Plagioclase lamellae in hypersthene,Tikkoatokhakh Bay,Labrador

Abundant lamellae of plagioclase are present in the (100) planes of hypersthene megacrysts in andesine anorthosite along Tikkoatokhakh Bay, northwest of Nain, Labrador. Spongy intergrowths of plagioclase in hypersthene also occur. Plagioclase lamellae have mean compositions ranging from An₄₃ to An₉₂, with extreme compositions from An₃₉ to An₉₇; the calcic compositions are the more abundant. Such lamellae are always accompanied in the hypersthene by grains or lamellar segments of magnetite, and rarely by lamellae of olivine, augite, magnetite, or ilmenite. Some calcic plagioclase lamellae contain antiperthitic spindles of orthoclase. The host rocks of the hypersthene megacrysts are layered leuconorites and anorthosites with mean plagioclase compositions ranging from An₄₁ to An₅₅. The plagioclase lamellae in hypersthene are characteristically much more calcic than the host-rock plagioclase. There is little doubt that the lamellae exsolved from a pyroxene host, dominantly by a coupled redox reaction which generated magnetite, thereby releasing silica to combine with the Ca-Tschermak and jadeite components of the precursor pyroxene. Rapid growth of megacrysts may account for their aluminous nature.     

The incorporation of Mg2+ and Sr2+ into calcite overgrowths: influences of growth rate and solution composition

The concentrations of Mg²⁺ and Sr²⁺ incorporated within calcite overgrowths precipitated from seawater and related solutions, determined at 25°C, were independent of the precipitation rate over approximately an order of magnitude. The saturation states used to produce this range of precipitation rates varied from 3 to 17 depending on the composition of the solution.The amount of Mg²⁺ incorporated in the overgrowths was not directly proportional to $\text{Mg}{}^{\mathrm{2+}}\text{Ca}{}^{\mathrm{2+}}$ in solution over the entire range (1–20) of ratios studied. Below a ratio of 7.5, the overgrowth was enriched in MgCO₃ relative to what is predicted by the constant distribution coefficient measured above a ratio of 7.5. This increased MgCO₃ correlates with the relative enrichment of adsorbed Mg²⁺. Above a ratio of 7.5 the concentration of MgCO₃ in the calcite overgrowths followed a classical thermodynamic behavior characterized by a constant distribution coefficient of 0.0123 (±0.008 std dev).The concentration of SrCO₃ incorporated in the overgrowths was linearly related to the MgCO₃ content of the overgrowths, and is attributed to increased solubility of SrCO₃ in calcite due to the incorporation of the smaller Mg²⁺ ions.The kinetic data indicate that the growth mechanism involves the adsorption of the cations on the surface of the calcite prior to dehydration and final incorporation. It is suggested that dehydration of cations at the surface is the rate controlling step.     

Evaluation of bioremediation strategies of a controlled oil release in a wetland

A controlled petroleum release was conducted to evaluate bioremediation in a wetland near Houston, Texas. The 140-day study was conducted using a randomized, complete block design to test three treatments with six replicates per treatment. The three treatment strategies were inorganic nutrients, inorganic nutrients with an alternative electron acceptor, and a no-action oiled control. Samples were analyzed for petroleum chemistry and inorganic nutrients. These results are discussed in the context of our related research involving toxicology and microbiology at the site during the experiment. To evaluate biodegradation, the targeted compounds were normalized to the conservative compound C3017alpha, 21beta-[H]hopane, thus reducing the effects of spatial heterogeneity and physical transport. The two biostimulation treatments demonstrated statistically-higher rates of biodegradation than the oiled no-action control. For the majority of the experiment, target nutrient levels were maintained. Further research may be warranted to optimize these bioremediation strategies as well as evaluating additional treatment strategies for wetlands and other shoreline systems.     

Estrogen in a karstic aquifer

Adverse impacts on the health of some fish populations, such as skewed sex distributions, have been noted in surface waters and in laboratory experiments with relatively low concentrations (above 25 ng/L) of natural estrogen (17 beta-estradiol--E2). Sources of E2 to surface and ground waters can include avian, human, and mammalian waste products. The Ozark Plateau Aquifer (OPA) is a karstic basin that receives a significant portion of its water through losing reaches of rivers. Thus, there is a direct connection between surface water and ground water. The OPA was targeted for an E2 study to assess the potential for adverse health effects to aquatic organisms living in the system. Eight springs, which drain the aquifer, were sampled quarterly. The concentrations of E2 in the OPA ranged from 13 to 80 ng/L. For any one sampling event, the concentrations of E2 at the spring waters were statistically similar; however, the concentrations of E2 at all springs varied throughout the year. At Maramec Spring, one of the larger springs, the E2 concentration, was correlated with discharge. Based on the correlation between discharge and E2 concentration, aquatic organisms living in the plateau or in its discharged waters, including the threatened southern cavefish T. subterraneus, are exposed to concentration of E2 above 25 ng/L approximately 60% of the time. This implies that organisms living in karst basins throughout the OPA are likely exposed to E2 concentrations that may adversely impact their reproductive success for a significant portion of each year.     

