Michael A. Velbel

Mineralogy and Petrology of Comet 81P/Wild 2 Nucleus Samples

The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.

Constancy of silicate-mineral weathering-rate ratios between natural and experimental weathering: implications for hydrologic control of differences in absolute rates

Abstract Inverse models which apportion watershed effluxes (geochemical mass balance) over estimated mineral surface area give weathering rates for individual silicate minerals (e.g., feldspar) 1–3 orders of magnitude slower than laboratory rates. Comparisons of rates calculated from several recent watershed geochemical mass-balance studies with experimentally determined rates reveal that the ratio of mineral weathering rates for a given silicate-mineral pair determined in an internally consistent field study equals the ratio of rates for the same minerals in any internally consistent set of laboratory experimental data. Thus, the coefficient which corrects the weathering rate for any mineral from a laboratory data-set to a field setting is identical to the corresponding coefficient for any other mineral in the same data sets. The “correction factor” linking experimental and natural rates must be independent of composition. Physical (e.g., hydrologic) controls, rather than compositional or chemical controls, cause the difference between weathering rates in nature and the laboratory. Because flow in natural weathering profiles is spatially heterogeneous, not all of the potentially available surface in natural systems actually participates in reactions with pore fluids.

The Influence of Climate and Topography on Rock-Fragment Abundance in Modern Fluvial Sands of the Southern Blue Ridge Mountains, North Carolina

Chemical weathering influences the detrital composition of sand-size sediment derived from source areas subject to different amounts of precipitation in the Coweeta Basin, North Carolina. Of the grain types studied, rock fragments are most sensitive to chemical degradation; therefore, their abundance is the best indicator of cumulative weathering effects. Destruction of sand-size rock fragments by chemical weathering is a function of both the intensity and duration of chemical weathering experienced by grains in regoliths of the source area. In the Coweeta Basin, the intensity of chemical weathering is directly related to the climate via effective precipitation in individual subbasins, whereas the duration of chemical weathering is inversely related to the relief ratio of the watershed. Therefore, soils in watersheds with low-relief ratios and high discharge per unit area experience the most extensive chemical weathering, and sediments derived from these watersheds contain the lowest percentage of rock fragments. The effects of climate alone cannot explain the systematic variation of rock fragment abundance in sediments from the Coweeta Basin. The compositional imprint left on these sediments by chemical weathering is a function of both climate and topographic slope in the sediment source area.

Terrestrial weathering of Antarctic stone meteorites: Formation of Mg-carbonates on ordinary chondrites

Abstract White efflorescences of weathering origin occur superposed on fusion crusts, or along fractures in the interiors, of approximately 5% of all meteorites in the US Antarctic collection. Efflorescences from equilibrated ordinary chondrites consist of the hydrous Mg-carbonates nesquehonite (±hydromagnesite). X-ray diffraction and scanning electron microscope studies of efflorescences from LEW 85320 (H5) show abundant elongate prismatic crystals of nesquehonite (idiomorphic, not pseudomorphous after lansfordite), with minor local encrustations of hydromagnesite. Abundances of Na, K, Ca, and Rb in efflorescences from LEW 85320 suggest that the observed contents of these elements would require only modest fractionation of chondritic composition, whereas extensive fractionation would be required to derive the observed cation ratios from terrestrial sea-salts. Therefore, cations in evaporite minerals on Antarctic meteorites are most likely not products of contamination by terrestrial (marine) salts. The Mg in the efflorescences probably originated from weathering of meteoritic olivine; other cations in the efflorescences are also of meteoritic provenance. Thermodynamic analysis of the reaction forsterite + water + carbon dioxide → nesquehonite + silica at Antarctic temperatures and pCO 2 indicates spontaneity for all water activities greater than 0.65, compatible with the presence of liquid water as brines and/or thin films.

Rapid Growth of Magnesium-Carbonate Weathering Products in a Stony Meteorite from Antarctica

Nesquehonite, a hydrous magnesium carbonate, occurs as a weathering product on the surface of the Antarctic meteorite LEW 85320(H5 chondrite). Antarctic meteorites have resided on the earth for periods of 104 to 106 years, but the time needed for weathering products to form has been uncertain. Isotopic measurements of δ13C and δ18O indicate that the nesquehonite formed at near freezing temperatures by reaction of meteoritic minerals with terrestrial water and carbon dioxide. Results from carbon-14 dating suggest that, although the meteorite has been in Antarctica for at least 3.2 x 104 to 3.3 x 104 years, the nesquehonite formed after A.D.1950.

Geochemical mass balances and weathering rates in forested watersheds of the southern Blue Ridge II. Effects of botanical uptake terms

Abstract Geochemical mass balance methods are commonly used in small-watershed studies to estimate rates of primary-mineral weathering and soil formation, and the contribution of these processes to cation budgets, nutrient cycling, and landscape susceptibility to acid deposition. Many researchers employ the “balance sheet” approach, a system of simultaneous linear equations with constant coefficients which represent the steady-state behavior of the modelled system. Most workers also assume that the steady-state assumption for the entire system means that botanical factors can be ignored; in other words, that there is no net elemental transfer between biomass and inorganic compartments of the system. However, the widely invoked assumption that biomass is at steady-state is mathematically a second, explicit assumption, not a built-in mathematical consequence of the assumption of overall steady-state. The common assumption that the biomass is necessarily at steady-state in a steady-state system ignores the possibility of uptake of elements into the forest biomass. To quantify the implications of the separate mathematical assumptions, weathering rates for seven forested watersheds in the southern Blue Ridge mountains are calculated twice, once allowing for net element exchange with biomass, and once assuming no net exchange with biomass. Our calculations show that misunderstanding the mathematical constraints which are built into the most widely used geochemical mass balance equations commonly causes errors (underestimates) of up to a factor of 4 in the calculated rates of mineral weathering and soil formation. This problem is most pronounced for minerals which contain major nutrient elements, and for rates calculated from mass balances of those nutrient elements.

