Showing papers in "Meteoritics & Planetary Science in 1997"

A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites

Abstract: — Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED mantle indicate that the parent body may have been completely molten during its early history. During cooling of a chondritic composition magma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded liquids are similar in composition to Main Group and Stannern trend eucrites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Subsequent fractional crystallization of a Main Group eucrite liquid, which has been isolated as a body of magma, produces the Nuevo Laredo trend and the cumulate eucrites. The predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta.

Mineralogy of meteorite groups

Abstract: Approximately 275 mineral species have been identified in meteorites, reflecting diverse redox environments, and, in some cases, unusual nebular formation conditions. Anhydrous ordinary, carbonaceous and R chondrites contain major olivine, pyroxene and plagioclase; major opaque phases include metallic Fe-Ni, troilite and chromite. Primitive achondrites are mineralogically similar. The highly reduced enstatite chondrites and achondrites contain major enstatite, plagioclase, free silica and kamacite as well as nitrides, a silicide and Ca-, Mg-, Mn-, Na-, Cr-, K- and Ti-rich sulfides. Aqueously altered carbonaceous chondrites contain major amounts of hydrous phyllosilicates, complex organic compounds, magnetite, various sulfates and sulfides, and carbonates. In addition to kamacite and taenite, iron meteorites contain carbides, elemental C, nitrides, phosphates, phosphides, chromite and sulfides. Silicate inclusions in IAB/IIICD and lIE iron meteorites consist of mafic silicates, plagioclase and various sulfides, oxides and phosphates. Eucrites, howardites and diogenites have basaltic to orthopyroxenitic compositions and consist of major pyroxene and calcic plagioclase and several accessory oxides. Ureilttes .are made up mainly of calcic, chromian olivine and low-Ca clinopyroxene embedded in a carbonaceous matrix; accessory phases include the C polymorphs graphite, diamond, lonsdaleite and chaoite as well as metallic Fe-Ni, troilite and halides. Angrites are achondrites rich in fassaitic pyroxene (i.e. , AI-Ti diopside); minor olivine with included magnesian kirschsteinite is also present. Martian meteorites comprise basalts, Iherzolites, a dunite and an orthopyroxenite. Major phases include various pyroxenes and olivine; minor to accessory phases include various sulfides, magnetite, chromite and Ca-phosphates. Lunar meteorites comprise mare basalts with major augite and calcic plagioclase and anorthositic breccias with major calcic plagioclase. Several meteoritic phases were formed by shock metamorphism. Martensite (a2-fe,Ni) has a distorted body-centered-cubic structure and formed by a shear transformation from taenite during shock reheating and rapid cooling. The C polymorphs diamond, lonsdaleite and chaoite formed by shock from graphite. Suessite formed in the North Haig ureilite by reduction of Fe and Si (possibly from olivine) via reaction with carbonaceous matrix material. Ringwoodite, the spinel form of (Mg,Fe)2Si04, and majorite, a polymorph of (Mg,Fe)Si03 with the garnet structure, formed inside shock veins in highly shocked ordinary chondrites. Secondary minerals in meteorite finds that formed during terrestrial weathering include oxides and hy-. droxides formed directly from metallic Fe-Ni by oxidation, phosphates formed by the alteration of schreibersite, and sulfates formed by alteration of troilite.

Vesta as the howardite, eucrite and diogenite parent body: Implications for the size of a core and for large‐scale differentiation

Abstract: — If Vesta is the parent body of the howardite, eucrite, and diogenite (HED) meteorites, then geo-chemical and petrologic constraints for the meteorites may be used in conjunction with astronomical constraints for the size and mass of Vesta to (1) determine the size of a possible metal core in Vesta and (2) model the igneous differentiation and internal structure of Vesta. The density of Vesta and petrologic models for HED meteorites together suggest that the amount of metal in the parent body is <25 mass%, with a best estimate of ∼5%, assuming no porosity. For a porosity of up to 5% in the silicate fraction of the asteroid, the permissible metal content is <30%. These results suggest that any metal core in the HED parent body and Vesta is not unusually large. A variety of geochemical and other data for HED meteorites are consistent with the idea that they originated in a magma ocean. It appears that diogenites formed by crystal accumulation in a magma ocean cumulate pile and that most noncumulate eucrites (excepting such eucrites as Bouvante and Statinem) formed by subsequent crystallization of the residual melts. Modelling results suggest that the HED parent body is enriched in rare earth elements by a factor of ∼2.5–3.5 relative to CI-chondrites and that it has approximately chondritic Mg/Si and Al/Sc ratios. Stokes settling calculations for a Vesta-wide, nonturbulent magma ocean suggest that early-crystallizing magnesian olivine, orthopyroxene, and pigeonite would have settled relatively quickly, permitting fractional crystallization to occur, but that later-crystallizing phases would have settled (or floated) an order of magnitude more slowly, allowing, instead, a closer approach to equilibrium crystallization for the more evolved (eucritic) melts. This would have inhibited the formation of a plagioclase-flotation crust on Vesta. Plausible models for the interior of Vesta, which are consistent with the data for HED meteorites and Vesta, include a metal core (<130 km radius), an olivine-rich mantle (∼65–220 km thick), a lower crustal unit (∼12–43 km thick) composed of pyroxenite, from which diogenites were derived, and an upper crustal unit (∼23–42 km thick), from which eucrites originated. The present shape of Vesta (with ∼60 km difference in the maximum and minimum radius) suggests that all of the crustal materials, and possibly some of the underlying olivine from the mantle, could have been locally excavated or exposed by impact cratering.

