Petrographic and geochemical evidence for multiphase formation of carbonates in the Martian orthopyroxenite Allan Hills 84001

TL;DR

This study uses petrographic and geochemical analyses to reveal that Martian carbonate formation in the Allan Hills 84001 meteorite occurred through multiple distinct precipitation and shock events over billions of years.

Contribution

It provides new evidence for multiphase carbonate formation in a Martian meteorite, highlighting multiple fluid pulses and impact-related fracturing processes.

Findings

Two distinct carbonate generations identified by compositional differences

Evidence of fracturing and corrosion between carbonate formation events

Carbonates formed through multiple fluid pulses over billions of years

Abstract

Martian meteorites can provide valuable information about past environmental conditions on Mars. Allan Hills 84001 formed more than 4 Gyr ago, and owing to its age and long exposure to the Martian environment, this meteorite has features that may record early processes. These features include a highly fractured texture, gases trapped during one or more impact events or during formation of the rock, and spherical Fe-Mg-Ca carbonates. Here we have concentrated on providing new insights into the context of these carbonates using a range of techniques to explore whether they record multiple precipitation and shock events. The petrographic features and compositional properties of these carbonates indicate that at least two pulses of Mg- and Fe-rich solutions saturated the rock. Those two generations of carbonates can be distinguished by a very sharp change in compositions, from being rich in Mg and poor in Fe and Mn, to being poor in Mg and rich in Fe and Mn. Between these two generations of carbonate is evidence for fracturing and local corrosion.