Thermal Histories Of Chondrules Research Articles

Chondrule radiative cooling in a non-uniform density environment

In an effort to better use thermal histories of chondrules in chondrites to test chondrule formation models, the present note addresses the formation of chondrules through the radiative cooling of a droplet cloud of non-uniform density. More generally, we show that the cooling rates obtained can be related not only to the different types of texture of the chondrules but also to their relative abundance in order to evaluate the relevance of models concerning the formation of chondrules.

The effect of multiple particle sizes on cooling rates of chondrules produced in large‐scale shocks in the solar nebula

AbstractChondrules represent one of the best probes of the physical conditions and processes acting in the early solar nebula. Proposed chondrule formation models are assessed based on their ability to match the meteoritic evidence, especially experimental constraints on their thermal histories. The model most consistent with chondrule thermal histories is passage through shock waves in the solar nebula. Existing models of heating by shocks generally yield a good first‐order approximation to inferred chondrule cooling rates. However, they predict prolonged heating in the preshock region, which would cause volatile loss and isotopic fractionation, which are not observed. These models have typically included particles of a single (large) size, i.e., chondrule precursors, or at most, large particles accompanied by micron‐sized grains. The size distribution of solids present during chondrule formation controls the opacity of the affected region, and significantly affects the thermal histories of chondrules. Micron‐sized grains evaporate too quickly to prevent excessive heating of chondrule precursors. However, isolated grains in chondrule‐forming regions would rapidly coagulate into fractal aggregates. Preshock heating by infrared radiation from the shock front would cause these aggregates to melt and collapse into intermediate‐sized (tens of microns) particles. We show that inclusion of such particles yields chondrule cooling rates consistent with petrologic and isotopic constraints.

The importance of experiments: Constraints on chondrule formation models

Abstract— We review a number of constraints that have been placed on the formation of chondrules and show how these can be used to test chondrule formation models. Four models in particular are examined: the “X‐wind” model (sudden exposure to sunlight <0.1 AU from the proto‐Sun, with subsequent launching in a magnetocentrifugal outflow); solar nebula lightning; nebular shocks driven by eccentric planetesimals; and nebular shocks driven by diskwide gravitational instabilities. We show that constraints on the thermal histories of chondrules during their melting and crystallization are the most powerful constraints and provide the least ambiguous tests of the chondrule formation models. Such constraints strongly favor melting of chondrules in nebular shocks. Shocks driven by gravitational instabilities are somewhat favored over planetesimal bow shocks.

On the origin of the “kleine Kügelchen” called Chondrules

In this paper we investigate two major issues: (1) What are chondrules, and (2) why do they exist? We review the literature pertaining to each question and explore answers. We discuss the diversity of chondrules, especially with regard to their igneous textures and compositions. We review the constraints that have been placed experimentally on the thermal histories of chondrules and list those fundamental properties that all chondrule formation models must explain quantitatively in order to be considered predictive, quantitative models. We provide background on the three major classes of chondrule formation models currently being discussed, and scrutinize each with respect to how well they adhere to the experimental constraints placed on chondrule formation. Finally, we list several unresolved issues that are now or will soon be at the forefront of chondrule research.

Deformation and thermal histories of chondrules in the Chainpur (LL3.4) chondrite

Abstract— Transmission‐electron‐microscopy (TEM) and optical data suggest that chondrules in the Chainpur (LL3.4) chondrite experienced varied thermal and deformation histories prior to the final agglomeration of the meteorite. Chainpur may be regarded as an agglomerate or breccia that experienced little deformation or heating during and after the final accumulation and compaction of its constituents.One chondrule in Chainpur was impact‐shocked to high pressures (∼ 20–50 GPa), almost certainly prior to final agglomeration, either while it was an independent entity in space or while it was in the regolith of a parent body. However, most (>85%) of the chondrules in Chainpur were evidently not significantly shock‐metamorphosed subsequent to their formation. The dearth of shock effects implies that most chondrules in Chainpur did not form by shock melting, although some chondrules may have formed by this process.Dusty‐metal‐bearing olivine grains, which are widely interpreted to have escaped melting during chondrule formation, contain moderate densities of dislocations (∼ 108 cm−2). The dislocations in these grains were introduced before or during the last episode of melting in at least one chondrule. This observation can be explained if olivine was impact‐deformed before or during chondrule formation, or if olivine was strained by reduction or thermally‐induced processes during chondrule formation.Low‐Ca pyroxene grains in chondrules are often strained. In most cases this strain probably arose as a by‐product of polytype transformations (protoenstatite → clinoenstatite/orthoenstatite and clinoenstatite → orthoenstatite) that occurred during the igneous crystallization and static annealing of chondrules.Droplet chondrules with glassy mesostases were minimally annealed, consistent with an origin as relatively rapidly cooled objects in an unconfined, cold environment. Some irregular chondrules and at least one droplet chondrule were thermally metamorphosed prior to final agglomeration, either as a result of moderately slow cooling (∼ 100 °C/hr) from melt temperatures (during autometamorphism) or as a result of reheating episodes. Two of the most annealed chondrules contain relatively abundant plagioclase feldspar, and one of these has a uniform olivine composition appropriate to that of an LL4 chondrite.

Thermoluminescence of meteorites: Shedding light on the cosmos

Abstract The TL sensitivity of meteorites reflects the abundance and nature of the feldspar. Thus intense shock, which destroys feldspar, causes the TL sensitivity to decrease by 1–2 orders of magnitude, while metamorphism, which generates feldspar through the devitrification of primary igneous glass, causes TL sensitivity to increase by a factor of ∼ 105. TL-metamorphism relationship is particularly strong for the lowest levels of metamorphism and has led to a new means of classification for little-metamorphosed meteorites. The TL emission characteristics of feldspar (peak temperature and peak width) are also related to the thermal history. Induced TL properties have thus found useful application to the paleothermometry of primitive meteorites, to the thermal histories of chondrules, to the ejection mechanism of martian meteorites and to arguments about different extraterrestrial sources for meteorites currently falling world-wide and those which fell 105–106 yr ago in the Antarctic. The level of natural TL in meteorites is determined by competition between build-up, due to exposure to cosmic radiation, and thermal decay. Terrestrial conditions should result in lower levels of natural TL compared to those expected in space, and, in principle, natural TL levels should be related to the period of time on earth (“terrestrial age”). Antarctic meteorites tend to have lower natural TL than non-Antarctic meteorites whose fall was observed, presumably because of the large terrestrial ages of the latter (105–106 yr). A few meteorites have especially low natural TL suggesting that they have recently been reheated. These may have come to earth on orbits with particularly small perihelia (≤ 0.8 astronomical units), and 26 meteorites with calculated orbits show a correlation between natural TL and perihelion distance. Dose-rate variations due to shielding, heating during atmospheric passage and anomalous fading also cause variations in natural TL levels, but the effects are either relatively small, occur infrequently or can be experimentally identified.