Monomict Ureilites Research Articles

Quantitative shock measurement of olivine in ureilite meteorites

AbstractUreilites are the second most abundant achondrite meteorite group, yet they are from an unknown source. They are ultramafic in composition and have undergone thermal metamorphism as well as shock deformation since their formation. In this work, olivine grains from six monomict ureilites, Northwest Africa 7059, Elephant Moraine 96042, Shişr 007 (all ˜shock level S3), Northwest Africa 2221 (S3‐S4), Larkman Nunatak 04315 (S5), and Allan Hills A81101 (˜S6) and one polymict ureilite, Elephant Moraine 87720 (S5), were examined by in situ micro‐X‐ray diffraction (XRD). It is observed in this study that with increasing shock, the samples develop more complicated 2‐D XRD patterns from diffracted spots, to streaks, to asterism, and “spotty” Debye rings, and these patterns correspond to shock metamorphic effects observed by optical microscopy from undulatory extinction to mosaicism and annealing or recrystallization textures. These 2‐D XRD patterns, representing olivine strain‐related mosaicity (SRM), as documented by given lattice planes, were each measured as the sum of the full‐width‐half‐maximum along the Debye ring or χ dimension (∑[FWHMχ]) to quantify the shock‐related deformation recorded in the olivine grains. In each meteorite, all olivine lattice planes show similar measured ∑(FWHMχ) values for the diffraction streaks. This work demonstrates an increasing trend of average ∑(FWHMχ) in degrees of arc, 4.0 ± 1.7°, 4.1 ± 1.4°, 3.7 ± 1.8° (all S3), 4.9 ± 2.1° (S3–S4), 7.0 ± 3.5° (S5), 13.4 ± 5.7° (S6), and 7.5 ± 3.3° (polymict, S5), which agrees well with petrographic observations. This quantitative method contributes to a more comprehensive shock classification system specifically for ureilites, complementing recently published shock classification systems for olivine‐bearing rocks.

Quenched primary melt in Ramlat as Sahmah 517 – Snapshot of ureilite anatexis in the early solar system

Ureilites are the second largest group of achondrite meteorites but consensus is still lacking on the nature of their precursors, melting processes, and the genetic relationship between monomict ureilites and brecciated ureilites. The recently found ureilite Ramlat as Sahmah 517 is of special interest in this context. This meteorite lacks shock features in its primary silicates and belongs to a rare augite- and chromite-bearing subset of the monomict ferroan ureilites. It hosts abundant intergranular glass veinlets speckled with pyroxene and metal globules. Detailed petrographic investigations show that the Si-Al-rich glass represents quenched anatectic melt that was present prior to formation of the reduced olivine rims by incomplete low-pressure equilibration (smelting) of carbon and silicates. The melt facilitated smelting which, along with rapid crystallization of secondary pyroxene, modified the originally trachyandesitic melt. Melt-silicate equilibrium preceding these events is constrained by modelling using MELTS and the first reported in-situ measurements of LREE-enriched glass that is largely complementary to the depleted mafic silicates in monomict ureilites. The inferred major element composition of the partial melt that formed in RaS 517 is similar to that of trachyandesite in Almahata Sitta but RaS 517 lacks phosphates which are abundant in the Almahata Sitta trachyandesite and in alkali-rich feldspathic clasts in polymict ureilites. The LREE-depletion in the dominant monomict ferroan ureilite population can be explained by the formation of melt fractions similar to the glass in RaS 517 after initial rapid melting of phosphates. These finds provide evidence for a genetic relationship between ferroan ureilites and lithologies similar to the Almahata Sitta trachyandesite and further suggest that these ureilites formed by partial melting of P- and alkali-rich precursors with trace element concentrations similar to equilibrated ordinary chondrites. Quenched Si-Al-rich glass also occurs in magnesian ureilites but has lower concentrations of alkalis and LREE-depleted trace element signatures which can reflect more depleted compositions at the onset of partial melting. The evidence presented here favors a scenario in which the primary ureilite differentiation was driven by gradual heating from radioactive decay with resulting temperatures (>1100 °C) being maintained until disruption of the ureilite parent asteroid.

