EFFECT OF METAMORPHISM ON COMPONENT SIZE WITHIN LL CHONDRITES NORTHWEST AFRICA 4910, 5660, 7981

Chondritic meteorites are composed of materials that condensed directly from the solar nebula and can help us better understand the formation of our universe. Chondritic meteorites are thought to originate from multiple asteroidal parent bodies. Due to localized heterogeneity within the solar nebula, each parent body has a unique composition and mineralogy. Within each chondrite group, there is also significant variation in abundance and size of components (e.g., chondrules, refractory inclusions, and metal). Though chondrites are derived from primitive (unmelted) asteroids, many experience secondary alteration, such as thermal metamorphism or aqueous alteration. Ordinary chondrites, the most abundant group of chondrites, have experienced varying degrees of metamorphism, from petrologic type 3 (minimal metamorphism) to type 6 (significant metamorphism). The least metamorphosed (unequilibrated) ordinary chondrites are divided into petrologic subtypes 3.0 – 3.9. Though there are significant chemical changes in chondrite components during low-temperature metamorphism, such as an increase in the FeO-content of chondrule olivine, the effects of metamorphism on the size and abundance of chondrite components has not been thoroughly evaluated.

To investigate how increasing temperature affects chondrite components at the earliest stages of metamorphism, we examined abundances and sizes of chondrule populations in three LL ordinary chondrites of increasing petrologic type: NWA 4910 (type 3.1), NWA 7981 (type 3.5), and NWA 5660 (type 3.7). We focused our study on chondrules because they are the most prominent component in ordinary chondrites. Following the methods of Ebel et al. (2015), we used ImageJ to combine Mg, Fa, and Al X-ray maps of the samples into RGB images, which allowed us to identify all chondrite components. Chondrules, metals, and CAI’s were then grouped into separate layers and classified by color. These images were exported to ImageJ where the area and diameters of the chondrules were measured. The measurements were then corrected for non-equatorial sectioning of chondrules and oversampling of larger chondrules. By comparing resulting data from chondrites of various petrologic types, we can determine whether size and abundance of chondrules change during progressive metamorphism.