Deformation and Thermal Histories of Ordinary Deformation and Thermal Histories of Ordinary Chondrites: Evidence for Postdeformation Annealing Chondrites: Evidence for Postdeformation Annealing and Syn-Metamorphic Shock and Syn-Metamorphic Shock

-- We show that olivine


INTRODUCTION
Collisions affected all solar system bodies and could have played a possibly 27 complex and important role in the geological evolution of low-gravity, asteroidal-sized 28 planetesimals (Scott et al., 1989;Stöffler et al., 1988). Chondritic meteorites are 29 samples of such planetesimals that potentially can record evidence in their 30 microstructures for collisional shock, during and shortly after accretion and in much later 31 events. The L chondrite parent body evidently suffered a major impact ~0.5 Ga ago that 32 largely destroyed it, and which caused much debris to be delivered to Earth shortly

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(1991) shock stage scheme, which is used widely. However, generally fast diffusion in 49 olivine might allow deformation features to preferentially "heal" in this mineral during 50 annealing (Rubin 1992(Rubin , 1994(Rubin , 1995(Rubin , 2002(Rubin , 2003(Rubin , 2004. If so, the shock stage of olivine 51 alone might lead to erroneous inferences. deformed (S1), were selected based on evidence that they could have experienced a 70 more complex shock history, involving warm ambient conditions during or after shock. 71 Each contains features that could be relicts of shock metamorphism which were not 72 obliterated during annealing (Rubin, 2004). Coarse metal veins in the H chondrites such  (Rubin, 2002(Rubin, , 2004. 80 Preliminary data were reported previously (Hutson et al., 2007(Hutson et al., , 2009Ruzicka 81 and Hugo, 2011, 2014).

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Samples were studied at Portland State University with optical microscopy using 84 a Leica DM 2500 petrographic microscope, and with TEM using an FEI Tecnai G3 85 transmission electron microscope. Sample designations for meteorites studied with 86 these techniques are given in Tables EA-1 and EA-2 (Electronic Annex). For each 87 meteorite, optical and TEM data were acquired from the same polished thin section. 88 Optical microscopy was used to perform a grain-by-grain assessment of olivine 89 deformation for a representative sampling of grains ≥50 µm across using the method of  Stöffler (1995) and Schmitt (2000), olivine grains were assigned to different shock 92 stages based on intragranular misorientation and the presence of parallel straight planar 93 fractures or planar deformation features (Table EA-1). Misorientation angles were the 94 primary categorization tool, and were determined in cross-polarized transmitted light by 95 the difference in extinction position between the main extinction portion of the grains 96 and their remaining portions. These data were used to determine the conventional 97 shock stage (the highest category shown by at least 25% of grains, as given by Stöffler 98 et al., 1991), the "weighted shock stage" (the mean of all grain shock stages), and the 99 non-uniformity of deformation (variation of grain shock stages).

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The TEM, operated at 200kV, was used to study all electron transparent areas of 101 olivine grains in ion-milled 3-mm-diameter discs cored from polished thin sections.

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Amplitude contrast imaging in both bright field (bf) and g-3g weak beam dark field 103 (wbdf) mode was used. Parameters were measured that have been found to be related 104 to extent and temperature of deformation, and the extent of annealing. Deformation deformation involves high strain rates (e.g., Spray, 2010), so temperature is probably 115 the main variable that determines which slip system will be activated in chondritic 116 olivine. The ratio of "free" dislocations (not "bound" in subgrain boundaries) to the total 117 number of dislocations (free/(free+bound) or FFB ratio) is related to annealing extent 118 (Goetze and Kohlstehdt, 1973). For each discrete olivine grain in our specimens we  Optical data are summarized in Table EA-1 (Electronic Annex) and by shock 130 stage histograms in Fig. 1. The meteorites fall into two overall groups: weakly deformed 131 S1 (Portales Valley, Kernouvé, Park, MIL 99301) and more strongly deformed S4 132 (Leedey, Bruderheim) and S5 (Morrow County) chondrites (Fig. 1). Leedey is 133 reclassified as shock stage S4, revised from the literature value of S3 (Rubin, 1994).

