17:15 〜 19:15
[PPS09-P11] Protolith Composition and Fluid Transport Controls on Ancient Martian Carbonate Formation: Insights from Thermochemical Modeling
キーワード：Mars, Carbonates, Thermochemical modeling, Alteration
Observed ancient carbonates on Mars broadly fall into two compositional groups: Mg-rich (including Mg-Fe, e.g., magnesite, MgCO3) and Ca/Fe-rich (e.g., siderite, FeCO3) [1]. This suggests different carbonate formation conditions were present on ancient Mars. The apparent spatial association between Ca/Fe carbonates and more felsic rocks on a global scale may suggest a causal relationship [1]. Also, multiple lines of evidence point to the presence of global groundwater circulation on ancient Mars [e.g., 2], which may have led to the formation of the carbonates. In this study, we conducted numerical thermochemical simulations to investigate the effects of protolith composition and fluid transfer on carbonate formation on ancient Mars.
Using the PHREEQC software, we simulated low temperature (5 °C) alteration of a mafic and a felsic protolith rock column by groundwater whose top surface is in contact with a 1 bar early Mars CO2 atmosphere. The mafic and felsic protolith compositions were derived from rover measurements of the Séítah Formation (Jezero Crater [3]) and the Meetinghouse rock (Gale Crater [4]), respectively. The simulations were run either with diffusive transport only (diffusive) or with both diffusion and downward advection (advective-dispersive, consistent with water percolation from the surface). Kinetics were implemented by including the dissolution rates of protolith phases in the simulations and suppressing high-temperature phases from precipitating. The diffusive simulations were run for both long (105 Earth years) and short (~3.2 Earth years) durations, while the advective-dispersive simulations were only run for ~3.2 Earth years.
The short-duration simulations all resulted in very low degrees of protolith dissolution (<20 wt. %), and the carbonates produced were predominantly Ca/Fe-rich (calcite and siderite). In all cases, calcite formed first. In cases with the mafic protolith, calcite was later replaced by siderite. In long-duration simulations, the protolith either substantially or completely dissolved, and the resulting carbonate composition depended on the protolith composition. The felsic protolith showed the same pure calcite to pure siderite evolution as in the short-duration simulations, while the mafic protolith produced a more complex evolution: a small amount of calcite formed first, a larger quantity of huntite (Mg3Ca(CO3)4) replaced it, and then the huntite was gradually replaced by siderite. The final alteration fluid contained an elevated Mg concentration, so although out of the scope of our simulations, Mg-carbonates could potentially precipitate as this fluid evaporates. Taken together, our results show that in a low temperature environment in contact with a thick CO2 atmosphere, Mg-rich carbonates can only form from a mafic protolith with a sufficiently long-term presence of liquid water to substantially dissolve the protolith, while Ca/Fe carbonates can be produced on a large scale either when the protolith is more felsic or when the extent of protolith dissolution is limited.
Among the short-duration simulations with the mafic protolith, the advective-dispersive simulation produced a much larger quantity of carbonates than the diffusive one, suggesting that advective transport greatly increases the efficiency of carbonate production. Also, the carbonates produced by the advective-dispersive simulation were all located at depth compared to at or near the surface in the diffusive case. This makes advective transport a potential mechanism to bury carbonates in Mars’s crust, and could explain the presence of kilometer-deep carbonates exposed by later craters [e.g, 1].
References
[1] Wray J. J. et al. (2016) J. Geophys. Res. Planets, 121, 652–677. [2] Salese F. et al. (2019) J. Geophys. Res. Planets, 124, 374– 395. [3] Wiens R. C. et al. (2022) Sci. Adv., 8, eabo3399. [4] Cousin A. et al. (2017) Icarus, 288, 265–283.
Using the PHREEQC software, we simulated low temperature (5 °C) alteration of a mafic and a felsic protolith rock column by groundwater whose top surface is in contact with a 1 bar early Mars CO2 atmosphere. The mafic and felsic protolith compositions were derived from rover measurements of the Séítah Formation (Jezero Crater [3]) and the Meetinghouse rock (Gale Crater [4]), respectively. The simulations were run either with diffusive transport only (diffusive) or with both diffusion and downward advection (advective-dispersive, consistent with water percolation from the surface). Kinetics were implemented by including the dissolution rates of protolith phases in the simulations and suppressing high-temperature phases from precipitating. The diffusive simulations were run for both long (105 Earth years) and short (~3.2 Earth years) durations, while the advective-dispersive simulations were only run for ~3.2 Earth years.
The short-duration simulations all resulted in very low degrees of protolith dissolution (<20 wt. %), and the carbonates produced were predominantly Ca/Fe-rich (calcite and siderite). In all cases, calcite formed first. In cases with the mafic protolith, calcite was later replaced by siderite. In long-duration simulations, the protolith either substantially or completely dissolved, and the resulting carbonate composition depended on the protolith composition. The felsic protolith showed the same pure calcite to pure siderite evolution as in the short-duration simulations, while the mafic protolith produced a more complex evolution: a small amount of calcite formed first, a larger quantity of huntite (Mg3Ca(CO3)4) replaced it, and then the huntite was gradually replaced by siderite. The final alteration fluid contained an elevated Mg concentration, so although out of the scope of our simulations, Mg-carbonates could potentially precipitate as this fluid evaporates. Taken together, our results show that in a low temperature environment in contact with a thick CO2 atmosphere, Mg-rich carbonates can only form from a mafic protolith with a sufficiently long-term presence of liquid water to substantially dissolve the protolith, while Ca/Fe carbonates can be produced on a large scale either when the protolith is more felsic or when the extent of protolith dissolution is limited.
Among the short-duration simulations with the mafic protolith, the advective-dispersive simulation produced a much larger quantity of carbonates than the diffusive one, suggesting that advective transport greatly increases the efficiency of carbonate production. Also, the carbonates produced by the advective-dispersive simulation were all located at depth compared to at or near the surface in the diffusive case. This makes advective transport a potential mechanism to bury carbonates in Mars’s crust, and could explain the presence of kilometer-deep carbonates exposed by later craters [e.g, 1].
References
[1] Wray J. J. et al. (2016) J. Geophys. Res. Planets, 121, 652–677. [2] Salese F. et al. (2019) J. Geophys. Res. Planets, 124, 374– 395. [3] Wiens R. C. et al. (2022) Sci. Adv., 8, eabo3399. [4] Cousin A. et al. (2017) Icarus, 288, 265–283.