Accretional and alterational differences in a carbonaceous chondrite parent body: Evidence from the NWA 5491 CV3 meteorite

The NWA 5491 CV3 meteorite is a CVoxA subtype, and composed of two substantially different units (titled “upper” and “lower” units) in the cm size range with original accreted material and also subsequent alteration produced features. Based on the large chondrules in the “upper” unit and the small chondrules plus CAIs in the “lower” unit, they possibly accreted material from different parts of the solar nebula and/or at different times, whereas substantial changes happened in the nebula's composition. Differences are observed in the level of early fragmentation too, which was stronger in the upper units. During later alteration oxidizing fluids possibly circulated only in the upper unit, mechanical fragmentation and resorption were also stronger there. In the last phase of the geological history these two rock units came into physical contact, but impact‐driven shock effects were not observed. The characteristics of this meteorite provide evidence that the same parent body might accrete substantially different material and also the later processes could differ spatially in the parent body.     

Coupled organic synthesis and mineral alteration on meteorite parent bodies

Abstract— The hypothesis that the soluble fraction of the organic compounds present in carbonaceous chondrite meteorites was formed during aqueous alteration of the parent body was tested with mass‐transfer, reaction‐path calculations. In these calculations, we start with likely compositions of the original parent body and asteroidal fluids that are far from thermodynamic equilibrium, and metastable and stable equilibrium constraints are imposed as the total Gibbs free energy of the parent body environment is minimized. The results of these calculations suggest that the classes of soluble organic compounds present in carbonaceous chondrite meteorites could have formed during relatively low temperature aqueous alteration of the meteorite parent body or bodies. The main controls on the potential for synthesis and transformation of organic compounds were the oxidation state of the rock/fluid system, the bulk composition of that system, and the temperatures that were achieved during the alteration event or events. It also appears that the alteration mineral assemblages were influenced by the presence of soluble organic compounds and reaction among them.     

Magnetization of celestial bodies with special application to the primeval Earth and Moon

The magnetic fields of celestial bodies are usually supposed to be due to a ‘hydromagnetic dynamo’. This term refers to a number of rather speculative processes which are supposed to take place in the liquid core of a celestial body. In this paper we shall follow another approach which is more closely connected with hydromagnetic processes well-known from the laboratory, and hence basically less speculative. The paper should be regarded as part of a general program to connect cosmical phenomena with phenomena studied in the laboratory. As has been demonstrated by laboratory experiments, a poloidal magnetic field may be increased by the transfer of energy from a toroidal magnetic field through kink instability of the current system. This mechanism can be applied to the fluid core of a celestial body. Any differential rotation will produce a toroidal field from an existing poloidal field, and the kink instability will feed toroidal energy back to the poloidal field, and hence amplify it. In the Earth-Moon system the tidal braking of the Earth's mantle acts to produce a differential angular velocity between core and mantle. The braking will be transferred to the core by hydromagnetic forces which at the same time give rise to a strong magnetic field. The strength of the field will be determined by the rate of tidal braking. It is suggested that the magnetization of lunar rocks from the period ?4 to ?3 Gyears derives from the Earth's magnetic field. As the interior of the Moon immediately after accretion probably was too cool to be melted, the Moon could not produce a magnetic field by hydromagnetic effects in its core. The observed lunar magnetization could be produced by such an amplified Earth field even if the Moon never came closer than 10 or 20 Earth's radii. This hypothesis might be checked by magnetic measurements on the Earth during the same period.     

