Showing papers in "Meteoritics & Planetary Science in 2000"

Source regions and timescales for the delivery of water to the Earth

Abstract: — In the primordial solar system, the most plausible sources of the water accreted by the Earth were in the outer asteroid belt, in the giant planet regions, and in the Kuiper Belt. We investigate the implications on the origin of Earth's water of dynamical models of primordial evolution of solar system bodies and check them with respect to chemical constraints. We find that it is plausible that the Earth accreted water all along its formation, from the early phases when the solar nebula was still present to the late stages of gas-free sweepup of scattered planetesimals. Asteroids and the comets from the Jupiter-Saturn region were the first water deliverers, when the Earth was less than half its present mass. The bulk of the water presently on Earth was carried by a few planetary embryos, originally formed in the outer asteroid belt and accreted by the Earth at the final stage of its formation. Finally, a late veneer, accounting for at most 10% of the present water mass, occurred due to comets from the Uranus-Neptune region and from the Kuiper Belt. The net result of accretion from these several reservoirs is that the water on Earth had essentially the D/H ratio typical of the water condensed in the outer asteroid belt. This is in agreement with the observation that the D/H ratio in the oceans is very close to the mean value of the D/H ratio of the water inclusions in carbonaceous chondrites.

Space weathering on airless bodies: Resolving a mystery with lunar samples

Abstract: — Using new techniques to examine the products of space weathering of lunar soils, we demonstrate that nanophase reduced iron (npFe0) is produced on the surface of grains by a combination of vapor deposition and irradiation effects. The optical properties of soils (both measured and modeled) are shown to be highly dependent on the cumulative amount of npFe0, which varies with different starting materials and the energetics of different parts of the solar system. The measured properties of intermediate albedo asteroids, the abundant S-type asteroids in particular, are shown to directly mimic the effects predicted for small amounts of npFe0 on grains of an ordinary chondrite regolith. This measurement and characterization of space weathering products seems to remove a final obstacle hindering a link between the abundant ordinary chondrite meteorites and common asteroids.

Primordial noble gases in “phase Q” in carbonaceous and ordinary chondrites studied by closed‐system stepped etching

Abstract: — The HF/HCI-resistant residues of the chondrites CM2 Cold Bokkeveld, CV3 (ox.) Grosnaja, CO3.4 Lance, CO3.7 Isna, LL3.4 Chainpur, and H3.7 Dimmitt have been measured by closed-system stepped etching (CSSE) in order to better characterise the noble gases in “phase Q”, a major carrier of primordial noble gases. All isotopic ratios in phase Q of the different meteorites are quite uniform, except for (20Ne/22Ne)Q. As already suggested by precise earlier measurements (Schelhaas et al., 1990; Wieler et al., 1991, 1992), (20Ne/22Ne)Q is the least uniform isotopic ratio of the Q noble gases. The data cluster ∼10.1 for Cold Bokkeveld and Lance and 10.7 for Chainpur, Grosnaja, and Dimmitt, respectively. No correlation of (20Ne/22Ne)Q with the classification or the alteration history of the meteorites has been found. The Ar, Kr, and Xe isotopic ratios for all six samples are identical within their uncertainties and similar to earlier Q determinations as well as to Ar-Xe in ureilites. Thus, an unknown process probably accounts for the alteration of the originally incorporated Ne-Q. The noble gas elemental compositions provide evidence that Q consists of at least two carbonaceous carrier phases “Q1” and “Q2” with slightly distinct chemical properties. Ratios (Ar/Xe)Q and (Kr/Xe)Q reflect both thermal metamorphism and aqueous alteration. These parent-body processes have led to larger depletions of Ar and Kr relative to Xe. In contrast, meteorites that suffered severe aqueous alteration, such as the CM chondrites, do not show depletions of He and Ne relative to Ar but rather the highest (He/Ar)Q and (Ne/Ar)Q ratios. This suggests that Q1 is less susceptible to aqueous alteration than Q2. Both subphases may well have incorporated noble gases from the same reservoir, as indicated by the nearly constant, though very large, depletion of the lighter noble gases relative to solar abundances. However, the elemental ratios show that Q1 and Q2 must have acquired (or lost) noble gases in slightly different element proportions. Cold Bokkeveld suggests that Q1 may be related to presolar graphite. Phases Q1 and Q2 might be related to the subphases that have been suggested by Gros and Anders (1977). The distribution of the 20Ne/22Ne ratios cannot be attributed to the carriers Q1 and Q2. The residues of Chainpur and Cold Bokkeveld contain significant amounts of Ne-E(L), and the data confirm the suggestion of Huss (1997) that the 22Ne-E(L) content, and thus the presolar graphite abundances, are correlated with the metamorphic history of the meteorites.

Argon-40/argon-39 age of the El'gygytgyn impact event, Chukotka, Russia

Abstract: of 3.58 2 0.04 Ma. The Ar step-heating method was critical in this study in identifying inherited Ar in the samples due to incomplete degassing of the Cretaceous volcanic rocks during impact melting. This age is consistent with, but more precise than, previous K-Ar and fission-track ages and indicates an "instantaneous" formation of the crater. This tight age control, in conjunction with the presence of impactites, shocked quartz, and other features, is consistent with an impact origin for the structure and seems to discount internal (volcanogenic) origin models.

