Physics
Scientific paper
Oct 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011jgre..11610006v&link_type=abstract
Journal of Geophysical Research, Volume 116, Issue E10, CiteID E10006
Physics
Planetary Sciences: Solid Surface Planets: Erosion And Weathering, Planetary Sciences: Solid Surface Planets: Origin And Evolution
Scientific paper
The cooccurrence of Mg-Fe-Ca carbonate and stratigraphically lower, phyllosilicate-bearing rock units, identified in the Nili Fossae region on Mars, may reveal a CO2-driven aqueous alteration process on ancient Mars. We reproduce hydrogeochemical conditions for concurrent carbonate and phyllosilicate formation in rock columns by using hydrogeochemical one-dimensional transport modeling, which is based on chemical equilibrium thermodynamics. Our models assume the isochemical, low-temperature equilibration of ultramafic rocks with pure water at varying atmospheric CO2 partial pressure, water-to-rock ratio, temperature, and rock composition. Equilibration among (1) mineral assemblages of olivine-rich rocks (18-28 wt% MgO), (2) pure water (water-to-rock ratio: 500-50 mol mol-1), (3) CO2 transferred from the atmosphere (CO2 partial pressure lower than ˜2 bars and higher than ˜0.01 bar) at ˜278 K, and (4) 100,000 years of diffusive mass transport create hydrogeochemical conditions enabling the concurrent formation of Mg-rich Mg-Fe-Ca solid-solution carbonate and stratigraphically lower phyllosilicate-rich alteration assemblages in rock columns. The modeled composition of alteration mineral assemblages, their depth distribution, and carbonate composition are similar to the observations from the Nili Fossae region. Higher equilibration temperature (373 K) results in the formation of Ca-dominated Ca-Mg carbonate instead of Mg-rich Mg-Fe-Ca carbonate and in an inverse stratification with phyllosilicate-rich units overlying carbonate-rich units. Modeling results suggest that rocks in the Nili Fossae region, hosting the carbonate and phyllosilicate mineral assemblage, have been exposed to a groundwater alteration process driven by atmospheric CO2 partial pressures ranging between ˜2 bars and ˜0.01 bar at low temperature (˜278 K).
Fu Yunjiao
van Berk Wolfgang
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