Constraints on the role of impact heating and melting in asteroids

Abstract: (Received 1996 August 30; accepted in revised form 1997 January 30) Abstract-Imaging of asteroids Gaspra and Ida and laboratory studies of asteroidal meteorites show that im- pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks. However, in recent years, impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids, such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events. The latter were proposed to explain the origin of ureilites, aubrites, mesosiderites, the Eagle Station pallasites, acapulcoites, lodranites, and the IAB, IIICD, and IIE irons. We considered fundamental observations from terrestrial impact craters, combined with results from laboratory shock experiments and theoretical consider- ations, to evaluate the efficiency of impact heating and melting of asteroids. Studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster- oids will produce two types of impact melts: (1) large-scale whole rock melts (total melts, not partial melts) at high shock pressure and (2) localized melts formed at the scale of the mineral constituents (mineral specific or grain boundary melting) at intermediate shock pressures. The localized melts form minuscule amounts of melt that quench and solidify in sifu, thus preventing them from pooling into larger melt bodies. Partial melting as defined in petrology has not been observed in natural and experimental shock metamor- phism and is thermodynamically impossible in a shock wave-induced transient compression of rocks. The total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater. Internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids, and impact melt rocks are usually clast-laden. Thermal metamorphism of country rocks by impact is extremely minor. Experimental and theoretical considerations suggest that (1) single dis- ruptive impacts cannot raise the average global temperature of strength- or gravity-dominated asteroids by more than a few degrees; (2) cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter; (3) small crater size, low gravity, and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small (-0.014.1%) fraction of the total displaced crater volume; (4) total impact melt volume formed during the typical lifetime of an asteroid is a small fraction (<0.001) of the volume of impact-generated debris; and (5) much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and, thus, not retained on the asteroid. The inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above, and we must turn to processes other than impact, such as decay of short-lived radionuclides or electromagnetic induction during an early T-tauri phase of the Sun to explain heating and melting of the parent bodies of these meteorites.

The Leonard Award Address - Presented 1996 July 25, Berlin, Germany - The Elemental Composition Of Stony Cosmic Spherules

Abstract: Five hundred stony cosmic spherules collected from deep-sea sediments, polar ice, and the strato- sphere have been analyzed for major and some minor element composition. Typical spherules are products of atmospheric melting of millimeter sized and smaller meteoroids. The samples are small and modified by atmospheric entry, but they are an important source of information on the composition of asteroids. The spherules in this study were all analyzed in an identical manner, and they provide a sampling of the solar system's asteroids that is both different and less biased than provided by studies of conventional meteorites. Volatile elements such as Na and S are depleted due to atmospheric heating, while siderophiles are depleted by less understood causes. The refractory nonsiderophile elements appear not to have been significantly dis- turbed during atmospheric melting and provide important clues on the elemental composition of millimeter sized meteoroids colliding with the Earth. Typical spherules have CM-like composition that is distinctively different than ordinary chondrites and most other meteorite types. We assume that C-type asteroids are the primary origin of spherules with this composition. Type S asteroids should also be an important source of the spherules, and the analysis data provide constraints on their composition. A minor fraction of the spher- ules are melt products of precursor particles that did not have chondritic elemental compositions. The most common of these are particles that are dominated by olivine. The observed compositions of spherules are in- consistent with the possibility that an appreciable fraction of the spherules are simply chondrules remelted during atmospheric entry.

Original size of the Vredefort Structure: Implications for the geological evolution of the Witwatersrand Basin

Abstract: — Historically, there have been a range of diameter estimates for the large, deeply eroded Vredefort impact structure within the Witwatersrand Basin, South Africa. Here, we estimate the diameter of the transient cavity at the present level of erosion as ∼124–140 km, based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted Transvaal outliers. Taking erosion into account (<6 km) and scaling to original final rim diameter, an estimate of close to 300 km for the rim diameter is obtained. Independent estimates of the final rim diameter, based on an empirical relation of central uplift diameter to rim diameter, spatial distribution of pseudotachylites, and concentric large scale structural patterns, give a similar estimate of close to 300 km for the original final rim diameter. An impact structure of this size is expected to have had an original multi-ring form. At this size, the Vredefort impact structure encompasses the bulk of the Witwatersrand Basin, which appears to owe its preservation to the Vredefort impact. In addition, the Vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area, which, in recent interpretations, is believed to be a component in the creation of the world-class gold deposits of the Witwatersrand Basin.

Origin of fayalitic olivine rims and lath‐shaped matrix olivine in the CV3 chondrite Allende and its dark inclusions

Abstract: — We have studied an Allende dark inclusion by optical microscopy, scanning electron microscopy, electron microprobe analysis and transmission electron microscopy. The inclusion consists of chondrules, isolated olivines and matrix, which, as in the Allende host, is mainly composed of 5–20 μm long lath-shaped fayalitic grains with a narrow compositional range (Fa42 ± 2) and nepheline. Olivine phenocrysts in chondrules and isolated olivine grains show various degrees of replacement by 5–10 μm wide fayalitic rims (Fa39 ± 2) and 100–1000 μm wide translucent zones, which consist of 5–20 μm long lath-shaped fayalitic grains (Fa41 ± 1) intergrown with nepheline. These fayalitic olivines, like those in the matrix of the dark inclusion, contain 10–20 nm sized inclusions of chromite, hercynite, and Fe-Ni sulfides. The fayalitic rims around remnant olivines are texturally and compositionally identical to those in Allende host, suggesting that they have similar origins. Chondrules are surrounded by opaque rims consisting of tiny lath-shaped fayalitic olivines (<1–3 μm long) intergrown with nepheline. As in the Allende host, fayalitic olivine veins may crosscut altered chondrules, fine-grained chondrule rims and extend into the matrix, indicating that alteration occurred after accretion. We infer that fayalitic olivine rims and lath-shaped fayalites in Allende and its dark inclusions formed from phyllosilicate intermediate phases. This explanation accounts for (1) the similarity of the replacement textures observed in the dark inclusion and Allende host to aqueous alteration textures in CM chondrites; (2) the anomalously high abundances of Al and Cr and the presence of tiny inclusions of spinels and sulfides in fayalitic olivines in Allende and Allende dark inclusions; (3) abundant voids and defects in lath-shaped fayalites in the Allende dark inclusion, which may be analogous to those in partly dehydrated phyllosilicates in metamorphosed CM/CI chondrites. We conclude that the matrix and chondrule rims in Allende were largely converted to phyllosilicates and then completely dehydrated. The Allende dark inclusions experienced diverse degrees of aqueous/hydrothermal alteration prior to complete dehydration. The absence of low-Ca pyroxene in the dark inclusion and its significant replacement by fayalitic olivine in Allende is consistent with the lower resistance of low-Ca pyroxene to aqueous alteration relative to forsteritic olivine. Hydro-thermal processing of Allende probably also accounts for the low abundance of planetary noble gases and interstellar grains, and the formation of nepheline, sodalite, salite-hedenbergite pyroxenes, wollastonite, kirschsteinite and andradite in chondrules and Ca,Al-rich inclusions.

