Knut Metzler

Accretionary dust mantles in CM chondrites: Evidence for solar nebula processes

Abstract The texture and modal composition of fourteen carbonaceous chondrites of the CM group were studied in detail to unravel the origin of their various components and the evolution of their parent body(ies). The analysis and “mapping” of complete thin sections by scanning electron and optical microscopy revealed textural units in most samples in which all coarse-grained “primordial” chondritic components (chondrules, chondrule fragments, refractory inclusions, particles rich in hydrous silicates (PCP), and various mineral fragments) are coated by fine-grained, serpentine- and tochilinite-rich dust. The dust-mantled particles have spheroidal shapes and form a special lithology in which they are densely aggregated and compacted without any interstitial clastic material. The dust mantles termed “accretionary dust mantles” and their host lithology called “primary accretionary rock” are interpreted as products of accretion processes in the solar nebula. An origin by processes on the parent body is excluded for textural and mineralogical reasons. Most CM chondrites are affected by secondary impact-induced brecciation of variable intensity except for Yamato 791198, which is composed entirely of a primary accretionary rock, obviously unaltered by secondary parent body processes such as aqueous alteration or brecciation. The CM chondrites Y74662, Haripura, Cold Bokkeveld, Kivesvaara, Mighei, Pollen, Nogoya, Y793321, ALH 83100, Murray, and Murchison consist of clasts of primary accretionary rock embedded in a fine-grained clastic matrix derived from primary rock material. The volume percentage of the clastic matrix ranges from 5% ( Y74662) to 76% (Murchison). Bells and Essebi are brecciated and affected by aqueous alteration to an extent that fragments of primary rock are not preserved. The evaluation of literature data indicates that solar wind—implanted noble gases are absent in the primary accretionary rock component of CM chondrites. They are only present in brecciated samples. It must be concluded from the relation between the content of trapped solar wind and degree of brecciation that ( 1 ) the solar noble gases are residing exclusively in the clastic matrix and ( 2 ) some CM chondrites are regolith breccias and others are fragmental or monomict breccias. Both types were formed on the parent body although the latter were never exposed to the solar wind. Based on the results of textural, chemical, and mineralogical investigations, we conclude that a major portion of the water-bearing phases in CM chondrites were produced by the hydration of refractory phases prior to the formation of dust mantles and prior to the formation of the recent CM parent body. This process requires special physicochemical conditions which allow the crystallization of OH-bearing silicates, e.g., higher density of the nebula gas or special precursor planetesimals of the CM chondrite parent body.

Thermal and impact metamorphism on the HED parent asteroid

Abstract The bulk texture and composition of four monomict eucrites, five polymict eucrites, and one howardite, as well as those of 16 separated clasts and lithological units from these samples were analyzed by optical and scanning electron microscopy and by electron microprobe. Bulk chemical compositions were obtained by INAA. The monomict eucrites Stannern, Millbillillie, Camel Donga, and Juvinas are recrystallized monomict breccias that probably originate from brecciated crater floors or ejecta blocks. The texture of igneous clasts from Juvinas can be explained by interactions of impact and igneous activity that led to disturbance of magma crystallization. Due to the presence of lithic clasts with highly variable chemical compositions and the occurrence of both equilibrated and unequilibrated pyroxenes, the monomict eucrite Pasamonte is redescribed as a polymict eucrite. Three clasts of impact-related lithologies in the polymict eucrite Pasamonte and the howardite EET 87503 contain considerable amounts of chondritic projectile contaminations. The textures of the investigated meteorites reflect a complex post-igneous history dominated by multistage thermal and impact metamorphism. The chronological sequence of thermal and impact events comprises up to six evolutionary phases. Phase I represents crystallization of primary magmas that led to the formation of unequilibrated basalts and other igneous rocks. Phase II represents slow subsolidus cooling or a period of reheating during which pyroxene equilibrated. Phases III and V represent periods of impact brecciation during which the rocks were brecciated in situ or, in the case of polymict HED breccias, mixed with various other rock types. During Phases IV and VI the breccias suffered annealing and recrystallization due to thermal metamorphism. The thermal events that caused recrystallization and equilibration of HED lithologies were active prior, during, and after the formation of impact breccias, indicating that the thermal input by impact might be responsible for thermal overprinting.

