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 ofthe 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 Chain pur 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 (-10' em -2). The dislocations in these grains were introduced before or during the last episode ofmelting 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 auto metamorphism) or as a result of reheating episodes. Two ofthe most annealed chondrules contain relatively abundant plagioclase feldspar, and one of these has a uniform olivine composition appropriate to that of an LlA chondrite.

Iodine-xenon analysis of Chainpur (LL3.4) chondrules

“…This suggests that the chondrules experienced different conditions and amounts of alteration on the parent body. This, in turn, suggests a mixing of chondrules from sources across the parent body, which is consistent with the suggestion of Ruzicka (1990) (based on the variation in shock levels between chondrules) that Chainpur is an agglomerate composed of chondrules with widely diverse histories before compaction. It is notable that, of our samples, only CR2 exhibits a mixing line requiring phases with different 129 Xe*/I ratios, which is suggestive of a contribution of iodine and planetary xenon from more than one event.…”

“…Although Ruzicka (1990) found at least one chondrule had suffered significant shock deformation, there is no requirement that any of the chondrules studied here should have experienced comparable levels of shock, given that Chainpur appears to be an agglomerate. In addition, shock is predominantly a process that is magnified at grain boundaries (Stöffler et al, 1988;Stöffler et al, 1991).…”

Enstatite chemical composition and microstructures in the La Villa H4 chondrite

“…The first one is the protoenstatite inversion to produce a range of ortho-clino-enstatite intergrowths, as observed in type 3 chondrites (TopelSchadt and Miiller, 1985;Ruzicka, 1990); the second one, primarily based on the evidence illustrated in Fig. 5, is the clinoenstatite inversion in the ortho-enstatite stability field during subsequent thermal metamorphism.…”

“…In turn, experiments for time periods up to 24 d at 800 up to 1045 "C performed by Brearley and Jones (1993) on clinoenstatite-richer grains revealed no or very minor inversion. Previous HRTEM investigations of unshocked or slightly shocked H3 to H6 chondrites and L3 and LL3 chondrites have shown that enstatite is mainly twinned clinoenstatite in unmetarnorphosed type 3 chondrites (Tieschtitz, Mezo-Madaras, and Chainpur), with rare orthoenstatite crystals (Ashworth, 1981;Topel-Schadt and Miiller, 1985;Ruzicka, 1990). In turn, the abundance of orthoenstatite increases with petrographic type, as in the Quenggouk, Allegan, and Butsura H4-6 series (Ashworth, 1980(Ashworth, , 1981.…”

“…High-resolution transmission electron microscope (HRTEM) studies of unshocked or slightly shocked H3 to H6 chondrites (Ashworth, 1980(Ashworth, , 1981TBpel-Schadt and Miiller, 1985;Jones, 1993) andL3 andLL3 chondrites (TBpel-Schadt andMiiller, 1985;Ruzicka, 1990) revealed that enstatite is mainly monoclinic in unmetamorphosed type 3 chondrites, whereas the orthoenstatite content increases with petrographic type. Whereas it is widely accepted that the enstatite originally formed through protoenstatite inversion during fast chondrule cooling in type 3 chondrites, there are two distinct interpretations for the nature of enstatite in type 4-6 chondrites.…”