Hydrocode modeling of oblique impacts: The fate of the projectile

Abstract: — All impacts are oblique to some degree. Only rarely do projectiles strike a planetary surface (near) vertically. The effects of an oblique impact event on the target are well known, producing craters that appear circular even for low impact angles (>15° with respect to the surface). However, we still have much to learn about the fate of the projectile, especially in oblique impact events. This work investigates the effect of angle of impact on the projectile. Sandia National Laboratories' three-dimensional hydrocode CTH was used for a series of high-resolution simulations (50 cells per projectile radius) with varying angle of impact. Simulations were carried out for impacts at 90, 60, 45, 30, and 15° from the horizontal, while keeping projectile size (5 km in radius), type (dunite), and impact velocity (20 km/s) constant. The three-dimensional hydrocode simulations presented here show that in oblique impacts the distribution of shock pressure inside the projectile (and in the target as well) is highly complex, possessing only bilateral symmetry, even for a spherical projectile. Available experimental data suggest that only the vertical component of the impact velocity plays a role in an impact. If this were correct, simple theoretical considerations indicate that shock pressure, temperature, and energy would depend on sin2θ, where θ is the angle of impact (measured from the horizontal). However, our numerical simulations show that the mean shock pressure in the projectile is better fit by a sin θ dependence, whereas shock temperature and energy depend on sin3/2 θ. This demonstrates that in impact events the shock wave is the result of complex processes that cannot be described by simple empirical rules. The mass of shock melt or vapor in the projectile decreases drastically for low impact angles as a result of the weakening of the shock for decreasing impact angles. In particular, for asteroidal impacts the amount of projectile vaporized is always limited to a small fraction of the projectile mass. In cometary impacts, however, most of the projectile is vaporized even at low impact angles. In the oblique impact simulations a large fraction of the projectile material retains a net downrange motion. In agreement with experimental work, the simulations show that for low impact angles (30 and 15°), a downrange focusing of projectile material occurs, and a significant amount of it travels at velocities larger than the escape velocity of Earth.

The production of cosmogenic nuclides in stony meteoroids by galactic cosmic‐ray particles

Abstract: (Received 1999 April 5; accepted in revised form 1999 October 12) ~~ ~~ Abstract-We present a purely physical model for the calculation of depth- and size-dependent production rates of cosmogenic nuclides by galactic cosmic-ray (GCR) particles. Besides the spectra of primary and secondary particles and the excitation hnctions of the underlying nuclear reactions, the model is based on only one free parameter-the integral number of GCR particles in the meteoroid orbits. We derived this value from analysis of radionuclide data in Knyahinya. We also show that the mean GCR proton spectrum in the meteoroid orbits has been constant over about the last 10 Ma. For the major target elements in stony meteoroids, we present depth- and size-dependent production rates for loBe, 14C, 26A1, 36Cl, and 53Mn as well as for the rare gas isotopes 3He, ZoNe, 21Ne, 22Ne, 36Ar, and 38Ar. The new data differ from semi- empirical estimates by up to a factor of 4 but agree within -20% with results obtained by earlier parametric or physical approaches. The depth and size dependence of the shielding parameter 22Ne/21Ne and the correlations 26AI vs. loge, 26AI vs. 53Mn, loBe/zlNe vs. 22Ne/21Ne, and 36Ar vs. 36Cl for deciphering preatmospheric sizes, shielding depths, terrestrial residence times, and exposure histories are also discussed.

Numbers, types, and compositions of an unbiased collection of cosmic spherules

Abstract: : Abstract-Micrometeorites collected from the bottom of the South Pole water well (SPWW) may represent a complete, well-preserved sample of the cosmic dust that accreted on Earth from 1100-1500 A.D. We classified 1588 cosmic spherules in the size range 50-800 pm. The collection has 41% barred olivine spherules, 17% glass spheres, 12% cryptocrystalline spherules, 11% porphyritic olivine spherules, 12% relic-grain- bearing spherules, 3% scoriaceous spherules, 2% I-type spherules, 1 % Ca-Al-Ti-rich (CAT) spherules, and 1% G-type spherules. We also found bubbly glass spherules, spherules with glass caps, and ones with sulfide coatings-particles that are absent from other collections. A classification sequence of the stony spherules (scoriaceous, relic-grain-bearing, porphyritic, barred olivine, cryptocrystalline, glass, and CAT) is consistent with progressive heating and evaporation of Fe from chondritic materials. The modem-day accretion rate and size distribution measured at the SPWW can account for the stony spherules present in deep-sea collection through preferential dissolution of glass and small stony spherules. However, weathering alone cannot account for the high accretion rate of I-type spherules determined for two deep-sea collections. The SPWW collection provides data to constrain models of atmospheric-entry heating and to assess the effects of terrestrial weathering.

The titanium contents of lunar mare basalts

Abstract: — Lunar mare basalt sample data suggest that there is a bimodal distribution of TiO2 concentrations. Using a refined technique for remote determination of TiO2, we find that the maria actually vary continuously from low to high values. The reason for the discrepancy is that the nine lunar sample return missions were not situated near intermediate basalt regions. Moreover, maria with 2–4 wt% TiO2 are most abundant, and abundance decreases with increasing TiO2. Maria surfaces with TiO2 >5 wt% constitute only 20% of the maria. Although impact mixing of basalts with differing Ti concentrations may smear out the distribution and decrease the abundance of high-Ti basalts, the distribution of basalt Ti contents probably reflects both the relative abundances of ilmenite-free and ilmenite-bearing mantle sources. This distribution is consistent with models of the formation of mare source regions as cumulates from the lunar magma ocean.

