『Abstract
Various iron-bearing primary phases and rocks have been weathered
experimentally to simulate possible present and past weathering
processes occurring on Mars. We used magnetite, monoclinic and
hexagonal pyrrhotites, and metallic iron as it is suggested that
meteoritic input to the martian surface may account for an important
source of reduced iron. The phases were weathered in two different
atmospheres: one composed of CO2 + H2O, to model the present and primary martian atmosphere,
and a CO2 + H2O + H2O2 atmosphere to simulate
the effect of strong oxidizing agents. Experiments were conducted
at room temperature and a pressure of 0.75 atm. Magnetite is the
only stable phase in the experiments and is thus likely to be
released on the surface of Mars from primary rocks during weathering
processes. Siderite, elemental sulfur, ferrous sulfates and ferric
(oxy)hydroxides (goethite and lepidocrocite) are the main products
in a water-bearing atmosphere, depending on the substrate. In
the peroxide atmosphere, weathering products are dominated by
ferric sulfates and goethite. A kinetic model was then developed
for iron weathering in a water atmosphere, using the shrinking
core model (SCM). This model includes competition between chemical
reaction and diffusion of reactants through porous layers of secondary
products. The results indicate that for short time scales, the
mechanism is dominated by a chemical reaction with second order
kinetics (k = 7.75×10-5 g-1/h), whereas
for longer time scales, the mechanism is diffusion-controlled
(DeA = 2.71×10-10 m2/h).
The results indicate that a primary CO2-
and H2O-rich atmosphere should favour sulfur,
ferrous phases such as siderite or Fe2+-sulfates, associated
with ferric (oxy)hydroxides (goethite and lepidocrocite). Further
evolution to more oxidizing conditions may have forced these precursors
to evolve into ferric sulfates and goethite/hematite.』
1. Introduction
2. Methods
2.1. Sample descriptions
2.2. Experimental protocol
2.3. Analytical methods
3. Results
3.1. Magnetite
3.2. Elemental iron
3.3. Pyrrhotites
3.4. Mixture of MPo and α-Fe
3.5. Control samples (490 days of weathering)
4. Weathering mechanisms
4.1. Elemental iron α-Fe
4.2. Pyrrhotites
4.2.1. Water atmosphere
4.2.2. Hydrogen peroxide atmosphere
4.3. Weathering kinetics: the shrinking core model SCM
4.4. Model uncertainties
5. Implications for mars
5.1. Weathering model and evolution of martian surface conditions
5.2. Evolution of iron (oxy)hydroxides on Mars
5.3. Magnetic phases on mars
5.4. Sulfur and sulfates in the martian regolith
6. Conclusions
Acknowledgments
References