unlike any previously recognized lunar lithology or glass group. Their likely provenance is within the Procellarum KREEP Terrane, but they are not found within the Apollo 16 ancient regolith breccias and therefore were likely deposited at the Apollo 16 site post-Imbrium. The basaltic-andesite glasses are the most ferroan variety of KREEP yet discovered.     

Experimental petrology of eucritic meteorites

Low pressure melting experiments on eucritic meteorites demonstrate that the compositions of most eucrites can be generated by low pressure fractionation of pigeonite and plagioclase from liquids similar in composition to the Sioux County and Juvinas eucrites. It is unlikely that the liquids with compositions similar to Sioux County and Juvinas were themselves residual liquids produced by extensive fractionation of more magnesian parental liquids. The compositions of Stannern and Ibitira cannot be produced by fractionation of liquids with compositions similar to other known eucrites. Liquid compositions similar to Stannern, Ibitira, and Sioux County could have been generated by increasing degrees of low pressure partial melting of source regions composed of olivine (~Fo65), pigeonite (~Wo5En65), plagioclase (~An94), Cr-rich spinel, and metal. These source assemblages may have been primitive, undifferentiated material of the basaltic achondrite parent body and the eucrites may represent melts produced in early stages of its melting and differentiation. Further melting in these source regions, after exhaustion of plagioclase, may have produced magnesian liquids from which the magnesian pyroxenes and olivines in howardites, diogenites, and mesosiderites crystallized in closed-system plutonic environments. Most of the cumulate eucrites (e.g. Moama, Moore County, Serra de Magé) could not have equilibrated with liquids similar in composition to known eucrites. These cumulates may have accumulated from liquids produced by extensive fractionation of advanced partial melts of the source regions of eucritic liquids. A depletion in Na, K, and Rb in Ibitira is noted.     

Enstatite achondrite meteorites (aubrites) and the histories of their asteroidal parent bodies

The aubrites are nearly monomineralic enstatite pyroxenites, consisting mostly of nearly FeO-free enstatite, with minor albitic plagioclase, nearly FeO-free diopside and forsterite, metallic Fe,Ni, troilite, and a host of rare accessory minerals, many unknown from Earth, that formed under highly reducing conditions. As a result, many of the normally lithophile elements such as Ti, Cr, Mn, Na, etc. behave partly as chalcophiles (i.e., occur in sulfides), and Si is partly siderophile and occurs in metallic Fe,Ni. Aubrites must therefore have formed in a very unique part of the solar nebula, possibly within 1 AU of the Sun. While of the 27 aubrites, 15 are fragmental breccias, 6 are regolith breccias, and 6 are described as non-brecciated, their ingredients are clearly of igneous origin and formed by melting and fractional crystallization, possibly of a magma ocean. This is indicated by the occurrence of a variety of lithic clasts of igneous origin, and by the REE and other trace element distributions. Their highly reduced nature and their oxygen isotopic compositions suggest close kinship to the enstatite chondrites. However, they did not form from known EH or EL chondrites on their parent bodies. Rather, they formed from enstatite chondrite-like material on at least two separate parent bodies, the Shallowater parent body and, for all other aubrites, on the aubrite parent body. Visible and near-infrared reflectance spetra of asteroids suggest that the aubrite parent bodies may be asteroids of the E-type and perhaps the E(II) sub-class, such as 3103 Eger and 2867 Steins (the target of the Rosetta Mission). If aubrites formed by the melting and fractional crystallization of enstatite chondrite-like parent lithologies, which should have contained ～10 vol% plagioclase, then meteorites of enstatite-plagioclase basaltic composition should exist, which is not the case. These early basaltic melts may have been removed from the aubrite parent body by explosive pyroclastic volcanism, and these small pyroclasts would have been destroyed in space long ago. Age dates suggest that the aubrites formed very early in the history of the solar system, within a few Ma of CAI formation, and that the heat sources for heating and melting of their parent bodies were, most likely, short-lived radionuclides such as ²⁶Al and, perhaps, ⁶⁰Fe. Finally, attention has been drawn to the surface composition of Mercury of low bulk FeO and of nearly FeO-free enstatite, perhaps with plagioclase, diopside and sulfide. While known aubrites clearly did not originate from Mercury, recent calculations suggest that several percent of high-speed ejecta from Mercury reach Earth. This is only factors of 2–3 less than typical launches from Mars and, since there are now 53 Martian meteorites in our collections, meteoriticists should be alert to the potential discovery of a genuine meteorite from Mercury which, superficially, should resemble aubrites. However, recent results from the Neutron Spectrometer of the Messenger Flyby of Mercury have been interpreted to suggest that the planet’s surface may, in fact, contain abundant Fe–Ti-oxides and, if true, a meteorite from Mercury should not resemble any currently known meteorite type.     