Near-infrared spectroscopy of 3:1 Kirkwood Gap asteroids: Mineralogical diversity and plausible meteorite parent bodies

The 3:1 Kirkwood gap asteroids are a mineralogically diverse set of asteroids located in a region that delivers meteoroids into Earth-crossing orbits. Mineralogical characterizations of asteroids in/near the 3:1 Kirkwood Gap can be used as a tool to “map” conditions and processes in the early Solar System. The chronological studies of the meteorite types provide a “clock” for the relative timing of those events and processes. By identifying the source asteroids of particular meteorite types, the “map” and “clock” can be combined to provide a much more sophisticated understanding of the history and evolution of the late solar nebula and the early Solar System.A mineralogical assessment of seven 3:1 Kirkwood Gap asteroids has been carried out using near-infrared spectral data obtained over the years 2006–2009 combined with visible spectral data (when available) to cover the spectral interval of 0.4–2.5 μm. We explore the diversity, uniqueness, and possible links between the asteroids (198) Ampella, (329) Svea, (495) Eulalia, (556) Phyllis, (623) Chimaera, (908) Buda, and (1772) Gagarin, which are located adjacent to the 3:1 resonance, and the meteorite types in the terrestrial collections.     

The ubiquitous presence of silica-rich glass inclusions in mafic minerals: Examples from Earth,Mars, Moon and the aubrite parent body

Abstract— Highly silicic glass inclusions are commonly present in mafic minerals of xenolithic terrestrial upper mantle rocks (Schiano and Clocchiatti, 1994). They are believed to be the products of volatile-rich silicic melts for which several sources have been proposed (Francis, 1976; Frey and Green, 1974; Schiano et al, 1995), but their origin(s) and, consequently, that of the glasses, remains unknown. However, in situ formation by very low-degree partial melting seems to be possible as has been shown by experiments (e.g., Baker et al, 1995; Draper and Green, 1997). Glass inclusions of silicic chemical composition are also present in some mafic minerals of achondritic meteorites (e.g., Fuchs, 1974; Okada et al, 1988; Johnson et al, 1991). The enstatite achondrites (aubrites) Aubres and Norton County, which record early planetesimal and planet formation in the solar nebula, and the olivine achondrite (chassignite) Chassigny, a rock believed to originate from Mars, contain abundant glass inclusions in their main minerals enstatite and olivine, respectively. Glasses of glass-bearing inclusions have a highly silicic and volatile-rich chemical composition similar, but not identical, to that of glass inclusions in terrestrial upper mantle peridotite minerals. Furthermore, glass inclusions in olivines from the Moon (e.g., Roedder and Weiblen, 1977) are also silica-rich. Because different physicochemical conditions prevail in the source regions of these rocks, the process of melting is, perhaps, not generally applicable for the generation of silica-rich glasses. Alternatively, the glasses could have been formed via precipitation from silicate-loaded fluids (Schneider and Eggler, 1986) or vapors. Another possible mechanism, not previously identified, could be dehydrogenation of nominally nonhydrous mafic minerals by heating or depressurization that should be accompanied by expulsion of excess silica and incompatible elements. This process will mimic low-temperature, very low-degree partial melting. It could account also for the highly variable glass/bubble ratios observed in glass inclusions in aubrite enstatites. We suggest that such a process could have been operating in the solar nebula, the Moon and Mars, and could be operating still on Earth.     

Mineralogy,petrology, chronology,and exposure history of the Chelyabinsk meteorite and parent body

Three masses of the Chelyabinsk meteorite have been studied with a wide range of analytical techniques to understand the mineralogical variation and thermal history of the Chelyabinsk parent body. The samples exhibit little to no postentry oxidation via Mössbauer and Raman spectroscopy indicating their fresh character, but despite the rapid collection and care of handling some low levels of terrestrial contamination did nonetheless result. Detailed studies show three distinct lithologies, indicative of a genomict breccia. A light‐colored lithology is LL5 material that has experienced thermal metamorphism and subsequent shock at levels near S4. The second lithology is a shock‐darkened LL5 material in which the darkening is caused by melt and metal‐troilite veins along grain boundaries. The third lithology is an impact melt breccia that formed at high temperatures (~1600 °C), and it experienced rapid cooling and degassing of S₂ gas. Portions of light and dark lithologies from Chel‐101, and the impact melt breccias (Chel‐102 and Chel‐103) were prepared and analyzed for Rb‐Sr, Sm‐Nd, and Ar‐Ar dating. When combined with results from other studies and chronometers, at least eight impact events (e.g., ~4.53 Ga, ~4.45 Ga, ~3.73 Ga, ~2.81 Ga, ~1.46 Ga, ~852 Ma, ~312 Ma, and ~27 Ma) are clearly identified for Chelyabinsk, indicating a complex history of impacts and heating events. Finally, noble gases yield young cosmic ray exposure ages, near 1 Ma. These young ages, together with the absence of measurable cosmogenic derived Sm and Cr, indicate that Chelyabinsk may have been derived from a recent breakup event on an NEO of LL chondrite composition.     

