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We retrace hydrogeochemical processes leading to the formation of Mg-Fe-Ca carbonate concretions (first distinct carbonate population, FDCP) in Martian meteorite ALH84001 by generic hydrogeochemical equilibrium and mass transfer modeling. Our simple conceptual models assume isochemical equilibration of orthopyroxenite minerals with pure water at varying water-to-rock ratios, temperatures and CO 2 partial pressures. Modeled scenarios include CO 2 partial pressures ranging from 10.1325 to 0.0001 MPa at water-to-rock ratios between 4380 and 43.8 mol mol )1 and different temperatures (278, 303 and 348 K) and enable the precipitation of Mg-Fe-Ca solid solution carbonate. Modeled range and trend of carbonate compositional variation from magnesio-siderite (core) to magnesite (rim), and the precipitation of amorphous SiO 2 and magnetite coupled to magnesite-rich carbonate are similar to measured compositional variation. The results of this study suggest that the early Martian subsurface had been exposed to a dynamic gas pressure regime with decreasing CO 2 partial pressure at low temperatures (approximately 1.0133 to 0.0001 MPa at 278 K or 6 to 0.0001 MPa at 303 K). Moderate waterto-rock ratios of ca. 438 mol mol )1 and isochemical weathering of orthopyroxenite are additional key prerequisites for the formation of secondary phase assemblages similar to ALH84001's 'FDCP'. Outbursts of water and CO 2(g) from confined ground water in fractured orthopyroxenite rocks below an unstable CO 2 hydrate-containing cryosphere provide adequate environments on the early Martian surface.

Redox conditions are seen as the key to controlling aqueous uranium concentrations (cU). Groundwater data collected by a state-wide groundwater quality monitoring study in Mecklenburg-Western Pomerania (Germany) reveal peak cU up to 75 μg L but low background uranium concentrations (median cU < 0.5 μg L). To characterize the hydrogeochemical processes causing such groundwater contamination by peak cU, we reanalyzed measured redox potentials and total concentrations of aqueous uranium, nitrate, and sulfate species in groundwater together with their distribution across the aquifer depth and performed semigeneric 2D reactive mass transport modeling which is based on chemical thermodynamics. The combined interpretation of modeling results and measured data reveals that high cU and its depth-specific distribution depending on redox conditions is a result of a nitrate-triggered roll-front mobilization of geogenic uranium in the studied aquifers which are unaffected by nuclear activities. The modeling results show that groundwater recharge containing (fertilizer-derived) nitrate drives the redox shift from originally reducing toward oxidizing environments, when nitrate input has consumed the reducing capacity of the aquifers, which is present as pyrite, degradable organic carbon, and geogenic U(IV) minerals. This redox shift controls the uranium roll-front mobilization and results in high cU within the redoxcline. Moreover, the modeling results indicate that peak cU occurring at this redox front increase along with the temporal progress of such redox conversion within the aquifer.

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[1] The cooccurrence of Mg-Fe-Ca carbonate and stratigraphically lower, phyllosilicatebearing rock units, identified in the Nili Fossae region on Mars, may reveal a CO 2 -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 CO 2 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) CO 2 transferred from the atmosphere (CO 2 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 CO 2 partial pressures ranging between ∼2 bars and ∼0.01 bar at low temperature (∼278 K).Citation: van Berk, W., and Y. Fu (2011), Reproducing hydrogeochemical conditions triggering the formation of carbonate and phyllosilicate alteration mineral assemblages on Mars (Nili Fossae region),