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MIL 99301 is evidently a breccia, containing mostly weakly deformed (S1) but also 135 some more deformed and even recrystallized (S6) grains (Fig. 1). Some meteorites 136 (e.g., Leedey) show less dispersion in shock stages, and on this basis provide no 137 evidence for being breccias or for having been impacted more than once. Morrow 138 County is a definite S5 based on olivine data, consistent with the presence in this 139 meteorite (alone among the samples studied) of all feldspar as maskelynite, a hallmark 140 of shock stage S5 (Stöffler et al., 1991). Morrow County also contains numerous 141 chromite-plagioclase (maskelynite) assemblages, a proposed shock indicator (Rubin,142 2003), as well as prominent shock veins.

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The latter correlation is the trend expected for microstructural recovery involving 214 dislocation climb and annihilation (Fig. 5b). This implies that the low dislocation  The correlation between shock stage and average dislocation density for the L6 286 chondrites (Fig. 5a) is what one would expect for a shock history uncomplicated by 287 extensive post-deformation recovery. The data imply characteristic dislocation densities 288 in olivine for substantially unannealed and quickly cooled S4 and S5 chondrites of ~3 x 289 10 9 to 1 x 10 10 cm -2 , and for S1 chondrites of ~3 x 10 8 cm -2 . These densities are

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The highest average dislocation densities were found in Bruderheim, despite this 294 meteorite having a lower conventional shock stage than Morrow County and a lower 295 weighted shock stage than Leedey (Fig. 1, 5). This probably indicates that the region in 296 Bruderheim observed with TEM was slightly more deformed than average for 297 Bruderheim. This variation is not unexpected, as the passage of a shock wave in rocks 298 will produce local variations in shock pressures and temperatures owing to the 299 heterogeneous nature of the rocks (e.g., Sharp and DeCarli, 2006 (Table 1). These densities are respectively similar to those in Park (S1), between Park 312 and Leedey (i.e., S2-S3), and between Morrow County and Bruderheim (i.e., S4-S5).

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Based on these comparisons, we suggest that post-deformation annealing 317 lowered apparent optical strain levels in olivine from Kernouvé, MIL 99301, and 318 (especially) Portales Valley. Kernouvé probably was S1 originally, though a more 319 deformed S1 than it is now. MIL 99301 originally could have been shock stage S2-S3 or 320 a more deformed S1, less than the S4 inferred by Rubin (2002) (Table 1). In MIL 99301, at least one well-studied kink band (Fig. 4h, i)   Portales Valley (Rubin et al., 2001;Rubin, 2002Rubin, , 2004. This weakly supports the idea 383 of impacting a warm target.

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Also characteristic of shock deformation in a warm chondritic target is 385 recrystallized troilite (Schmitt, 2000). If the pre-shock temperature is elevated (i.e.,~650 386 °C), troilite recrystallizes readily during shock to form polycrystalline troilite even under 387 conditions that result in little deformation of olivine and plagioclase (Schmitt, 2000).

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Polycrystalline troilite is not prevalent in any of the S1 chondrites but has been reported 389 for MIL 99301 (Rubin, 2002(Rubin, , 2004  followed by minimal post-shock annealing (Fig. 7). Park has an ancient 39 Ar- 40  admixture of more highly shocked material (Fig. 1), possibly caused by impact mixing or 438 heterogeneity introduced during a shock event. The first possibility is favored by a 439 comparatively narrow range of shock effects seen in other chondrites such as Leedey, 440 Bruderheim, and Morrow County (Fig. 1), which suggests that single shock events do 441 not produce an overly large range of shock stages. In any case, given the evidence for  The traditional view of chondrite formation is that 1) "secondary" thermal 474 metamorphism pre-dated "tertiary" shock processes, and that 2) thermal metamorphism    annealed H and LL chondrites. Contrary to the situation for meteorites as a whole, there is no correlation between dislocation density and FFB ratio on the scale of individual TEM areas. If dislocation annihilation occurred during the formation of subgrain boundaries, the dislocation densities of areas with lower FFB would have been higher initially. Together with maximum dislocation densities and dislocation densities in regions with high FFB, these data can be interpreted to indicate that overall dislocation densities prior to recovery were higher initially for the annealed S1 chondrites.
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