Dark Clasts Research Articles

Mineralogy and petrology of dark clasts in the Allan Hills 76005 polymict eucrite pairing group

AbstractThe Allan Hills 76005 polymict eucrite pairing group consists of 15 paired masses recovered during six different field seasons in the Transantarctic Mountains. Although this group has been well studied in general, most of the meteorites contain a significant portion of dark clasts that have not been well characterized. The Dawn mission to Vesta discovered dark materials that provide insight into its evolution. The ALH dark clasts are thus of great interest to understanding the evolution of Vesta. Here, 45 different dark clasts from 15 different thin sections from the pairing group are characterized in detail to better understand their nature and origin. Five different textural types of dark clasts are recognized among this group—skeletal, vitrophyric, pilotaxitic, fan spherulitic, and troilite‐silica‐plagioclase‐rich clasts with aphyric or blobby textures. Mineralogy of the clasts is dominated by plagioclase and pyroxene, with minor troilite, silica, ilmenite, chromite, and rare Fe‐Ni metal. All of the textures can be produced by rapid cooling rates on the order of 60–2500°C h−1. Bulk compositions of the clasts are demonstrably eucritic, and not chondritic, howarditic, or diogenitic. The combination of mineralogy, composition, and textures strongly suggests that the dark clasts are eucritic impact melts. Several craters on Vesta have associated orange deposits that have been proposed as impact melt breccias. The ALH pairing group may thus represent material that originated near Oppia or Octavia craters.

Temperature constraints by Raman spectroscopy of organic matter in volatile-rich clasts and carbonaceous chondrites

An important question regarding the formation of the solar system is how planetary bodies developed from dust and ice into the planets and planetary bodies. A particularly interesting topic is the thermal evolution of carbonaceous chondrites and volatile-rich clasts that could have originated from CM- and CI-like parent bodies. Two types of these volatile-rich clasts, which are a particular type of dark clasts, can be found. These clasts are mineralogically very similar to CM and CI chondrites and can occasionally be found in achondritic meteorites. Mineral assemblages suggest that both CM and CI chondrites as well as volatile-rich clasts experienced low peak temperatures. However, these mineral assemblages only offer large estimated temperature ranges to describe the thermal history of CM and CI chondrites, and the thermal history of volatile-rich clasts has not been previously described. In this study, to gain a better understanding of the thermal history of both CM and CI chondrites and volatile-rich clasts, we estimated peak temperatures of 30 volatile-rich clasts (16 CI-, and 14 CM-like) in 10 different host meteorites (4 polymict ureilites, 5 polymict eucrites and 1 howardite) by Raman carbon thermometry. An automated method was developed in order to describe over 4000 collected Raman spectra using four pseudovoigt functions. The full width half maximum (FWHM) of the D1-band was then used to calculate peak temperatures. Results were then compared to Raman data of 8 different well-studied carbonaceous chondrites (including CI and CM chondrites) to evaluate the suggestion that volatile-rich clasts are composed of similar material to the equivalent CI and CM chondrites. Our results show that the peak temperatures experienced by CI-like clasts range between 30–110 °C with an average of about 65 ± 25 °C; the peak temperatures experienced by CM-like clasts range from 50 to 110 °C with an average of about 70 ± 25 °C. Six of the 8 studied carbonaceous chondrites (CM, CI, CR or C2ungr) also plot in the same low-temperature range between 50 °C and 75 °C and can thus be considered to have formed under similar temperature conditions as the volatile-rich clasts. This is in agreement with previous suggestions, based on their mineral compositions that volatile-rich clasts and CI and CM carbonaceous chondrites are composed of similar materials. The peak temperatures for carbonaceous chondrites determined in this study considerably reduce the range of temperature estimates proposed previously for these chondrites by different methods. By highlighting the ability of our methodology to evaluate data in an automated way, this study shows that Raman carbon thermometry is a good analytical technique for obtaining information about peak temperatures in small and delicate samples.