Dissolution Kinetics of Calcite in NaCl–CaCl2–MgCl2 Brines at 25 °C and 1 bar pCO2

Calcite dissolution rates were measured as a function of saturation state in NaCl–CaCl₂–MgCl₂ solutions at 1 bar (0.1 MPa) pCO₂ and 25 °C. Rates measured in phosphate- and sulfate-free pseudo-seawater (Ca²⁺:Mg²⁺= 0.2, I= 0.7) were compared with those in synthetic brines. The brines were prepared by co-varying calcium and magnesium (Ca²⁺:Mg²⁺= 0.9; 2.0; 2.8; 3.1; 4.8; 5.8) along with ionic strength (I= 0.9; 1.1; 1.6; 2.1; 3.0; 3.7; 4.4 m) to yield solutions approximating those of subsurface formation waters. The rate data were modeled using the equation, R = k(1 ? Omega;) ⁿ , where k is the empirical rate constant, n describes the order of the reaction and ω is saturation state. For rates measured in the pseudo-seawater, n= 1.5 and k= 4.7 × 10^?2 mol m^?2 hr^?1. In general, rates were not significantly faster in the synthetic brines (n= 1.4 ± 0.2 and k= 5.0 ± 7 × 10^?2 mol m^?2 hr^?1). The rate coefficients agree within experimental error indicating that they are independent of ionic strength and Ca²⁺:Mg²⁺ over a broad range of brine compositions. These findings have important application to reaction-transport modeling because carbonate bearing saline reservoirs have been identified as potential repositories for CO₂ sequestration.     

Measurement of water content as a control of particle entrainment by wind

Of all controls on particle transport by wind, which include texture, crusting, vegetation cover and roughness, the role of water content is one of the most difficult to parameterize because of its high degree of spatial and temporal variability and its operation at a particle‐scale level directly at the surface. This study demonstrates that measurement of the distribution of brightness for all pixels in an image, now routinely employed in digital photography, is strongly correlated with gravimetric water content. Wind tunnel experiments further suggest that measurement of the distribution of β, as normalized against the brightness of the dry sand surface, is very useful in determining the order of magnitude of the mass transport rate (q). Finer resolution will likely never be achieved because of the heterogeneity of the particle transport phenomenon. Analysis of the variability in surface brightness does suggest that q is governed by the partitioning of momentum to particle motion that terminates in adhesion to surrounding areas of the surface that remain relatively wet. The proportion of surface particles that becomes dry appears to be of less importance. Preliminary work suggests that field application of digital photography in tracking spatial and temporal changes in the water content of beach deposits looks promising. Copyright © 2006 John Wiley & Sons, Ltd.     