Impact origin of the Vesta family

Abstract: — The compelling petrographic link (Consolmagno and Drake, 1977; Gaffey, 1983) between basaltic achondrite meteorites and the ∼530 km diameter asteroid 4 Vesta has been tempered by a perceived difficulty in launching rocks from this asteroid's surface at speeds sufficient to bring them to Earth (Wasson and Wetherill, 1979) without obliterating Vesta's signature crust. I address this impasse in response to recent imaging (Zellner et al, 1996; Dumas and Hainaut, 1996) of a ∼450 km impact basin across Vesta's southern hemisphere (Thomas et al., 1997) and model the basin-forming collision using a detailed two-dimensional hydrocode with brittle fracture including self-gravitational compression (cf., Asphaug and Melosh, 1993). A ∼42 km diameter asteroid striking Vesta's basaltic crust (atop a denser mantle and iron core) at 5.4 km/s launches multikilometer fragments up to ∼600 m/s without inverting distal stratigraphy, according to the code. This modeling, together with collisional, dynamical, rheological and exposure-age timescales (Marzari et al., 1996; Welten et al., 1996), and observations of V-type asteroids (Binzel and Xu, 1993) suggests a recent (<∼1 Ga) impact origin for the Vesta family and a possible Vesta origin for Earth-approaching V-type asteroids (Cruik-shank et al., 1991).

The Barringer Award Address Presented 1996 July 25, Berlin, Germany: Impact experiments related to the evolution of planetary regoliths

Abstract: — Impact-induced comminution of planetary surfaces is pervasive throughout the solar system and occurs on submillimeter to global scales, resulting in comminution products that range from fine-grained surface soils, to massive, polymict ejecta deposits, to collisionally fragmented objects. Within this wide range of comminution products, we define regoliths in a narrow sense as materials that were processed by repetitive impacts to dimensional scales comparable to or smaller than that of component minerals of the progenitor rock(s). In this paper, we summarize a wide variety of impact experiments and other observations that were primarily intended to understand the evolution of regoliths on lunar basalt flows, and we discuss some of their implications for asteroidal surfaces. Cratering experiments in both rock and noncohesive materials, combined with photogeologic observations of the lunar surface, demonstrate that craters <500 m in diameter contribute most to the excavation of local bedrock for subsequent processing by micrometeorites. The overall excavation rate and, thus, growth rate of the debris layer decreases with time, because the increasingly thicker fragmental layer will prevent progressively larger projectiles from reaching bedrock. Typical growth rates for a 5 m thick lunar soil layer are initially (∼≥3 Ga ago) a few mm/Ma and slowed to <1 mm/Ma at present. The coarse-grained crater ejecta are efficiently comminuted by collisional fragmentation processes, and the mean residence time of a 1 kg rock is typically 10 Ma. The actual comminution of either lithic or monomineralic detritus is highly mineral specific, with feldspar and mesostasis comminuting preferentially over pyroxene and olivine, thus resulting in mechanically fractionated fines, especially at grain sizes <20 μm. Such fractionated fines also participate preferentially in the shock melting of lunar soils, thus giving rise to “agglutinate” melts. As a consequence, agglutinate melts are systematically enriched in feldspar components relative to the bulk composition of their respective host soil(s). Compositionally homogeneous, impact derived glass beads in lunar soils seem to result from micrometeorite impacts on rock surfaces, reflecting lithic regolith components and associated mineral mixtures. Cumulatively, experimental and observational evidence from lunar mare soils suggests that regoliths derive substantially from the comminution of local bedrock; the addition of foreign, exotic components is not necessary to explain the modal and chemical compositions of diverse grain size fractions from typical lunar soils. Regoliths on asteroids are qualitatively different from those of the Moon. The modest impact velocities in the asteroid belt, some 5 km s−1, are barely sufficient to produce impact melts. Also, substantially more crater mass is being displaced on low-gravity asteroids compared to the Moon; collisional processing of surface boulders should therefore be more prominent in producing comminuted asteroid surfaces. These processes combine into asteroidal surface deposits that have suffered modest levels of shock metamorphism compared to the Moon. Impact melting does not seem to be a significant process under these conditions. However, the role of cometary particles encountering asteroid surfaces at presumably higher velocities has not been addressed in the past. Unfortunately, the asteroidal surface processes that seemingly modify the spectral properties of ordinary chondrites to match telescopically obtained spectra of S-type asteroids remain poorly understood at present, despite the extensive experimental and theoretical insights summarized in this report and our fairly mature understanding of lunar surface processes and regolith evolution.