Correlated accretion ages and ε54Cr of meteorite parent bodies and the evolution of the solar nebula

We look at the relationship between the value of ε⁵⁴Cr in bulk meteorites and the time (after calcium‐aluminum‐rich inclusion, CAI) when their parent bodies accreted. To obtain accretion ages of chondrite parent bodies, we estimated the maximum temperature reached in the insulated interior of each parent body, and estimated the initial ²⁶Al/²⁷Al for this temperature to be achieved. This initial ²⁶Al/²⁷Al corresponds to the time (after CAI formation) when cold accretion of the parent body would have occurred, assuming ²⁶Al/²⁷Al throughout the solar system began with the canonical value of 5.2 × 10^?5. In cases of iron meteorite parent bodies, achondrite parent bodies, and carbonaceous chondrite parent bodies, we use published isotopic ages of events (such as core formation, magma crystallization, and growth of secondary minerals) in each body's history to obtain the probable time of accretion. We find that ε⁵⁴Cr correlates with accretion age: the oldest accretion ages (1 ± 0.5 Ma) are for iron and certain other differentiated meteorites with ε⁵⁴Cr of ?0.75 ± 0.5, and the youngest ages (3.5 ± 0.5 Ma) are for hydrated carbonaceous chondrites with ε⁵⁴Cr values of 1.5 ± 0.5. Despite some outliers (notably Northwest Africa [NWA] 011 and Tafassasset), we feel that the correlation is significant and we suggest that it resulted from late, localized injection of dust with extremely high ε⁵⁴Cr.     

Impact histories of angrites,eucrites, and their parent bodies

Abstract– Eucrites, which are probably from 4 Vesta, and angrites are the two largest groups of basaltic meteorites from the asteroid belt. The parent body of the angrites is not known but it may have been comparable in size to Vesta as it retained basalts and had a core dynamo. Both bodies were melted early by ²⁶Al and formed basalts a few Myr after they accreted. Despite these similarities, the impact histories of the angrites and eucrites are very different: angrites are very largely unshocked and none are breccias, whereas most eucrites are breccias and many are shocked. We attribute the lack of shocked and unbrecciated angrites to an impact, possibly at 4558 Myr ago—the radiometric age of the younger angrites—that extracted the angrites from their original parent body into smaller bodies. These bodies, which may have had a diameter of approximately 10 km, suffered much less impact damage than Vesta during the late heavy bombardment because small bodies retain shocked rocks less efficiently than large ones and because large bodies suffer near‐catastrophic impacts that deposit vastly more impact energy per kg of target. Our proposed history for the angrites is comparable to that proposed by Bogard and Garrison (2003) for the unbrecciated eucrites with Ar‐Ar ages of 4.48 Gyr and that for unbrecciated eucrites with anomalous oxygen isotopic compositions that did not come from Vesta. We infer that the original parent bodies of the angrites and the anomalous eucrites were lost from the belt when the giant planets migrated and the total mass of asteroids was severely depleted. Alternatively, their parent bodies may have formed in the terrestrial planet region and fragments of these bodies were scattered out to the primordial Main Belt as a consequence of terrestrial planet formation.     