Petrology of the mafic sill of Narshingpur-Lakhnadon section,Eastern Deccan volcanic province

Deccan volcanism with a tremendous burst of volcanic activity marks a unique episode in Indian geological history and covers nearly two third of Peninsular India. Occurrences of mafic sill in the continental basalts are rather rare throughout the flood basalt provinces and only few sporadic reports have been described from different Continental Flood Basalts of the world. In the present article, petrology of mafic sill from the Narshingpur-Lakhnadon section of Eastern Deccan province of India has been presented. The mafic sill in the field is found to occur in a relatively deep valley amidst Gondwana rocks, which occur as the basement of the extrusion. The sill is spatially associated with three initial flows viz. flow I, II and III of adjacent Narshingpur-Harrai-Amarwara section. The sill in its central part is a medium grained rock and petrographically corresponds to dolerite containing augite, plagioclase and rare olivine grains; the chilled facies of the sill is characterized by phenocrysts of olivine, plagioclase and augite that are set in groundmass consisting predominantly of plagioclase, olivine and glass. Mineral chemistry indicates that olivine phenocrystal phase is magnesian (Fo61). Plagioclase phenocrystal composition ranges from An 51 to An 71 whereas the same variation of the groundmass plagioclase composition corresponds to An 31 to An 62. The overlap in the compositions for groundmass and phenocrystal plagioclase may be explained due to fluctuating PH2O condition. The pyroxene compositions (both groundmass and phenocryst) in majority of the cases are clubbed well within the augite field, however, in a few cases, groundmass compositions are found to fall in the sub-calcic augite and pigeonite field. Some zoned pyroxene phenocrysts, characteristically display different types of zoning patterns. Opaque minerals in the mafic sill are found to be magnetite and ilmenite and this coexisting iron-oxide composition helps to constrain the prevalent fO2 condition in the parent magma. The geochemistry of the mafic sill and associated basaltic lava flows indicates close genetic link amongst them. Critical consideration of trace elements indicates a distinct enriched mantle source (EM-I/EM-II/HIMU) for the parental magma. Trace element modeling indicates that equilibrium batch-melting of plume source followed by fractionation of olivine, clinopyroxene and plagioclase and subsequent heterogeneous mixing of melt and settled crystals can very well explain the genesis of the mafic sill and the associated basaltic flows.     

