Aqueous alteration on early Mars: New insights from the aqueous geochemistry and secondary mineralogy of meteorite ALH 84001

In the wake of the discovery of clay minerals in Gale crater (an ancient martian crater) by NASA's Curiosity rover, and numerous remote and in situ detections of hydrous minerals on the early surfaces of Mars, the absence of clays in the oldest martian meteorite, ALH 84001, remains conspicuous. ALH 84001 is an orthopyroxenite that contains carbonate minerals which were formed on Mars - probably as a result of low temperature, near-surface aqueous alteration of the rock - but it does not contain clays. This body of work comprises a number of studies and different analytical techniques used to understand and constrain the composition of early martian fluids that lead to the alteration of the meteorite when it was emplaced near the surface of Mars ~3.9 Ga ago. Petrological and mineralogical studies; carbon, nitrogen and noble gas stepped-combustion analyses; geochemical modelling under different conditions; and aqueous alteration experiments were all carried out here for this purpose. A young martian meteorite, shergottite NW A 6234, was also investigated in the context of this work to understand whether aqueous alteration persisted in time. Compositional clues described here point to varying degrees of aqueous alteration in ALH 8400 I, and possibly even in a shock melt vein in NW A 6234. The results of the extensive geochemical modelling strongly suggest that the fluid that altered ALH 84001 was near-neutral (pH≈7), low temperature (-20°C), and that the atmospheric CO2 pressure required to produce the carbonates was under 1 bar. The fluid was probably transient, and evaporated after precipitating the ubiquitous carbonates. A suite of other secondary products are predicted to have formed neighbouring ALH 84001, a few of which have been detected on Mars (such as clays), yet others are more surprising (such as talc and zeolites, and a possible pathway to the abiotic production of methane)