A petrologic study of the IAB iron meteorites: Constraints on the formation of the IAB‐Winonaite parent body

Abstract: — We studied 26 IAB iron meteorites containing silicate-bearing inclusions to better constrain the many diverse hypotheses for the formation of this complex group. These meteorites contain inclusions that fall broadly into five types: (1) sulfide-rich, composed primarily of troilite and containing abundant embedded silicates; (2) nonchondritic, silicate-rich, comprised of basaltic, troctolitic, and peridotitic mineralogies; (3) angular, chondritic silicate-rich, the most common type, with approximately chondritic mineralogy and most closely resembling the winonaites in composition and texture; (4) rounded, often graphite-rich assemblages that sometimes contain silicates; and (5) phosphate-bearing inclusions with phosphates generally found in contact with the metallic host. Similarities in mineralogy and mineral and O-isotopic compositions suggest that IAB iron and winonaite meteorites are from the same parent body. We propose a hypothesis for the origin of IAB iron meteorites that combines some aspects of previous formation models for these meteorites. We suggest that the precursor parent body was chondritic, although unlike any known chondrite group. Metamorphism, partial melting, and incomplete differentiation (i.e., incomplete separation of melt from residue) produced metallic, sulfide-rich and silicate partial melts (portions of which may have crystallized prior to the mixing event), as well as metamorphosed chondritic materials and residues. Catastrophic impact breakup and reassembly of the debris while near the peak temperature mixed materials from various depths into the re-accreted parent body. Thus, molten metal from depth was mixed with near-surface silicate rock, resulting in the formation of silicate-rich IAB iron and winonaite meteorites. Results of smoothed particle hydrodynamic model calculations support the feasibility of such a mixing mechanism. Not all of the metal melt bodies were mixed with silicate materials during this impact and reaccretion event, and these are now represented by silicate-free IAB iron meteorites. Ages of silicate inclusions and winonaites of 4.40-4.54 Ga indicate this entire process occurred early in solar system history.

Bleached chondrules: Evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites

Abstract: — We present the first detailed study of a population of texturally distinct chondrules previously described by Kurat (1969), Christophe Michel-Levy (1976), and Skinner et al. (1989) that are sharply depleted in alkalis and Al in their outer portions. These “bleached” chondrules, which are exclusively radial pyroxene and cryptocrystalline in texture, have porous outer zones where mesostasis has been lost. Bleached chondrules are present in all type 3 ordinary chondrites and are present in lower abundances in types 4–6. They are most abundant in the L and LL groups, apparently less common in H chondrites, and absent in enstatite chondrites. We used x-ray mapping and traditional electron microprobe techniques to characterize bleached chondrules in a cross section of ordinary chondrites. We studied bleached chondrules from Semarkona by ion microprobe for trace elements and H isotopes, and by transmission electron microscopy. Chondrule bleaching was the result of low-temperature alteration by aqueous fluids flowing through finegrained chondrite matrix prior to thermal metamorphism. During aqueous alteration, interstitial glass dissolved and was partially replaced by phyllosilicates, troilite was altered to pentlandite, but pyroxene was completely unaffected. Calcium-rich zones formed at the inner margins of the bleached zones, either as the result of the early stages of metamorphism or because of fluid-chondrule reaction. The mineralogy of bleached chondrules is extremely sensitive to thermal metamorphism in type 3 ordinary chondrites, and bleached zones provide a favorable location for the growth of metamorphic minerals in higher petrologic types. The ubiquitous presence of bleached chondrules in ordinary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories, but there is no relationship between the degree of alteration and metamorphic grade. A correlation between the oxidation state of chondrite groups and their degree of aqueous alteration is consistent with the source of water being either accreted ices or water released during oxidation of organic matter. Ordinary chondrites were probably open systems after accretion, and aqueous fluids may have carried volatile elements with them during dehydration. Individual radial pyroxene and cryptocrystalline chondrules were certainly open systems in all chondrites that experienced aqueous alteration leading to bleaching.

Petrology and chemistry of the new shergottite Dar al Gani 476

Abstract: — In 1998, Dar al Gani (DaG) 476 was found in the Libyan desert. The meteorite is classified as a basaltic shergottite and is only the 13th martian meteorite known to date. It has a porphyritic texture consisting of a fine-grained groundmass and larger olivines. The groundmass consists of pyroxene and feldspathic glass. Minor phases are oxides and sulfides as well as phosphates. The presence of olivine, orthopyroxene, and chromite is a feature that DaG 476 has in common with lithology A of Elephant Moraine (EET) A79001. However, in DaG 476, these phases appear to be early phenocrysts rather than xenocrysts. Shock features, such as twinning, mosaicism, and impact-melt pockets, are ubiquitous. Terrestrial weathering was severe and led to formation of carbonate veins following grain boundaries and cracks. With a molar MgO/(MgO + FeO) of 0.68, DaG 476 is the most magnesian member among the basaltic shergottites. Compositions of augite and pigeonite and some of the bulk element concentrations are intermediate between those of lherzolitic and basaltic shergottites. However, major elements, such as Fe and Ti, as well as LREE concentrations are considerably lower than in other shergottites. Noble gas concentrations are low and dominated by the mantle component previously found in Chassigny. A component, similar to that representing martian atmosphere, is virtually absent. The ejection age of 1.35 ± 0.10 Ma is older than that of EETA79001 and could possibly mark a distinct ejection. Dar al Gani 476 is classified as a basaltic shergottite based on its mineralogy. It has a fine-grained groundmass consisting of clinopyroxene, pigeonite and augite, feldspathic glass and chromite, Ti-chromite, ilmenite, sulfides, and whitlockite. Isolated olivine and single chromite grains occur in the groundmass. Orthopyroxene forms cores of some pigeonite grains. Shock-features, such as shock-twinning, mosaicism, cracks, and impact-melt pockets, are abundant. Severe weathering in the Sahara led to significant formation of carbonate veins crosscutting the entire meteorite. Dar al Gani 476 is distinct from other known shergottites. Chemically, it is the most magnesian member among known basaltic shergottites and intermediate in composition for most trace and major elements between Iherzolitic and basaltic shergottites. Unique are the very low bulk REE element abundances. The CI-normalized abundances of LREEs are even lower than those of Iherzolitic shergottites. The overall abundance pattern, however, is similar to that of QUE 94201. Textural evidence indicates that orthopyroxene, as well as olivine and chromite, crystallized as phenocrysts from a magma similar in composition to that of bulk DaG 476. Whether such a magma composition can be a shergottite parent melt or was formed by impact melting needs to be explored further. At this time, it cannot entirely be ruled out that these phases represent relics of disaggregated xenoliths that were incorporated and partially assimilated by a basaltic melt, although the texture does not support this possibility. Trapped noble gas concentrations are low and dominated by a Chassigny-like mantle component. Virtually no martian atmosphere was trapped in DaG 476 whole-rock splits. The exposure age of 1.26 ± 0.09 Ma is younger than that of most shergottites and closer to that of EETA79001. The ejection age of 1.35 ± 0.1 Ma could mark another distinct impact event.