Mineralogical records of early planetary processes on the howardite, eucrite, diogenite parent body with reference to Vesta

Abstract: — Mineralogical information recovered from the howardite, eucrite, diogenite (HED) meteorites was employed to reconstruct the history of the parent body and relate it to 4 Vesta. These interpreted crustal evolution processes were then compared to the expected geological features on the surface of a likely proto-planet, 4 Vesta. The original crustal materials of the HED parent body were preserved as mineral grains and lithic clasts, but in many eucrites, Fe/Mg ratios in pyroxenes were homogenized by diffusion after crystallization. The crystallization trend of the protocrust has been deciphered by (1) examining monomict and crystalline samples and using their mineralogical and chemical information to formulate a sequence of crystallization and cooling trends; and by (2) reconstructing the original crust prior to cratering events from lithic clasts and mineral fragments in polymict breccias such as howardites and polymict eucrites. Mineral components are identical, both in the individual HED and in polymict breccias, and no remnants of primitive materials were preserved in the polymict breccias. A layered crust model reconstructed from such breccias consists of an upper crust with extrusive lava-like eucrites that have been brecciated and metamorphosed, diogenite mantle, and cumulate eucrites of varying thickness between them. This model can be used to explain the surface geological features of Vesta observed from the Hubble space telescope. A large crater with diogenitic orthopyroxene at the crater floor is consistent with the deepest diogenitic layer of the layered crust model; and an underlying olivine layer is expected from early crystallized olivine in the crystal fractionation model. The old terrain of eucritic surface materials of Vesta can be howardites, polymict eucrites, or regolith-like eucrites produced from eucrites extruded and impacted on the surface. Partial melting models of eucrites seem to be favored by the rare-earth element (REE) chemistry and experimental studies. Unfortunately, partial melting models have not demonstrated how the HED parent body is converted to a layered crust without producing any metamorphosed primitive material in the layered crust. The origin of cumulate eucrites with systematic variation of textures and chemistries of pyroxene can be explained by the layered crust model with excavation and mixing of trapped liquid. Discovery of basaltic materials with Na-rich plagioclase and augite in iron meteorites, which are the products of partial melting, suggests that eucrites may be unique to a body that underwent large-scale differentiation and metamorphism.

Suevite breccia of the Ries impact crater, Germany: Petrography, chemistry and shock metamorphism of crystalline rock clasts

Abstract: — Clasts of deep-seated crystalline basement rocks in suevites of the Ries crater, Germany, were catalogued lithologically and classified with regard to their degree of shock metamorphism. The sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior. These clasts can be grouped into seven types of metamorphic and nine types of igneous rocks. One hundred forty-three clasts, representing these lithologies, were analyzed for major element bulk composition. The fallout suevite contains on average 4 vol% of crystalline basement clasts, 0.4 vol% of sedimentary rocks, 16 vol% of glass bodies (some of them aerodynamically shaped), and 79 vol% of groundmass. On average, 52% of all crystalline clasts are from metamorphic sources and 42% are of igneous origin. Using the shock classification of Stoffler (1974), 8% of all crystalline clasts appear unshocked (<10 Gpa), and 34, 30 and 27% of clasts are shocked to stages I (10–35 Gpa), II (35–45 GPa) and III (45–60 GPa), respectively. The bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations. This indicates that the glasses originate by shock-fusion of a similarly composed basement. The crater suevite contains the same crystalline rock types that occur in the fallout suevites. The bore hole “Nordlingen 1973” yields an average of 62 vol% metamorphic and 38 vol% igneous rocks. The crater suevite differs from fallout suevites by a higher clast/glass ratio, by preponderance (65–95%) of clasts shocked to stage I only, and by the absence of aerodynamically shaped glass bodies. The source of crystalline clasts and melt particles of suevites is a volume of rocks, located deep in the crystalline basement, to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 GPa (deep-burst impact model). Fallout suevites were ejected, propelled by an expanding plume of vaporized rock, and withdrew preferentially from this volume melt and highly shocked clasts, leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt.

The porosity and permeability of chondritic meteorites and interplanetary dust particles

Abstract: — We have investigated the porosity of a large number of chondritic interplanetary dust particles (IDPs) and meteorites by three techniques: standard liquid/gas flow techniques, a new, noninvasive ultrasonic technique, and image processing of backscattered images The latter technique is obviously best-suited to sub-kilogram sized samples We have also measured the gas and liquid permeabilities of some chondrites by two techniques: standard liquid/gas flow techniques, and a new, nondestructive pressure release technique We find that chondritic IDPs have a somewhat bimodal porosity distribution Peaks are present at 0 and 4% porosity; a tail then extends to 53% Type 1–3 chondrite matrix porosities range up to 30%, with a peak at 2% The bulk porosities for type 1–3 chondrites have the same approximate range as exhibited by the matrix, which indicates that other components of the bulk meteorites (including chondrules and aggregates) have the same average porosity as the matrix These results reveal that the porosities of primitive materials at scales ranging from nanogram to kilogram are similar, which implies that similar accretion dynamics operated through 12 orders of size magnitude Permeabilities of the investigated chondrites vary by several orders of magnitude, and there appears to be no simple dependence of permeability with degree of aqueous alteration, chondrite type or porosity

Some things we can infer about the Moon from the composition of the Apollo 16 regolith

