Chondrule Size Distributions: What Does It Mean?

One primary characteristic of chondrite groups is their distinctive size-frequency chondrule distributions [1,2, 3]. The restricted ranges of chondrule size has been interpreted as indicative of a sorting mechanism, such as some form of aerodynamic drag [3, 4]. We re-examine this concept in light of the current database. The database and its limitations. Chondrule sizes are typically determined by disaggregation or thinsection measurements. Sedimentalogy laboratory measurements are based almost entirely on disaggregation [5]. Samples of Bjurbole and Chainpur [6] and Qingzhen [1] have been disaggregated, and chondrule sizes estimated from photographs. Application is limited by the need to destroy significant quantities of material and the resistance of many meteorites to physical disaggregation. It is possible that disaggregation of meteorites results in undercounting of very small chondrules [1] and it is likely that some relatively friable chondrule types are underrepresented [7], and typically statistics are <100 chondrules per meteorite [6]. Another limitation to disaggregation studies of meteorites is that it is difficult to classify significant numbers of chondrules after separation [7]. Measurement of chondrule sizes in thin-section overcomes some of the limitations of disaggregation studies, being less subject to undercounting of friable or small chondrules [1,7], and easily applicable to a wide array of meteorites. Thin-section size measurements are not as a substitute for disaggregation techniques due to significant problems in estimating true grain sizes from random sections and measurement biases [5, 8]. Without extensive correction procedures, the data can only be compared to each other, the apparent means and standard deviations being inaccurate. Thin-section data size distributions are often based on very small numbers of chondrules, typically <100 [1,3], and are available for only ~50 meteorites (Fig. 1). Major chemical groups tend to exhibit fairly uniform mean chondrule sizes, with ordinary chondrites having the largest and CM/CO having the lowest average size [Fig. 1; see also ref. 1]. With the exception of CV chondrites, major chondrite classes have similar ranges of standard deviation. Data for Apollo 14 crystalline spherules (CLS) [9] are shown for comparison. Chondrule Size Distributions Data Boundaries. Chondrule Size Limits: Size distributions are the primary guide to the upper and lower limits to chondrule size in a given meteorite class, an estimate of boundaries being obtained from a fitted mathematical function. Chondrule size data from thin-section estimates tend to fit best to a Weibull distribution, in which there is a non-zero minimum chondrule size, typically about 10-20% of the mean size (40-150 μm) [8]; this may be an artifact of thin-section measurement [8], but disaggregation data also tends to support a minimum chondrule size [6, 10]. However, it is possible that both disaggregation and thin-section analysis undercount very small “microchondrules” (5-40 μm in diameter), which are found in the matrix or in rare clasts in at least a few chondrites, and “megachondrules” (>1 cm in diameter), which occur as fragments [11]. Multiple Chondrule Populations in Chondrites: It is often assumed that chondrules in a given meteorite are a uniform size-distribution population. However, there are significant differences in chondrule sizes and degree of sorting with textural and chemical class within individual meteorites. Rubin [12] noted significant differences in diameters of textural types in CO chondrites, with PO>PP and POP, PP>POP, and BO>porphyrtic chondrules. Textural groups tend to exibit the same degree of heterogeneity as meteorites in general. Chemical group A chondrules tend to be smaller than group B chondrules in highly unequilibrated ordinary chondrites [7]. In the CM chondrite Murchison, group B chondrules tend to be smaller than group A chondrules [13]. Chemical groups tend to exhibit less heterogeneity than chondrules as a whole.