Molecular Realism in Default Models for Information Theories of Hydrophobic Effects

This letter considers several physical arguments about contributions to hydrophobic hydration of inert gases, constructs default models to test them within information theories, and gives information theory predictions using those default models with moment information drawn from simulation of liquid water. Tested physical features include: packing or steric effects, the role of attractive forces that lower the solvent pressure, and the roughly tetrahedral coordination of water molecules in liquid water. Packing effects (hard sphere default model) and packing effects plus attractive forces (Lennard-Jones default model) are ineffective in improving the prediction of hydrophobic hydration free energies of inert gases over the previously used Gibbs and flat default models. However, a conceptually simple cluster Poisson model that incorporates tetrahedral coordination structure in the default model is one of the better performers for these predictions. These results provide a partial rationalization of the remarkable performance of the flat default model with two moments in previous applications. The cluster Poisson default model thus will be the subject of further refinement.Comment: 5 pages including 3 figure

Mesoarchaean Aluminous Rocks at Storø, Southern West Greenland: New Age Data and Evidence of Premetamorphic Seafloor Weathering of Basalts

Metamorphosed Meso- to Neoarchaean supracrustal rocks in the central part of the island of Storø (Nuuk region, southern West Greenland), show field- and geochemical evidence of premetamorphic chemical alteration. This alteration changed basaltic precursors into aluminous lithologies, and following amphibolite grade metamorphism and penetrative ductile deformation, these garnet biotite schists now resemble adjacent metapelitic schists of sedimentary origin. Mass balance calculations (isocon method), suggests that most major elements (Si, Fe, Mg, Ca, Na and P) were leached during alteration. The calculated overall net mass changes are between -18% and -45%, consistent with breakdown of olivine, pyroxenes, plagioclase and apatite in the basaltic precursor rocks. Major and trace elements such as, K, Cs, Rb, Ba, Pb, Zn, La, Ce were added during this alteration process, whereas high field strength elements (Ti, Al, Zr, Hf and Nb) remained essentially immobile and were thus residually enriched. Interestingly, Th which is generally assumed to be immobile in fluids, was also added during this process. These chemical changes reflect interaction between a basaltic protolith and hydrous fluids that established a new equilibrium and thus a different mineral assemblage. It is proposed that the premetamorphic alteration at Storø was due to low-temperature interaction between seawater and oceanic crust, and thus essentially represents in situ submarine seafloor weathering. This interpretation is consistent with the mass balances reported from well-documented examples in younger settings. New U-Pb zircon geochronology from the arc-related mafic sequences at Storø shows that they comprise at least two distinct age groups: an older anorthosite complex dated at 3051.3 plus or minus 2.6 Ma and a younger supracrustal sequence with age brackets between 2840-2710 Ma. The allochthonous nature of these two mafic igneous to sedimentary stacks is consistent with accretionary processes in island arc complexes and a compressional Archaean tectonic setting

Precambrian mineralising events in central West Greenland (66°–70°15´N)

During the past decade the Geological Survey of Denmark and Greenland (GEUS) has carried out two major resource evaluations in the Precambrian basement terranes of South and West Greenland in order to locate potential areas of mineral deposits (Steenfelt et al. 2000, 2004; Stendal & Schønwandt 2003; Stendal et al. 2004). Based on geological field work and geochemical and geophysical data, these evaluations have assessed the interplay between the magmatic, tectonic and metamorphic evolution in the study areas and their mineralising events. As a result of the second of these evaluations it is now possible to outline a succession of mineralising events in the northern part of the Nagssugtoqidian orogen and in the Disko Bugt area of central West Greenland (Fig. 1), and relate them to the general Archaean and Palaeoproterozoic geological evolution of this region. However, uncertainties still exist concerning the age and detailed setting of many epigenetic mineralisations