Magnesium and chromium isotope evidence for initial melting by radioactive decay of 26Al and late stage impact-melting of the ureilite parent body

Polymict ureilites are meteoritic breccias that provide insights into the differentiation history of the ureilite parent body. We have sampled a total of 24 clasts from the polymict ureilite Dar al Gani 319, representing a variety of lithologies such as mantle residues, cumulates and crustal fragments that are genetically related to monomict ureilites. In addition, we sampled four non-indigenous dark clasts and two chondrule-containing clasts from the same meteorite. We report on the petrology and the bulk mass-dependent and mass-independent magnesium and chromium isotope systematics of these clasts. The DaG 319 polymict ureilite consists predominantly of clasts related to Main Group ureilite residues (MG clasts) with varying Mg#s (0.74–0.91), as well as a significant fraction of olivine-orthopyroxene clasts related to Hughes Type ureilites (HT clasts) with consistently high Mg#s (∼0.89). In addition, DaG 319 contains less abundant feldspathic clasts that are thought to represent melts derived from the ureilite mantle. A significant mass-dependent Mg-isotope fractionation totaling Δμ25Mg=∼450ppm was found between isotopically light feldspathic clasts (μ25Mg=−305±25 to 15±12ppm), MG clasts (μ25Mg=−23±51ppm) and HT clasts (μ25Mg=157±21ppm). We suggest that this isotopic offset is the result of equilibrium isotope fractionation during melting in the presence of an isotopically light magnesite component. We propose Mg-carbonates to be stable in the upper ureilite mantle, and pure carbon phases such as graphite to be stable at higher pressures. This is consistent with HT clasts lacking carbon-related phases, whereas MG clasts contain abundant carbon. The timing of differentiation events for the ureilitic clasts are constrained by high precision 53Mn-53Cr systematics and 26Al-26Mg model ages. We show that a dichotomy of ages exist between the differentiation of main group ureilite residues and HT cumulates rapidly after CAI formation and later remelting of cumulates with corresponding feldspathic melts, at 3.8±1.3Myr after CAI formation. Assuming an initial 26Al/27Al abundance [(26Al/27Al)0=1.33-0.18+0.21×10−5] similar to the angrite parent body, the early melting event is best explained by heat production from 26Al whereas the late event is more likely caused by a major impact. Variations in 54Cr between MG clasts and HT clasts agree with a carbonaceous chondrite impactor onto the ureilite parent body. This impactor may be represented by abundant dark clasts found in polymict ureilites, which have μ26Mg∗ and μ54Cr signatures similar to CI chondrites. Similar volatile-rich dark clasts found in other meteorite breccias provide insights into the timing of volatile influx to the accretion region of the terrestrial planets.

New Raman Spectroscopic Data of the Almahata Sitta Meteorite

ABSTRACT Raman spectroscopy is a convenient method to classify polytypes plus polymorphs and to investigate different compositions, structures, and modifications of minerals. This information is required to classify meteorites of different mineral compositions. Our intention was to examine different graphite modifications and to determine the opaque phases within the Almahatta Sitta (AS) meteorite. Our investigation focused on specimens AS39 and AS4. Both are multicomponent breccias, composed of different meteoritic lithologies, that is, anomalous polymict ureilite material, different types of chondrites, and an iron meteorite.[ 1-5 ] The mix of material implicates a crash of variable asteroids within time and space. This means that a primary ureilitic asteroid body has been the target of multiple meteorite or asteroid impacts, which did not completely destroy the asteroid but added new material (from chondritic to iron) to the main mass. For instance, Gabriel and Pack[ 6 ] suggested that the vein metal of monomict ureilites is introduced by the impact of a Ni-poor iron meteorite in a similar scenario as described above. Raman spectroscopy helped to investigate different carbon materials, that is, graphite and diamond. We used single-spectrum acquisition and spatial mappings to identify the different modifications. Furthermore, we classified minor opaque phases such as schreibersite, suessite, daubreelite, cohenite, and kamacite. The investigation of these opaque phases is dealt with in further papers.[1,2]

CHROMIUM ISOTOPE SYSTEMATICS OF ACHONDRITES: CHRONOLOGY AND ISOTOPIC HETEROGENEITY OF THE INNER SOLAR SYSTEM BODIES

The standard planetary formation models assume that primitive materials, such as carbonaceous chondrites, are the precursor materials of evolved planetesimals. Past chronological studies have revealed that planetesimals of several hundred kilometers in size, such as the Howardite-Eucrite-Diogenite (HED) parent body (Vesta) and angrite parent body, began their differentiation as early as ~3 million years of the solar system formation, and continued for at least several million years. However, the timescale of planetesimal formation in distinct regions of the inner solar system, as well as the isotopic characteristics of the reservoirs from which they evolved, remains unclear. Here we present the first report for the precise 53Mn-53Cr ages of monomict ureilites. Chemically separated phases from one monomict ureilite (NWA 766) yielded the Mn-Cr age of 4564.60 ± 0.67 Ma, identical within error to the oldest age preserved in other achondrites, such as angrites and eucrites. The 54Cr isotopic data for this and seven additional bulk ureilites show homogeneous e54Cr of ~–0.9, a value distinct from other achondrites and chondrites. Using the e54Cr signatures of Earth, Mars, and Vesta (HED), we noticed a linear decrease in the e54Cr value with the heliocentric distance in the inner region of the solar system. If this trend can be extrapolated into the outer asteroid belt, the e54Cr signatures of monomict ureilites will place the position of the ureilite parent body at ~2.8 AU. These observations imply that the differentiation of achondrite parent bodies began nearly simultaneously at ~4565 Ma in different regions of the inner solar system. The distinct e54Cr value between ureilite and carbonaceous chondrite also implies that a genetic link commonly proposed between the two is unlikely.

