Origin of Archean migmatites from the Gwenoro Dam area,Zimbabwe-Rhodesia

Archean migmatites in the vicinity of Gwenoro Dam in Zimbabwe-Rhodesia are composed chiefly of trondhjemite gneiss (TR), mafic tonalite (MT), amphibolite (AM), leuco-trondhjemite veins (LTR), and pegmatites. The gneiss is intruded in nearby areas with small tonalite plutons (TN). Geochemical model studies together with field relationships are consistent with the following model for migmatite production: AM is produced by partial melting of a partly depleted ultramafic parent in which neither garnet nor amphibole remain in the residue; TR and TN are produced by partial melting of undepleted to variably depleted amphibolite in which garnet does not remain in the residue; MT is produced by mixing of plagioclase-rich TR with AM; and LTR represents the solid residue after fractional crystallization of TR.     

Removal of230Th and231Pa from the open ocean

Concentrations of²³⁰Th and²³¹Pa were measured in particulate matter collected by sediment traps deployed in the Sargasso Sea (Site S₂), the north equatorial Atlantic (site E), and the north equatorial Pacific (Site P) as well as in particles collected by in situ filtration at Site E. Concentrations of dissolved Th and Pa were determined by extraction onto manganese dioxide adsorbers at Site P and at a second site in the Sargasso Sea (site D).Dissolved²³⁰Th/²³¹Pa activity ratios were 3–6 at Sites P and D. In contrast, for all sediment trap samples from greater than 2000 m, unsupported²³⁰Th/²³¹Pa ratios were 22–35 (average 29.7). Ratios were lower in particulate matter sampled at shallower depths. Particles filtered at 3600 m and 5000 m at Site E had ratios of 50 and 40. Results show that suspended particulate matter in the open ocean preferentially scavenges Th relative to Pa. Most of the²³⁰Th produced by decay of²³⁴U in the open ocean is removed by adsorption to settling particulate matter. In contrast, less than 50% of the²³¹Pa produced by decay of²³⁵U is removed from the water column by this mechanism. Mixing processes transport the remainder to other sinks.     

Removal of 230Th and 231Pa at ocean margins

Uranium, thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the Panama Basin and by in-situ filtration of large volumes of seawater in the Panama and Guatemala Basins. Concentrations of dissolved Th and Pa isotopes were determined by extraction onto MnO₂ adsorbers placed in line behind the filters in the in-situ pumping systems.Concentrations of dissolved ²³⁰Th and ²³¹Pa in the Panama and Guatemala Basins are lower than in the open ocean, whereas dissolved ²³⁰Th/²³¹Pa ratios are equal to, or slightly greater than, ratios in the open ocean. Particulate ²³⁰Th/²³¹Pa ratios in the sediment trap samples ranged from 4 to 8, in contrast to ratios of 30 or more at the open ocean sites previously studied. Particles collected by filtration in the Panama Basin and nearest to the continental margin in the Guatemala Basin contained ²³⁰Th/²³¹Pa ratios similar to the ratios in the sediment trap samples. The ratios increased with distance away from the continent.Suspended particles near the margin show no preference for adsorption of Th or Pa and therefore must be chemically different from particles in the open ocean, which show a strong preference for adsorption of Th. Ocean margins, as typified by the Panama and Guatemala Basins, are preferential sinks for ²³¹Pa relative to ²³⁰Th. Furthermore, the margins are sinks for ²³⁰Th and, to a greater extent, ²³¹Pa transported by horizontal mixing from the open ocean.     

Effects of pollution on the benthos of the Firth of Forth

A long-term benthic study has been undertaken in the Firth of Forth to assess the environmental impact of a new sewage treatment scheme for the city of Edinburgh. The Sewage Scheme reduces the suspended solids content of the liquid effluent by approximately 60% and achieves a lowering of the concentration of materials in solution through more efficient dilution and dispersion. Marked changes in the intertidal benthic flora and fauna have been recorded along the Edinburgh coastal zone. These changes have been apparent as the decline and disappearance of populations of certain pollution indicator species and the appearance and establishment of several species previously unrecorded along the more polluted parts of the Edinburgh shoreline. Study of the sublittoral benthos has shown no deleterious effect arising from the new effluent outfall, although there does appear to be some enhancement of the fauna in the area.     

Are anharmonicity corrections needed for temperature-profile calculations of interiors of terrestrial planets?

It is shown that there is linearity between the thermal pressure P_TH and T between the Debye temperature θ and some high temperature $\text{T}{}^1$. $\text{T}{}^1$ has been measured at 1 atm and is reported for several minerals including, for example, MgO (1300 K) and forsterite (1200 K). The change in thermal pressure from room temperature for five solids, so far measured, indicate striking linearity with T at high temperatures.It is further shown that the value of $\text{T}{}^1$ increases greatly as the pressure increases. It is therefore concluded that P_TH is probably linear with T for mantle minerals under mantle conditions. The proportionality constant is derived from the measurements of thermal expansivity and bulk modulus at high temperature and zero pressure.The argument is then reversed. Assuming that the thermal pressure is in fact linear with T for the various shells in a planet, the resulting density and temperature profile of the planet is derived. The resulting density profile of the Earth compares favorably with corresponding values of recent seismic profiles.     