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.

Characterization of carbonaceous matter in xenolithic clasts from the Sharps (H3.4) meteorite: Constraints on the origin and thermal processing

Primitive xenolithic clasts, often referred to as “dark clasts”, are well known in many regolith breccias. The Sharps H3.4 ordinary chondrite contains unusually large dark clasts up to ∼1cm across. Poorly-graphitized carbon (PGC), with Fe, Ni metal and described as “carbon-rich aggregates”, has been reported in these clasts (Brearley, 1990). We report detailed analyses of carbonaceous matter in several identical Sharps clasts using FTIR, Raman, C-XANES, and TEM that provide insight on the extent of thermal processing and possible origin of such clasts. We also prepared acid residues of the clasts using the HCl/HF method and conducted mass spectrometric analysis of the entrained noble gases.Carbonaceous matter is often used to infer thermal history due to its sensitivity to thermal processes. The FTIR spectra of the acid residue from the Sharps clast suggest that carbonaceous matter in the clast contains less hydrogen and oxygen compared to acid residues from typical type 3.4 ordinary chondrites. The metamorphic temperatures obtained by Raman spectroscopy ranges between ∼380°C and ∼490°C. TEM observations indicate that the clasts experienced a peak temperature of 300°C to 400°C, based on the carbon d002 layer lattice spacing of C-rich aggregates. These estimates are consistent with an earlier estimate of 330±50°C, that is also estimated by the d002 layer lattice spacing (Brearley, 1990). It should be noted that the lattice spacing thermometer is based on terrestrial metamorphose rocks, and thus temperature was probably underestimated. Meanwhile, the C-XANES spectra of the C-rich aggregates show high exciton intensities, indicative of graphene structures that developed at around 700–800°C following an extensive period of time (millions of years), however, the surrounding matrix areas experienced lower temperatures of less than 300–500°C. Noble gas analysis of the acid residue from the Sharps clasts shows that the residue is almost identical with some material reported in carbonaceous chondrites, i.e., heavily enriched in the Q-gas component as well as HL-gas from presolar diamonds and Ne-E(H) from presolar SiC.These results indicate that the C-rich aggregates in the Sharps clasts formed under relatively high temperature conditions, up to 800°C, and were subsequently mixed with lower temperature matrix, probably in a different parent body, before they were incorporated into the final Sharps lithology by collision.

Allende CV隕石中の特異なDark clast:前駆物質と変成履歴の解明

Allende CV隕石中の特異なDark clast:前駆物質と変成履歴の解明

A POSSIBLE MESOARCHAEAN IMPACT STRUCTURE AT SETLAGOLE, NORTH WEST PROVINCE, SOUTH AFRICA: AEROMAGNETIC AND FIELD EVIDENCE