The differentiation of eucrites: The role of in situ crystallization

Abstract: We report on major and trace element analyses of 17 eucrites, including three cumulate eucrites (Binda, Moore County, and Serra de MagC), determined by, respectively, inductively-coupled plasma atomic emission spectrometry and inductively-coupled plasma mass spectrometry. The results obtained for Binda and Moore County are consistent with the model of Treiman (1997) for the formation of cumulate eucrites, which holds that these meteorites were produced from a eucritic melt. Our sample of Serra de Mage contains unusually large amounts of pyroxene and probably an accessory phase rich in heavy rare earth elements and is therefore not representative of this eucrite as known from literature data.

The lack of potassium‐isotopic fractionation in Bishunpur chondrules

Abstract: — In a search for evidence of evaporation during chondrule formation, the mesostases of 11 Bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and K-isotopic compositions. Except for six points, all areas of the chondrules that were analyzed had δ41K compositions that were normal within error (typically ±3%, 2s). The six “anomalous” points are probably all artifacts. Experiments have shown that free evaporation of K leads to large 41K enrichments in the evaporation residues, consistent with Rayleigh fractionation. Under Rayleigh conditions, a 3% enrichment in δ41K is produced by ∼12% loss of K. The range of L-chondrite-normalized K/Al ratios (a measure of the K-elemental fractionation) in the areas analyzed vary by almost three orders of magnitude. If all chondrules started out with L-chondrite-like K abundances and the K loss occurred via Rayleigh fractionation, the most K-depleted chondrules would have had compositions of up to δ41K ≅ 200%. Clearly, K fractionation did not occur by evaporation under Rayleigh conditions. Yet experiments and modeling indicate that K should have been lost during chondrule formation under currently accepted formation conditions (peak temperature, cooling rate, etc.). Invoking precursors with variable alkali abundances to produce the range of K/Al fractionation in chondrules does not explain the K-isotopic data because any K that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation. If K loss and isotopic fractionation was inevitable during chondrule formation, the absence of K-isotopic fractionation in Bishunpur chondrules requires that they exchanged K with an isotopically normal reservoir during or after formation. There is evidence for alkali exchange between chondrules and rim-matrix in all unequilibrated ordinary chondrites. However, melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules, which suggests that they at least remained closed since formation. If it is correct that some or all melt inclusions remained closed since formation, the absence of K-isotopic fractionation in them requires that the K-isotopic exchange took place during chondrule formation, which would probably require gas-chondrule exchange. Potassium evaporated from fine-grained dust and chondrules during chondrule formation may have produced sufficient K-vapor pressure for gas-chondrule isotopic exchange to be complete on the timescales of chondrule formation. Alternatively, our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation.

Isotopic properties of silicon carbide X grains from the Murchison meteorite in the size range 0.5-1.5 μm

Abstract: — We report isotopic abundances for C, N, Mg-Al, Si, Ca-Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 μm. Carbon was measured in 41 X grains, n in 37 grains, Mg-Al in 18 grains, Si in 87 grains, Ca-Ti in 25 grains, and Fe in 8 grains. These X grains have 12C/13C ratios between 18 and 6800, 14N/15n ratios from 13 to 200, δ29Si/28Si between −750 and +60%0, δ30Si/28Si from −770 to −10%0, and 54Fe/56Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic 26Mg (from the radioactive decay of 26Al, half-life ≅ 7 times 105 years) and radiogenic 44Ca (from the radioactive decay of 44Ti, half-life = 60 years). While all X grains but one have radiogenic 26Mg, only ∼20% of them have detectable amounts of radiogenic 44Ca. Initial 26Al/27Al ratios of up to 0.36 and initial 44Ti/48Ti ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf-Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct 44Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for 26Al/27Al ratios, except for the highest 26Al/27Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed 29Si/30Si ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured 14n/15n ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 Modot; type II SN if rotational mixing, preferential trapping of N, or both from 15n-rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial 26Al/27Al ratios in small X grains are remarkably similar to those of large X grains (2–10 μm). Titanium-44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti-bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain-size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources.

Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years

Abstract: We analyzed noble gases from 18 samples of weathering products ("iddingsite") from the Lafayette meteorite. Potassium-argon ages of 12 samples range from near zero to 670 +/- 91 Ma. These ages confirm the martian origin of the iddingsite, but it is not clear whether any or all of the ages represent iddingsite formation as opposed to later alteration or incorporation of martian atmospheric Ar-40. In any case, because iddingsite formation requires liquid water, this data requires the presence of liquid water near the surface of Mars at least as recently as 1300 Ma ago, and probably as recently as 650 Ma ago. Krypton and Xe analysis of a single 34 microg sample indicates the presence of fractionated martian atmosphere within the iddingsite. This also confirms the martian origin of the iddingsite. The mechanism of incorporation could either be through interaction with liquid water during iddingsite formation or a result of shock implantation of adsorbed atmospheric gas.