Hadean Ocean Carbonate Geochemistry

Relatively soon (0.2 Ga) after the Earthformed, it is likely that major oceans appeared in ahot (100°C) reducing environment where carbondioxide was probably the dominant atmospheric gas,with PCO2, values reaching perhaps in excess of 10atm. During the Hadean Eon between 4.3 and 3.8 Ga BP,major changes in the concentration of atmosphericCO2 and associated temperature changes had aprofound influence on the carbonate geochemistry ofthe Hadean Ocean. Although no rocks are known to havesurvived prior to the Archean Eon, it is stillpossible to calculate approximate values for importantseawater parameters during the Hadean Eon based onother sources of information and reasonableassumptions about processes such as weatheringreactions.Our calculations are based on a linear temperaturechange from 100°C to 70°C and logPCO2 change from 1 to -1.5 over the Hadean Eon. Over this range in temperature and P CO2, theinfluence of T is relatively small, but changes inP CO2 result in large compositional variations inthe carbonate chemistry of Hadean seawater. In theearly Hadean, seawater pH was probably about5.8 ± 0.2, DIC may have reached close to 130 mM,and alkalinity was perhaps close to 30 mM. By thelate Hadean, seawater pH probably had changed to closeto neutral (6.8), and DIC and alkalinity were closerto present-day values. Even large uncertainties inNa+ + Cl-, K+ and Mg2+concentrations produce relatively small uncertaintiesin our calculated values for the carbonic acid system. However, larger uncertainties result from reasonableranges for Ca2+ concentrations and the saturationstate of Hadean seawater with respect to calcite.Our calculations support the hypothesis that acarbonate chemistry of seawater roughly similar tothat of modern oceans could have been acquired veryearly in Earth history. If seawater composition werebuffered by reactions involving carbonates andsilicates, then the composition of late Hadean-earlyArchean seawater was not vastly different from that oftoday. Thus, by the conclusion of the Hadean Eon, ifnot before, environmental conditions at the Earth's surface, including temperature and seawatercomposition, were sufficiently equable for theevolution of life, including the Archaebacteria: theextreme halophiles and thermophiles and methanogens.Contrary to the hypothesis of an early Na-bicarbonateocean, our calculations suggest the possibility thatthe early oceans of Earth were a NaCl-dominatedaqueous solution, with somewhat higher DIC andalkalinity concentrations, higher saturation state,and the possibility of lower calcium concentrations.The time course of approach of Hadean seawater to acarbonate composition closer to that of today isdifficult to predict. It is distinctly possible thatthe concentration of calcium in seawater did not reachlevels like that of modern seawater until the latePrecambrian and thus constrained the timing of the"Big Bang" of organic evolution, the emergence of theshelled invertebrates at the beginning of thePhanerozoic.     

An experimentally verified model for calcite precipitation in veins

Calcite is commonly found as a vein-filling mineral in rocks. However, the factors controlling its deposition are complex and not well understood in quantitative terms. In order to advance our understanding of the processes involved, we have refined the model for calcium carbonate mass transport in subsurface carbonate rocks of Morse and Mackenzie (1993) and developed a new experimental technique to test it. This technique uses a flow-through reactor that simulates a vein opening. Agreement was observed between model predictions and experimental observations for the deposition of calcite in synthetic veins. The influences of surface area to solution volume ratio, solution saturation state with respect to calcite, and flow velocity were well predicted by the model.

The model predicts that in order to have a fairly uniform deposition of calcite within a vein, solution flow must be quite rapid (tens to thousands of cm h⁻¹) or the solution must be only slightly supersaturated with respect to calcite. A low degree of solution supersaturation demands what may be unreasonably large volumes of solution flow to achieve vein filling for the vein configuration we have studied.     

Liquid collection in sidewall crystallization of magma: A comment on “liquid fractionation. Part I”, by A.R. McBirney, B.H. Baker and R.H. Nilson

The term liquid fractionation is inappropriate for a process driven by crystallization; liquid collection is proposed instead. The amount of evolved residual liquid produced by sidewall crystallization is not greater than that produced by any perfect fractional crystallization process and the fractionation products must be balanced somewhere by less evolved crystal deposits. There is no free lunch in petrology. Sidewall heating limits the local duration of liquid collection and the complementary crystal products should be sought in conduits. Flux ratios rather than diffusivities control the differences between thermal and chemical boundary layers.     

The dissolution kinetics of shallow marine carbonates in seawater: A laboratory study

The dissolution kinetics of shallow water marine carbonates (low-Mg calcite, aragonite and Mg-calcites) were investigated in seawater (S = 35) at 25°C and a P_CO2 of 10^?2.5 atm. using the pH-stat method. Carbonate dissoluton rates (μmoles g^?1 hr^?1) fit the empirical kinetic expression, R = k(1 - Ω)ⁿ, where R = dissolution rate, k = rate constant, Ω = saturation state, and n = order of reaction. Reaction orders were near 2.9 for low-Mg calcites, 2.5 for aragonites and 3.4 for Mg-calcites.The rate constant, k, expressed as μmoles g^?1 hr^?1, varied by nearly a factor of ten for the different samples, reflecting differences in amount of reactive surface area. Reactive surface area of the biogenic phases ranged from 0.3% to 66% of the total surface area determined by the BET gas adsorption method. The discrepancy between reactive and total surface area was greatest for samples with high BET surface areas (> 1 m² g^?1) and delicate microstructures.Relative dissolution rates of the various biogenic carbonates as a function of seawater calcium carbonate ion molal product (IMP) were related to both mineral stability and grain microstructure. In seawater undersaturated with respect to aragonite, finely crystalline aragonites dissolved more rapidly than thermodynamically less stable high Mg-calcites (15–18 mole% MgCO₃) with lower reactive surface areas. Therefore, under certain conditions, differences in grain microstructural complexity can override thermodynamic constraints and lead to selective dissolution of a thermodynamically more stable mineral phase.