Late Ordovician island‐arc volcanic rocks,northern Capertee Zone,Lachlan Fold Belt,New South Wales

The Cape Hoskins volcanoes form part of the Quaternary volcanic island arc that extends from Rabaul in the east to the Schouten Islands in the west, and they overlie the northerly dipping New Britain Benioff Zone. The products of the volcanoes range in composition from basalt to rhyolite, and are normative in quartz and hypersthene. They contain phenocrysts of plagioclase and subordinate augite, hypersthene, and in most samples iron‐titanium oxides; some samples also contain olivine or quartz or both, and some pumice contains hornblende and, rarely, biotite.

Chemical analyses of 29 volcanic rocks are presented; 22 were also analysed for 17 minor elements — Rb, Ba, Sr, Pb, Zn, Cu, Zr, Y, Ni, Co, Sc, Cr, V, Ga, B, U, and Th.

Chemically the rocks have many of the characteristics of the ‘island arc tholeiitic series’, but do not show a pronounced relative enrichment in iron and appear to be relatively enriched in Sr. Compared with volcanic rocks from the northern part of the Willaumez Peninsula, they are lower in K (but not Na), Ti, Rb, Ba, Zr, Pb, Th, Ni, and probably also V, Cu, and Zn: these differences are attributed to the greater depth of the Benioff Zone beneath the Willaumez Peninsula. The more basic of the Cape Hoskins rocks are similar in most respects to lavas of comparable composition from Ulawun volcano to the east.     

Aggregation of montmorillonite and organic matter in aqueous media containing artificial seawater

Background  

The dispersion-aggregation behaviors of suspended colloids in rivers and estuaries are affected by the compositions of suspended materials (i.e., clay minerals vs. organic macromolecules) and salinity. Laboratory experiments were conducted to investigate the dispersion and aggregation mechanisms of suspended particles under simulated river and estuarine conditions. The average hydrodynamic diameters of suspended particles (representing degree of aggregation) and zeta potential (representing the electrokinetic properties of suspended colloids and aggregates) were determined for systems containing suspended montmorillonite, humic acid, and/or chitin at the circumneutral pH over a range of salinity (0 – 7.2 psu).     

Electron microscopic observations of shale diagenesis, offshore Louisiana, USA, Gulf of Mexico

Scanning and transmission electron microscopic analyses of shale samples from offshore Louisiana, USA, Gulf of Mexico, reveal the relationship between mineralogical and microfabric changes during burial diagenesis. The local geopressured zone begins at 2200-m depth. Above that depth the shales are smectite-rich, generally lack particle orientation, and contain appreciable pores. Below the 2200-m depth, the shales become more illite-rich with increasing burial, more crystalline, and less porous. Microfabric changes are mainly caused by compaction during burial diagenesis; mineralogical changes (smectite-to-illite) and crystal growth also play an important role in fabric alteration during deep burial diagenesis.     

Temperatures and thermophysical properties of the lunar outermost layer

Comparisons of calculated diurnal and eclipse temperatures of the lunar outermost layer are made with Earth-based infrared and millimeter data. The thermophysical model upon which the calculations are based incorporates variable physical properties. The thermal conductivity is a function of both density (depth) and temperature; the specific heat is a function of temperature; the density is a function of depth; and the dielectric constant and loss tangent are functions of density (depth). Laboratory measurements and Apollo sample results are incorporated in the property data. Calculational cases are based largely upon different density profiles. The model is consistent with the data, and the comparisons of theoretical and observational temperatures are very favorable. For such comparisons, further sophistication of the thermophysical model of the outermost layer is probably not justified.     

Palaeomagnetism and chronology of Caledonian igneous episodes in the Cross Fell Inlier and northern Lake District

This study employs palaeomagnetism to differentiate between Palaeozoic minor intrusions in northern England. The lamprophyres of the Cross Fell Inlier subdivide petrographically into kersantites and minettes. All the kersantites have steep directions of magnetisation probably correlating with an interval of anomalous geomagnetic field in Caradocian times. The Threlkeld–St John's microgranodiorite (445 m.y.) and Carrock Fell dolerite dyke swarm also possess steep magnetisations assigned to the same interval. The minettes have shallow directions of magnetisation similar to the Shap Granite (393 m.y.) and correlate with the later Caledonian intrusions of northern England. These episodes of intrusion are separated by late-Silurian tectonism and define a trend of increasing K₂O/Na₂O in the later magmas of the Caledonian orogeny. Miscellaneous palaeomagnetic results from the Borrowdale Volcanic Group, Embleton diorites and Haweswater dolerite are also reported.     