182Hf– 182W chronometry and early differentiation of the ureilite parent body

Eight samples spanning the bulk chemical range of monomict ureilites, have been analyzed for their W isotopic composition and Hf/W in order to elucidate the differentiation history of the ureilite parent body. The ureilites studied here show sub-chondritic Hf and W concentrations and low Hf/W ratio, and all show clear 182W deficits relative to bulk chondrites. The W isotope data provide evidence that the ureilite parent body differentiated within 1 to 2 million years after the start of the solar system. Such early accretion and differentiation is comparable to the parent bodies of various magmatic iron meteorites, as well as the HED parent body. However, the trace element fractionations and by inference melting processes are very different. An early smelting/partial melting event coupled with metal-silicate segregation, however, can reproduce the observed Hf–W data in ureilites, and is consistent with petrological and other chemical evidence for a missing basaltic component and with primitive oxygen and noble gases compositions.

Petrology of an unusual monomict ureilite, NWA1241

NWA1241 is an unusual monomict ureilite, consisting mainly of pigeonite, olivine, suessite, and carbonaceous materials. The modal ratio of pyroxene/(olivine + pyroxene) is higher than those of common monomict ureilites. Metallic phases in NWA1241 are mainly suessite, although metals in common ureilites are mainly kamacite. The high ratio of pyroxene to olivine and the abundant occurrence of suessite make the meteorite unusual. The suessite coexists with magnesian olivine and pyroxene in NWA1241, and this mineral assemblage indicates that the suessite was produced from FeO-bearing olivine and Ni-bearing kamacite at temperatures around 1500–1400K and oxygen fugacity of 10 −18–10 −20 bars. Most ureilites experienced two-stage reduction. Stage-I reduction was a smelting process in ureilite parent bodies (UPB's) to form the framework of monomict ureilites. Stage-II reduction took place after catastrophic destruction of the UPB's to eject many fragmental blocks into space, where suessite (NWA1241) or Si-bearing kamacite (common ureilites) were produced together with the formation of reversely zoned olivines.

Northwest Africa 1500: Plagioclase-bearing monomict ureilite or ungrouped achondrite?

Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (∼100- 500 μm) olivine (95-96%), augite (2-3%), and chromite (0.6-1.6%) in an equilibrated texture. Plagioclase (0.7-1.8%) occurs as poikilitic grains (up to ∼3 mm) in vein-like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl-apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65-73, with a strong peak at Fo 68-69. Most grains are reverse- zoned, and also have ∼10-30 μm reduction rims. In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo ≥75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite-bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augite- bearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800-1000 °C, from two-pyroxene and olivine-chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history.

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.

Feldspathic clast populations in polymict ureilites: Stalking the missing basalts from the ureilite parent body

Polymict ureilites DaG 164/165, DaG 319, DaG 665, and EET 83309 are regolith breccias composed mainly of monomict ureilite-like material, but containing ∼2 vol% of feldspathic components. We characterized 171 feldspathic clasts in these meteorites in terms of texture, mineralogy, and mineral compositions. Based on this characterization we identified three populations of clasts, each of which appears to represent a common igneous (generally basaltic) lithology and whose mafic minerals show a normal igneous fractionation trend of near-constant Fe/Mn ratio over a range of Fe/Mg ratios that extend to much higher values than those in monomict ureilites. The melts represented by these populations are unlikely to be impact melts, because the ubiquitous presence of carbon in polymict ureilites (the regolith of the ureilite parent body) implies that impact melts would have crystallized under conditions of carbon redox control and therefore have highly magnesian mafic mineral compositions with constant Mn/Mg ratio. Therefore, these melts appear to be indigenous products of igneous differentiation on the ureilite parent body (UPB), complementary to the olivine-pigeonite residues represented by the majority of monomict ureilites. The most abundant population is characterized by albitic plagioclase in association with pyroxenes, phosphates, ilmenite, silica, and incompatible-element enriched glass. Model calculations suggest that it formed by extensive fractional crystallization of the earliest melt(s) of precursor materials from which the most magnesian (shallowest) olivine-pigeonite ureilites formed. A less abundant population, characterized by labradoritic plagioclase, may have formed from melts complementary to more ferroan olivine-pigeonite ureilites, and derived from deeper in the UPB. The third population, characterized by the presence of olivine and augite, could only have formed from melts produced at greater depths in the UPB than the olivine-pigeonite ureilites. Many other feldspathic clasts cannot be positively associated with any of these three populations, because their mafic mineral compositions exhibit carbon redox control. However, they may be products of early crystallization of basaltic melts produced on the UPB, before carbon was exhausted by reduction. Partial melting on the ureilite parent body was a fractional (or incremental) process. Melts were produced early in UPB history, and most likely extracted rapidly, thus preserving primitive chemical and oxygen isotopic signatures in the residues.

