Carbonates in the Martian meteorite Allan Hills 84001 formed at 18 ± 4 °C in a near-surface aqueous environment

Abstract

Despite evidence for liquid water at the surface of Mars during the Noachian epoch, the temperature of early aqueous environments has been impossible to establish, raising questions of whether the surface of Mars was ever warmer than today. We address this problem by determining the precipitation temperature of secondary carbonate minerals preserved in the oldest known sample of Mars' crust-the approximately 4.1 billion-year-old meteorite Allan Hills 84001 (ALH84001). The formation environment of these carbonates, which are constrained to be slightly younger than the crystallization age of the rock (i.e., 3.9 to 4.0 billion years), has been poorly understood, hindering insight into the hydrologic and carbon cycles of earliest Mars. Using "clumped" isotope thermometry we find that the carbonates in ALH84001 precipitated at a temperature of approximately 18°C, with water and carbon dioxide derived from the ancient Martian atmosphere. Furthermore, covarying carbonate carbon and oxygen isotope ratios are constrained to have formed at constant, low temperatures, pointing to deposition from a gradually evaporating, subsurface water body-likely a shallow aquifer (meters to tens of meters below the surface). Despite the mild temperatures, the apparently ephemeral nature of water in this environment leaves open the question of its habitability.T he extreme difficulty in achieving mild surface temperatures in early Mars climate models (1) is in disagreement with widespread geomorphological evidence for surface water runoff during the Noachian epoch (2, 3). A robust determination of the temperature at or near the surface of Noachian Mars would provide insight into this apparent paradox, but has been challenging to establish. Carbonate minerals can record and preserve information regarding the temperature and chemistry of their formation environment through aspects of their oxygen and carbon isotopic composition (δ 18 O and δ 13 C, respectively). These minerals have been observed in a range of Martian geological materials, including dust, bedrock outcrops, and several meteorites (4-9). Collectively, these observations highlight a role for carbonate formation in the global Martian carbon cycle and the evolution of its atmosphere. The oldest known carbonate from Mars, or any other planetary body, occurs as a minor constituent (approximately 1% by weight) in the meteorite Allan Hills 84001 (ALH84001), which has a crystallization age of approximately 4.1 billion years (10). The carbonate in ALH84001, geochronologically constrained to be slightly younger in age [between 3.9 and 4.0 billion years old; (11)], provides a unique window into the hydrologic cycle, carbon cycle, and climate of Noachian Mars.Texturally, the carbonates in ALH84001 occur as (i) chemically and isotopically zoned ovoid concretions, veins, and void fillings and (ii) regions of massive carbonate, variably intergrown with unweathered feldspathic glass and orthopyroxene, the latter of which makes up the bulk of the meteorite (12)(13)(14) The carbonates in ALH84...

Carbonate formation events in ALH 84001 trace the evolution of the Martian atmosphere

“…1). The 3-h acid extraction step at 150°C yielded CO 2 with δ 13 C = 38‰ and δ 18 O = 21‰, similar to what has been previously reported for the Fe-and Mg-rich carbonate rosettes (9,22) (Fig. 1).…”

“…If the Ca-rich phase precipitated at room temperature similar to the rosettes (22), it suggests that the atmosphere δ 13 C was near 13‰. This atmospheric composition is very similar to what is predicted by the degassing/evaporation models (22,33) outlined in the case where the Ca-rich carbonate formed in close association with the slab/ rosettes, presented earlier (Fig. 3).…”

“…Data from this study and other stepped dissolution measurements of ALH 84001 showing the variation in δ 13 C observed in this study for Martian phases (9,22). Gray crosses indicate the range of values from SIMS studies correlated by Mg contents.…”

“…However, the covariant trend indicated by the rest of the ALH 84001 data potentially points to an initial atmospheric composition near 15‰, which would also be consistent with the Ca-rich carbonates measured in this study. These studies have suggested that the Ca-rich carbonates formed from high-temperature hydrothermal fluids (13) Finally, it has also been suggested that the carbonates formed from a single CO 2 -rich fluid that began degassing and evaporating as it was exposed near the surface (22,33). This would suggest an ancient atmosphere that possessed a δ 13 C of 15-20‰, whereas the carbonates obtained their heavier and covariant carbon and oxygen isotopic compositions through Rayleigh distillation effects during evaporation and degassing.…”

Mars: a small terrestrial planet

“…Local outcrops of magnesite or calcite have also been reported from central peaks of impact craters (Michalski and Niles 2010;Wray et al 2016), and siderite (Fe-carbonate) has been detected on the Columbia Hills by the Spirit rover (Morris et al 2010). Martian meteorite ALH84001 contains carbonates formed as a result of fluid circulation at depth, tough the oxygen isotopic ratio suggests a meteoric origin for the water (e.g., Halevy et al 2011;Shaheen et al 2015). The presence of carbonates is fundamental as it is potentially a significant sink for CO 2 which has broad implications for evolution of Mars' atmosphere.…”