Shock experiments with the H6 chondrite Kernouvé: Pressure calibration of microscopic shock effects

Abstract: — Shock-recovery experiments were carried out on samples of the H6 chondrite Kernouve at shock pressures of 10, 15, 20, 25, 30, 35, 45, and 60 GPa and preheating temperatures of 293 K (low-temperature experiments) and 920 K (high-temperature experiments). Using a calculated equation of state of Kernouve, pressure-pulse durations of 0.3 to 1.2 μs were estimated. The shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine, orthopyroxene, oligoclase, and troilite. The following pressure calibration is proposed for silicates: (1) undulatory extinction of olivine 35 GPa; whereas in the high-temperature experiments, troilite shows (1) complete recrystallization from 10 up to 45 GPa and (2) melting and crystallization above 45 GPa. Localized shock-induced melting is observed in samples shocked to pressures >15 GPa in the high-temperature experiments and >30 GPa for the low-temperature experiments in the form of FeNi metal and troilite melt injections and intergrowths and as pockets and veins of whole-rock melt. Obviously, the onset and abundance of shock-induced localized melting strongly depends on the initial temperature of the sample.

Noble gas record, collisional history, and pairing of CV, CO, CK, and other carbonaceous chondrites

Abstract: — Concentration and isotopic composition of the light noble gases as well as of 84Kr, 129Xe, and 132Xe have been measured in bulk samples of 60 carbonaceous chondrites; 45 were measured for the first time. Solar noble gases were found in nine specimens (Arch, Acfer 094, Dar al Gani 056, Graves Nunataks 95229, Grosnaja, Isna, Mt. Prestrud 95404, Yamato (Y) 86009, and Y 86751). These meteorites are thus regolith breccias. The CV and CO chondrites contain abundant planetary-type noble gases, but not CK chondrites. Characteristic features of CK chondrites are high 129Xe/132Xe ratios. The petrologic type of carbonaceous chondrites is correlated with the concentration of trapped heavy noble gases, similar to observations shown for ordinary chondrites. However, this correlation is disturbed for several meteorites due to a contribution of atmospheric noble gases, an effect correlated to terrestrial weathering effects. Cosmic-ray exposure ages are calculated from cosmogenic 21Ne. They range from about 1 to 63.5 Ma for CO, CV, and CK classes, which is longer than exposure ages reported for CM and CI chondrites. Only the CO3 chondrite Isna has an exceptionally low exposure age of 0.15 Ma. No dominant clusters are observed in the cosmic-ray exposure age distribution; only for CV and CK chondrites do potential peaks seem to develop at ∼9 and ∼29 Ma. Several pairings among the chondrites from hot deserts are suggested, but 52 of the 60 investigated meteorites are individual falls. In general, we confirm the results of Mazor et al. (1970) regarding cosmic-ray exposure and trapped heavy noble gases. With this study, a considerable number of new carbonaceous chondrites were added to the noble gas data base, but this is still not sufficient to obtain a clear picture of the collisional history of the carbonaceous chondrite groups. Obviously, the exposure histories of CI and CM chondrites differ from those of CV, CO, and CK chondrites that have much longer exposure ages. The close relationship among the latter three is also evident from the similar cosmic-ray exposure age patterns that do not reveal a clear picture of major breakup events. The CK chondrites, however, with their wide range of petrologic types, form the only carbonaceous chondrite group which so far lacks a solar-gas-bearing regolith breccia. The CK chondrites contain only minute amounts of trapped noble gases and their noble gas fingerprint is thus distinguishable from the other groups. In the future, more analyses of newly collected CK chondrites are needed to unravel the genetic and historic evolution of this group. It is also evident that the problems of weathering and pairing have to be considered when noble gas data of carbonaceous chondrite are interpreted.

Experimental generation of shock‐induced pseudotachylites along lithological interfaces

Abstract: — To understand the mechanism of formation of shock-induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation, shock recovery experiments were carried out on samples consisting of two distinct lithologies (dunite and quartzite). It was possible to generate melt veins of 1–6 μm width along lithological interfaces at moderate shock pressures (6 to 34 GPa). The magnitudes of displacement along the interface, strain rate, and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins. The experimentally produced veins resemble natural S-type pseudotachylites. The geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness, formation of melt pockets and injection veins, sudden changes in vein orientation, and sharp vein margins. Two types of melt were observed: vesicle-free and vesicular melts. Dense vesicle-free melt rock is likely to represent high-pressure melts. Vesicular melts were also generated during shock compression, but they remained in a molten state during pressure release and continued shearing. Intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting. Shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine. The composition of the frictional melts ranges from olivine-normative to enstatite-normative and is, thus, largely determined by olivine melting. The validity of the sequence of friction melting susceptibilities of rock-forming minerals inferred from tectonically-produced pseudotachylites is confirmed and can now be applied to ultra-high strain rates during shock compression.

