Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites

TEM, HRTEM, HVEM and SEM methods, coupled with energy dispersive X-ray analysis, have been used to study the microstructure and the phases comprising the matrix of carbonaceous chondrites Murchison, Cold Bokkeveld, Nawapali and Cochabamba. A wide variety of phyllosilicate morphologies occurs in each. Very small crystals and clasts of olivine, pyroxene and other unhydrated minerals are mixed intimately with the phyllosilicates. Intergrowths of carbonates and Sulfides within the phyllosilicates also occur, as well as a ubiquitous spongey material which is difficult to characterize, but contains elementary phyllosilicate units and embryo crystals. The identifiable large crystalline phyllosilicates are principally Fe-rich serpentine-group minerals and intermediate more Mg-rich chrysotilelike group members, with characteristic ~ 7.0–7.4 Å basal layer spacings. Complex interlayered and intergrown hydrous minerals also occur associated with the spongey material, and other poorly crystalline silicates and finely divided Fe-Ni sulphides. Fe/Si and Mg/Si ratios vary on a sub-micron scale, and the morphologies of the larger phyllosilicate crystals correlate broadly with these variations. Small crystals of sodium chloride and potassium chloride have been identified, occluded within a predominantly organic mass.The matrix minerals have a multistage history of formation in which the effects of aqueous alteration are dominant. Few, if any, of matrix minerals can be unmodified nebular condensates, although some clasts and inclusions have escaped alteration and predate the alteration process.     

Electron petrography of shock effects in a gas-rich enstatite-achondrite

The enstatite-achondrite Khor Temiki has been studied by high-voltage electron microscopy. The normal Khor Temiki lithology has a fine-grained matrix in which individual grains show the well-known effects of unshielded solar irradiation. Intensity of deformation varies greatly from grain to grain; this material has a varied history of impact deformation, and must have formed in an environment similar to that of the lunar regolith. The meteorite is traversed by veins of extra-dark material. This was produced in situ from the normal lithology by intense shock, sufficient to erase its irradiation record almost completely. Instead of the enstatite that dominates the bulk meteorite, optically twinned clinoenstatite is found to be the major mineral in such a shock-vein. It is highly defective, and its electron diffraction patterns contain diffuse orthopyroxene maxima. It is interpreted in terms of inversion of protoenstatite produced by the thermal effect of the shock. Recrystallization phenomena, and the occurrence of enstatite in close association with the clinoenstatite, are described. This occurrence of twinned clinoenstatite is contrasted with those due to quenching of primary protopyroxene in small bodies such as chondrules. The effects of shock have eliminated porosity from the vein material, and indurated it. Less severe such effects, outside the veins themselves, must have contributed to the lithification of the meteorite.     

Electron petrography of shock-deformed olivine in stony meteorites

We have studied olivine by high-voltage electron microscopy in meteorites showing shock effects, namely the ordinary chondrites Olivenza and Hedjaz, the ureilite Goalpara and the carbonaceous chondrite Allende. The observations are compared with published data on experimentally deformed and annealed olivine.Olivenza and Hedjaz have suffered low-temperature shock, which has produced high densities of [001] screw dislocations and cracks. Arrays of cracks are found to correspond to planar features which can be seen optically and to be responsible for mosaicism. Goalpara has suffered very heavy shock, of which the final result is extensive recrystallization of the olivine, and the new grains have distinctive low-density distributions of dislocations. Allende has not been shocked as a whole, but individual parts have deformational and heating histories dating from before the aggregation of the meteorite.     

Lithification of gas-rich meteorites

High-voltage electron microscopy has been used to study the fine structure of four gas-rich meteorites, with particular reference to the cementation and compaction processes that have affected the fine-grained matrix. The observed features are compared with similar effects in lunar breccias. Lithification is attributed to the passage of shock waves through porous aggregates, causing deformation whose intensity varied spatially on a small scale, the most intense deformation and heating effects being concentrated at the edges of large grains and in the matrix between them. It is inferred that relatively mild shocks have produced amorphous cement between matrix grains in the achondrite Khor Temiki and the chondrite Weston. Relatively intense shock is inferred for specimens of the chondrites St. Mesmin and Pantar. These have non-porous, completely crystalline matrices, and fine-grained black veins which fill cracks in relatively large deformed grains. Recrystallization of some deformed material is attributed to shock-heating, which was not sustained long enough to erase the irradiation record from all the relatively large grains. Matrix recrystallization without extensive melting constitutes a metamorphic event, and the observations indicate that shock was responsible for the metamorphism experienced by these chondrites at relatively late stages of the evolution of their parent bodies.     

The impact of land use change and check‐dams on catchment sediment yield

Extensive land use changes have occurred in many areas of SE Spain as a result of reforestation and the abandonment of agricultural activities. Parallel to this the Spanish Administration spends large funds on hydrological control works to reduce erosion and sediment transport. However, it remains untested how these large land use changes affect the erosion processes at the catchment scale and if the hydrological control works efficiently reduce sediment export. A combination of field work, mapping and modelling was used to test the influence of land use scenarios with and without sediment control structures (check‐dams) on sediment yield at the catchment scale. The study catchment is located in SE Spain and suffered important land use changes, increasing the forest cover 3‐fold and decreasing the agricultural land 2·5‐fold from 1956 to 1997. In addition 58 check‐dams were constructed in the catchment in the 1970s accompanying reforestation works. The erosion model WATEM‐SEDEM was applied using six land use scenarios: land use in 1956, 1981 and 1997, each with and without check‐dams. Calibration of the model provided a model efficiency of 0·84 for absolute sediment yield. Model application showed that in a scenario without check dams, the land use changes between 1956 and 1997 caused a progressive decrease in sediment yield of 54%. In a scenario without land use changes but with check‐dams, about 77% of the sediment yield was retained behind the dams. Check‐dams can be efficient sediment control measures, but with a short‐lived effect. They have important side‐effects, such as inducing channel erosion downstream. While also having side‐effects, land use changes can have important long‐term effects on sediment yield. The application of either land use changes (i.e. reforestation) or check‐dams to control sediment yield depends on the objective of the management and the specific environmental conditions of each area. Copyright © 2008 John Wiley & Sons, Ltd.