Differentiation associated with discrete crenulation cleavages

Differentiation, both mineralogical and chemical, plays a significant part in the development of discrete crenulation cleavages in low to medium grade metamorphic rocks. These cleavages are therefore not iron stained fractures and microfaults as was previously hypothesized. They are thin differentiated zones (seams or mica films) which truncate a crenulated pre-existing fabric. Their mineralogy is white mica-opaques-iron oxides particularly where the host rock mineralogy is white mica-quartz/feldspar-opaques-iron oxides-(chlorite/biotite). The chemistry of discrete crenulation cleavages can be extremely variable both along individual cleavages and between different cleavages in the same rock (due to variable amounts of Fe and Ti oxides along the cleavages). However, all the cleavages analysed show a consistent decrease in SiO₂ and increases in Al₂O₃ and K₂O with respect to the host rock. This chemical differentiation is due largely to the observed mineralogical variations which are related to the development of the cleavages. The differentiated nature, the microfabrics and the microtextural relations of discrete crenulation cleavages suggest that they are ‘solution planes’ characterized by relatively insoluble minerals. Solution and removal of quartz along the cleavages resulting in a passive concentration of phyllosilicates, is considered responsible for the development of this variety of cleavage.     

Lewis Cliff 87223, an anomalous enstatite chondrite and the diversity of enstatite chondrites

We studied a thin section of Lewis Cliff (LEW) 87223, an unusual EL3-related, enstatite chondrite (EC) that has primary and secondary features not observed in other ECs. We studied its metal-rich nodules, possible shock features, and chondrules, eight of which are Al-rich chondrules (ARCs). LEW 87223 has petrologic and compositional features similar to EL3s. Enstatite is the dominant mineral; chondrule boundaries are well defined; Si content of metal (0.5–0.6 wt%) is consistent with typical EL3; it has Cr-bearing troilite, oldhamite, and alabandite; and its O-isotopic composition is similar to other ECs. However, metal abundance in LEW 87223 (~13 vol%) is slightly higher than in other EL3s and its metal nodules are texturally and mineralogically different from other ECs. Both high and low Ni metals are present, and its alabandite has higher Fe (27.8 wt% Fe) than in other EL3s. Silicates appear darkened in plane polarized light, largely due to reduction of Fe from silicate. A remarkable feature of LEW 87223 is the high abundance of ARCs, which contain Ca-rich plagioclase and varying amounts of Na-rich plagioclase along chondrule edges and as veins. This suggests Na metasomatism and the possibility of hydrothermal fluids, potentially related to an impact event. LEW 87223 expands the range of known EC material. It shows that ECs are more diverse and record a wider range of parent body processes than previously known. LEW 87223 is an anomalous EL3, potentially the first member of a new EC group should similar samples be discovered.     

Dense Gas Diagnostics: Maser Excitation and Variability

I review a number of diagnostic techniques, based upon the excitation and variability of astrophysical masers, which can be applied to dense gas. In each case, I consider the observations necessary and the amount of interpretation, or modelling, required to complete the analysis of a source, presenting examples of each diagnostic method.     

Estimation of parameters in petrologic materials balance equations

A rigorous treatment of the least-squares estimation of the parameters in petrologic materials balance equations is developed to take into account the uncertainties inherent in the chemical analyses. The choice of the optimal estimation procedure for a particular problem is dependent upon the extent of the petrogenetic understanding of the exact nature of the materials balance involved. Some of the complications entailed in obtaining least-squares estimates are illustrated by the examples of magmatic differentiation and metamorphic reactions.     

Transmission of CO2 in the atmosphere of Venus for the spectral region near 7 microns

The transmission in the 7-μm “window” opf Venus was calculated for a 7-layer model atmosphere. The synthetic spectra show that radiation from the layer 20–30 km above the surface would reach the top of the atmosphere between 6.2 and 7.0 μm if there were no absorption besides the isotopic CO₂ bands; for the 7.0- to 8.2-μm region, the radiating level would be located 40–50 km above the surface of Venus. The brightness temperature for the entire region is 430°K; for the above two regions it is 494 and 341°K, respectively.     

Particle dynamics in reconnection field configurations

Trajectories of test particles are studied numerically in two types of reconnection magnetic field configurations, a single X-line magnetic field configuration and a tearing magnetic field configuration. Both adiabatic and nonadiabatic motions are examined, with special emphasis on net energy gain and time spent in the neutral line regions. They spend typically one characteristic gyroperiod in the X-line region and are ejected predominantly along field lines in the vicinity of the separatrix. Both adiabatic and nonadiabatic test particles in the tearing-type field configuration are channelled into and accelerated along the O-line region. It may be inferred from these test particle results that particle energizations are significant along the O-line region, but not along the X-line region. These results are in qualitative agreement with those obtained by a self-consistent particle simulation.