Bulk density of ordinary chondrite meteorites and implications for asteroidal internal structure

Abstract: — The bulk densities of 82 samples of 72 ordinary chondrites (OCs) were measured to an accuracy of ∼1% using a modified Archimedian method. We found the average bulk density to be 3.44 ± 0.19 g/cm3 for the H group, 3.40 ± 0.15 g/cm3 for the L group, and 3.29 ± 0.17 g/cm3 for the LL group (± represent 1σ). Bulk density measurements of 11 pieces of one fall, Pultusk (H group), were also found to vary considerably (3.31 to 3.63 g/cm3). To investigate controls on bulk density within the OCs, we compared density with bulk chemical composition (the ratio of Fe metal to total Fe; the ratio of total Fe to SiO2; the ratio of FeO to total Fe and MgO). Within each OC group, bulk chemical composition is nearly invariant whereas bulk density varies from about 3.0 to 3.8 g/cm3. Slight but systematic differences in average density between the H, L, and LL groups presumably relate to differences in metallic Fe abundance. However, considerable overlap between OC groups and the wide range of bulk densities within each group suggest differences in porosity dominantly control variations of density within the OC subgroups.

Radial transpression ridges: A new structural feature of complex impact craters

Abstract: — The gravity-driven collapse of complex impact craters induces the mass transfer of large rock volumes. In distal parts of a crater, inward movements dominate; whereas in the centre of an impact structure, outward movements occur if the central uplift collapses. The particle trajectory field is centro-symmetric, which signifies that the conditions for plane-strain deformation are not fullfilled. Converging particle trajectories can be compensated either by a bulk thickening of inward sliding masses (folding, repetition of rock units along thrust faults, plastic flow) or by the formation of localised radial transpression ridges (RTR) at the edges of individual landslides. In these transpression ridges, material is uplifted to accommodate the converging mass flow. Different modes of uplift are possible including radial folding, lateral overthrusting, and the formation of positive flower structures. A simple geometric model is used to estimate the amount of transpression thickening and bulk thickening of inward sliding masses on the base of volumetric considerations. The existence of RTRs is confirmed by structural investigations at the Siljan impact structure, Sweden, as well as at other complex impact craters on Earth.

The oxygen‐isotopic record in enstatite meteorites

Abstract: — Oxygen-isotopic compositions were determined for a suite of enstatite chondrites and aubrites. In agreement with previous work (Clayton et al., 1984), most samples have O-isotopic compositions close to the terrestrial fractionation line (TFL), and there appear to be no significant differences in O-isotopic compositions between individual EH and EL chondrites and aubrites. Five enstatite meteorites have O-isotopic compositions that are significantly different from the other samples and >0.2% away from the TFL. Two of these have petrographic evidence of brecciation and interaction between other meteorite types; for the other three, similar scenarios are suggested. There appears to be a systematic increase in δ18O from enstatite chondrites (both EH and EL) of petrologic type 3 to those of type 6. There is also good evidence that the EH meteorites do not fall along a mass fractionation line but along a line slope 0.66. At the present time, detailed understanding of the origin of these O-isotopic systematics remain elusive but clearly point to a complex accretion history, parent-body evolution, or both.

A petrographic, chemical, and isotopic study of calcium‐aluminum‐rich inclusions and aluminum‐rich chondrules from the Axtell (CV3) chondrite

Abstract: Petrographic, compositional, and isotopic characteristics were studied for three calcium-aluminum-rich inclusions (CAIs) and four plagioclase-bearing chondrules (three of them Al-rich) from the Axtell (CV3) chondrite. All seven objects have analogues in Allende (CV3) and other primitive chondrites, yet Axtell, like most other chondrites, contains a distinctive suite of CAIs and chondrules. In common with Allende CAIs, CAIs in Axtell exhibit initial ^(26)Al/^(27)Al ratios ((^(26)Al/^(27)Al)0) ranging from ∼5 × 10^(−5) to 2 Ma after the first CAIs. As in other CV3 chondrites, some objects in Axtell show evidence of isotopic disturbance. Axtell has experienced only mild thermal metamorphism (<600 °C), probably not enough to disturb the Al-Mg systematics. Its CAIs and chondrules have suffered extensive metasomatism, probably prior to final accretion. These data indicate that CAIs and chondrules in Axtell (and other meteorites) had an extended history of several million years before their incorporation into the Axtell parent body. These long time periods appear to require a mechanism in the early solar system to prevent CAIs and chondrules from falling into the Sun via gas drag for several million years before final accretion. We also examined the compositional relationships among the four plagioclase-bearing chondrules (two with large anorthite laths and two barred-olivine chondrules) and between the chondrules and CAIs. Three processes were examined: (1) igneous differentiation, (2) assimilation of a CAI by average nebular material, and (3) evaporation of volatile elements from average nebular material. We find no evidence that igneous differentiation played a role in producing the chondrule compositions, although the barred olivine compositions can be related by addition or subtraction of olivine. Methods (2) and (3) could have produced the composition of one chondrule, AXCH-1471, but neither process explains the other compositions. Our study indicates that plagioclase-bearing objects originated through a variety of processes.

Refractory forsterite in primitive meteorites: Condensates from the solar nebula?

Abstract: — All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements such as Ca, Al, and Ti. Ten such grains (52 to 754 μg) with minor amounts of adhering matrix were separated from the Allende meteorite. After bulk chemical analysis by instrumental neutron activation analysis (INAA), some samples were analyzed with an electron microprobe and some with an ion microprobe. Matrix that accreted to the forsterite grains has a well-defined unique composition, different from average Allende matrix in having higher Cr and lower Ni and Co contents, which implies limited mixing of Allende matrix. All samples have approximately chondritic relative abundances of refractory elements Ca, Al, Sc, and rare-earth elements (REE), although some of these elements, such as Al, do not quantitatively reside in forsterite; whereas others (e.g., Ca) are intrinsic to forsterite. The chondritic refractory element ratios in bulk samples, the generally high abundance level of refractory elements, and the presence of Ca-Al-Ti-rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine. The Ca-Al-Ti-rich glasses may have acted as nuclei for forsterite condensation. Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chondrules: (a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. (b) The low FeO content of refractory forsterite of 0.2-0.4 wt% imposes an upper limit of ∼1 wt% of FeO on the parental melt, too low for ordinary and carbonaceous chondrule melts, (c) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivines in all classes of meteorites. The observed Mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition, (d) As shown earlier, refractory forsterites have O-isotopic compositions different from chondrules (Weinbruch et al., 1993a). Refractory olivines in carbonaceous chondrites are found in matrix and in chondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 1992). As refractory forsterite cannot have formed by crystallization from chondrule melts, we conclude that refractory forsterite from chondrules are relic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of refractory forsterite by condensation from an oxidized nebular gas.

