A liquid-supported condensation of major minerals in the solar nebula: Evidence from glasses in the Kaba (CV3) chondrite

Glasses, in the Kaba CV3 chondrite, occur as mesostasis in chondrules and aggregates and as inclusions in olivines, both confined or open and connected to the mesostasis. The inclusions in olivine and the glassy mesostasis of aggregates seem to have formed contemporaneously. The confined glass inclusions and open inclusions in olivine were formed during olivine growth and the mesostasis glass during olivine aggregation. All glasses have high trace element contents (10-20×CI) with unfractionated CI-normalized abundances of refractory trace elements. In contrast, V, Mn, Li, and Cr are depleted in all glasses with respect to the refractory trace elements, as is Rb in the glass inclusions in olivine but not in the mesostasis glass. This abundance pattern indicates vapor fractionation and a common condensation origin for both glasses. Glasses of confined glass inclusions in olivine have a SiAlCa-rich composition with a chondritic Ca/Al ratio. Glasses of open glass inclusions and mesostasis are poor in Ca and enriched in alkalis. However, Ca contents of olivines indicate crystallization from a Ca-rich melt of a composition similar to that of the glass inclusions. In addition, trace element abundances indicate that these glasses (liquids) probably had an original composition similar to that of the inclusion glass. They apparently lost Ca in exchange for alkalis in a metasomatic exchange reaction, presumably with the vapor. There is now growing evidence that liquids can indeed condense from a solar nebula gas, provided the gas/dust ratio is sufficiently low. In these regions with enhanced oxygen fugacity as compared to that of a nebula of solar composition, liquids (the glass precursor) probably played an important role in growing crystals from the vapor by liquid-phase epitaxy. The glasses appear to be the remnants of this thin liquid layer interface that supported the growth of olivine from the vapor following the Vapor-Liquid-Solid process. This liquid will have a refractory composition and will have trace element contents which are in equilibrium with the vapor, and, therefore, will not change much during the time of olivine growth. The composition of the liquid seems to be unconstrained by the phases it is in contact with. Samples of this liquid will be retained as glass inclusions in olivine. The glassy mesostasis could also be a sample of this liquid that got trapped in inter-crystal spaces. The mesostasis glass subsequently behaved as an open system and its Ca was exchanged—presumably with the vapor—for the alkali elements Na, K, and Rb. In contrast, glass inclusions in olivine were protected by the host, could not react, and thus preserved the original composition of this liquid.     

Chemical and mineralogical size segregation in the impact disruption of inhomogeneous, anhydrous meteorites

We performed impact disruption experiments on pieces from eight different anhydrous chondritic meteorites—four weathered ordinary chondrite finds from North Africa (NWA791, NWA620, NWA869 and MOR001), three almost unweathered ordinary chondrite falls (Mbale, Gao, and Saratov), and an almost unweathered carbonaceous chondrite fall (Allende). In each case the impactor was a small (1/8 or 1/4 in) aluminum sphere fired at the meteorite target at , comparable to the mean collision speed in the main-belt. Some of the ∼5 to debris from each disruption was collected in aerogel capture cells, and the captured particles were analyzed by in situ synchrotron-based X-ray fluorescence. For each meteorite, many of the smallest particles ( up to in size, depending on the meteorite) exhibit very high Ni/Fe ratios compared to the Ni/Fe ratios measured in the larger particles , a composition consistent with the smallest debris being dominated by matrix material while the larger debris is dominated by fragments from olivine chondrules. These results may explain why the interplanetary dust particles (IDPs) collected from the Earth's stratosphere are C-rich and volatile-rich compared to the presumed solar nebula composition. The IDPs may simply sample the matrix of an inhomogeneous parent body, structurally and mineralogically similar to the chondritic meteorites, which are inhomogeneous assemblages of compact, strong, C- and volatile-poor chondrules that are distributed in a more porous, C- and volatile-rich matrix. In addition, these results may explain why the micrometeorites, which are to millimeters in size, recovered from the polar ices are Ni- and S-poor compared to chondritic meteorites, since these polar micrometeorites may preferentially sample fragments from the Ni- and S-poor olivine chondrules. These results indicate that the average composition of the IDPs may be biased towards the composition of the matrix of the parent body while the average composition of the polar micrometeorites may be more heavily weighted towards the composition of the chondrules and clasts. Thus, neither the IDPs nor the polar micrometeorites may sample the bulk composition of their respective parent bodies.We determined the threshold collisional specific energy for these chondritic meteorites to be 1419 J/kg, about twice the value for terrestrial basalt. Comparison of the mass of the largest fragment produced in the disruption of an sample of the porous ordinary chondrite Saratov with the largest fragment produced in the disruption of an sample of the compact ordinary chondrite MOR001 when each was struck by an impactor having approximately the same kinetic energy confirms that it requires significantly more energy to disrupt a porous target than a non-porous target.These results may also have important implications for the design of spacecraft missions intended to sample the composition and mineralogy of the chondritic asteroids and other inhomogeneous bodies. A Stardust-like spacecraft intended to sample asteroids by collecting only the small debris from a man-made impact onto the asteroid may collect particles that over-sample the matrix of the target and do not provide a representative sample of the bulk composition. The impact collection technique to be employed by the Japanese HAYABUSA (formerly MUSES-C) spacecraft to sample the asteroid Itokawa may result in similar mineral segregation.     