A 25 to 30 km wide magnetic anomaly within the >2.79 Ga granite-greenstone rocks of the northwestern Kaapvaal Craton is spatially associated with megabreccia outcrops near the village of Setlagole in the North West Province, South Africa. The breccia comprises angular to rounded clasts of TTG gneisses, granites and granodiorites, with lesser amounts of amphibolite, calc-silicate rock and banded iron-formation as well as unusual dark grey to black, irregular, centimetre- to decimetre-sized clasts that show evidence of fluidal behaviour and plastic deformation during incorporation into the breccia. The largest clasts reach up to several metres in size. Evidence of fluvial transport is found in rare thin sandy to gritty layers that show crude bedding and upward-fining with layers dipping gently to the northeast. The breccia matrix is highly variable but is dominated by angular mineral clasts (mainly quartz and feldspar, with subsidiary biotite, amphibole and epidote) with interstitial chlorite. The clasts show variable amounts of alteration (saussuritization, sericitization, chloritization of biotite and amphibole). The dark clasts contain angular quartz and feldspar and small biotite fragments in a cryptocrystalline chlorite-dominant matrix. Textures indicate a lower greenschist facies metamorphic overprint. The absence of lava, dolomite or quartzite clasts suggests that the breccia formed prior to the deposition of the Neoarchaean Ventersdorp and Eoproterozoic Transvaal Supergroups, whereas the metamorphic grade indicates that it postdates the ca. 2.79 Ga amphibolite-facies metamorphic peak in the region. This suggests a late Mesoarchaean or early Neoarchaean (ca. 2.79 to 2.71 Ga) age for the breccia. A similar age is inferred for the magnetic anomaly based on postulated cross-cutting dyke ages. Despite a comprehensive search, unequivocal shock-diagnostic microdeformation features have not yet been found in either the breccia or the highly-weathered granitic gneiss outcrops in the central parts of the anomaly. The unusual plastically-deformed dark clasts may represent chloritized mud clasts or impact melt clasts. Geochemical data on these clasts and other components of the megabreccia provide no conclusive support for a meteoritic origin, but the unparalleled composition of the clasts and their high trace element abundances of Ni, Cr, V, Zn and Co relative to rocks of the Kraaipan granite-greenstone basement, suggests an unusual origin for this matrix material. Given the distinctive nature of the breccia and its proximity to a large circular magnetic anomaly, it is postulated that the megabreccia could represent a mass or debris flow in a marine setting triggered by an impact tsunami or resurge. Subsequent faulting may have led to the preferential preservation of these deposits. This interpretation of the Setlagole megabreccia and geophysical anomaly is evaluated in terms of other possible modes of origin and it is concluded that a meteoritic source best fits the available data.

The Stannern trend eucrites: Contamination of main group eucritic magmas by crustal partial melts

We report on the petrology of a new eucrite belonging to the Stannern trend and discuss the origin of this trend. The eucrite Northwest Africa 4523 (NWA 4523) is an equilibrated eucrite consisting of dark clasts embedded in a fine-grained crystallized matrix. Two types of clasts have been observed: medium-grained ophitic/subophitic clasts, and very fine-grained clasts. Despite textural differences, the clasts display the same mineralogy, in particular the same kind of pyroxenes with pigeonitic cores containing sparse exsolution lamellae, and augitic rims, zoned plagioclases and the occurrence of K-feldspar. The major and trace element abundances of a large medium-grained clast are very similar to Stannern or Bouvante. The Stannern trend eucrites are characterized by high incompatible trace element abundances. Their trace element patterns normalized to a representative Main Group eucrite, exhibit significant Eu, Sr and Be negative anomalies. In this paper, we show that contamination of Main Group eucritic magmas by melts derived by partial melting of the asteroid’s crust can successfully explain both the high incompatible trace elements concentrations and the distinctive Eu, Sr, Be anomalies shown by the Stannern trend eucrites. This model is in agreement with the view that Stannern and some Main Group-Nuevo Laredo trend eucrites have been contemporaneously erupted, and with the probable assumption that Stannern trend eucrites formed rather late in the history of the 4-Vesta’s crust.

Delving into Allende's dark secrets

What are the strange dark clasts within carbonaceous chondrites? How did they form? And what do they tell us about the early solar system? Mark Sephton, one of a team investigating the Allende meteorite, summarizes their findings. Asteroids are remnants of the planetary construction process that occurred in the early solar system. By studying asteroids and their collision-induced fragments that fall to Earth as meteorites, we approach an understanding of the physical and chemical mechanisms that marked nascent planet formation. It is in the oldest meteorites that a record of events that began the journey from nebula material to planet-sized bodies is contained. Consequently, much attention is directed towards the most ancient class of meteorite: carbonaceous chondrites. These meteorites contain a diverse mixture of mineralogical components including high-temperature silicate “chondrules”, formed by rapid heating then cooling, but also low-temperature materials such as carbonates and phylosilicates, which are generally considered to be evidence of liquid water on the meteorite parent body. Yet the role of water and heat in the generation of other components remains enigmatic.