Abstract: Characteristics of the regolith of Cayley plains as sampled at the Apollo 16 lunar landing site are reviewed and new compositional data are presented for samples of less than 1 mm fines ('soils') and 1-2 mm regolith particles. As a means of determining which of the many primary (igneous) and secondary (crystalline breccias) lithologic components that have been identified in the soil are volumetrically important and providing an estimate of their relative abundances, more than 3 x 10(exp 6) combinations of components representing nearly every lithology that has been observed in the Apollo 16 regolith were systematically tested to determine which combinations best account for the composition of the soils. Conclusions drawn from the modeling include the following. At the site, mature soil from the Cayley plains consists of 64.5% +/- 2.7% components representing 'prebasin' materials: anorthosites, feldspathic breccias, and a small amount (2.6% +/- 1.5% of total soil) of nonmare, mafic plutonic rocks, mostly gabbronorites. On average, these components are highly feldspathic, with average concentrations of 3l-32% Al2O3 and 2-3% FeO and a molar Mg/(Mg+Fe) ratio of O.68. The remaining 36% of the regolith is syn- and postbasin material: 28.8% +/- 2.4% mafic impact-melt breccias (MIMBS, i.e., 'LKFM' and 'VHA basalts') created at the time of basin formation, 6.0% +/- 1.4% mare-derived material (impact and volcanic glass, crystalline basalt) with an average TiO2 concentration of 2.4%, and 1% postbasin meteoritic material. The MIMBs are the principal (80-90%) carrier of incompatible trace elements (rare earths, Th, etc.) and the carrier of about one-half of the siderophile elements and elements associated with mafic mineral phases (Fe, Mg, Mn, Cr, Sc). Most (71 %) of the Fe in the present regolith derives from syn- and postbasin sources (MIMBS, mare-derived material, and meteorites). Thus, although the bulk composition of the Apollo 16 regolith is nominally that of noritic anorthosite, the noritic part (the MIMBs) and anorthositic parts (the prebasin components) are largely unrelated.

Magnesium oxide-iron oxide mass balance constraints and a more detailed model for the relationship between eucrites and diogenites

Abstract: — According to a currently popular model for petrogenesis on the howardite, eucrite, and diogenite (HED) parent asteroid, the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite-dominated plutons. This model can be tested for consistency with mass balance for MgO and FeO, assuming the overall diogenite/(diogenite + eucrite) ratio, d, of the parent asteroid is at least comparable to the average d for the eucrite + diogenite dominated howardite regolith breccias. Average mg# (=MgO/[MgO + FeO]) is much lower for eucrites, especially noncumulate eucrites, than for diogenites. Unless the diogenite parent magmas eventually produced a large proportion of low-mg# residual basalt and gabbro (RBG), the implied initial magma's mg# is vastly higher than that of any noncumulate eucrite. Starting from a source previously depleted by putative primary eucrite genesis, melt mg# can be estimated as a function of the exchange reaction KD and degree of melting. Using several very conservative assumptions (e.g., assuming that the total [MgO + FeO] concentration is nearly the same in the nascent melt as in the residual solids), the degree of melting required to yield a melt with mg# high enough to satisfy mass balance, without implying an RBG component that accounts for >50% of all eucrites, is an implausibly high 60–80 wt%. The separate orthopyroxenitic plutons (SOP) model also seems inconsistent with the uniform density of melts across the diogenite-eucrite compositional spectrum (2.77–2.82 g/cm3), which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas. This difficulty cannot be reduced by simply assuming that later-formed magmas were systematically both more plutonic and more MgO-rich than earlier ones, because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg# than noncumulate eucrites. Conceivably, the bulk mg# of the asteroid's silicate system was increased between primary-melt eucrite genesis and SOP diogenite genesis by graphite-fueled reduction of FeO. However, the graphite oxidation process generates a huge proportion of gas, which would have enhanced the buoyancy of the nascent diogenite-parent magmas, thus exacerbating the difficulty of achieving the implied high degrees of partial melting. To avoid these difficulties but still form most eucrites as rapidly cooled extrusives, I propose the NERD (noncumulate eucrites as extruded residua of diogenites) model. In this model, the diogenites form as early cumulates from a large magma system (probably a global “magma ocean”) that yields a large proportion of eucritic melt as residuum. This residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions, dikes and sills of rapidly cooled noncumulate eucrites. Slight (∼5–10%) porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone. The common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone. The NERD model's greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the HED asteroid.

Thoroughly anomalous chromium in Orgueil

Abstract: — Stepwise dissolution of bulk Orgueil reveals that all of the Cr in the whole rock is isotopically anomalous, with an anomaly pattern that is thus far unique. Most of the Cr (along with other major and minor cations) is dissolved by acetic and nitric acids; it is deficient in 54Cr by ∼5 ɛ. Subsequent treatment with hydrochloric acid dissolves a small fraction of the Cr with positive 54Cr anomalies, up to ∼210 ɛ. Mass balance indicates that whole rock Cr is isotopically normal within analytical uncertainties. The least extravagant interpretation of these results is that some mineral phase is enriched in a heavy-Cr nucleosynthetic component, while most of the Cr is a homogenized mixture of diverse nucleosynthetic components that would be normal except for lack of the postulated heavy Cr carrier. The carrier is likely, but not necessarily, presolar interstellar grains. Its identity is unknown and constrained only circumstantially: it must be relatively rich in Cr, it is substantially soluble in hydrochloric acid, and it is not magnetite or spinel/chromite. Scanning electron microscope (SEM) examination of Orgueil reveals candidate Cr-rich oxides, silicates, sulfides and phosphides, but none of these can be identified yet as the heavy Cr carrier. Whether presolar or not, the carrier is not chemically resistant and likely not thermally refractory, thereby differing from most other phases known to host isotopic anomalies. Its survival (or production) thus establishes constraints on a different regime of nebular history.

A new empirical cooling rate indicator for meteorites based on the size of the cloudy zone of the metallic phases

Abstract: A new empirical cooling rate indicator for metal particles is proposed. The cooling rate indicator is based on the relationship between the size of the island phase in the cloudy zone, which abuts the outer taenite rim (clear taenite I), and the cooling rate of the host meteorite as obtained by conventional metal- lographic techniques. The size of the island phase was measured by high-resolution scanning electron micros- copy (SEM) in 26 meteorites and decreases fiom 470 nm to 17 nm, while the cooling rate of the host meteorite increases from 0.5 Ma to 325 Ma. This island phase size vs. cooling rate relationship is independent of whether the host is an iron, stony-iron, or stony meteorite and can be used to estimate the low-temperature cooling rate of the host meteorite. The measurement of the size of the island phase in the cloudy zone can also be applied to a large number of meteorites.