Dynamical History of the Earth–Moon System

Differential equations describing the tidal evolution of the earth's rotation and of the lunar orbital motion are presented in a simple close form. The equations differ in form for orbits fixed to the terrestrial equator and for orbits with the nodes precessing along the ecliptic due to solar perturbations. Analytical considerations show that if the contemporary lunar orbit were equatorial the evolution would develop from an unstable geosynchronous orbit of the period about 4.42 h (in the past) to a stable geosynchronous orbit of the period about 44.8 days (in the future). It is also demonstrated that at the contemporary epoch the orbital plane of the fictitious equatorial moon would be unstable in the Liapunov's sense, being asymptotically stable at early stages of the evolution. Evolution of the currently near-ecliptical lunar orbit and of the terrestrial rotation is traced backward in time by numerical integration of the evolutional equations. It is confirmed that about 1.8 billion years ago a critical phase of the evolution took place when the equatorial inclination of the moon reached small values and the moon was in a near vicinity of the earth. Before the critical epoch t _cr two types of the evolution are possible, which at present cannot be unambiguously distinguished with the help of the purely dynamical considerations. In the scenario that seems to be the most realistic from the physical point of view, the evolution also has started from a geosynchronous equatorial lunar orbit of the period 4.19 h. At t < t _cr the lunar orbit has been fixed to the precessing terrestrial equator by strong perturbations from the earth's flattening and by tidal effects; at the critical epoch the solar perturbations begin to dominate and transfer the moon to its contemporary near-ecliptical orbit which evolves now to the stable geosynchronous state. Probably this scenario is in favour of the Darwin's hypothesis about originating the moon by its separation from the earth. Too much short time scale of the evolution in this model might be enlarged if the dissipative Q factor had somewhat larger values in the past than in the present epoch. Values of the length of day and the length of month, estimated from paleontological data, are confronted with the results of the developed model.     

Complete fragmentation of the parent bodies of Themis,Eos, and Koronis families

The fragmentation of the parent asteroids of the Themis, Eos, and Koronis families is investigated by considering mutual gravitational effects among the fragmented bodies. The masses of the parent asteroids and the kinetic and gravitational energies of the fragmented bodies are estimated. Comparison of these results and data from the laboratory impact experiments leads to the conclusion that the parent asteroids of the three families were completely fragmented at E_p/M of 10⁸ erg/g or more (E_p, impact energy; M, parent mass). However, since most of the fragments had low relative velocities many reaccumulated through mutual gravitation. The larger members in these families should have the rubble pile structures and hydrostatic equilibrium figures.     

An intrinsic volatility scale relevant to the Earth and Moon and the status of water in the Moon

The notion of a dry Moon has recently been challenged by the discovery of high water contents in lunar apatites and in melt inclusions within olivine crystals from two pyroclastic glasses. The highest and most compelling water contents were found in pyroclastic glasses that are not very common on the lunar surface. To obtain more representative constraints on the volatile content of the lunar interior, we measured the Zn content, a moderately volatile element, of mineral and rock fragments in lunar soils collected during Apollo missions. We here confirm that the Moon is significantly more depleted in Zn than the Earth. Combining Zn with existing K and Rb data on similar rocks allows us to anchor a new volatility scale based on the bond energy of nonsiderophile elements in their condensed phases. Extrapolating the volatility curve to H shows that the bulk of the lunar interior must be dry (≤1 ppm). This contrasts with the water content of the mantle sources of pyroclastic glasses, inferred to contain up to approximately 40 ppm water based on H₂O/Ce ratios. These observations are best reconciled if the pyroclastic glasses derive from localized water‐rich heterogeneities in a dominantly dry lunar interior. We argue that, although late addition of 0.015% of a chondritic veneer to the Moon seems required to explain the abundance of platinum group elements (Day et al. 2007), the volatile content of the added material was clearly heterogeneous.     