Liquid composition-dependence of calcium isotope fractionation during diffusion in molten silicates

Liquid phase diffusion experiments were carried out to determine whether diffusive isotopic fractionation of a major chemical element (Ca) varies with chemical composition in high-temperature molten silicates. The objective was to determine how differences in silicate liquid structure, such as the ratio of bridging to non-bridging oxygen atoms, as well as bulk transport properties such as viscosity, relate to isotope discrimination during diffusion. This information, in turn, may relate to the lifetimes and sizes of multi-atom structures in the liquid. Diffusion couples consisting of juxtaposed natural mafic and felsic liquids were held at T = 1450 °C and P = 1.0 GPa for durations of 12-24 h in a standard piston-cylinder assembly. Experiments were done using different mafic endmember compositions (two tholeiitic basalts and a ugandite) and a single rhyolite composition. Major-element diffusion profiles and Ca isotope profiles were measured on the recovered quenched glasses. The starting materials were isotopically indistinguishable, but ⁴⁴Ca/⁴⁰Ca variations of ca. 5‰ arose due to a mass dependence of the Ca diffusion coefficients. Results indicate that the mass dependence of Ca diffusion coefficients varies with the magnitude and direction of aluminum gradients and the viscosity of the liquid. Some Ca fractionations result mainly from Al gradients.A simplified multicomponent diffusion model was used to model the experimental results. The model allows for diffusion of Ca in response to gradients in the concentrations of both CaO as well as Al₂O₃, and the model results are consistent with the inferred existence of at least two distinct species of Ca. The magnitude of isotopic discrimination during diffusion also appears to be stronger on the rhyolite versus the basalt/ugandite side of diffusion couples. The results can largely be accounted for by an adaptation of the model of Dingwell (1990), whereby in high silica liquids, Ca diffuses largely by site hopping through a quasi-stationary aluminosilicate matrix, producing strong isotopic effects because the Ca diffusion is not strongly correlated with the movement of the framework atoms. In low-silica liquids, Ca diffusion is correlated with the movement of the other components and there is less mass discrimination. Combining our Ca results with Ca, Mg, and Li data from previous studies, we show that this model can explain most of the cation- and composition-dependence of diffusive isotopic fractionations observed thus far. A key parameter controlling isotopic discrimination is the ratio of the elemental (Ca, Mg, Li) diffusivity to the Eyring (or Si) diffusivity. However, all experiments done so far also exhibit isotopic features that are not yet fully explained; some of these may relate to small temperature gradients in the capsules, or to more complex coupling effects that are not captured in simplified diffusion models.     

The Southwest Pacific GPS Project: Geodetic results from Burst 1 of the 1990 Field Campaign

The Southwest Pacific GPS Project (SWP) is using the Global Positioning System (GPS) to monitor crustal motion across and within a plate boundary complex between the Australian and Pacific plates. GPS field campaigns were conducted in 1988, 1989 and 1990, to observe networks of increasing size and complexity. The 1990 campaign consisted of two periods, or Bursts, and this paper focuses on the analysis of data collected during the nine day Burst 1 in July, 1990, a period in which GPS Selective Availability was activated. During Burst 1, baselines that spanned the Tonga Trench and the Lau Basin were observed, and only one station (Espiritu Santo, Vanuatu) was located west of Fiji in the network. The lengths of the baselines observed fall mainly between 300 km and 1600 km, but some lines are as long as 3500 km. A total of 78 station-days of field site data and approximately 150 station-days of global fiducial data were processed from predominantly codeless receivers. A global fiducial network of 20 sites was used to provide orbit control and accuracy assessment for the 13 available satellites. The daily solutions for 45 baselines between 10 SWP sites have an RMS scatter in the length of 24 mm plus 6 parts per billion. This scatter provides an estimate of baseline precision for the Burst 1 nominal solution. Experiments were conducted to investigate a variety of possible effects on the SWP Network baseline estimates, including the influence of a reduced global fiducial network for the purpose of assessing the quality of results obtained in 1988 and 1989 in which the fiducial network was smaller than in 1990. These experiments produced results that agreed with the nominal solution at the level of the precision estimate. Furthermore, estimates for selected baselines in Australia, the Central Pacific, North America and Europe, also measured by VLBI and SLR, were used for an external accuracy evaluation. The GPS and VLBI or SLR determinations of length agreed at a level consistent with the nominal solution precision estimate.     

A detailed paleomagnetic survey of the type location for the Gilsa geomagnetic polarity event

A total of 163 cores have been taken from a maximum of 40 separate lavas in three separate sections of the Jökuldalur, southwest of Egilsstadir, Iceland, and subjected to paleomagnetic analysis and some K-Ar dating. Previous work on the sections by McDougall and Wensink (1966) led to the establishment of the Gilságeomagnetic polarity event, with an age of about 1.60 m.y., during the reversed polarity Matuyama epoch. This earlier study described a possible reversely magnetized lava separating the Gilsáevent from a second normal polarity lava, perhaps representing the Olduvai event. Such a possibility was subsequently a source of speculation from diverse sources concerning the polarity history for the lower Matuyama. The present study indicates clearly that there is no second normal polarity event represented in the sections. Only one normal polarity event is therefore represented in the lower Matuyama of the Jökuldalur, and the age of the lavas involved is confirmed to be approximately 1.58 ± 0.08 m.y. Because of uncertainties in the interpretation of the original K-Ar results from Olduvai Gorge, it is still not possible to be certain that the Olduvai and Gilsáevents are separated in time. An incidental result of the survey is evidence to show that, contrary to recent suggestions by Einarsson (1972), there is no substantial hiatus between the major lower parts of the section and lavas believed to represent extrusions after a regional tilting and peneplanation.