Deformation and thermal histories of ordinary chondrites: Evidence for post-deformation annealing and syn-metamorphic shock

Alex Ruzicka,Richard Hugo,Melinda Hutson

doi:10.1016/j.gca.2015.04.030

Abstract

We show that olivine microstructures in seven metamorphosed ordinary chondrites of different groups studied with optical and transmission electron microscopy can be used to evaluate the post-deformation cooling setting of the meteorites, and to discriminate between collisions affecting cold and warm parent bodies. The L6 chondrites Park (shock stage S1), Bruderheim (S4), Leedey (S4), and Morrow County (S5) were affected by variable shock deformation followed by relatively rapid cooling, and probably cooled as fragments liberated by impact in near-surface settings. In contrast, Kernouvé (H6 S1), Portales Valley (H6/7 S1), and MIL 99301 (LL6 S1) appear to have cooled slowly after shock, probably by deep burial in warm materials. In these chondrites, post-deformation annealing lowered apparent optical strain levels in olivine. Additionally, Kernouvé, Morrow County, Park, MIL 99301, and possibly Portales Valley, show evidence for having been deformed at an elevated temperature (⩾800–1000°C). The high temperatures for Morrow County can be explained by dynamic heating during intense shock, but Kernouvé, Park, and MIL 99301 were probably shocked while the H, L and LL parent bodies were warm, during early, endogenically-driven thermal metamorphism. Thus, whereas the S4 and S5 chondrites experienced purely shock-induced heating and cooling, all the S1 chondrites examined show evidence for static heating consistent with either syn-metamorphic shock (Kernouvé, MIL 99301, Park), post-deformation burial in warm materials (Kernouvé, MIL 99301, Portales Valley), or both. The results show the pitfalls in relying on optical shock classification alone to infer an absence of shock and to construct cooling stratigraphy models for parent bodies. Moreover, they provide support for the idea that “secondary” metamorphic and “tertiary” shock processes overlapped in time shortly after the accretion of chondritic planetesimals, and that impacts into warm asteroidal bodies were common.

Highlights

Collisions affected all solar system bodies and could have played a possibly complex and important role in the geological evolution of low-gravity, asteroidal-sized planetesimals (Scott et al, 1989; Stöffler et al, 1988)
(e.g., Leedey) show less dispersion in shock stages, and on this basis provide no evidence for being breccias or for having been impacted more than once
The L6 chondrites Park, Leedey, Bruderheim and Morrow County show the effects of varying shock intensity and all could have cooled quickly in ejecta fragments

Summary

Introduction

Collisions affected all solar system bodies and could have played a possibly complex and important role in the geological evolution of low-gravity, asteroidal-sized planetesimals (Scott et al, 1989; Stöffler et al, 1988). Chondritic meteorites are samples of such planetesimals that potentially can record evidence in their microstructures for collisional shock, during and shortly after accretion and in much later events. Impact redistribution could have placed chondrites of different metamorphic grades (petrographic types) at different depths, accounting for an overall lack of correlation between these grades and cooling rates based on metallographic techniques (Scott and Rajan, 1981; Grimm, 1985; Taylor et al, 1987; Scott et al, 2014). Optical petrographic properties of olivine provide the main way to assess the shock histories of chondrites (and other meteorites) with the Stöffler et al

Results

Discussion

Conclusion