Paired nakhlites MIL 090030, 090032, 090136, and 03346: Insights into the Miller Range parent meteorite

Abstract– Miller Range (MIL) 03346 is the most oxidized and least equilibrated nakhlite known and displays the highest amount of intercumulus phase. The discovery of three new nakhlites, MIL 090030, MIL 090032, and MIL 090136, in the Miller Range, Antarctica, geographically close to the location of MIL 03346, suggests that they may come from the same parent meteorite. In this study, we investigate the mineralogy and texture of cumulus and intercumulus phases, in situ major and trace element compositions for the cumulus phases, as well as pyroxene crystal size distribution patterns and spatial distribution patterns of MIL 090030, 090032, and 090136. Using these combined results, we conclude that the three nakhlites studied here are paired with MIL 03346. However, modal mineral abundances of MIL 090030, 090032, 090136, and 03346 exhibit variations indicating that a single sample of a Miller Range nakhlite is not modally representative of the parent meteorite and that analyses of multiple samples for a single nakhlite may be necessary to obtain representative modal data for placement in the cumulate pile. Our calculated parental melt composition based on all the paired samples confirms a previous study suggesting that the nakhlite parent melt crystallized as a closed system.     

Differentiation and evolution of the IVA meteorite parent body: Clues from pyroxene geochemistry in the Steinbach stony‐iron meteorite

Abstract— We analyzed the Steinbach IVA stony‐iron meteorite using scanning electron microscopy (SEM), electron microprobe analysis (EMPA), laser ablation inductively‐coupled‐plasma mass spectroscopy (LA‐ICP‐MS), and modeling techniques. Different and sometimes adjacent low‐Ca pyroxene grains have distinct compositions and evidently crystallized at different stages in a chemically evolving system prior to the solidification of metal and troilite. Early crystallizing pyroxene shows evidence for disequilibrium and formation under conditions of rapid cooling, producing clinobronzite and type 1 pyroxene rich in troilite and other inclusions. Subsequently, type 2 pyroxene crystallized over an extensive fractionation interval. Steinbach probably formed as a cumulate produced by extensive crystal fractionation (?60–70% fractional crystallization) from a high‐temperature (?1450–1490 °C) silicate‐metallic magma. The inferred composition of the precursor magma is best modeled as having formed by ≥30–50% silicate partial melting of a chondritic protolith. If this protolith was similar to an LL chondrite (as implied by O‐isotopic data), then olivine must have separated from the partial melt, and a substantial amount (?53–56%) of FeO must have been reduced in the silicate magma. A model of simultaneous endogenic heating and collisional disruption appears best able to explain the data for Steinbach and other IVA meteorites. Impact disruption occurred while the parent body was substantially molten, causing liquids to separate from solids and oxygen‐bearing gas to vent to space, leading to a molten metal‐rich body that was smaller than the original parent body and that solidified from the outside in. This model can simultaneously explain the characteristics of both stony‐iron and iron IVA meteorites, including the apparent correlation between metal composition and metallographic cooling rate observed for metal.     

Impacts on the CV parent body: A coordinated,multiscale fabric analysis of the Allende meteorite

Evidence of impact-induced compaction in the carbonaceous chondrites, specifically CMs and CVs, has been widely investigated utilizing microscopy techniques and impact experiments. Here, we use high-resolution photography and large area and high-resolution electron backscattered diffraction (EBSD) mapping analyses in tandem, to explore the effects of impact-induced compaction at both the meso- and micro-scales in the Allende CV3.6 carbonaceous chondrite. Macro-scale photography images of a ~25 cm slab of Allende captured meso-scale features including calcium-aluminum inclusions (CAIs) and chondrules. CAIs have a long-axis shape-preferred orientation (SPO). Examination of such meso-scale features in thin section revealed the same trend. Matrix grains from this section display a large amount of heterogeneity in petrofabric orientation; microscale, high-resolution, large area EBSD mapping of ~300,000 olivine matrix grains; high-resolution large area EBSD map across an elongate CAI; and a series of high-resolution EBSD maps around two chondrules and around the CAI revealed crystallographic preferred orientations (CPOs) in different directions. Finally, internal grains of the CAI were found to demonstrate a weak lineation CPO, the first crystallographic detection of possible CAI “flow.” All results are consistent with multiple, gentle impacts on the Allende parent body causing hemispheric compaction. The larger, more resistant components are likely to have been compressed and oriented by earlier impacts, and the matrix region petrofabrics and CAI “flow” likely occurred during subsequent impacts. Meteoritic components respond differently to impact events, and consequently, it is likely that different components would retain evidence of different impact events and angles.     