A new martian meteorite from the Sahara: The shergottite Dar al Gani 489

Abstract: — Dar al Gani 489 (DaG 489) is a meteorite fragment of 2146 g found in the Libyan Sahara by a meteorite finder during one of his search campaigns in 1997–98. It is a porphyritic rock with millimetersized olivine crystals (Fo79–59) set in a fine-grained groundmass (average grain size 0.1 mm) consisting of pigeonite (En75–57 Wo5–15) crystals and interstitial feldspathic glass (An67–56 Or0–1). Minor phases include enstatite (En82–71 Wo2–4), augite (En48–52 Wo29–32), chromite, Ti-chromite, ilmenite, pyrrhotite, merrillite, and secondary calcite and iron oxides. On the basis of mineralogical, petrographic, bulk chemical, O-isotopic, and noble gas data, DaG 489 can be classified as a highly shocked martian meteorite (e.g., Fe/Mn(bulk) = 42.1, Ni/Mg(bulk) = 0.002; δ17O = 2.89, δ18O = 4.98, and Δ17O = 0.305), belonging to the basaltic shergottite subgroup. The texture and modal composition of DaG 489 are indeed those of basalts; nonetheless, the bulk chemistry, the abundance of large olivine and chromite crystals, and enstatitic pyroxene suggest some relationship with lherzolitic shergottites. As such, DaG 489 is similar to the hybrid shergottite Elephant Moraine (EET) A79001 lithology A; however, there are some relevant differences including a higher olivine content (20 vol%), the lack of orthopyroxene megacrysts, a higher molar Mg/(Mg + Fe)(molar) = 0.68, and a lower rare earth element content in the bulk sample. Therefore, DaG 489 has the potential of providing us with a further petrogenetic link between the basaltic and lherzolitic shergottites. Noble gases data show that DaG 489 has an ejection age of ∼1.3 Ma. This young age lends support to the requirement of several ejection events to produce the current population of shergottites, nakhlites, and chassignites (SNC) meteorites. In terms of texture, mineral and bulk compositions, shock level, and weathering features, DaG 489 is essentially identical to DaG 476, another basaltic shergottite independently found ∼25 km due northnortheast of DaG 489. Because DaG 489 also has the same exposure history as DaG 476, it is very likely that both meteorites are fragments of the same fall. In addition to the existing hypotheses on the petrogenesis of the similar EETA79001 lithology A and the identical DaG 476, we propose that DaG 489 could have formed through high-degree partial melting of a lherzolite-like material.

Australasian microtektites in the South China Sea and the West Philippine Sea: Implications for age, size, and location of the impact crater

Abstract: -Microtektites from two deep-sea cores in the South China Sea and the West Philippine Sea are identified as belonging to the Australasian tektite strewn field based on the morphology, chronostratigraphic occurrence, and geographical location of these microtektites. The higher concentrations of microtektites (> 1000/cm2) in the marginal seas of the western Pacific, with the peak concentration in the South China Sea, support the hypothesis of a large impact crater in Indochina. These two new occurrences lead to a more precise dating of the impact event at 793 ka, whereas the size of the Australasian source crater on the Indochina Peninsula is estimated to be 90-1 16 km. INTRODUCTION The Australasian strewn field, covering at least one-tenth of the Earth's surface, is the largest and the youngest (0.77 -+ 0.02 Ma; Izett and Obradovich, 1992) of the four known tektite strewn fields. Microtektites belonging to the Australasian strewn field have been found in more than 40 deep-sea cores throughout much of the Indian Ocean, the western equatorial Pacific Ocean, and the Sulu, Celebes, and Philippine Seas (Glass

Noble Gases in Interplanetary Dust Particles I: The Excess Helium-3 Problem and Estimates of the Relative Fluxes of Solar Wind and Solar Energetic Particles in Interplanetary Space

Abstract: (Received 1999 July 3; accepted in revised form 2000 January 5) Abstract-We report mass-spectrometric measurements of light noble gases pyrolytically extracted from 28 interplanetary dust particles (IDPs) and discuss these new data in the context of earlier analyses of 44 IDPs at the University of Minnesota. The noble gas database for IDPs is still very sparse, especially given their wide mineralogic and chemical variability, but two intriguing differences from isotopic distributions observed in lunar and meteoritic regolith grains are already apparent. First are puzzling overabundances of 3He, manifested as often strikingly elevated 3He/4He ratios-up to >40x the solar-wind value-and found primarily but not exclusively in shards of some of the larger IDPs ("cluster particles") that fragmented on impact with the collectors carried by high-altitude aircraft. It is difficult to attribute these high ratios to 3He production by cosmic-ray-induced spallation during estimated space residence times of IDPs, or by direct implantation of solar-flare He. Minimum exposure ages inferred from the 3He excesses range from -50 Ma to an impossible >I0 Ga, compared to Poynting-Robertson drag lifetimes for low-density 20-30 pm particles on the order of -0.1 Ma for an asteroidal source and -10 Ma for origin in the Kuiper belt. The second difference is a dominant contribution of solar-energetic-particle (SEP) gases, to the virtual exclusion of solar- wind (SW) components, in several particles scattered throughout the various datasets but most clearly and consistently observed in recent measurements of a group of individual and cluster IDPs from three different collectors. Values of the SEP/SW fluence ratio in interplanetary space from a simple model utilizing these data are -1% of the relative SEP/SW abundances observed in lunar regolith grains, but still factors of approximately 10-1 00 above estimates for this ratio in low-energy solar particle emission.