Fullerenes, fulleranes and polycyclic aromatic hydrocarbons in the Allende meteorite

Abstract: In this paper, we confirm our earlier observations of fullerenes (C60 and C70) in the Allende meteorite (Becker et al., 1994a, 1995). Fullerene C60 was also detected in two separate C-rich (approximately 0.5-1.0%) dark inclusions (Heymann et al., 1987) that were hand picked from the Allende sample. The amounts of C60 detected were approximately 5 and approximately 10 ppb, respectively, which is considerably less than what was detected in the Allende 15/21 sample (approximately 100 ppb; Becker et al., 1994a, 1995). This suggests that fullerenes are heterogeneously distributed in the meteorite. In addition, we present evidence for fulleranes, (C60Hx), detected in separate samples by laser desorption (reflectron) time-of-flight (TOF) mass spectrometry (LDMS). The LDMS spectra for the Allende extracts were remarkably similar to the spectra generated for the synthetic fullerane mixtures. Several fullerane products were synthesized using a Rh catalyst (Becker et al., 1993a) and separated using high-performance liquid chromatography (HPLC). Polycyclic aromatic hydrocarbons (PAHs) were also observed ppm levels) that included benzofluoranthene and corannulene, a cup-shaped molecule that has been proposed as a precursor molecule to the formation of fullerenes in the gas phase (Pope et al., 1993).

Water in tektites and impact glasses by fourier‐transformed infrared spectrometry

Abstract: — To improve the scarce data base of H2O content in tektites and impact glasses, we analyzed 26 tektites from all four strewn fields and 25 impact glass samples for their H2O content We used the fourier-transformed infrared (FTIR) spectrometry method, which permits measurement of areas of ∼40 μm in diameter Our results show that the tektites have H2O contents ranging from 0002 to 0030 wt% (average 0014 ± 0008 wt%) Ivory Coast tektites have the lowest H2O abundances (0002–0003 wt%), and Muong Nong-type indochinites and some North American tektites having the highest contents (up to ∼003 wt%) Impact glass samples (from the Zhamanshin, Aouelloul, and Rio Cuarto craters) yielded H2O contents of 0008 to 013 wt% H2O Typical impact glasses from the Aouelloul and Zhamanshin craters have low H2O contents (0008 to 0063 wt%) Libyan Desert Glasses and Rio Cuarto glasses have higher H2O contents (∼011 wt%) We also analyzed glasses of unknown origin (eg, urengoites; glass fragments from Tikal), which showed very low H2O contents, in agreement with an origin by impact Our data confirm that all tektites found on land have very low H2O contents (<003 wt% H2O), while impact glasses have slightly higher H2O contents Both glass types are very dry compared to volcanic glasses This study confirms that the low H2O contents (<005 wt%) of such glasses can be considered good evidence for an origin by impact

Vesta fragments from v6 and 3:1 resonances: Implications for V‐type near‐Earth asteroids and howardite, eucrite and diogenite meteorites

Abstract: — A large body of evidence, including the presence of a dynamical family associated with 4 Vesta, suggests that this asteroid might be the ultimate source of both the V-type near-Earth asteroids (NEAs) and howardite, eucrite and diogenite (HED) meteorites. Dynamical routes from Vesta to the inner regions of the solar system are provided by both the 3:1 mean-motion resonance with Jupiter and the V6, secular resonance. For this reason, numerical integrations of the orbits of fictitious Vesta fragments injected in both of these resonances have been performed. At the same time, the orbital evolution of the known V-type NEAs has been investigated. The results indicate that the dynamical half lifetimes of Vesta fragments injected in both the 3:1 and the V6, resonances are rather short ('2 Ma). The present location of the seven known V-type NEAs is better explained by orbital evolutions starting from the v6 secular resonance. The most important result of the present investigation, however, is that we now face what we call the “Vesta paradox.” Roughly speaking, the paradox consists of the fact that the present V-type NEAs appear to be too dynamically young to have originated in the event that produced the family, but they are too big to be plausible second-generation fragments from the family members. The cosmic-ray exposure (CRE) age distribution of HED meteorites also raises a puzzle, since we would expect an overabundance of meteorites with short CRE ages. We propose different scenarios to explain these paradoxes.

Cosmic‐ray exposure ages of diogenites and the recent collisional history of the howardite, eucrite and diogenite parent body/bodies

Abstract: ~~~ Abstract-We determined the cosmic-ray exposure age of 20 diogenites from measured cosmogenic noble gas isotopes and calculated production rates of 3He, 21Ne and 38Ar. The production rates were calculated on the basis of the measured chemical composition and the cosmogenic 22Ne/21Ne ratio of each sample. The shielding conditions of each sample were also checked on the basis of the measured I0Be and 26A1 concen- trations. The exposure ages range from 6 to 50 Ma but do not form a continuous distribution: ten ages clus- ter at 2 1-25 Ma and four at 3542 Ma. The two diogenite clusters coincide with the 22 Ma and 38 Ma peaks in the exposure age distribution of eucrites and howardites. After the selection from literature data of 32 eu- crites and 11 howardites with reliable ages, we find a total of 23 howardite, eucrite and diogenite (HED) group meteorites at 20-25 Ma and 10 at 3542 Ma. The shape of the two peaks is consistent with single im- pact events, and random number statistics show that they are statistically significant at the 99% level. Alto- gether, this provides strong evidence for two major impact events 22 Ma and 39 Ma ago. Although these two events can explain more than half of all HED exposure ages, it takes at least five impact events to ex- plain all ages <50 Ma. An impact frequency of one per 10 Ma corresponds to projectiles of at least 24 km in diameter for Vesta and of 60-300 m for the lOOx smaller Vesta-derived "vestoids." Based on the HED exposure-age distribution, the size distribution of the main-belt asteroids and the difference in size between Vesta and the kilometer size vestoids, we favor Vesta as the major source of HED meteorites, although some of the meteorites may have been ejected from the vestoids rather than directly from Vesta.

