Phase relations of pyroxene and amphibole in greenstone,blueschist and eclogite of the Franciscan Complex,California

The phase relations of pyroxenes, amphiboles and associated minerals in metamorphic rocks of the Franciscan Complex can be graphically depicted on a ternary diagram which has at its apices the metamorphic clinopyroxene end members, viz NaAl-NaFe³⁺-Ca(Fe²⁺, Mg). Phases are plotted by projection from a constant subassemblage of minerals. This analysis allows interpretation of the effects of pressure, temperature, bulk rock composition and fluid composition on stability of minerals within the Franciscan.Pyroxenes in meta-igneous rocks and metagraywackes have a limited compositional range and fall into two groups: the omphacites, with 50±5% diopside +hedenbergite component; and the jadeitic pyroxenes with 10±5% diopside+hedenbergite. Pyroxenes intermediate between these two groups are unstable relative to assemblages containing Na-amphibole+other minerals.Coexisting pyroxenes and amphiboles in eclogites and associated coarse blueschists comprise equilibrium assemblages, and the proportion of pyroxene to amphibole is a function of rock composition. Eclogites are stable at higher temperature than regionally developed fine-grained greenstones and blueschists in the Franciscan, and at higher pressure than amphibolites. X _H2O ^fluid is not an important factor in the stability of Franciscan eclogite relative to amphibolite.     

Structure energies of the alkali feldspars

Structural energetics of the alkali feldspars have been studied using a “lattice” or structure energy model. Electrostatic energies, U _e,for 20 well-refined, non-intergrown alkali feldspars were calculated using Bertaut's (1952) summation procedure and average about ?13,400 kcal/mol; the repulsive energies of the alkali site in each structure (～15 kcal/mol) were calculated using repulsive parameters for K-O and Na-O interactions estimated from bulk modulus data for NaF and KF and the exponential form of the repulsive potential. Using a procedure in which the position of the alkali cation was varied while the oxygen cage was kept fixed, structure energy gradients for the alkali sites of high albite and a hypersolvus Ab₄₂Or₅₈ structure were computed. In both cases, a broad structure energy well, elongated approximately parallel to c and subparallel to the observed split Na positions, was found. In both structures there is a single energy minimum corresponding closely with the observed single alkali positions. Comparison of U _e values for the alkali feldspars with different K/Na ratios shows that intermediate compositions are predicted to be less “stable” than either endmember and that the potassic end-member is predicted to be less “stable” than the sodic one, assuming that all other factors contributiong to the free energies of each phase are approximately the same. Comparison of U _e values for the high albite and low sanidine structures with different Al/Si distributions and a fixed tetrahedral framework indicates that the ordered charge distributions are 63.0 and 54.8 kcal/mol, respectively, more “stable” than the disordered distributions. Smaller, more realistic energy differences were obtained by using U _evalues averaged from four separate calculations with a +3 charge on a different T site in each and with +4 charges on the other T sites. If, in addition, the charges on cations and oxygen are reduced to half their nominal formal charges, in agreement with Pauling's electroneutrality principle and the results of recent molecular orbital calculations on silicates, the predicted electrostatic energy differences are reduced to 3.6 and 1.6 kcal/mol, respectively. These calculations also indicate that the T₁O site in the high albite structure energetically favors Al and that the Al/Si distribution determines the Na position within the alkali site.     

Space-time variations in British Caledonian Granites: some geophysical correlations

The characteristics of 30 “granites” from the postulated Caledonian (Iapetus Ocean) suture zone of mainland Britain are discussed. Geophysical (gravity, aeromagnetic), geochemical (U/Pb and initial⁸⁷Sr/⁸⁶Sr) and isotopic age data indicate that these British Caledonian intrusions (390–600 Ma) can be divided into two distinct groups temporally, each of which is further subdivided spatially.The temporal division applies throughout the British province and separates a pre-Silurian (group 1) suite of low-volume, low-mobility magmas, which were intruded under compressive conditions, from a Siluro-Devonian (group 2) suite of large volume, mobile magmas intruded under tensional conditions.The spatial subdivisions of groups 1 and 2 are made between intrusions emplaced to either side of the postulated ENE-WSW Iapetus Ocean suture which runs through the Solway Firth. First, the group 1 granites northwest of the suture probably were produced by partial fusion involving Proterozoic continental crust (group 1N) whereas those to the southeast have isotopic characteristics simply indicating a mantle or ocean crustal source (group 1S). Second, the Siluro-Devonian granites, which were all derived largely at the expense of Caledonian mantle, have different aeromagnetic expressions depending on their position in the northwest (2N) or southeast (2S) Caledonides. These aeromagnetic characteristics are probably related to the differences in basement structure recently identified by seismic surveys and they provide further evidence for the former existence and Siluro-Devonian closure of Iapetus. Another significant implication may be that Proterozoic basement is lacking from beneath the region immediately adjacent to and southeast of the suture — southern Scotland and most of England.     