The halite‐bearing Zag and Monahans (1998) meteorite breccias: Shock metamorphism, thermal metamorphism and aqueous alteration on the H‐chondrite parent body

Abstract— Zag and Monahans (1998) are H‐chondrite regolith breccias comprised mainly of light‐colored metamorphosed clasts, dark clasts that exhibit extensive silicate darkening, and a halite‐bearing clastic matrix. These meteorites reflect a complex set of modification processes that occurred on the H‐chondrite parent body. The light‐colored clasts are thermally metamorphosed H5 and H6 rocks that were fragmented and deposited in the regolith. The dark clasts formed from light‐colored clasts during shock events that melted and mobilized a significant fraction of their metallic Fe‐Ni and troilite grains. The clastic matrices of these meteorites are rich in solar‐wind gases. Parent‐body water was required to cause leaching of chondritic minerals and chondrule glass; the fluids became enriched in Na, K, Cl, Br, Al, Ca, Mg and Fe. Evaporation of the fluids caused them to become brines as halides and alkalies became supersaturated; grains of halite (and, in the case of Monahans (1998), halite with sylvite inclusions) precipitated at low temperatures (≤100 °C) in the porous regolith. In both meteorites fluid inclusions were trapped inside the halite crystals. Primary fluid inclusions were trapped in the growing crystals; secondary inclusions formed subsequently from fluid trapped within healed fractures.

The Richfield LL3 chondrite

Abstract— Richfield is a moderately shocked (shock stage S4) LL3.7 genomict breccia find consisting mainly of light‐colored recrystallized clasts and dark clasts exhibiting significant silicate darkening; a few impact‐melt‐rock clasts and LL5 chondrite clasts also occur. The cosmic‐ray exposure age of 14.5 Ma is indistinguishable from the main exposure peak for LL chondrites (15 Ma). Although the exposure ages indicate little He loss, the gas‐retention ages indicate high gas losses that must have occurred prior to or during ejection from the LL parent body.

Dark clasts in the Khor Temiki aubrite: Not basalts

Abstract— The enstatite achondrite meteorites (aubrites) are ultramafic assemblages with highly variable bulk rare earth element (REE) compositions. An enrichment of REE in a dark clast from the Khor Temiki aubrite led Wolf et al. (1983) to suggest that such dark clasts could be the basaltic (i.e., enstatite‐plagioclase) complements to the ultramafic aubrites, with the relatively high REE contents resulting from the presence of plagioclase, which is a common carrier of the REEs. We have studied several dark clasts from the Khor Temiki aubrite and find no evidence for a basaltic character for such material. The microscopic character of the dark clasts is not significantly different from the main portions of Khor Temiki and consists either of highly brecciated material, containing a fine‐grained matrix, or of enstatite grains with abundant inclusions. We suggest that the dark clasts are shock‐darkened, heterogeneous Khor Temiki material that, by chance, contained variable trace contents of oldhamite (CaS), which has been shown to be a major carrier of REE in aubrites. We find that the REE contents of the clasts range from 0.1 to ∼20× CI. Most have negative Eu anomalies, but one has a small positive anomaly. Extensive searches have failed to identify basaltic material in Khor Temiki and other aubrites. The absence of basaltic material is consistent with, but does not prove, the model of Wilson and Keil (1991). They calculate that, on an asteroidal parent body < ∼100 km in radius, a volatile‐rich basaltic partial melt erupted with a velocity greater than the escape velocity of the asteroid and, thus, was lost into space ∼ 4.55 Ga ago.