Osmium isotope systematics of ureilites

The ¹⁸⁷Os/¹⁸⁸Os for 22 ureilite whole rock samples, including monomict, augite-bearing, and polymict lithologies, were examined in order to constrain the provenance and subsequent magmatic processing of the ureilite parent body (or bodies). The Re/Os ratios of most ureilites show evidence for a recent disturbance, probably related to Re mobility during weathering, and no meaningful chronological information can be extracted from the present data set. The ureilite ¹⁸⁷Os/¹⁸⁸Os ratios span a range from 0.11739 to 0.13018, with an average of 0.1258 ± 0.0023 (1σ), similar to typical carbonaceous chondrites, and distinct from ordinary or enstatite chondrites. The similar mean of ¹⁸⁷Os/¹⁸⁸Os measured for the ureilites and carbonaceous chondrites suggests that the ureilite parent body probably formed within the same region of the solar nebula as carbonaceous chondrites. From the narrow range of the ¹⁸⁷Os/¹⁸⁸Os distribution in ureilite meteorites it is further concluded that Re was not significantly fractionated from Os during planetary differentiation and was not lost along with the missing ureilitic melt component. The lack of large Re/Os fractionations requires that Re/Os partitioning was controlled by a metal phase, and thus metal had to be stable throughout the interval of magmatic processing on the ureilite parent body.     

Nitrogen components in ureilites

Abundances and isotopic compositions of nitrogen and argon have been investigated in bulk samples as well as in acid-resistant C-rich residues of a suite of ureilites consisting of six monomict (Haverö, Kenna, Lahrauli, ALH81101, ALH82130, LEW85328), three polymict (Nilpena, EET87720, EET83309), and the diamond-free ureilite ALH78019. Nitrogen in bulk ureilites varies from 6.3 ppm (in ALH 78019) to ∼55 ppm (in ALH82130), whereas C-rich acid residues have ∼65 to ∼530 ppm N, showing approximately an order of magnitude enrichment, compared with the bulk ureilites, somewhat less than trapped noble gases. Unlike trapped noble gases that show uniform isotopic composition, nitrogen shows a wide variation in δ¹⁵N values within a given ureilite as well as among different ureilites. The variations observed in δ¹⁵N among the ureilites studied here suggest the presence of at least five nitrogen components. The characteristics of these five N components and their carrier phases have been identified through their release temperature during pyrolysis and combustion, their association with trapped noble gases, and their carbon (monitored as CO + CO₂ generated during combustion). Carrier phases are as follows: 1) Amorphous C, as found in diamond-free ureilite ALH78019, combusting at ≤500°C, with δ¹⁵N = -21‰ and accompanied by trapped noble gases. Amorphous C in all diamond-bearing ureilites has evolved from this primary component through almost complete loss of noble gases, but only partial N loss, leading to variable enrichments in ¹⁵N. 2) Amorphous C as found in EET83309, with similar release characteristics as component 1, δ¹⁵N ≥ 50‰ and associated with trapped noble gases. 3) Graphite, as clearly seen in ALH78019, combusting at ≥700°C, δ¹⁵N ≥ 19‰ and devoid of noble gases. 4) Diamond, combusting at 600-800°C, δ¹⁵N ≤ -100‰ and accompanied by trapped noble gases. 5) Acid-soluble phases (silicates and metal) as inferred from mass balance are expected to contain a large proportion of nitrogen (18 to 75%) with δ¹⁵N in the range -25‰ to 600‰. Each of the ureilites contains at least three N components carried by acid-resistant C phases (amorphous C of type 1 or 2, graphite, and diamond) and one acid-soluble phase in different proportions, resulting in the observed heterogeneity in δ¹⁵N. In addition to these five widespread components, EET83309 needs an additional sixth N component carried by a C phase, combusting at <700°C, with δ¹⁵N ≥ 153‰ and accompanied by noble gases. It could be either noble gas-bearing graphite or more likely cohenite. Some excursions in the δ¹⁵N release patterns of polymict ureilites are suggestive of contributions from foreign clasts that might be present in them.Nitrogen isotopic systematics of EET83309 clearly confirm the absence of diamond in this polymict ureilite, whereas the presence of diamond is clearly indicated for ALH82130. Amorphous C in ALH78019 exhibits close similarities to phase Q of chondrites.The uniform δ¹⁵N value of −113 ± 13 ‰ for diamond from both monomict and polymict ureilites and its independence from bulk ureilite δ¹⁵N, Δ¹⁷O, and %Fo clearly suggest that the occurrence of diamond in ureilites is not a consequence of parent body-related process. The large differences between the δ¹⁵N of diamond and other C phases among ureilites do not favor in situ shock conversion of graphite or amorphous C into diamond. A nebular origin for diamond as well as the other C phases is most favored by these data. Also the preservation of the nitrogen isotopic heterogeneity among the carbon phases and the silicates will be more consistent with ureilite formation models akin to “nebular sedimentation” than to “magmatic” type.     

