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

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., <5 GPa) 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.

Alkali‐halogen metasomatism of theCMcarbonaceous chondrites

“…This classification is consistent with Lindgren et al. (), who estimated that its pair, MET 01072, has a petrologic subtype of CM2.3. From the concentration of H in water/OH, Alexander et al.…”

“…) and so did not experience high impact pressures, the petrofabrics could have formed from one or more lower intensity collisions (Lindgren et al. ). There are, however, no clear petrographic indicators of whether impact compaction took place before, during, or after aqueous alteration.…”

Testing the genetic relationship between fluid alteration and brecciation in CM chondrites

“…; Rubin ; Lindgren et al. ). Although alteration has been shown to clearly predate brecciation in a few cases (Metzler et al.…”

Absence of olivine orientation fabric in highly shocked Martian dunite

“…; Rubin and Swindle ; Lindgren et al. ; Ruzicka and Hugo ); the latter seemingly in contrast with the low shock stage classifications (S1–S2) of many of the studied chondrites. This paradox is resolved by the results of matrix‐based structural studies (Scott et al.…”

“…Nevertheless, an essential feature in rock deformation is conspicuously absent in the list of shock criteria in meteorites: Preferred orientation fabrics, such as shape-preferred orientation (SPO) involving the orientation of the grains themselves and lattice-preferred orientation (LPO) involving the slip of specific crystallographic planes in a preferred direction. Several structural studies of chondrites have identified deformed chondrules and grain alignment fabrics and have attributed these to burial (Cain et al 1986;Zolensky et al 1997) or shock impact (Scott et al 1992;Rubin and Swindle 2011;Lindgren et al 2015;Ruzicka and Hugo 2018); the latter seemingly in contrast with the low shock stage classifications (S1-S2) of many of the studied chondrites. This paradox is resolved by the results of matrix-based structural studies (Scott et al 1992;Nakamura et al 1995;Watt et al 2006;Forman et al 2016Forman et al , 2017 concluding that chondrule deformation and alignment occurs through impact-induced reduction in matrix porosity through pore collapse with consequential matrix compaction.…”