A chondrule origin for dusty relict olivine in unequilibrated chondrites

Abstract: We address the origin of ''dusty,'' metal-bearing relict olivine grains in chondrules. It has been suggested previously that these grains may be either primitive condensates or derived from a previous generation of chondrules. In this paper, we infer the original composition of dusty olivine grains, before they were reduced, and examine the possibility that they were derived from a previous generation of chondrules. Original compositions of dusty grains, including their estimated initial FeO contents and their minor element contents, match closely with compositions of olivines from chondrules in unequilibrated chondrites. In addition, the cores of some dusty grains are unaltered, and the compositions of these cores are also consistent with a chondrule origin. Therefore, we conclude that a derivation from a previous generation of chondrules is a plausible origin for these relicts. Although alternative origins, such as condensates or interstellar grains, cannot be ruled out on the basis of the available data, chondrules are an obvious source, and we suggest that this is the most likely interpretation. If this is the case, it is additional evidence for the importance of recycling of chondrule material in the chondrule-forming region.

Spectral characteristics of iron‐bearing phyllosilicates: Comparison to Orgueil (CI1), Murchison and Murray (CM2)

Abstract: — Phyllosilicate alteration minerals are commonly found in low petrologic types of carbonaceous chondrites. Previous spectral studies have examined Mg-bearing phyllosilicates with limited success in matching the spectral properties of CM and CI chondrites. Transmission electron microscope and other analytical techniques suggest that Fe-bearing clays are more abundant in CI and CM chondrites than magnesian varieties. Here, we present the results of an examination of the reflectance spectra of Fe-phyllosilicates, including serpentines and berthierines, of which the latter were formerly known as septechlorites. We have measured the diffuse reflectance spectra of powdered samples from 0.3 to 25 μn. We find that these minerals provide a better spectral match to many of the features seen in CI and CM chondrites, and simple linear combinations of the spectra of both Fe- and Mg-phyllosilicates closely approximate the spectra of CM and CI chondrites.

Planetary cratering 2: Studies of saturation equilibrium

Abstract: — A realistic computer model has been developed to display images of imaginary cratered surfaces, taking into account empirically measured input size distributions of primary and secondary craters, ejecta blanket morphology including feathering with distance, obliteration due to ejecta from outside the imaged area, lighting effects, etc. The model allows us to track surface evolution of morphology as new craters are added. Using the model as well as lunar photos, we have studied the approach to saturation equilibrium (defined as a condition when no further proportionate increase in crater density occurs as input cratering increases). We find that an identifiable saturation equilibrium occurs close to a level previously identified for this state (Hartmann, 1984), typically fluctuating around a crater density from ∼0.4 to 2 times that level. This result is fairly robust vis-a-vis the range of model parameters we have chosen. Flooding, basin ejecta blankets, and other obliterative effects can introduce structure and oscillations within this range, even after saturation equilibrium is achieved. These findings may constrain or revise certain earlier interpretations of satellite and planet surface evolution and impactor populations, which were predicated on the assumed absence of saturation equilibrium. In our fourth experimental run, we found that suppression of “sandblasting” by subresolution impacts allows the smallest secondaries to rise above the saturation equilibrium line, a result that might be relevant to a similar situation on Gaspra and perhaps some other asteroids.

Mineralogy of Meteorite Groups: An Update

Abstract: Twenty minerals that were not included in the most recent list of meteoritic minerals have been reported as occurring in meteorites. Extraterrestrial anhydrous Ca phosphate should be called menillite, not whitlockite.

Complex exposure histories for meteorites with “short” exposure ages

Abstract: — We report measurements of 26Al and 10Be activities in nine ordinary chondrites and of the light noble gas concentrations and 36Cl and 41Ca activities in subsets of those meteorites. All but Murray have low 21Ne concentrations (<1.0 × 10−8cm3STP/g) and have previously been used to estimate 21Ne production rates. Ladder Creek, Murchison, Sena, and Timochin have inventories of cosmogenic radionuclides that are compatible with a single stage of irradiation and give 21Ne production rates that are consistent with the standard L-chondrite value of 0.33 × 10−8cm3STP/g/Ma. In contrast, Cullison, Guenie, Shaw, and Tsarev experienced complex irradiation histories. They and several other meteorites with low nominal exposure ages also have lower 3He/21Ne ratios than expected based on their 22Ne/21Ne ratios. A general association between low 21Ne contents and 3He losses suggests that meteorites with short lifetimes often occupy orbits with small perihelia. However, meteorites with low 21Ne contents, one-stage exposure histories, and losses of cosmogenic 3He are rare. Possible explanations for the scarcity are (1) statistical, (2) that it is harder for more deeply buried protometeoroids to lose gas in a liberating collision, and (3) that it is harder to insert more deeply buried protometeoroids directly into orbits with small perihelia.

Absorption bands near three micrometers in diffuse reflectance spectra of carbonaceous chondrites: Comparison with asteroids

Abstract: — We measured infrared diffuse reflectance spectra of several carbonaceous chondrites in order to obtain additional information on the surface materials of their presumed parent bodies, C-type asteroids. The presence and intensity of absorption bands near 3 μm in the reflectance spectra are due to the presence and abundance of hydrates and/or hydroxyl ions. The absorption features of the 3 μm hydration bands of carbonaceous chondrites were compared with those of asteroids 1 Ceres and 2 Pallas. They are commonly classified into separate subtypes, G- and B-type. The spectral shapes of Pallas and Renazzo (CR2 chondrite) around the 3 μm absorption band are an excellent match. This result may suggest that the amount of hydrous minerals in the surface material of Pallas is smaller than that in the CM2 or CI chondrites, and the hydrous minerals on the surface of Pallas may be similar to those found in Renazzo. The spectral features around the 3 μm band of Ceres are different from those of carbonaceous chondrites studied in this paper.