Investigation into the petrogenesis of Apollo 14 high-Al basaltic melts through crystal stratigraphy of plagioclase

The petrogenesis of Apollo 14 high-Al basaltic melts was studied using crystal stratigraphy, which involves textural (crystal size distributions — CSDs) and chemical analyses (electron microprobe and laser ablation inductively coupled plasma mass spectrometry). The samples studied here include pristine basalt 14072 and basaltic clasts from breccia 14321, and impact-generated crystalline samples 14073, 14276 and 14310. Plagioclase was the focus of this study because of its relatively high modal abundances and because it was on the liquidus for much of the melt cooling histories. Plagioclase crystals were analyzed (core-to-rim compositions where possible) to test and refine petrogenetic models based upon whole-rock compositions (Groups A, B, and C designations) and to investigate basalt 14072 and impact-melt crystallization. Textural studies have shown that each basalt group has distinctive plagioclase CSDs, which are in turn distinctive from those of the impact melts. Evolution of the individual basaltic melts was studied by comparing the equilibrium-melt compositions (calculated from plagioclase compositions using relevant partition coefficients) to fractional crystallization (FC) and assimilation and fractional crystallization (AFC) models. Petrogenetic modeling of trace elements in Group A basalts revealed that petrogenesis continued beyond 40% total crystallization required to model whole-rock compositions, and that there were open-system processes that affected the magma during plagioclase crystallization. Petrogenetic modeling of pristine high-Al basalts (14072 and Groups A, B and C) using trace elements shows that the equilibrium-melt compositions do not fall on a single AFC or FC trajectory. This is consistent with fluctuating degrees of assimilation (i.e., variable r-values) and/or variable assimilant compositions during petrogenesis. Petrogenetic modeling reveals that the impact melts experienced only closed-system fractional crystallization. This work demonstrates the importance of crystal stratigraphy in revealing the intricacies of lunar basalt petrogenesis.     

The variety of lithologies in the Yamato-86032 lunar meteorite: Implications for formation processes of the lunar crust

We performed a petrologic, mineralogical, geochemical, and isotopic study of several lithologies in the Y-86032 feldspathic breccia. This study leads us to conclude that Y-86032 likely originated on the lunar farside. Y-86032 is composed of several types of feldspathic clasts, granulitic breccias, and minor basaltic clasts set in a clastic matrix. We identify an “An97 anorthosite” that has An contents similar to those of nearside FANs. Mg′ (= molar Mg/(Mg + Fe) × 100) values vary significantly from ∼45 to ∼80 covering the ranges of both nearside FANs and the Mg′ gap between FANs and the Mg-suite. A light-gray feldspathic (LG) breccia making up ∼20% of the investigated slab (5.2 × 3.6 cm²) mainly consists of fragments of anorthosites (“An93 anorthosite”) more sodic than nearside FANs. LG also contains an augite-plagioclase clast which either could be genetically related to the An93 anorthosite or to slowly-cooled basaltic magma intruded into the precursor rock. The Na-rich nature of both An93 anorthosite and this clast indicates that the LG breccia was derived from a relatively Na-rich but incompatible-element-poor source. The Mg′ variation indicates that the “An97 anorthosite” is a genomict breccia of several types of primary anorthosites. Granulitic breccias in Y-86032 have relatively high Mg′ in mafic minerals. The highest Mg′ values in mafic minerals for the “An97 anorthosite” and granulitic breccias are similar to those of Mg-rich lithologies recently described in Dhofar 489. Basaltic clasts in the dark-gray matrix are aluminous, and the zoning trends of pyroxene are similar to those of VLT or LT basalts. The crystallization of these basaltic clasts pre-date the lithification age of the clastic matrix at ∼3.8 Ga. The low K contents of plagioclase in both the anorthositic and basaltic clasts and generally low incompatible element abundances in all the lithologies in Y-86032 indicate that KREEP was not involved during the formation of the precursor lithologies. This observation further suggests that urKREEP did not exist in the source regions of these igneous lithologies. All these facts support the idea that Y-86032 was derived from a region far distant from the PKT and that the lithic clasts and fragments are indigenous to that region. An An97 anorthositic clast studied here has distinct Sm-Nd isotopic systematics from those previously found for another An97 anorthositic clast and “An93 anorthosite”, and suggests either that An97 anorthosites come from isotopically diverse sources, or that the Sm-Nd isotopic systematics of this clast were reset ∼4.3 Ga ago. These lines of geochemical, isotopic, and petrologic evidence suggest that the lunar crust is geochemically more heterogeneous than previously thought.     

