SHOCK METAMORPHISM, MINERALOGY, AND PETROGRAPHY OF PECORA ESCARPMENT 82500, A CK4/5 CHONDRITE

Pecora Escarpment 82500 (PCA 82500) is an equilibrated (petrologic type 4/5) CK chondrite, originally classified as an LL ordinary chondrite. Though PCA 82500 is clearly equilibrated, as indicated by its recrystallized matrix and absence of well-defined chondrules, compositions of magnetite differ significantly from magnetite in other equilibrated CK chondrites. Instead, magnetite in PCA 82500 is compositionally similar to magnetite in unequilibrated (petrologic type 3) chondrites. Additionally, exsolution lamellae of ilmenite and spinel, which are common in magnetite found in other equilibrated CK chondrites, are absent in magnetite from PCA 82500. Since exsolution lamellae form during slow cooling, their absence may suggest that PCA 82500 experienced rapid cooling compared to other equilibrated CK chondrites.

This study aims to characterize the mineralogy and petrography of PCA 82500 using techniques including Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) and Electron Microprobe Phase Analysis (EMPA). In addition, we intend to find evidence of shock heating/metamorphism. To accomplish this, we first classified the shock stage of the sample using Stöffler’s progressive stages of shock metamorphism developed for ordinary chondrites and shown to be applicable to carbonaceous chondrites. Scott (1992) characterized the shock level of PCA 82500 as S1 (unshocked), while Rubin (1992) suggested it was S2 (very weakly shocked). To resolve the discrepancy in the literature, we examined a random sampling of 40 olivine grains > 50 um in size. The majority of grains exhibit undulatory extinction and irregular fractures, confirming a shock stage of S2. We will also look for other evidence of shock metamorphism, such as shock blackening and melt veins, using high-magnification imaging.

Our ultimate goal is to determine the shock history in addition to the mineralogy and petrography of PCA 82500 in order to further understand the thermal history of the CK chondrite parent body.