Al-rich Chondrules Research Articles

Al-rich chondrules from the Ybbsitz H4-chondrite: evidence for formation by collision and splashing

Three Al-rich chondrules (Al 2O 3 > 10 wt.%) from the H4-chondrite Ybbsitz were chemically and mineralogically studied. All three clearly crystallized from molten droplets as individual objects. They consist predominantly of olivine, spinel, and a feldspar-rich mesostasis. The three Al-rich chondrules have high concentrations of incompatible elements. The rare earth element patterns are fractionated with enrichment of light rare earths and a strong positive Eu anomaly. This type of fractionated refractory element pattern is indicative of igneous processes. Ca,Al-rich inclusions from carbonaceous chondrites also show high enrichments in refractory elements. Their patterns are, however, either flat or fractionated according to variable volatilities of refractory elements in the solar nebula (e.g. Yb anomaly). The high level of refractory siderophiles observed in Ca,Al-rich inclusions from carbonaceous chondrites is not found in the Al-rich chondrules. We therefore conclude that the Al-rich Ybbsitz chondrules formed by different processes than the Ca,Al-rich inclusions from carbonaceous chondrites. Melting of inhomogeneous precursors, generally assumed for the formation of ferromagnesian chondrules, is also unlikely to produce such extreme compositions. The possibility that metamorphism is the cause of the high content of incompatible elements can be excluded. We therefore propose a new model for the formation of the Al-rich chondrules. They may have formed by collisions of partly molten ferromagnesian chondrules with other objects (chondrules). During this process Al-rich melt fractions (residual liquids) may be ejected and form independent Al-rich chondrules. This model explains the high enrichment and the fractionated patterns of incompatible elements in Al-rich chondrules as well as their generally much smaller sizes compared with ferromagnesian chondrules. The Eu anomaly is probably due to diffusive exchange of Eu between matrix and the Al-rich chondrules. Eu is decoupled from the other REE in many ferromagnesian chondrules of equilibrated ordinary chondrites, including three ferromagnesian chondrules from Ybbsitz analyzed in this study.

The origin of type C inclusions from carbonaceous chondrites

Type C inclusions are plagioclase-rich, Ca-, Al-rich inclusions found in carbonaceous chondrites. They formed as solid condensates which were later melted in an event that destroyed the original condensate grains. Neither the melting event nor secondary alteration had a significant effect on bulk composition. Two stages in the condensation history can be discerned on the basis of major element bulk compositions. As in type A inclusions, the condensate phase assemblage originally consisted of melilite + spinel + perovskite ± hibonite. Type C's were, however, significantly enriched in spinel relative to unaltered portions of most type A's. In contrast to type A's, condensate grains of spinel and melilite in type C's reacted partially with a coexisting gas to produce anorthite + diopside. One-half to two-thirds of the silica now in type C inclusions was introduced by this process. The reactions involving melilite and spinel that are predicted by equilibrium condensation calculations for a cooling gas of solar composition did not occur, probably due to kinetic constraints. Type C's may be related to Al-rich chondrules in ordinary chondrites by the addition of olivine and albite. Bulk compositions of Al-rich chondrules in enstatite chondrites are consistent with the addition of orthopyroxene and albite to type C inclusions. Thus, types A and C inclusions and Al-rich chondrules could represent sequences of condensates removed from interaction with the primitive solar nebula at progressively lower temperatures. If Al-rich chondrules represent the high-temperature component in chondritic material, then type C inclusions rather than the more common subgroups of CAIs could be the true parents of chondrites.

Al-rich objects in ordinary chondrites: Related origin of carbonaceous and ordinary chondrites and their constituents

Ca-Al-rich inclusions (CAIs), common constituents of carbonaceous chondrites, have been studied extensively because they contain refractory, high-temperature minerals, suggesting that they formed at high temperatures during the early history of the solar system. Few CAIs have previously been discovered in ordinary chondrites. Here we describe 169 Al-rich objects (arbitrarily defined as having ≥ 10 wt.% Al 2O 3) from 24 types 3 and 4 ordinary chondrites, five regolith breccias containing unequilibrated material, the unique meteorite Kakangari, and a few types 5 and 6 ordinary chondrites. Based on shape and texture, we divide Al-rich objects into chondrules (round, with igneous textures), irregularly-shaped inclusions (similar to type F and spinel-rich complex CAIs), and fragments (probably fragments of Al-rich chondrules and inclusions). For descriptive purposes, we further subdivide Al-rich chondrules into compositional subgroups, although they are entirely transitional. Conclusions: 1. a) Al-rich chondrules and irregularly-shaped inclusions and fragments are wide-spread, albeit rare, constituents of ordinary chondrites and are similar to CAIs (except the coarse-grained ones) in carbonaceous chondrites, thus providing strong support for a related origin of ordinary and carbonaceous chondrites and their constituents, 2. b) Al-rich chondrules formed as independent, freely-floating, molten droplets and cooled rather rapidly, 3. c) Al-rich chondrules, like the common Mg-Fe-rich chondrules, formed by melting of millimeter- to submillimeter-sized dust-balls. In the case of Al-rich chondrules, the dust-balls consisted largely of mixtures of variable amounts of at least two solid precursor materials, one refractory-rich and the other volatile-rich. Phases which we interpret as unmelted relicts in some of these objects support this model, 4. d) The wide ranges in bulk compositions of Al-rich chondrules from refractory-rich to volatile-rich ones, yet complete transition between them, and the existence of chondrules transitional in composition between the Al-rich and common Mg-Fe-rich chondrules of ordinary chondrites, suggest that all chondrules formed by related processes. Differences in bulk composition result from mixing of different proportions of refractory-, volatile-, and Mg-Fe-rich (and other) solid precursor materials into dust-balls which were then melted, 5. e) Most irregularly-shaped inclusions show no indication of a once molten origin. Secondary alteration has probably changed the original mineral assemblage. Mainly volatile-rich materials entered the refractory objects as a gas, reacted with the constituents, and produced secondary phases. Rims probably formed both by decomposition and reaction with solar gas as well as by accretion, 6. f) Coarse-grained inclusions as found in Allende were not observed in ordinary chondrites.

Ca–Al-rich chondrules and inclusions in ordinary chondrites

Ca–Al-rich objects, hitherto mostly found in carbonaceous chondrites, are shown to be widespread, albeit rare, constituents of type 3 ordinary chondrites. Widespread occurrence and textural similarities of Ca–Al-rich chondrules to common, Mg–Fe-rich chondrules suggest that they formed by related processes. It is suggested in this article that Ca–Al-rich chondrules were formed by total melting and crystallization of heterogeneous, submillimetre- to millimetre-sized dustballs made up of mixtures of high-temperature, Ca–Al-rich and lower-temperature, Na–K-rich components.

U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The 206Pb/ 204Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206Pb/ 204Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the 207Pb/ 204Pb vs 204Pb/ 204Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed 87Sr/ 86Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970). The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.

Zoned Ca [sbnd]Al-rich chondrule in Bali: New evidence against the primordial condensation model

One large (8.5 mm) chondrule in the Bali carbonaceous chondrite is strongly enriched in refractory elements similarily to the white objects in Allende which are widely believed to be “primitive” condensates. However, detailed investigations show the Bali chondrule to have an element distribution opposite to that predicted by the “primitive condensation” models. Refractory elements like Ti and Al are enriched at or near the surface of the chondrule. The contents of some volatile elements, e.g. Na, Si, and Cr, are higher in the central portion. It is suggested that this chondrule - and similar objects in other C-chondrites - originated in one, or more likely several, impact events and that the element distribution is the result of volatilization-condensation.