Lithologic and stratigraphic setting of zinc mineralization in Grenville metasediments,Quebec

For many years, high-grade metamorphism and multiphase deformation have discouraged mineral exploration in the Grenville terrain of the Canadian Shield. However, detailed studies of known zinc occurrences in the Grenville Supergroup metasediments of Quebec now reveal that mineralization is stratigraphically controlled and consistently limited to mapable lithologic units. Within the vast regions composed predominantly of calcitic marbles, zinc mineralization is found to be confined to the much more limited areas of dolomitic marble, and more precisely, to the lithologic transition from metaclastic beds to meta-dolomites. Massive sphalerite mineralization of the Balmat-type is very closely restricted to this contact zone, while disseminated sphalerite with sparse amounts of galena are commonly observed in the adjacent dolomite. With these guidelines, and with evidence that mineralization is fundamentally stratiform in nature, it has now been demonstrated that mineralized stratigraphic horizons can be located beyond the areas of previously known sphalerite showings, and consequently the potential for important discoveries of zinc mineralization in the Grenville Supergroup has become very attractive to the present-day mining industry.     

Evidence for zonally-trapped propagating waves in the eastern atlantic from satellite sea surface temperature observations

It has been hypothesized (Moore et al., 1978; O'Brien et al., 1978), that equatorial upwelling and subsequent coastal upwelling on the eastern boundary of the Atlantic Ocean are the result of eastward propagating equatorially trapped Kelvin waves in the Atlantic. Concurrent satellite and ship sea surface temperature observations taken during the GATE experiment permit validation of the satellite data as well as relating sea surface temperature (SST) variability to the local current dynamics. A method based on cross-correlations and cross-spectra of the SST field at various locations is utilized to test the Kelvin wave hypothesis. Significant periodic variation of time lags in the SST variability in the eastern Atlantic is observed by the spectral techniques. Satellite data for the 1974 summer show periodic variability which fits either eastward or westward propagating waves with 1 m s^-1 phase speed, i.e., SST supports the quasi-continuous presence of Kelvin or Yanai waves. We find no evidence for a seasonally solitary eastward propagating signal in the eastern Atlantic from SST.     

Polymerization of silicate and aluminate tetrahedra in glasses,melts, and aqueous solutions—I. Electronic structure of H6Si2O7, H6AlSiO71−, and H6Al2O72−

As part of a study of the effect of geologically common network modifiers on polymerization in silicate melts, glasses, and silica-rich aqueous solutions, we have studied the energies, electronic structures, and inferred chemical properties of ^IVT-O-^IVT linkages in the tetrahedral dimers H₆,Si₂O₇, H₆AlSiO₇^1?, and H₆Al₂O₇^2? using semi-empirical molecular orbital theory (CNDO/2). Our results indicate that the electron donating character of the bridging oxygen, O(br), linking two tetrahedra increases with increasing T-O(br) bond length but decreases with decreasing T-O(br)-T angles and increasing O-T-O(br) angles. This increase or decrease of the donor character of O(br) coincides with an increase or decrease of the affinity of O(br) for hard acceptors. The calculated electronic structure for the H₆Si₂O₇ molecule is compared with the observed X-ray emission, absorption, and photoelectron spectra of quartz and vitreous silica; the reasonable match between calculated and observed oxygen Kα emission spectra of vitreous silica supports our assertion that non-bonded O(br) electron density energetically at the top of the valence band controls the chemical reactivity of ^IVT-O-^IVT linkages in polymerized tetrahedral environments.     

Pyrolysis-high resolution gas chromatography and pyrolysis gas chromatography-mass spectrometry of kerogens and kerogen precursors

A series of kerogens and kerogen precursors isolated from DSDP samples, oil shales and Recent algal mats have been examined by Curie point pyrolysis-high resolution gas chromatography and gas chromatography-mass spectrometry. This study has shown that the three main types of kerogens (marine, terrestrial and mixtures of both) can be characterized using these techniques. The marine (algal) kerogens yield principally aliphatic products and the terrestrial kerogens yield more aromatic and phenolic products with some n-alkanes and n-alkenes. The yields of n-alkanes and n-alkenes increase and phenols decrease with increasing geologic age, however, pyrolysis-GC cannot be used to characterize the influence of short term diagenesis on the kerogen structure.     

Determination of predevelopment denudation rates of an agricultural watershed (Cayaguás River, Puerto Rico) using in-situ-produced Be in river-borne quartz

Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ-produced cosmogenic ¹⁰Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguás River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared using ¹⁰Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguás. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguás Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos.     

International liability for weather modification

Can a State be held liable to another State or to citizens of another State for damage caused by (a) the State's own weather modification activities; (b) weather modification activities conducted by a nongovernmental party within its borders, or (c) weather modification activities conducted by persons within its control but outside its borders? What measures can be taken by a State to minimize its liability for weather modification activities? This article tries to answer these questions. It focuses on liability under international law, and emphasizes issues related to cloud-seeding to affect precipitation and to hurricane modification.