Macrochondrules in chondrites—Formation by melting of mega‐sized dust aggregates and/or by rapid collisions at high temperatures?

Abstract

Abstract-Seventy-four macrochondrules with sizes >3 mm were studied. Considering the extraordinary size of the chondrules (occasionally achieving a mass of 1000 times (and more) the mass of a normal-sized chondrule), the conditions in the formation process must have been somewhat different compared with the conditions for the formation of the common chondrules. Macrochondrules are typically rich in olivine and texturally similar to specific chondrule types (barred, radial, porphyritic, and cryptocrystalline) of normal-sized chondrules. However, our studies show that most of the macrochondrules are fine-grained or have elongated crystals (mostly BO, RP, and C), which lead to the assumption that they were once totally molten and cooled quite rapidly. Porphyritic chondrules belong to the least abundant types of macrochondrules. This distribution of chondrule types is highly unusual and just a reverse of the distribution of chondrule types among the typical-sized chondrules in most chondrite groups except for the CH and CB chondrites. New chondrule subtypes (like radialolivine [RO] or multi-radial [MR] chondrules) are defined to better describe the textures of certain large chondrules. Macrochondrules may have formed due to melting of huge precursor dust aggregates or due to rapid collisions of superheated melt droplets, which led to the growth of large molten spherules in regions with high dust densities and high electrostatic attraction.

Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion

“…Particles of large enough size to concentrate by streaming instabilities could have been present in the early stages of the protoplanetary disk when planetesimals formed. Such particles include macrochondrules (16), chondrule aggregates (17) and ice-rich pebbles. All of these drift rapidly towards the Sun because of gas drag.…”

“…2) Large chondrules. Another possibility is that the first planetesimal seeds formed out of centimeter-sized macrochondrules (16). These could have drifted out of the asteroid belt subsequent to planetesimal formation and left the stage for accretion of their millimeter-sized counterparts.…”

Chondrule size and related physical properties: A compilation and evaluation of current data across all meteorite groups

“…However, we do report on 10-100 m microchondrules (e.g., Rubin et al, 1982;Bigolski et al, 2014) and macrochondrules and/or megachondrules (e.g., Weisberg et al, 1988a;Ruzicka et al, 1998;Weyrauch and Bischoff, 2012) in chondritic meteorites. However, the origins of some of these may be different than chondrules more typical in size.…”

Constraints on compound chondrule formation from laboratory high-temperature collisions

“…Bouncing Plastic We roughly mark different temperatures where changes in collisions occur from bouncing elastic to plastic (T p ) to sticking (T s ) to compound formation (T c ). If compounds fused together completely, the result might be a single, larger chondrule, so frequent collisions might lead to the formation of macrochondrules (Weyrauch and Bischoff, 2012;Bischoff et al, 2017). We mark this as T m .…”