30 research outputs found
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Basalt weathering rates on Earth and the duration of liquid water on the plains of Gusev Crater, Mars
Where Martian rocks have been exposed to liquid water, chemistry versus depth profiles could elucidate both Martian climate history and potential for life. The persistence of primary minerals in weathered profiles constrains the exposure time to liquid water: on Earth, mineral persistence times range from {approx}10 ka (olivine) to {approx}250 ka (glass) to {approx}1Ma (pyroxene) to {approx}5Ma (plagioclase). Such persistence times suggest mineral persistence minima on Mars. However, Martian solutions may have been more acidic than on Earth. Relative mineral weathering rates observed for basalt in Svalbard (Norway) and Costa Rica demonstrate that laboratory pH trends can be used to estimate exposure to liquid water both qualitatively (mineral absence or presence) and quantitatively (using reactive transport models). Qualitatively, if the Martian solution pH > {approx}2, glass should persist longer than olivine; therefore, persistence of glass may be a pH-indicator. With evidence for the pH of weathering, the reactive transport code CrunchFlow can quantitatively calculate the minimum duration of exposure to liquid water consistent with a chemical profile. For the profile measured on the surface of Humphrey in Gusev Crater, the minimum exposure time is 22 ka. If correct, this estimate is consistent with short-term, episodic alteration accompanied by ongoing surface erosion. More of these depth profiles should be measured to illuminate the weathering history of Mars
Mars Sample Return: The Value of Depth Profiles
Sample return from Mars offers the promise of data from Martian materials that have previously only been available from meteorites. Return of carefully selected samples may yield more information about the history of water and possible habitability through Martian history. Here we propose that samples collected from Mars should include depth profiles of material across the interface between weathered material on the surface of Mars into unweathered parent rock material. Such profiles have the potential to yield chemical kinetic data that can be used to estimate the duration of water and information about potential habitats on Mars
Nanometer to micrometer scale characterization of pore networks in fine-grained rocks using electron microscopy and small angle neutron scattering
Scaling of Geochemical Reaction Rates via Advective Solute Transport
Transport in porous media is quite complex, and still yields occasional surprises. In geological porous media, the rate at which chemical reactions (e.g., weathering and dissolution) occur is found to diminish by orders of magnitude with increasing time or distance. The temporal rates of laboratory experiments and field observations differ, and extrapolating from laboratory experiments (in months) to field rates (in millions of years) can lead to order-of-magnitude errors. The reactions are transport-limited, but characterizing them using standard solute transport expressions can yield results in agreement with experiment only if spurious assumptions and parameters are introduced. We previously developed a theory of non-reactive solute transport based on applying critical path analysis to the cluster statistics of percolation. The fractal structure of the clusters can be used to generate solute distributions in both time and space. Solute velocities calculated from the temporal evolution of that distribution have the same time dependence as reaction-rate scaling in a wide range of field studies and laboratory experiments, covering some 10 decades in time. The present theory thus both explains a wide range of experiments, and also predicts changes in the scaling behavior in individual systems with increasing time and/or length scales. No other theory captures these variations in scaling by invoking a single physical mechanism. Because the successfully predicted chemical reactions include known results for silicate weathering rates, our theory provides a framework for understanding changes in the global carbon cycle, including its effects on extinctions, climate change, soil production, and denudation rates. It further provides a basis for understanding the fundamental time scales of hydrology and shallow geochemistry, as well as the basis of industrial agriculture. VC 2015 AIP Publishing LLC
Kinetic Metal Release from Competing Processes in Aquifers
Understanding groundwater time scales wherein kinetic
metal-desorption
and mineral-dissolution are important mechanisms is essential for
realistic modeling of metal release. In this study, release rate constants
were compiled and the Damköhler number was applied to calculate
residence times where kinetic formulations are relevant. Desorption
rate constants were compiled for arsenic, barium, cadmium, copper,
lead, mercury, nickel, and zinc, and span 6 orders of magnitude, while
mineral-dissolution rate constants compiled for calcite, kaolinite,
smectite, anorthite, albite, K-feldspar, muscovite, quartz, goethite,
and galena ranged over 13 orders of magnitude. This Damköhler
analysis demonstrated that metal-desorption kinetics are potentially
influential at residence times up to about two years, depending on
the metal and groundwater conditions. Kinetic mineral-dissolution
should be considered for nearly all residence times relevant to groundwater
modeling, provided the rate, solubility, and availability of the mineral
generates a non-negligible concentration. Geochemical models of competitive
desorption and dissolution for an illustrative metal demonstrate total
metal concentrations may be sensitive to dissolution rate variations
despite the predominance of release from desorption. Ultimately, this
analysis provides constraints on relevant processes for incorporation
into transport models