The Early Carboniferous paleomagnetic field of North America and its bearing on tectonics of the Northern Appalachians

We have obtained additional evidence for the Early Carboniferous paleomagnetic field for cratonic North America from study of the Barnett Formation of central Texas. A characteristic magnetization of this unit was isolated after thermal demagnetization at four sites (36 samples) out of eight sites (65 samples) collected. The mean direction of declination = 156.3°, inclination = 5.8° (N = 4 ,k = 905 , α₉₅ = 3.0°), corresponds to a paleomagnetic pole position at lat. = 49.1°N,long. = 119.3°E (dp = 1.5° , dm = 3.0°). Field evidence suggests that characteristic magnetization was acquired very early in the history of the rock unit whereas the rejected sites are comprised of weakly magnetized limestones dominated by secondary components near the present-day field direction. Comparison of the Barnett pole with other Early Carboniferous (Mississippian) paleopoles from North America shows that it lies close to the apparent polar wander path for stable North America and that the divergence of paleopoles from the Northern Appalachians noted previously for the Devonian persisted into the Early Carboniferous. We interpret this difference in paleopoles as further evidence for the Northern Appalachian displaced terrain which we refer to here as Acadia, and the apparent coherence of Late Carboniferous paleopoles as indicating a large (～1500 km) motion of Acadia with respect to stable North America over a rather short time interval in the Carboniferous.     

Late glacial and postglacial pollen and plant macrofossils from Lake West Okoboji, Northwestern Iowa

Pollen and plant macrofossils preserved in lake sediment from Lake West Okoboji, Dickinson County, Iowa, indicate how the vegetation of that area changed during the late glacial and postglacial. A closed coniferous forest, dominated by spruce and larch trees, produced the Picea-Larix pollen assemblage zone. Fir trees were a minor constituent of this forest; pine trees were probably absent. Black ash trees increased in abundance at Lake West Okoboji and by 13,500 yr ago were an important constituent of the forest. The sediment accumulation rate and the pollen influx were low throughout this time. Birch and alder pollen peaked in abundance approximately 11,800 yr ago. Pollen influx increased rapidly as birch and alder replaced coniferous trees on the uplands. A deciduous forest, containing abundant oak and elm trees, replaced the birch-alder-coniferous forest. This forest inhabited northwestern Iowa from approximately 11,000 to 9000 yr B.P. Nonarboreal species became prevalent between approximately 9000 and 7700 yr B.P. as prairie began to replace deciduous forest on the uplands. Charred remains of Amorpha canescens and other upland species attest to the presence of prairie fires as an aid in establishing prairie and destroying the forest. The pollen influx declined. The warmest, driest part of the postglacial occurred in northwestern Iowa from approximately 7700 to 3200 yr ago. Lake level fell 9 to 10 m, and prairie extended to the edge of the lake. Wet-ground weeds inhabited areas near lake level which were alternately flooded, then dry. Pollen influx was approximately 100 grains/cm²/yr during the driest time in this dry interval.Deciduous trees, particularly oaks, returned after approximately 3200 yr B.P. Prairie continued to occupy the uplands but trees were more common in the lowlying wet areas. Settlement by Europeans in northwestern Iowa about 1865 is marked by an increase in weed pollen. Macrofossil deposition changed in 1910 in response to the stabilization of lake level.     

The role of magma vapors in volcanic tremors and rapid eruptions

A fracture dynamics model in which an igneous intrusion of magma within a crack occurs is used to describe the psysical processes of magma transport. A symbiotic relationship exists between the crack and the fluid. The crack tip cannot accelerate faster than the fluid within it can flow in the channel provided by the crack, and the speed of the fluid is limited by its own viscosity. A volatile phase at the tip of the crack at lithostatic pressures will allow the crack to accelerate to high speeds, since the viscosity of a volatile is small. It is proposed that periods of quiescence in volcanic activities may not in fact be quiet, but only periods where the crack velocity (and therefore the magma transport rate) is slow. At rapid crack velocities, where there is sufficient kinetic energy for the generation of abundant acoustic radiation, the crack generates a detectable seismic signal. From this point of view, seismic methods always underestimate the size of cracks. The analyses here apply fracture dynamics to the Kilauea cruption of 1963.     

The role of topography in controlling throughflow generation

Previous studies of the downslope movement of soil moisture have been largely confined to two-dimensional slope profiles—the influence of contour curvature (three-dimensional) on moisture movement has received little attention. An automatic tensiometer system has been used to monitor soil moisture status continuously in a single hollow and adjacent spurs. Maximum saturated hillslope flow is shown to coincide with the stream discharge ‘throughflow peak’. Relative to the hollow, little saturation is evident at the base of the spurs. This subcatchment model is applied to a major portion of the catchment, showing the general importance of topography in the control of stream discharge.