What Solutions Caused Noachian Weathering on Mars?

Abstract:

The stratified sequence of Al-rich and Fe/Mg phyllosilicates in Noachian formations indicates widespread chemical weathering of mafic materials [1]. The composition of alteration solutions could be inferred from the mineralogy of weathering profiles and models for water-rock interaction. We have developed numerical models for basalt weathering by acidic solutions with different concentrations of weak (H₂CO₃) and strong (H₂SO₄, HCl) acids equilibrated with low- and high-pressure CO₂ atmospheres. The results show that the observed clay stratigraphies could be produced by neutral to strongly acidic solutions. Weathering by solutions equilibrated with dense CO₂ atmospheres produces abundant carbonates at depth, which are not observed in clay stratigraphies. The development of weathering profiles by S-, Cl-free solutions equilibrated with 6 mbar CO₂ requires large volumes of water. These volumes are inconsistent with density and orientation of Noachian valley networks and climate models [3–5]. Weathering by sulfate-free fluids does not produce abundant Ca sulfates reported in the Mawrth Valley region [2]. Weathering by low-pH H₂SO₄-bearing solutions does not require elevated water/rock ratios, a warm climate, or a dense CO₂ atmosphere. It leads to formation of Ca sulfates in middle parts of weathering profiles together with neutralized Mg-rich sulfate solutions at depth. The weathering could have occurred through transient volcanism- and impact-generated supply of strong acids, and volcanism-, impact-, obliquity-related warming and partial ice melting, consistent with climate models [3–5].

Refs: [1] Carter, J. et al. (2015), Icarus 248, 373–382. [2] Wray, J. J. et al. (2010), Icarus 209, 416–421. [3] Wordsworth, R. et al. (2013), Icarus 222, 1–19. [4] Mischna, M. A. et al. (2013), J. Geophys. Res. Planets 118, 518–576. [5] Halevy, I., Head, J.W. (2014), Nature Geosci. 7, 865–868.