Origin of dark clasts in the Acfer 059/El Djouf 001 CR2 chondrite

Abstract— The ten specimens of the paired Acfer 059/El Djouf 001 CR2 chondrite contain abundant lithic fragments which we refer to as dark clasts. Petrological and mineralogical studies reveal that they are not related to the CR2 host meteorite but are similar to dark clasts in other CR2 chondrites. Dark clasts consist of chondrule and mineral fragments, phyllosilicate fragments and clusters, magnetite, sulfides and accessory phases, embedded into a very fine‐grained, phyllosilicate‐rich matrix. Magnetite has morphologies known from CI chondrites: spherules, framboids and platelets. Average abundances of major elements in the dark clasts are mostly in the range of both CR and CV chondrites, but strong depletions in Na and S are apparent. Oxygen isotopic compositions of two dark clasts suggest relationships to type 3 carbonaceous chondrites and dark inclusions in Allende. The dark clasts are clearly different in texture and mineralogical composition from the host matrix of Acfer 059/El Djouf 001. Therefore, these dark clasts are xenoliths and are quite unlike the Acfer 059/El Djouf 001 CR2 host meteorite. We suggest that dark clasts accreted at the same time with all other components during the formation of Acfer 059/El Djouf 001 whole rock.

Unusual dark clasts in the Vigarano CV3 carbonaceous chondrite: Record of parent body process

Abstract— Two unusual dark clasts found in the Vigarano CV3 chondrite were examined using an optical microscope and a scanning electron microscope (SEM). Both clasts lack chondrules, Ca‐Al‐rich inclusions, and coarse‐grained mineral fragments; they, instead, contain abundant inclusions that consist of fine grains (<1 μm) of homogeneous Fe‐rich olivine, thus resembling the fine‐grained variety of dark inclusions in CV3 chondrites. The external shapes of inclusions in the clasts bear a close resemblance to those of chondrules and chondrule fragments; some of the inclusions are surrounded by dark rims similar to chondrule rims. Our SEM observations reveal the following unusual characteristics: 1) the inclusions are not mere random aggregates of olivine grains but have peculiar internal textures, that is, assemblages of round or oval shaped outlines, which are suggestive of pseudomorphs after porphyritic olivine chondrules; 2) one of thick inclusion rims contains a network of vein‐like strings of elongated olivine grains; 3) an Fe‐Ni metal aggregate in one of the clasts has an Fe‐, Ni‐, S‐rich halo suggesting a reaction between its precursor and the surrounding matrix; and 4) olivine in the clasts commonly shows a swirly, fibrous texture similar to that of phyllosilicate. These characteristics suggest that the dark clasts in Vigarano are not primary aggregates of dust in the solar nebula but were affected by aqueous alteration and subsequent dehydration by heating after accretion to the meteorite parent body. The fine olivine grains in these clasts were presumably produced by thermal transformation of phyllosilicate, as is the case with those in the two thermally metamorphosed Antarctic CM chondrites, Belgica‐7904 and Yamato‐86720. From textural and mineralogical similarities, some of the dark inclusions and clasts previously reported from CV3 chondrites and other types of meteorites may have origins common with these clasts in Vigarano.

Chemical studies of h chondrites: III. Regolith evolution of the fayetteville chondrite parent

Data for Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, and Zn in consortium samples of solar-gas- and track-rich dark matrix, and dark and light clasts in the Fayetteville H chondrite regolith breccia were determined by radiochemical neutron activation analysis. The matrix proved chemically homogeneous at a level never before observed in an ordinary chondrite. Most dark clasts had similar mean trace element concentrations as matrix but were compositionally much more heterogeneous. Two clasts were unusual, one having apparently formed from impact-melted soil, the other representing a mixture of 90% matrix-10% C2M chondrite. The single light clast proved compositionally similar to H4–6 chondrite falls except for its higher contents of labile Bi, Cd, In, and Tl. Relative to matrix, all other samples seemed Cs-deficient. Other H chondrite regolith breccias studied in the past had shown peculiar Cs and/or Rb contents so that these alkalis seem diagnostic of such breccias. Dark portions of such breccias apparently represent compacted regolith soil produced by isochemical comminution of “normal” dark and light clasts. The dark clasts, in turn, likely represent primordial nebular condensate, although it is possible that they represent planetary “cold traps” that garnered labile elements generated elsewhere in the parent body.

