Exsolution and shock microstructures of igneous pyroxene clasts in the Northwest Africa 7533 Martian meteorite

Research areas:
Year:
2016
Authors:
  • Hugues Leroux
  • Damien Jacob
  • Maya Marinova
  • Roger H. Hewins
  • Brigitte Zanda
  • Sylvain Pont
  • Jean-Pierre Lorand
  • Munir Humayun
Journal:
METEORITICS & PLANETARY SCIENCE
Volume:
51
Number:
5
Pages:
932-945
Month:
May
ISSN:
1086-9379
Abstract:
Northwest Africa (NWA) 7533 is a Martian regolith breccia. This
meteorite (and its pairings) offers a good opportunity to study (near-)
surface processes that occurred on early Mars. Here, we have conducted a
transmission electron microscope study of medium- and coarse-grained (a
few tens to hundreds of micrometers) Ca-rich pyroxene clasts in order to
define their thermal and shock histories. The pyroxene grains have a
high-temperature (magmatic) origin as revealed by the well-developed
pigeonite-augite exsolution microstructure. Exsolution lamella
characteristics (composition, thickness, and spacing) indicate a
moderately slow cooling. Some of the pyroxene clasts display evidence
for local decomposition into magnetite and silica at the submicron
scale. This phase decomposition may have occurred at high temperature
and occurred at high oxygen fugacity at least 2-3 log units above the
QFM buffer, after the formation of the exsolution lamellae. This
corresponds to oxidizing conditions well above typical Martian magmatic
conditions. These oxidizing conditions seem to have prevailed early and
throughout most of the history of NWA 7533. The shock microstructure
consists of (100) mechanical twins which have accommodated plastic
deformation. Other pyroxene shock indicators are absent. Compared with
SNC meteorites that all suffered significant shock metamorphism, NWA
7533 appears only mildly shocked. The twin microstructure is similar
from one clast to another, suggesting that the impact which generated
the (100) twins involved the compacted breccia and that the pyroxene
clasts were unshocked when they were incorporated into the NWA 7533
breccia.