Altered basaltic glass: A terrestrial analog to the soil of Mars

Analyses of Martian surface soil by Viking and Earth-based telescopes have been interpreted as indicating a regolith dominated by the weathering products of mafic or ultramafic rocks. Basaltic glass has previously been proposed as a more likely precursor than crystalline rock, given the low efficiency of surface weathering under present Martian conditions. On Earth large volumes of basaltic glass formed by quenching of magma by water. A similar interaction, between magma and ground ice, may have been a common occurrence on Mars. On the basis of this scenario palagonite, the alteration product of basaltic sideromelane glass, was studied as a possible analog to Martian soil. Samples from Iceland, Alaska, Antarctica, Hawaii, and the desert of New Mexico and Mexico were examined by optical and scanning electron microscopy, electron microprobe analysis, X-ray diffraction, spectrophotometry, and magnetic and thermogravimetric analysis. We suggest that palagonite is a good analog to the surface soil of Mars in chemical composition, particle size, spectral signature, and magnetic properties. Our model for the formation of fine-grained Martian surface soil begins with eruptions of basaltic magma through ground ice, forming deposits of glassy tuff. Individual glass shards are then altered by low-temperature hydrothermal systems to palagonitic material. Dehydration and aeolian abrasion strip the alteration rinds from the glass, and wind storms distribute the silt-sized palagonitic fragments in a planet-wide deposit.     

PLAGIOCLASE TWIN LAWS IN LUNAR HIGHLAND ROCKS; POSSIBLE PETROGENETIC SIGNIFICANCE

Plagioclases in different types of lunar highland rocks (all highly feldspathic) are twinned according to different laws and in different styles. Carlsbad and Carlsbad-albite twins, presumed to be growth twins, occur mainly in rocks which show igneous texture, and which have not been severely brecciated. These two twin laws appear to be absent from cataclastic rocks, including cataclastic anorthosite, possibly because the original twins were preferentially broken up in cataclasis (the composition plane being a plane of weakness). Pericline and lamellar albite twins, presumed to be deformation twins (except for some albite growth twins) occur in all types of rocks, and obvious deformation features, such as bending of lamellae, are well shown in many cataclastic rocks. Surprisingly, some Carlsbad and Carlsbad-albite twins are found in rocks with granoblastic texture, which presumably recrystallized in the solid state. This contrasts with previous observations on terrestrial metamorphic rocks, in which such growth twins are scarce or absent. The difference may be due to a higher rate of temperature change in the shock-heated lunar rocks. Further study of plagioclase twin laws in lunar rocks may assist in deciphering the complicated history of the highlands.     

http://www.sciencedirect.com/science/article/pii/S1674987114000395

The study of fluid inclusions in high-grade rocks is especially challenging as the host minerals have been normally subjected to deformation, recrystallization and fluid-rock interaction so that primary in- clusions, formed at the peak of metamorphism are rare. The larger part of the fluid inclusions found in metamorphic minerals is typically modified during uplift. These late processes may strongly disguise the characteristics of the ＂original＂ peak metamorphic fluid. A detailed microstructural analysis of the host minerals, notably quartz, is therefore indispensable for a proper interpretation of fluid inclusions. Cathodoluminescence （CL） techniques combined with trace element analysis of quartz （EPMA, LA- [CPMS） have shown to be very helpful in deciphering the rock-fluid evolution. Whereas high-grade metamorphic quartz may have relatively high contents of trace elements like Ti and A1, low- temperature re-equilibrated quartz typically shows reduced trace element concentrations. The result- ing microstructures in CL can be basically distinguished in diffusion patterns （along microfractures and grain boundaries）, and secondary quartz formed by dissolution-reprecipitation. Most of these textures are formed during retrograde fluid-controlled processes between ca. 220 and 500 ℃, i.e. the range of semi-brittle deformation （greenschist-facies） and can be correlated with the fluid inclusions. In this way modified and re-trapped fluids can be identified, even when there are no optical features observed under the microscope.     

Carbon dioxide sequestration: how much and when?