TEM investigations on the monomict ureilites Jalanash and Hammadah al Hamra 064

Abstract— We studied the petrography and mineralogy of two monomict ureilites, Hammadah al Hamra 064 (HH064) and Jalanash, by using reflected light and scanning electron microscopy. Quantitative analyses were performed by electron microprobe and the microstructures were investigated with transmission electron microscopy (TEM). HH064 features two different textures, a poikilitic and a typical one, whereas Jalanash shows only the typical ureilite texture. Our synergetic chemical and microstructural investigations reveal a complex cooling history for both ureilites. The temperature for the first equilibrium deduced from the pigeonite‐augite assemblage in HH064 is ∼1200°C. The presence of antiphase domains in low‐Ca pyroxenes proves that they are clearly pigeonite. The occurrences of tweed micro structure and orthopyroxene lamellae, which are incompletely developed, imply a faster cooling rate from the first equilibrium with a sudden end. Although both ureilites contain shock induced diamonds, dislocations in silicates are rare. This observation suggests that the meteorites were hot at the time of strong shock metamorphism or that they were heated after strong shock metamorphism. After this event, new microstructural features were generated by different cooling processes and were frozen by a final rapid decrease in temperature possibly due to excavation from the ureilite parent body, or bodies.

Microdistributions and petrogenetic implications of rare earth elements in polymict ureilites

Abstract— Polymict ureilites contain various mineral and lithic clasts not observed in monomict ureilites, including plagioclase, enstatite, feldspathic melt clasts and dark inclusions. This paper investigates the microdistributions and petrogenetic implications of rare earth elements (REEs) in three polymict ureilites (Elephant Moraine (EET) 83309, EET 87720 and North Haig), focusing particularly on the mineral and lithic clasts not found in monomict ureilites.As in monomict ureilites, olivine and pyroxene are the major heavy (H)REE carriers in polymict ureilites. They have light (L)REE‐depleted patterns with little variation in REE abundances, despite large differences in major element compositions.The textural and REE characteristics of feldspathic melt clasts in the three polymict ureilites indicate that they are most likely shocked melt that sampled the basaltic components associated with ureilites on their parent body. Simple REE modeling shows that the most common melt clasts in polymict ureilites can be produced by 20–30% partial melting of chondritic material, leaving behind a ureilitic residue. The plagioclase clasts, as well as some of the high‐Ca pyroxene grains, probably represent plagioclase‐pyroxene rock types on the ureilite parent body. However, the variety of REE patterns in both plagioclase and melt clasts cannot be the result of a single igneous differentiation event. Multiple processes, probably including shock melting and different sources, are required to account for all the REE characteristics observed in lithic and mineral clasts.The C‐rich matrix in polymict ureilites is LREE‐enriched, like that in monomict ureilites. The occurrence of Ce anomalies in C‐rich matrix, dark inclusions and the presence of the hydration product, iddingsite, imply significant terrestrial weathering.A search for 26Mg excesses, from the radioactive decay of 26Al, in the polymict ureilite EET 83309 was negative.

Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite

Abstract— Magmatic inclusions occur in type II ureilite clasts (olivine‐orthopyroxene‐augite assemblages with essentially no carbon) and in a large isolated plagioclase clast in the Dar al Gani (DaG) 319 polymict ureilite. Type I ureilite clasts (olivine‐pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al.(2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component‐enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields.DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene‐metal and pyroxene‐plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene‐metal crystallization trend corresponds to the magmatic inclusions in the type II ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene‐plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body.The felsic clasts may have formed mainly from albite component‐rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali‐bearing chondritic precursors. However, type II ureilites may have formed as cumulates from a basaltic melt.