K–Ar ages of meteorites: Clues to parent-body thermal histories

Whereas most radiometric chronometers give formation ages of individual meteorites >4.5 Ga ago, the K–Ar chronometer rarely gives times of meteorite formation. Instead, K–Ar ages obtained by the ³⁹Ar–⁴⁰Ar technique span the entire age of the solar system and typically measure the diverse thermal histories of meteorites or their parent objects, as produced by internal parent body metamorphism or impact heating. This paper briefly explains the Ar–Ar dating technique. It then reviews Ar–Ar ages of several different types of meteorites, representing at least 16 different parent bodies, and discusses the likely thermal histories these ages represent. Ar–Ar ages of ordinary (H, L, and LL) chondrites, R chondrites, and enstatite meteorites yield cooling times following internal parent body metamorphism extending over ∼200 Ma after parent body formation, consistent with parent bodies of ∼100 km diameter. For a suite of H-chondrites, Ar–Ar and U–Pb ages anti-correlate with the degree of metamorphism, consistent with increasing metamorphic temperatures and longer cooling times at greater depths within the parent body. In contrast, acapulcoites–lodranites, although metamorphosed to higher temperatures than chondrites, give Ar–Ar ages which cluster tightly at ∼4.51 Ga. Ar–Ar ages of silicate from IAB iron meteorites give a continual distribution across ∼4.53–4.32 Ga, whereas silicate from IIE iron meteorites give Ar–Ar ages of either ∼4.5 Ga or ∼3.7 Ga. Both of these parent bodies suffered early, intense collisional heating and mixing. Comparison of Ar–Ar and I–Xe ages for silicate from three other iron meteorites also suggests very early collisional heating and mixing. Most mesosiderites show Ar–Ar ages of ∼3.9 Ga, and their significantly sloped age spectra and Ar diffusion properties, as well as Ni diffusion profiles in metal, indicate very deep burial after collisional mixing and cooling at a very slow rate of ∼0.2 °C/Ma. Ar–Ar ages of a large number of brecciated eucrites range over ∼3.4–4.1 Ga, similar to ages of many lunar highland rocks. These ages on both bodies were reset by large impact heating events, possibly initiated by movements of the giant planets. Many impact-heated chondrites show impact-reset Ar–Ar ages of either >3.5 Ga or <1.0 Ga, and generally only chondrites show these younger ages. The younger ages may represent orbital evolution times in the asteroid belt prior to ejection into Earth-crossing orbits. Among martian meteorites, Ar–Ar ages of nakhlites are similar to ages obtained from other radiometric chronometers, but apparent Ar–Ar ages of younger shergottites are almost always older than igneous crystallization ages, because of the presence of excess (parentless) ⁴⁰Ar. This excess ⁴⁰Ar derives from shock-implanted martian atmosphere or from radiogenic ⁴⁰Ar inherited from the melt. Differences between meteorite ages obtained from other chronometers (e.g., I–Xe and U–Pb) and the oldest measured Ar–Ar ages are consistent with previous suggestions that the ⁴⁰K decay parameters in common use are incorrect and that the K–Ar age of a 4500 Ma meteorite should be possibly increased, but by no more than ∼20 Ma.