Carbon dioxide (CO₂) sequestration has been proposed as a key component in technological portfolios for managing anthropogenic climate change, since it may provide a faster and cheaper route to significant reductions in atmospheric CO₂ concentrations than abating CO₂ production. However, CO₂ sequestration is not a perfect substitute for CO₂ abatement because CO₂ may leak back into the atmosphere (thus imposing future climate change impacts) and because CO₂ sequestration requires energy (thus producing more CO₂ and depleting fossil fuel resources earlier). Here we use analytical and numerical models to assess the economic efficiency of CO₂ sequestration and analyze the optimal timing and extent of CO₂ sequestration. The economic efficiency factor of CO₂ sequestration can be expressed as the ratio of the marginal net benefits of sequestering CO₂ and avoiding CO₂ emissions. We derive an analytical solution for this efficiency factor for a simplified case in which we account for CO₂ leakage, discounting, the additional fossil fuel requirement of CO₂ sequestration, and the growth rate of carbon taxes. In this analytical model, the economic efficiency of CO₂ sequestration decreases as the CO₂ tax growth rate, leakage rates and energy requirements for CO₂ sequestration increase. Increasing discount rates increases the economic efficiency factor. In this simple model, short-term sequestration methods, such as afforestation, can even have negative economic efficiencies. We use a more realistic integrated-assessment model to additionally account for potentially important effects such as learning-by-doing and socio-economic inertia on optimal strategies. We measure the economic efficiency of CO₂ sequestration by the ratio of the marginal costs of CO₂ sequestration and CO₂ abatement along optimal trajectories. We show that the positive impacts of investments in CO₂ sequestration through the reduction of future marginal CO₂ sequestration costs and the alleviation of future inertia constraints can initially exceed the marginal sequestration costs. As a result, the economic efficiencies of CO₂ sequestration can exceed 100% and an optimal strategy will subsidize CO₂ sequestration that is initially more expensive than CO₂ abatement. The potential economic value of a feasible and acceptable CO₂ sequestration technology is equivalent – in the adopted utilitarian model – to a one-time investment of several percent of present gross world product. It is optimal in the chosen economic framework to sequester substantial CO₂ quantities into reservoirs with small or zero leakage, given published estimates of marginal costs and climate change impacts. The optimal CO₂ trajectories in the case of sequestration from air can approach the pre-industrial level, constituting geoengineering. Our analysis is silent on important questions (e.g., the effects of model and parametric uncertainty, the potential learning about these uncertainties, or ethical dimension of such geoengineering strategies), which need to be addressed before our findings can be translated into policy-relevant recommendations.     

Spinel-bearing,Al-rich chondrules in two chondrite finds from Roosevelt County,New Mexico: Indicators of nebular and parent body processes

Abstract— Two rare, spinel-bearing, Al-rich chondrules have been identified in new chondrite finds from Roosevelt County, New Mexico—RC 071 (L4) and RC 072 (L5). These chondrules have unusual mineralogies, dominated by highly and asymmetrically zoned, Al-Cr-rich spinels. Two alternatives exist to explain the origin of this zoning—fractional crystallization or metamorphism. It appears that fractional crystallization formed the zoning of the trivalent cations (Al, Cr) and caused a localized depletion in chromites around the large Al-Cr-rich spinels. The origin of the zoning of the divalent cations (Fe, Mg, Zn) is less certain. Diffusive exchange and partitioning of Fe and Mg between olivine and spinel during parent body metamorphism can explain the asymmetric zoning of these elements. Unfortunately, appropriate studies of natural and experimental systems to evaluate the formation of zoning of the divalent cations by fractional crystallization have not yet been conducted. The bulk compositions of the chondrules suggest affinities with the Na-Al-Cr-rich chondrules, as would be expected from the abundance of Al-Cr-rich spinels. Melting of rare and unusual precursors produced the compositions of Na-Al-Cr-rich chondrules, possibly including a spinel-rich precursor enriched in Cr₂O₃ and ZnO. The two chondrules we studied have larger modal abundances of Al-Cr-rich spinels than reported in other Na-Al-Cr-rich chondrules of similar composition, and Al-rich chondrules even more enriched in spinel are reported in the literature. These differences indicate that factors other than bulk composition control the mineralogy of the chondrules. The most important of these factors are the temperature to which the molten chondrule was heated and the cooling rate during crystallization. These two chondrules cooled rapidly from near the liquidus, as indicated by the zoning, occurrence and sizes of spinels, radiating chondrule textures and localized chromite depletions. The range of mineralogies in other Al-rich chondrules of similar composition reflect a range of peak temperatures and cooling rates. We see no reason to believe that this range is fundamentally different from the range of thermal histories experienced by “normal” Fe-Mg-rich chondrules.     

First attempts towards an integrative concept for the ecological assessment of groundwater ecosystems

Healthy aquifers deliver important ecosystem services, e.g. the purification of infiltrating water and the storage of high-quality water over decades in significant quantities. The functioning of terrestrial and surface aquatic ecosystems directly depends on groundwater and vice versa. Nowadays, legislation has started to consider groundwater not only as a resource but as a living ecosystem. The assessment of ecosystems requires consideration of ecological criteria, which, so far, are not available for groundwater systems. In the framework of a project supported by the German Federal Environment Agency (UBA), a first concept for the ecological assessment of groundwater ecosystems is developed. Steps to be taken are introduced, with a strong focus on microbes as potential bioindicators. These include (1) the typology of groundwater ecosystems, (2) the derivation of natural background values, (3) the identification of potential bioindicators, and (4) the development of an assessment model. First successes and difficulties associated with these challenges, e.g. the lack of simple correlations between abiotic and biotic variables, are discussed on the basis of a data set from a local and regional aquifer in NE Germany. The need for collaboration between ecologists, hydrogeologists and geochemists, as well as the application of modern approaches such as multivariate statistics, is emphasized.