Formation Of Refractory Inclusions Research Articles

A unified chronology of dust formation in the early solar system

The chronology of dust formation in the early solar system remains controversial. Chondrules are the most abundant high-temperature objects formed during the evolution of the circumsolar disk. Considering chondrule formation, absolute lead‑lead (PbPb) ages and aluminum‑magnesium (26Al26Mg) ages relative to calcium‑aluminum-rich inclusions (CAIs) provide inconsistent chronologies, with PbPb ages showing early and protracted chondrule formation episodes whereas 26Al26Mg ages suggest that chondrule production was delayed by >1.5 Ma. Here, we develop a new method to precisely determine in situ26Al26Mg ages of spinel-bearing chondrules, which are not affected by secondary asteroidal processes. Our data demonstrate that 26Al26Mg chondrule formation ages are actually 1 Ma older than previously thought and extend over the entire lifetime of the disk. This shift in chondrule formation ages relative to CAIs, however, is not sufficient to reconcile the PbPb and 26Al26Mg chronologies of chondrule and achondrite formation. Thus, either chondrules' PbPb ages and volcanic achondrites' 26Al26Mg ages are incorrect or the age of CAIs should be reevaluated at 4,568.7 Ma to ensure consistency between chronometers. We favor the second hypothesis, given that (i) the canonical age of CAIs was determined using only 4 specimens and (ii) older ages of 4,568.2 Ma have also been measured. We show that the adoption of 4,568.7 Ma as the new canonical age of CAIs and the use of our new spinel-derived 26Al26Mg ages enable reconciling the PbPb and 26Al26Mg ages of chondrules and achondrites. This new chronology implies the existence of a 0.7–1 Ma gap between the formation of refractory inclusions and chondrules, and supports the homogeneous distribution of 26Al in the circumsolar disk.

Analytical Developments for High-Precision Measurements of W Isotopes in Iron Meteorites

A procedure was developed to accurately measure the W isotopic compositions of iron meteorites with a precision of better than +/-0.1 epsilon on epsilon182W and epsilon184W (normalized to 186W/183W). Purification of W was achieved through a two-step, ion-exchange procedure. In most cases, the yield is better than 80%, and purified W solutions are clear of matrix elements and direct isobars of W. The final W solutions were analyzed using a Micromass Isoprobe multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Tests performed on mixtures of terrestrial standards and meteorite samples demonstrate that the method is accurate and that epsilon182W variations as small as approximately 0.1 epsilon can be detected. Analyses of three different aliquots of the Gibeon (IVA) iron meteorite obtained over a period of 6 months show identical epsilon182W values with a weighted mean of 3.38 +/- 0.05, consistent with literature data for IVA iron meteorites, and indicating that the metal-silicate differentiation event in its parent body was either contemporaneous with or slightly postdated (by up to approximately 2.5 My) the formation of refractory inclusions. We demonstrate our ability to measure epsilon184W accurately and precisely (within +/-0.1 epsilon), which is useful for characterizing cosmogenic and nucleosynthetic effects that may be present in iron meteorites. We also report for the first time measurements of epsilon180W, albeit with large error bars (<+/-4 epsilon, in most cases).

Extremely rapid cooling of a carbonaceous-chondrite chondrule containing very 16O-rich olivine and a 26Mg-excess

We describe a phenocryst in a CO-chondrite type-II chondrule that we infer to have formed by melting an amoeboid olivine aggregate (AOA). This magnesian olivine phenocryst has an extremely 16O-rich composition Δ 17O (=δ 17O – 0.52 · δ 18O) = −23‰. It is present in one of the most pristine carbonaceous chondrites, the CO3.0 chondrite Yamato 81020. The bulk of the chondrule has a very different Δ 17O of −1‰, thus the Δ 17O range within this single chondrule is 22‰, the largest range encountered in a chondrule. We interpret the O isotopic and Fe-Mg distributions to indicate that a fine-grained AOA assemblage was incompletely melted during the flash melting that formed the chondrule. Some Fe-Mg exchange but negligible O-isotopic exchange occurred between its core and the remainder of the chondrule. A diffusional model to account for the observed Fe-Mg and O-isotopic exchange yields a cooling rate of 10 5 to 10 6 K hr −1. This estimate is much higher than the cooling rates of 10 1 to 10 3 K hr −1 inferred from furnace simulations of type-II chondrule textures (e.g. Lofgren, 1996); however, our cooling-rate applies to higher temperatures (near 1900 K) than are modeled by the crystal-growth based cooling rates. We observed a low 26Al/ 27Al initial ratio ((4.6 ± 3.0) · 10 −6) in the chondrule mesostasis, a value similar to those in ordinary chondrites (Kita et al., 2000). If the 26Al/ 27Al system is a good chronometer, then chondrule I formed about 2 Ma after the formation of refractory inclusions.

