Size-frequency distributions and physical properties of chondrules from x-ray computed microtomography and digital data extraction.

Abstract

Within this work, we present the first true three-dimensional (3D) analysis of chondrule size. Knowledge about the physical properties of chondrules is important for validating astrophysical theories concerning chondrule formation and their aggregation into the chondritic meteorites (known as chondrites) that contain them. The classification of chondrites into chemical groups also relies on chondrule properties, including their dimensions. Within this work, we quantify the diameters of chondrules in five ordinary chondrites (OCs; comprised of the H, L, and LL chondrites) andone low-iron enstatite (EL) chondrite. To extract the chondrule size data, we use x-ray computed microtomography to image small (~1-2 cm3 ) chondrite samples followed by manual digital segmentation to isolate chondrules within the volumes or subvolumes. Our data yield true 3D results without stereographic corrections necessary for two-dimensional (2D) or petrographic thin section-based determinations of chondrule sizes. Our results are completely novel, but are consistent with previous surface analysis (2D) data for OCs. Within our OC chondrule diameter data, we find the trend of mean chondrule diameters increasing in the order H < L < LL. We also present the first detailed EL chondrite chondrule size-frequency distribution. Finally, we examine the shapes and collective orientations of the chondrules within the chondrites and show that chondrite petrofabrics can be explored with our methodology. Chondrule shape-preferred orientations are identical to the orientations of the metal and sulfide grains in the chondrites and this is likely due to impact-related compaction. HIGHLIGHTS: We present a first true three-dimensional analysis of chondrule size. Our ordinary chondrite chondrule diameter data demonstrate the trend of mean chondrule diameters increasing in the order H chondrites < L chondrites < LL chondrites. We also present the first detailed low-iron enstatite chondrite chondrule size-frequency distribution. We examine the shapes and collective orientations of the chondrules and show that chondrite petrofabrics can be explored with our methodology.