Possible relationship between the Farmington meteorite and a seismically detected swarm of meteoroids impacting the Moon

Abstract— The Farmington ordinary L5 chondrite with its uniquely short cosmic-ray exposure age of less than 25 000 years may have been a member of a large meteoroid swarm which was detected by the Apollo seismic network when it encountered the Moon in June 1975. The association implies that the parent body of the Farmington meteorite was in an Earth-crossing orbit at the time the swarm was formed. This supports the idea that at least some meteorites are derived from the observable population of Earth-crossing asteroids.     

Interplanet variations in scale of crater morphology—Earth,Mars, Moon

Craters on the Earth, Mars, and the Moon show a spectrum of morphologies with diameter increasing from simple, bowl-shaped craters through craters with increasingly complex central peaks, to craters with “peak rings” and basins with multiple concentric scarps. In each category there is a range of diameters, centered around a characteristic diameter, D_c. It is found that D_c decreases as the size of the planet increases. Several possible explanations are considered. It is suggested that the effect results from a gravity scaling law derived here and having approximately from the D_c∂ 1/g^1.25, where g is the surface gravity. All geological structures in which gravity is the dominant parameter affecting the morphology should follow such a law.     

Gravitational attenuation and the internal heating of planetary bodies

The hypothesis of gravitational attenuation is discussed and several theoretical problems with possible solutions are noted. It is pointed out that, if energy were attenuated out of the gravitational field by matter, it would ultimately be thermalized. Geothermal data are then employed to establish an upper limit to the gravitational attenuation coefficient, some fifteen orders of magnitude smaller than the currently accepted value. The internal heating of other planetary bodies is investigated to see if a yet smaller upper limit is deducible therefrom – with negative results.     

Distribution of Impact Locations and Velocities of Earth Meteorites on the Moon

Following the analytical work of Armstrong et al. (Icarus 160:183–196, 2002), we detail an expanded N-body calculation of the direct transfer of terrestrial material to the Moon during a giant impact. By simulating 1.4 million particles over a range of launch velocities and ejecta angles, we have derived a map of the impact velocities, impact angles, and probable impact sites on the moon over the last 4 billion years. The maps indicate that the impacts with the highest vertical impact speeds are concentrated on the leading edge, with lower velocity/higher-angle impacts more numerous on the Moon’s trailing edge. While this enhanced simulation indicates the estimated globally averaged direct transfer fraction reported in Armstrong et al. (Icarus 160:183–196, 2002) is overestimated by a factor of 3–6, local concentrations can reach or exceed the previously published estimate. The most favorable location for large quantities of low velocity terrestrial material is 50 W, 85 S, with 8.4 times more impacts per square kilometer than the lunar surface average. This translates to 300–500 kg km⁻², compared to 200 kg km⁻² from the previous estimate. The maps also indicate a significant amount of material impacting elsewhere in the polar regions, especially near the South Pole-Aiken basin, a likely target for sample return in the near future. The magnitudes of the impact speeds cluster near 3 km/s, but there is a bimodal distribution in impact angles, leading to 43% of impacts with very low (<1 km/s) vertical impact speeds. This, combined with the enhanced surface density of meteorites in specific regions, increases the likelihood of weakly shocked terrestrial material being identified and recovered on the Moon.