Magnetic properties of Martian meteorites: Implications for an ancient Martian magnetic field

Abstract: — The magnetic properties of samples of seven Martian meteorites (EET 79001, Zagami, Nakhla, Lafayette, Governador Valadares, Chassigny and ALH 84001) have been investigated. All possess a weak, very stable primary natural remanent magnetization (NRM), and some have less stable secondary components. In some cases, the latter are associated with magnetic contamination of the samples, imparted since their recovery, and with viscous magnetization, acquired during exposure of the meteorites to the geomagnetic field since they fell. The magnetic properties are carried by a small content (<1%) of titanomagnetite and, in ALH 84001, possibly by magnetite as well. The most likely source of the primary NRM is a thermoremanent magnetization acquired when the meteorite material last cooled from a high temperature in the presence of a magnetic field. Current evidence is that this was 1.3 Ga ago for the nakhlites and Chassigny and 180 Ma for shergottites: the time of the last relevant cooling of ALH 84001 is not presently known. Preliminary estimates of the strength of the magnetizing field are in the range 0.5–5 uT, which is at least an order of magnitude greater than the present field. It is tentatively concluded that the magnetic field was generated by a dynamo process in a Martian core with appropriate structure and properties.

Automated energy dispersive spectrometer modal analysis applied to the diogenites

Abstract: — We have analyzed the modal abundances of 23 of the known 24 diogenites in 31 thin sections using an energy dispersive spectrometer (EDS) and automated phase distribution analysis software. Orthopyroxene is predictably the most abundant phase, ranging from 27.7 vol% to 99.8 vol% in these samples. The grand average mode of all the analyzed diogenites includes the “olivine diogenites” but not ALH 85015, a probable howardite, and ALHA81208, a sample with an abundant silica phase. The grand average of these 21 diogenites is: orthopyroxene 92.2 vol%, olivine 4.2 vol%, clinopyroxene 1.2 vol%, chromite 0.9 vol%, plagioclase 0.4 vol%, FeNi metal 0.1 vol%, troilite 0.6 vol%, and silica phase 0.4 vol%. Plagioclase feldspar is extremely depleted in all samples, with modal abundance from none detected to 4.6 vol% in range. Such a low volume of plagioclase may indicate that the diogenite parental melts originated in a source region depleted in Al (Warren, 1985; Stolper, 1975), which is consistent with crystallization from a melt derived from material that had previously experienced extraction of a eucrite-type melt.

Petrogenetic models for magmatism on the eucrite parent body: Evidence from orthopyroxene in diogenites

Abstract: Diogenites are recognized as a major constituent of the howardite, eucrite and diogenite (HED) meteorite group. Recently, several papers (Mittlefehldt, 1994; Fowler et al., 1994, 1995) have identified trace- element systematics in diogenites that appeared to mimic simple magmatic processes that involved large degrees of crystallization (up to 95% orthopyroxene) of basalt with extremely high normative hypersthene. Such a crystallization scenario linking all the diogenites is highly unlikely. The purpose of this study is to explore other possible models relating the diogenites. Computational major-element melting models of a variety of different potential bulk compositions for the eucrite parent body (EPB) mantle indicate that these compositions show a similar sequence in residuum min- eral assemblage with increasing degrees of partial melting. Numerous bulk compositions would produce melts with Mg# appropriate for diogenitic parent magmas at low to moderate degrees of partial melting (1 5% to 30%). These calculations also show that melts with similar Mg# and variable incompatible element con- centrations may be produced during small to moderate degrees of EPB mantle melting. The trace-element characteristic of the orthopyroxene in diogenites does not support a model for large amounts of fractional crystallization of a single "hypersthene normative" basaltic magma following either small-scale or large-scale EPB mantle melting. Small degrees of fractional crystallization of a series of dis- tinct basaltic magmas are much more likely. Only two melting models that we considered hold any promise for producing different batches of "diogenitic magmas." The first model involves the fractional melting of a homogeneous source that produces parental magmas to diogenites with an extensive range of incompatible elements and limited variations in Mg#. There are several requirements for this model to work. The first requirement of this model is that the DorthoPYroxene/melt must change during melting or crystallization to compress the range of incompatible elements in the calculated diogenitic magmas. The second prerequisite is that either some of the calculated diogenitic magmas are parental to eucrites or the Mg# in diogenitic magmas are influenced by slight changes in oxygen fugacity during partial melting. The second model in- volves batch melting of a source that reflects accretional heterogeneities capable of generating diogenitic magmas with the calculated Mg# and incompatible element contents. Both of these models require small to moderate degrees of partial melting that may limit the efficiency of core separation.

A late Triassic age for the Rochechouart impact structure, France

Abstract: Argon-40/Argon-39 laser spot fusion dating of pseudotachylyte from the -25 km diameter Roche- chouart impact structure of western-central France yields a matrix age of 214 -C 8 Ma (20). Field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process, prob- ably during transient cavity collapse. This new age is considerably older than the previously accepted age of 186 & 8 Ma for this structure, which was obtained from hydrothermally-altered melt sheet samples. The new age is in accordance with earlier paleomagnetic and fission track data, which indicated that Rochechouart was formed during the late Triassic. Moreover, the new determination is in agreement with the regional geolog- ical setting and field relations of the structure. The new age of 214 2 8 Ma falls within the Norian stage of the Triassic system.