The paradox between low shock-stage and evidence for compaction in CM carbonaceous chondrites explained by multiple low-intensity impacts

Paula Lindgren,Romy D Hanna,Katherine J Dobson,Tim Tomkinson,Martin R Lee

doi:10.1016/j.gca.2014.09.014

Abstract

Petrographic analysis of eight CM carbonaceous chondrites (EET 96029, LAP 031166, LON 94101, MET 01072, Murchison, Murray, SCO 06043, QUE 93005) by electron imaging and diffraction, and X-ray computed tomography, reveals that six of them have a petrofabric defined by shock flattened chondrules. With the exception of Murchison, those CMs that have a strong petrofabric also contain open or mineralized fractures, indicating that tensional stresses accompanying the impacts were sufficient to locally exceed the yield strength of the meteorite matrix. The CMs studied span a wide range of petrologic subtypes, and in common with Rubin (2012) we find that the strength of their petrofabrics increases with their degree of aqueous alteration. This correspondence suggests that impacts were responsible for enhancing alteration, probably because the fracture networks they formed tapped fluid reservoirs elsewhere in the parent body. Two meteorites that do not fit this pattern are MET 01072 and Murchison; both have a strong petrofabric but are relatively unaltered. In the case of MET 01072, impact deformation is likely to have postdated parent body aqueous activity. The same may also be true for Murchison, but as this meteorite also lacks fractures and veins, its chondrules were most likely flattened by multiple low intensity impacts. Multiphase deformation of Murchison is also revealed by the microstructures of calcite grains, and chondrule-defined petrofabrics as revealed by X-ray computed tomography. The contradiction between the commonplace evidence for impact-deformation of CMs and their low shock stages (most belong to S1) can be explained by most if not all having been exposed to multiple low intensity (i.e., <5GPa) shock events. Aqueous alteration was enhanced by those impacts that were of sufficient intensity to open high permeability fracture networks that could connect to fluid reservoirs.

Highlights

The aim of the present study is to reconcile this contradiction between the evidence for compaction of the CMs and their low shock stage, and we test three possible explanations: (i) the petrofabrics formed by lithostatic compaction rather than by hypervelocity impacts (Table 1); (ii) the petrofabrics formed by low shock pressures, which themselves may be due to attenuation of a higher magnitude shock wave by the porous meteorite matrix (Rubin, 2012); (iii) the petrofabrics formed by multiple episodes of low intensity shock, rather than by a single higher intensity event
We have focused our attention on chondrule deformation because Tomeoka et al (1999) and Nakamura et al (2000) showed experimentally that carbonaceous chondrite chondrules are flattened in the plane of the shock wave, and that their aspect ratios change in a regular manner as pressures increase
The sample is mounted on a stage with rotation axis perpendicular to the X-ray beam, and a series of 2D projections are collected at different angles as the sample is rotated

Summary

Introduction

Impacts have been important in the evolution of asteroids throughout solar system history. With regards to individual bodies, impacts have comminuted and mixed material within their regoliths (i.e., regolith ‘gardening’), and have led to compaction, fracturing, fragmentation, heating and possibly aqueous alteration of their interiors (e.g., Kerridge and Bunch, 1979; Metzler et al, 1992)

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