Rates and time scales of clay-mineral formation by weathering in saprolitic regoliths of the southern Appalachians from geochemical mass balance

Rates of clay formation in three watersheds located at the Coweeta Hydrologic Laboratory, western North Carolina, have been determined from solute flux-based mass balance methods. A system of mass balance equations with enough equations and unknowns to allow calculation of secondary-mineral formation rates as well as the more commonly determined primary-mineral dissolution rates was achieved by including rare earth elements (REE) in the mass balance. Rates of clay-mineral formation determined by mass balance methods have been used to calculate the time needed for a 5% (50 g kg−1) change in relative clay abundance in the saprolite at Coweeta; this corresponds to the “response time” of the clay mineral to, for example, a change in climate. The 5% change in relative clay abundance is the smallest change that can generally be detected using X-ray diffraction (XRD). Response times range from tens of thousands to hundreds of thousands of years. Extrapolating the Coweeta clay formation rates to other southern Appalachian regoliths, the time required to form measured clay abundances (“production times”) in eastern Blue Ridge and Inner Piedmont regolith have been calculated. The production times of clay-mineral assemblages range from 2 k.y. to 2 m.y., with mean values ranging from 50 k.y. to 1 m.y. The results of this study are consistent with the arguments of [Thiry (2000)][1] that the best resolution of the paleoclimatic record in marine clay-rich sediments and mudrocks is ∼1 or 2 m.y. [1]: #ref-107

Temporal variations in parameters reflecting terminal-electron-accepting processes in an aquifer contaminated with waste fuel and chlorinated solvents

Abstract A fundamental issue in aquifer biogeochemistry is the means by which solute transport, geochemical processes, and microbiological activity combine to produce spatial and temporal variations in redox zonation. In this paper, we describe the temporal variability of TEAP conditions in shallow groundwater contaminated with both waste fuel and chlorinated solvents. TEAP parameters (including methane, dissolved iron, and dissolved hydrogen) were measured to characterize the contaminant plume over a 3-year period. We observed that concentrations of TEAP parameters changed on different time scales and appear to be related, in part, to recharge events. Changes in all TEAP parameters were observed on short time scales (months), and over a longer 3-year period. The results indicate that (1) interpretations of TEAP conditions in aquifers contaminated with a variety of organic chemicals, such as those with petroleum hydrocarbons and chlorinated solvents, must consider additional hydrogen-consuming reactions (e.g., dehalogenation); (2) interpretations must consider the roles of both in situ (at the sampling point) biogeochemical and solute transport processes; and (3) determinations of microbial communities are often necessary to confirm the interpretations made from geochemical and hydrogeological measurements on these processes.

Effect of chemical affinity on feldspar hydrolysis rates in two natural weathering systems

Abstract Recent studies of alkali-feldspar hydrolysis kinetics have shown that, in the near-neutral pH range, weathering rates in natural systems are up to three orders of magnitude slower than laboratory rates. It has been hypothesized that decelerated rates may result from lower thermodynamic affinities for the hydrolysis reaction in natural systems than in laboratory systems. However, the chemical affinities for the feldspar hydrolysis reaction in two well-constrained natural systems are significantly higher than the threshold value at which affinity would exert detectable influences on the reaction rates. Thus, the hypothesis is rejected, and closer proximity to thermodynamic equilibrium in natural weathering systems does not account for the observed discrepancy between natural and laboratory rates of feldspar hydrolysis. Differences in feldspar weathering rates between natural and laboratory systems are most likely due to a combination of experimental preparation artifacts, loss of reactive surface to the formation of etch pits in naturally weathered feldspars, and inhomogeneous access of reactive fluids to those surfaces.

Natural weathering mechanisms of almandine garnet

The weathering of almandine garnet in the oxidized, vadose zone of saprolite near Otto, North Carolina, begins at grain boundaries and along fractures traversing the garnet grains. Radially oriented fibrous intergrowths of gibbsite and goethite form layers of uniform thickness, which grow by centripetal replacement as weathering proceeds. The contact between the garnet surface and the layer of weathering products is sharp and smooth, and garnet corners are rounded. Large, well-defined etch pits are absent on the underlying garnet surface. These observations suggest that diffusion (transport) of reactants and/or products through the gibbsite-goethite layer is the rate-limiting step in the weathering of almandine garnet in the oxidizing environment of the saprolite. In soils overlying the saprolite, garnet surfaces are directly exposed to weathering solutions without the intervening surface layer. Such “unprotected” grains in soils (and stream sediments) exhibit numerous large, well-developed etch pits. Surface-reaction control, rather than transport control, prevails during weathering of almandine garnet in the absence of protective surface layers of weathering products. Biochemical or biological processes in the soil apparently prevent the gibbsite-goethite layer from forming or persisting. Soil solutions can therefore react directly with the garnet surface, rather than requiring reactants or products to diffuse through the gibbsite-goethite coating as in the saprolite. The mechanism, and rate-limiting step, of almandine garnet weathering is apparently strongly dependent on the chemical environment in which weathering occurs.