Olivine Microstructures in the Miller Range 99301 (LL6) Ordinary Chondrite

Introduction: Miller Range 99301 (MIL 99301) was classified as an LL6 ordinary chondrite based on its recrystallized texture in thin section, chemical homogeneity of olivine and low-Ca pyroxene grains, and the presence of coarse (>50 μm) interstitial grains of plagioclase [1]. Determination of shock stage was problematic as various shock indicators gave contradictory information: olivine and plagioclase grains showed sharp optical extinction indicative of shock stage S1, whereas other indicators such as the presence of polycrystalline troilite and large grains of low-Ca clinopyroxene suggested a shock stage of S4 or higher [1]. To account for these observations, Rubin [1] proposed that MIL 99301 experienced a complex thermal history with metamorphism to petrographic type 6, a later shock event equivalent to shock stage S4 or higher, and annealing to metamorphic levels equivalent to petrographic type 4 to remove defects in olivine and plagioclase. Additionally, MIL 99301 records Ar-Ar evidence for two impact events, one at ~4.52 and another at ~4.23 Ga ago [2]. We used transmission electron microscope (TEM) imaging to examine microstructures in MIL 99301 olivine grains in order to understand more fully this meteorite’s deformation and thermal history. Methods: A small bulk sample of MIL 99301 was lapped to ~100 μm thickness, cored and affixed to a 3 mm copper support grid. The sample was ion milled, with a milling angle of 5°, until a perforation was detected. The final TEM sample contained five perforations in an area approximately 500 μm x 500 μm, with electron transparent areas adjacent to these perforations. TEM work was done at Portland State University using an FEI Tecnai F-20 G2 TEM/STEM equipped with an EDAX energy dispersive detector (EDS) operated at 200 kV. Olivine grains were identified using EDS analysis. Results: At present, ten olivine grains have been imaged. All of the grains contain very low dislocation densities. Figure 1 is typical of two of the areas studied, with grains that lack any evidence of dislocations. The remaining grains contain some dislocations, but also show evidence that more than one slip system was activated (Fig. 2). In two grains, dislocations are present in arrays (kink bands) (Fig. 2) which lack the regular spacing seen in subgrain boundaries. One area contained pinned and bowed dislocations (Fig. 3), while in another area 120° triple junctions were observed (Fig. 4).