The Bholghati howardite: Chemical study

Bholghati is homogeneous on a cm scale in major element composition, but heterogeneous in trace elements such as the rare earth elements (REEs). The REEs are concentrated in accessory phosphates, and their sampling can influence the REE patterns of eucritic clasts which, in turn, can govern the bulk REE patterns. Bholghati can be reasonably modeled with 56% eucritic, 45% diogenitic, and 3% dark clast components; however, an additional component (feldspathic) is required to match the bulk composition. Dark clasts represent carbonaceous CM type material that was admixed with other components during regolith formation on the eucrite parent body (EPB). One eucritic clast (fine-grained) is light REE (LREE) depleted relative to heavy REE (HREE), which is unusual in normal eucrites. The LREE depletion pattern cannot be modeled from one-stage partial melting of a chondritic source; however, it can be modeled from a non-chondritic source depleted in LREEs or multi-stage melting of a chondritic source, provided that the LREE depletion is not from undersampling of phosphates. Bholghati has a complex history that involved early multiple magmatic events, later annealing, and fragmentation and low-temperature mixing of eucritic, diogenitic, and carbonaceous components on the howardite parent body.

The Bholghati (howardite) consortium: An overview

The Bholghati (howardite) has had a complex history. The consortium studies indicate that eucrite clasts show evidence of rapid crystallization followed by prolonged subsolidus annealing. Dark clasts are carbonaceous CM2 type. Bholghati bulk composition can be modeled by 55% eucritic, 45% diogenitic, and 3% dark clast components. The eucritic clasts show a LREE depleted pattern relative to HREEs, which is not typical of a normal eucrite. The LREE depletion requires two-stage melting from a chondritic source. The volatile/mobile trace element patterns in dark and eucritic clasts are highly variable, probably due to volatile redistribution. The eucritic clast shows excess fission Xe, which is attributed to in situ decay of Pu-244. The cosmic-ray exposure age is 10–17 Ma. The Rb-Sr, Sm-Nd, and K-Ar ages of Bholghati bulk and eucritic clasts indicate 4.53 Ga as crystallization age, 2–3 Ga as metamorphic event, and probably a minor thermal event < 1 Ga. The Bholghati evolution scenario is 1) early multiple magmatic events (4.53 Ga ago), producing eucrites and diogenites; 2) a metamorphic event (2–3 Ga ago) and prolonged subsolidus annealing; 3) fragmentation and low-temperature mixing of eucrites and diogenites; 4) low-velocity impact (< 1 Ga. ago) and admixing of carbonaceous material; 5) disruption of regolith and ejection of Bholghati 10–17 Ma ago; and 6) Bholghati fell on the Earth in 1905.

Chemical compositions of refractory inclusions from the Vigarano and Leoville carbonaceous chondrites

Major and trace elements were determined by INAA in two hibonite-rich refractory inclusions and a dark clast from Vigarano, three refractory inclusions from Leoville, and a hibonite-rich and a melilite-rich fraction from a composite refractory inclusion from Allende. The refractory inclusions from Vigarano and Leoville are all similar to their counterparts in Allende in that they have groups I, II, and III refractory element fractionation patterns, but a large proportion of these Vigarano and Leoville inclusions also contain refractory element features that are very infrequently encountered in Allende inclusions. For example, Vigarano 1623-2 is a group I inclusion with a large Ce anomaly ( La Ce = 2.34 ± 0.32 relative to Cl); Vigarano 1623-3 is a group III in which Os is strongly fractionated relative to other refractory siderophiles ( Os Re = 2.13 ± 0.08 relative to Cl); and Leoville 3537-1 is a modified group II in which removal of refractory REEs and condensation of the remaining REEs both occurred at unusually low temperatures. All samples from Vigarano and Leoville, including a fine-grained inclusion, have Na, Au, and other volatile element concentrations at or below the low end of the concentration ranges for these elements in Allende coarse-grained inclusions. This is consistent with the much lower amounts of Na-rich, secondary alteration products in the Vigarano and Leoville inclusions than in the ones from Allende. Even the Vigarano dark clast has a lower Na content than Allende dark clasts. Secondary alteration of refractory inclusions in the reduced subgroup of C3 V meteorites occurred in a part of the solar nebula where grain-gas separation processes removed the inclusions from chemical communication with the gas at a higher temperature or after a shorter time than in the case of Allende inclusions. The hibonite- and melilite-rich parts of the Allende inclusion could have formed in separate parts of the same chemical reservoir by separation of early condensing hibonite from melilite. The relatively hibonite-rich Vigarano inclusions may have accumulated originally from condensates preferentially enriched in hibonite by gravitational settling of grains of this early condensate or by their transport in convection cells.

