AN OVERVIEW OF THE TYPE II CHONDRULES IN THE CM CHONDRILTE JBILET WINSELWAN

Jbilet Winselwan is a largely unweathered CM2 breccia from Morocco. It contains a range of lithologies. Most are typical of CMs with others showing brief but significant impact heating and brecciation. Lithologies have varying degrees of both chondrule alteration to phyllosilicates and degradation of chondrule rims, with strong rims being present in some lithologies while completely absent in others. Type II chondrules are little understood at the moment, with multiple theories on their formation relative to type I chondrules ranging from evaporation and reduction processes to separate nebular reservoirs. Here, type II chondrules from Jbilet Winselwan were examined with a particular focus on relict grains with high-Mg cores to examine the hypothesis that type II chondrules were derived from type I chondrule material.

A total of 39 type II chondrules were identified between the two thin sections, ranging from ~40µm-1100µm in diameter. Not a single type II chondrule contained crystalline pyroxene with each being some subset of porphyritic olivine, contrary to the mean composition within the entire sample. Both individual grain size and degree of alteration apparently had no bearing on chondrule type. The primary difference observed between type I and II chondrules was the ubiquitous zoning in the latter. In a significant portion of type II chondrules, these relict core compositions roughly matched that of type I chondrule material while rim compositions ranged from roughly 15-45% less Mg. “Dusty” type zoned relict grains were also observed in a few chondrules, but they were not visually distinct enough from type I chondrules rich in metal blebs to be confidently labeled as more than just zoned examples of these chondrules.

The observations in this overview of type II chondrules in Jbilet Winselwan works to support the hypothesis that type II chondrules were derived from type I chondrite material. The ubiquitous zoning suggests either growth in a depleting system or external alteration of the crystals. This combined with the measurements showing the correlation between the cores of zoned relict grains and type I chondrule material, the relict fine grained aggregates, and the visual link between dusty relicts and Fe/Ni bleb-rich type I grains shows that the latter case is much more likely.