Interrelationships among meteoric metals, meteors, interplanetary dust, micrometeorites, and meteorites

Abstract: — Meteor science, aeronomy, and meteoritics are different disciplines with natural interfaces. This paper is an effort to integrate the chemistry and mineralogy of collected interplanetary dust particles (IDPs), micrometeorites, and meteorites with meteoric data and with atmospheric metal abundances. Evaporation, ablation, and melting of decelerating materials in the Earth's atmosphere are the sources of the observed metal abundances in the upper atmosphere. Many variables ultimately produce the materials and phenomena we can analyze, such as different accretion and parent-body histories of incoming extraterrestrial materials, different interactions of meteors with the Earth's middle atmosphere, meteor data reduction, and complex chemical interactions of the metals and ions with the ambient atmosphere. The IDP-like and unequilibrated ordinary chondrite matrix materials are reasonable sources for observed meteoric and atmospheric metals. The hypothesis of hierarchical dust accretion predicts that low, correlated refractory element abundances in cometary meteors may be real. It implies that the CI or cosmic standard is not useful to appreciate the chemistry of incoming petrologically heterogeneous cometary matter. The quasi steady-state metal abundances in the lower thermosphere and upper mesosphere are derived predominantly from materials with cometary orbital characteristics and velocities such as comets proper and near-Earth asteroids. The exact influence of atmospheric chemistry on these abundances still needs further evaluation. Metal abundances in the lower mesosphere and upper stratosphere region are mostly from materials from the asteroidal belt and the Kuiper belt.

An experimental study on kinetically-driven precipitation of calcium-magnesium-iron carbonates from solution: Implications for the low-temperature formation of carbonates in martian meteorite Allan Hills 84001

Abstract: — Spherical carbonate globules of similar composition, size, and radial Ca-, Mg-, and Fe-zonation to those in martian meteorite Allan Hills (ALH) 84001 were precipitated from Mg-rich, supersaturated solutions of Ca-Mg-Fe-CO2-H2O at 150 °C. The supersaturated solutions (pH ≅ 6–7) were prepared at room temperature and contained in TeflonTM-lined stainless steel vessels, which were sealed and heated to 150 °C for 24 h. Experiments were also conducted at 25 °C and no globules comparable to those of ALH 84001 were precipitated. Instead, amorphous Fe-rich carbonates were formed after 24 h and Mg-Fe calcites formed after 96 h. These experiments suggest a possible low-temperature inorganic origin for the carbonates in martian meteorite ALH 84001.

The Meteoritical Bulletin, No. 84, 2000 August

Abstract: — Meteoritical Bulletin No 84 lists information for 1341 newly classified meteorites, comprising 842 from Antarctica, 341 from Africa, 66 from Australia, 48 from Asia (including 42 from the Arabian peninsula), 38 from North America, 4 from Europe, and 2 from South America Information is provided for 11 recent falls (Bilanga, Devri-Khera, Djoumine, Guangmingshan, Kitchener, Kobe, Leighlinbridge, Sabrum, Songyuan, Tagish Lake, and Vissannapeta); 4 martian meteorites (Dar al Gani 670/735, Dhofar 019, Los Angeles, and Sayh al Uhaymir 005/008); 3 lunar meteorites (Northwest Africa 032, Dhofar 025 and 026); an ungrouped enstatite-rich meteorite (Zaklodzie); 11 iron meteorites; and a wide variety of other interesting stony meteorites, including CK, CM, CO, CR, CV, R, enstatite, and unequilibrated ordinary chondrites, primitive achondrites, HED achondrites, ureilites, and aubrites

The Bosumtwi meteorite impact structure, Ghana: A magnetic model

Abstract: — A magnetic model is proposed for the Bosumtwi meteorite impact structure in Ghana, Africa. This relatively young (∼1.07 Ma) structure with a diameter of ∼10.5 km is exposed within early Proterozoic Birimian—Tarkwaian rocks. The central part of the structure is buried under postimpact lake sediments, and because of lack of drill cores, geophysics is the only way to reveal its internal structure. To study the structure below and beyond the lake, a high-resolution, low altitude (∼70 m) airborne geophysical survey across the structure was conducted, which included measurements of the total magnetic field, electromagnetic data, and gamma radiation. The magnetic data show a circumferential magnetic halo outside the lakeshore, ∼12 km in diameter. The central-north part of the lake reveals a central negative magnetic anomaly with smaller positive side-anomalies north and south of it, which is typical for magnetized bodies at shallow latitudes. A few weaker negative magnetic anomalies exist in the eastern and western part of the lake. Together with the northern one, they seem to encircle a central uplift. Our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact-melt rock or melt-rich suevite bodies. Petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites. Suevites have a higher magnetization and have low densities and high porosities compared to the target rocks. In suevites, the remanent magnetization dominates over induced magnetization (Koenigsberger ratio > 3). Preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the Jaramillo normal polarity epoch. This interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake. The reverse polarity remanence component, superimposed on the normal component, is probably acquired during subsequent reverse polarity events.