Tungsten isotopic constraints on the age and origin of chondrules

Significance The origin of chondrules—millimeter-sized silicate-rich spherules that dominate the most primitive meteorites, the chondrites—is a long-standing puzzle in cosmochemistry. Here, we present isotopic evidence that chondrules and matrix from the Allende chondrite contain different and complementary proportions of presolar matter, indicating that chondrules and matrix formed together from a single reservoir of solar nebula dust. This finding rules out formation of chondrules in protoplanetary impacts and demonstrates that chondrules are the product of localized melting events in the solar protoplanetary disk. Moreover, the isotopic complementarity of chondrules and matrix requires that chondrules formed in a narrow time interval and were rapidly accreted to a parent body, implying that chondrule formation was a critical step toward forming planetesimals. Chondrules may have played a critical role in the earliest stages of planet formation by mediating the accumulation of dust into planetesimals. However, the origin of chondrules and their significance for planetesimal accretion remain enigmatic. Here, we show that chondrules and matrix in the carbonaceous chondrite Allende have complementary 183W anomalies resulting from the uneven distribution of presolar, stellar-derived dust. These data refute an origin of chondrules in protoplanetary collisions and, instead, indicate that chondrules and matrix formed together from a common reservoir of solar nebula dust. Because bulk Allende exhibits no 183W anomaly, chondrules and matrix must have accreted rapidly to their parent body, implying that the majority of chondrules from a given chondrite group formed in a narrow time interval. Based on Hf-W chronometry on Allende chondrules and matrix, this event occurred ∼2 million years after formation of the first solids, about coeval to chondrule formation in ordinary chondrites.

Heterogeneous distribution of solar and cosmogenic noble gases in CM chondrites and implications for the formation of CM parent bodies

Abstract Distribution of solar, cosmogenic, and primordial noble gases in thin slices of Murchison, Murray, and Nogoya CM carbonaceous chondrites was determined by the laser microprobe analysis so as to put some constraints on the parent-body processes in the CM chondrite formation. The main lithological units of the three meteorite slices were located by electron microscope observations and classified into clastic matrix and clasts of primary accretionary rocks (PARs) based on the classification scheme of texture of CM chondrites. All sample slices contain both clastic matrix and PARs. Clastic matrix shows a comminuted texture formed by fragmentation and mechanical mixing of rocks due to impacts, whereas PARs preserve the original textures prior to the mechanical disruption. Solar-type noble gases are detected in all sample slices. They are located preferentially in clastic matrix. The distribution of solar gases is similar to that in ordinary chondrites where these gases reside in clastic dark portions of these meteorites. The heterogeneous distribution of solar gases in CM chondrites suggests that these gases were acquired not in a nebular accretion process but in parent body processes. Solar energetic particles (SEP) are predominant in CM chondrites. The low abundance of low energy solar wind (SW) component relative to SEP suggests preferential loss of SW from minerals comprising the clastic matrix, due to aqueous alteration in the parent bodies. Cosmogenic noble gases are also enriched in some portions in clastic matrix, indicating that some parts of clastic matrix were exposed to solar and galactic cosmic rays prior to the final consolidation of the CM parent bodies. Primordial noble gases are rich in fine-grained rims around chondrules in all three meteorites. However, average concentrations of heavy primordial gases in the rims differ among meteorites and correlate inversely to the degree of aqueous alteration that the meteorites have experienced. This appears to have been caused by aqueous alteration reactions between fluids and carbonaceous carrier phases of noble gases.

CODAG - Dust agglomeration experiment in micro-gravity

A micro-gravity experiment to study the growth of dust particles has been proposed to be flown on one of the Columbus precursor flights. The micro-gravity environment will allow for low collision velocities (of order of mm s−1) of the dust grains and for a large Knudsen number of the embedding gas; conditions expected in the early solar nebula. The outcome of the experiment will yield estimates of the sticking efficiency and the critical velocity for agglomeration. The values of these two parameters will provide substantial improvements in the constraints for models of the formation of planetesimals. In particular the questions related to growth rate and mode of the aggregation process will be answered. The range of material type, collision velocities, properties of the environment in which growth takes place, and other factors permit a natural extension of this experiment to take advantage of the capabilities of the space station Columbus. Other astrophysical applications, such as processes in Saturnian rings, with somewhat different regimes could also be investigated.