Formation of refractory inclusions by evaporation of condensate precursors

Berman’s (1983) activity-composition model for CaO-MgO-Al2O3-SiO2 liquids is used to calculate the change in bulk chemical and isotopic composition during simultaneous cooling, evaporation, and crystallization of droplets having the compositions of reasonable condensate precursors of Types A and B refractory inclusions in CV3 chondrites. The degree of evaporation of MgO and SiO2, calculated to be faithfully recorded in chemical and isotopic zoning of individual melilite crystals, is directly proportional to evaporation rate, which is a sensitive function of PH2, and inversely proportional to the droplet radius and cooling rate. When the precursors are partially melted in pure hydrogen at peak temperatures in the vicinity of the initial crystallization temperature of melilite, their bulk chemical compositions evolve into the composition fields of refractory inclusions, mass-fractionated isotopic compositions of Mg, Si, and O are produced that are in the range of the isotopic compositions of natural inclusions, and melilite zoning profiles result that are similar to those observed in real inclusions. For droplets of radius 0.25 cm evaporating at PH2 = 10−6 bar, precursors containing 8 to 13 wt.% MgO and 20 to 23% SiO2 evolve into objects similar to compact Type A inclusions at cooling rates of 2 to 12 K/h, depending on the precise starting composition. Precursors containing 13 to 14 wt.% MgO and 23 to 26% SiO2 evolve into objects with the characteristics of Type B1 inclusions at cooling rates of 1.5 to 3 K/h. The relatively SiO2-poor members of the Type B2 group can be produced from precursors containing 14 to 16 wt.% MgO and 27 to 33% SiO2 at cooling rates of <1 K/h. Type B2’s containing 27 to 35 wt.% SiO2 and <12% MgO require precursors with higher SiO2/MgO ratios at MgO > 15% than are found on any condensation curve. The characteristics of fluffy Type A inclusions, including their reversely zoned melilite, can only be understood in the context of this model if they contain relict melilite.

Stable Isotope Variations in Extraterrestrial Materials

The materials of the planets and the small bodies of the solar system contain a rich record of stable isotope variations in the light elements. As in terrestrial isotope geochemistry, this record reflects physical and chemical processes involving isotopic mixing among different reservoirs as well as fractionations arising in chemical reactions. The processes that influence the isotopic records of extraterrestrial materials range widely in environmental conditions from very high-energy events such as formation of refractory inclusions and chondrules by evaporation, condensation and melting in the solar nebula to lower temperature fluid-rock interactions in asteroids and planets. In addition, however, stable isotope cosmochemistry must consider issues that are beyond the scope of isotope geochemistry. For example, in the terrestrial sphere one may assume the existence of an isotopic reservoir that was originally homogenized during planet formation and the actual isotope compositions of the bulk earth do not need to be known in order to study the differences in stable isotope compositions that have been generated subsequently by various geochemical processes. This assumption of homogenization cannot be made for extraterrestrial samples, and in fact stable isotopes in meteorites preserve some of the most dramatic evidence for the incomplete nature of the mixing of distinct presolar materials during formation of the solar system. Such ‘isotopic anomalies’ are present in the isotopic distributions of H, C, N, and O on all spatial scales—from microscopic zoning in certain meteoritic minerals to the bulk compositions of asteroids and planets. Thus, some of the isotopic heterogeneities of ‘primitive’ solar system materials represent vestiges of primordial differences that could not be fully erased during the processing of presolar materials. In other cases, isotopic heterogeneities reflect the preservation of unique clues to processes occurring during formation of the solar system and planetary accretion, including early ‘geologic activity’ on planetesimals …

Refractory Phases in Primitive Meteorites Devoid of [TSUP]26[/TSUP]A[CLC]l[/CLC] and [TSUP]41[/TSUP]C[CLC]a[/CLC]:Representative Samples of First Solar System Solids?

A large number of Ca-Al-rich refractory inclusions in primitive meteorites incorporated the short-lived nuclide 26Al at the time of their formation with an initial 26Al/27Al ratio of 5×10−5. However, there exist inclusions and refractory phases like corundum and hibonite in these meteorites that have no evidence of 26Al. A suite of refractory inclusions from these meteorites also have well-defined initial 26Al/27Al ratios that are lower by factors of 5-1000 compared to the canonical value of 5×10−5. Refractory phases free of 26Al are also devoid of the short-lived nuclide,41Ca. Heterogeneous distribution of the short-lived nuclides in the early solar system and possible formation of refractory inclusions from large clumps of interstellar matter devoid of 26Al are among the suggestions made to explain these observations. In this Letter, we suggest that the refractory phases devoid of 26Al and 41Ca represent some of the first solar system solids that formed in the inner region of the solar nebula prior to the injection of these freshly synthesized nuclides from a stellar source into this region.