Carbon, Oxygen, Silicon, Sulphur, Calcium, and Iron Determinations in Twenty‐four Dark Clasts and One Dark Inclusion of the Allende Meteorite by 12C(d,p)13C Nuclear Reaction and PIXE

Abstract— A study of carbon contents of millimeter‐sized dark clasts and of one dark inclusion of the Allende meteorite with the 13C(d,P0)13C method has been undertaken. The results show that C in dark Allende areas varies, perhaps in a “clustered” fashion, from near 0.3%, which is essentially the bulk C‐content of the meteorite, to at least 2.1% in the dark rim of a dark inclusion. At least one cluster has now been identified with C‐contents in the range 0.4%‐0.5%. Raman spectra of C in the dark inclusion ADIR 4E1‐fu and in matrix have also been determined. These are identical. Our results are consistent with a comparatively low temperature origin of essentially all elemental carbon in Allende.

Post-Laramide Uplift and Erosional History of Northern Wind River Basin, Wyoming: ABSTRACT

Landsat Thematic Mapper (TM) multispectral scanner images together with aerial photographs have been used to infer Laramide to Holocene tectonic events along the northern fringe of Wind River basin near Wind River Canyon, Wyoming. TM images reveal the presence of a large system of alluvial fans, terraces, and residual tongue-shaped debris deposits covering an area of 90 mi2 at the base of Copper Mountain. The debris system contains predominantly dark metasedimentary clasts. Both Eocene (Wind River and Wagon Bed Formations) and Quaternary deposits are present, and some Eocene gravel has been reworked into the later units. These deposits contrast sharply in brightness and color with rocks in adjacent areas. Detailed topographic analysis of the terraces and fan remnants disclosed an episodic history of post-Wagon Bed (upper to middle Eocene) End_Page 245------------------------------ uplift and pediment cutting. At least 3 principal stages covering a vertical interval possibly as great as 1,300 ft have been identified. Soil profiles in Quaternary gravels capping the pediments show increase in maturity consistent with age inferred from topographic elevations. These local erosional stages may record tectonic events of regional significance. Their absolute ages need to be determined. End_of_Article - Last_Page 246------------

Impact melt‐rock clasts in the Hvittis Enstatite chondrite breccia: Implications for a genetic relationship between El chondrites and aubrites

The Hvittis enstatite chondrite breccia contains 5±2 vol% dark clasts which appear to be impact melt‐rocks. The largest clast (Clast A) is 2.5 cm in diameter; it has an igneous texture, no chondrules, and is depleted in metallic Fe, Ni and sulfide. Its fine grain size, low‐Ni kamacite and Schreibersite, and rare martensite resulted from rapid cooling. The abundance of melt‐rock clasts in Hvittis suggests that the breccia resided at its parent body surface. However, the apparent absence of EL chondrites with solar wind‐implanted rare gases suggests that the EL parent body was too small to develop a substantial regolith. The bulk composition of Clast A is very similar to that of average aubrites, except for a sevenfold enrichment in S, Cr, and Mn. These elements are concentrated in the sulfides. It thus is possible that the clast was derived from one of the sulfide‐rich regions of the Hvittis matrix. If so, then aubrites may be genetically related to EL chondrites.