Petrography of isotopically-dated clasts in the Kapoeta howardite and petrologic constraints on the evolution of its parent body

Detailed mineralogic and petrographic data are presented for four isotopically-dated basaltic rock fragments separated from the howardite Kapoeta. Clasts C and ρ have been dated at ~4.55 AE and ~ 4.60 AE respectively, and Clast ρ contains ²⁴⁴Pu and ¹²⁹I decay products. These are both igneous rocks that preserve all the features of their original crystallization from a melt. They thus provide good evidence that the Kapoeta parent body produced basaltic magmas shortly after its formation (< 100 m.y.). Clast A has yielded a Rb-Sr age of ~ 3.89 AE and a similar ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age. This sample is extensively recrystallized, and we interpret the ages as a time of recrystallization, and not the time of original crystallization from a melt. Clast B has yielded a Rb-Sr age of ~ 3.63 AE, and an ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age of ? 4.50 AE. This sample is moderately recrystallized, and the Rb-Sr age probably indicates a time of recrystallization, whereas the ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age more closely approaches the time of crystallization from a melt. Thus, there is no clearcut evidence for ‘young’ magmatism on the Kapoeta parent body.Kapoeta is a ‘regolith’ meteorite, and mineral-chemical and petrographic data were obtained for numerous other rock and mineral fragments in order to characterize the surface and near-surface materials on its parent body. Rock clasts can be grouped into two broad lithologic types on the basis of modal mineralogy—basaltic (pyroxene- and plagioclase-bearing) and pyroxenitic (pyroxenebearing). Variations in the compositions of pyroxenes in rock and mineral clasts are similar to those in terrestrial mafic plutons such as the Skaergaard, and indicate the existence of a continuous range in rock compositions from Mg-rich orthopyroxenites to very iron-rich basalts. The FeO and MnO contents of all pyroxenes in Kapoeta fall near a line with FeO/MnO ~ 35, suggesting that the source rocks are fundamentally related. We interpret these observations to indicate that the Kapoeta meteorite represents the comminuted remains of differentiated igneous complexes together with ‘primary’ undifferentiated basaltic rocks. The presently available isotopic data are compatible with the interpretation that this magmatism is related to primary differentiation of the Kapoeta parent body. In addition, our observations preclude the interpretation that the Kapoeta meteorite is a simple mixture of eucrites and diogenites.The FeO/MnO value in lunar pyroxenes (~60) is distinct from that of the pyroxenes in Kapoeta. Anorthositic rocks were not observed in Kapoeta, suggesting that plagioclase was not important in the evolution of the Kapoeta parent body, in contrast to the Moon. Both objects appear to have originated in chemically-distinct portions of the solar system, and to have undergone differentiation on different time scales involving differing materials.