Geology and tectonics of Beta Regio,Guinevere Planitia,Sedna Planitia,and Western Eistla Regio,Venus: Results from Arecibo image data

New radar images (resolution 1.5–2.0 km) obtained from the Arecibo Observatory are used to assess the geology of a portion of the equatorial region of Venus (1 S to 45 N and from 270 eastward to 30). Nine geologic units are mapped on the basis of their radar characteristics and their distribution and correspondences with topography are examined. Plains are the most abundant unit, make up 80%; of the area imaged, and are divided into bright, dark, and mottled. Mottled plains contain abundant lava flows and domes suggesting that volcanism forming plains is a significant process in the equatorial region of Venus. Tesserae are found primarily on Beta Regio and its eastern flank and are interpreted to be locally stratigraphically older units, predating episodes of faulting and plains formation. Isolated regions of tesserae concentrated to the north of Western Eistla Regio are interpreted to predate the formation of plains in this area. The volcanoes Sif Mons, Gula Mons, Sappho, Theia Mons, and Rhea Mons, are found exclusively in highland regions and their deposits are interpreted as contributing only a small percentage to the overall volume of the regional topography. The northern 15 of the image data overlap with Venera 15/16 images making it possible to examine the characteristics of geologic units mapped under various illumination directions and incidence angles. Surface panoramas and geochemical data obtained from Venera landers provide ground truth for map units, evidence that plains are made up of basaltic lava flows, and that linear deformation zones contain abundant blocks and cobbles. On the basis of spatial and temporal relationships between geologic units, the highlands of Beta Regio and Western Eistla Regio are interpreted to have formed in association with areas of mantle upwelling which uplift plains, cause rifting, and in the case of Beta Regio, disrupt a large region of tessera. Zones of linear belt deformation in Beta Regio and Western Eistla Regio are interpreted to be extensional and indicate that at least limited extension has occurred in both regions. The images reveal for the first time that southern Devana Chasma is a region of overlapping rift valleys separated by a distance of 600 km. Linear deformation zones in Guinevere Planitia, separating Beta Regio and Eistla Regio, converge at a region of ovoids forming a discontinuous zone of disruption and completes an equatorial encompassing network of highlands and tectonic features. The similarity between ovoids and coronae suggests a mechanism of formation associated with hotspots or mantle plumes. Analysis of the distribution and density of impact craters suggests a surface age for this part of the planet similar to or slightly less than that determined for the northern high latitudes from Venera 15/16 data (0.3 to 1.5 by) and comparable to that calculated for the southern hemisphere.     

Subsidence rates for the Southeastern Hawaiian Islands determined from submerged terraces

Detailed bathymetric maps of the southeastern Hawaiian Islands reveal terraces west of Lanai and west of Hawaii; these can be correlated with glacial maxima to provide island subsidence rates of 1.9 and 2.4 mm/yr, respectively. These data agree in general with subsidence rates determined by other means. Plots of present depth versus age for these terraces show that subsidence of the island of Hawaii has remained relatively constant since 600 ka; thus the magma production of the Hawaiian hotspot has been nearly steady for this period. These subsidence rates also explain why evidence of sea levels higher than present is not found on the islands southeast of Oahu.     

Comparative advantage and distant water fishing fleets: The Australian east coast tuna longline fishery

A coastal state benefits from importing the services of a distant water fishing nation (DWFN) fleet if the DWFN has a comparative advantage in exploiting the fishery, based on factors such as market access, costs or technology. The paper compares the activities of the Australian and Japanese vessels in the east coast tuna longline fishery and finds the Australian vessels receive higher prices, have higher costs and use a different technology from the Japanese fleet. These differences suggest a pattern of comparative advantage which can lead to Australia benefitting from importing the services of the Japanese fleet.     

Radar detection of Vesta

Asteroid 4 Vesta was detected on 1979 November 6 with the Arecibo Observatory's S-band (12.6-cm-wavelength) radar. The echo power spectrum, received in the circular polarization opposite to that transmmited,, yields a radar cross section of (0.2 ± 0.1)πa², for a = 272 km. The data are too noisy to permit derivation of Vesta's rotation period.     

Chemical evolution of metal in refractory inclusions in CV3 chondrites

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure distributions of the siderophile elements V, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, Pt, and Au in Fremdlinge with a spatial resolution of 15 to 25 μm. A sulfide vein in a refractory inclusion in Allende (CV3-oxidized) is enriched in Rh, Ru, and Os with no detectable Pd, Re, Ir, or Pt, indicating that Rh, Ru, and Os were redistributed by sulfidation of the inclusion, causing fractionation of Re/Os and other siderophile element ratios in Allende CAIs. Fremdlinge in compact Type-A inclusions from Efremovka (CV3-reduced) exhibit subsolidus exsolution into kamacite and taenite and minimal secondary formation of V-magnetite and schreibersite. Siderophile element partitioning between taenite and kamacite is similar to that observed previously in iron meteorites, while preferential incorporation of the light PGEs (Ru, Rh, Pd) relative to Re, Os, Ir, and Pt by schreibersite was observed. Fremdling EM2 (CAI Ef2) has an outer rim of P-free metal that preserves the PGE signature of schreibersite, indicating that EM2 originally had a phosphide rim and lost P to the surrounding inclusion during secondary processing. Most Fremdlinge have chondrite-normalized refractory PGE patterns that are unfractionated, with PGE abundances derived from a small range of condensation temperatures, ∼1480 to 1468 K at P_tot = 10⁻³ bar. Some Fremdlinge from the same CAI exhibit sloping PGE abundance patterns and Re/Os ratios up to 2 × CI that likely represent mixing of grains that condensed at various temperatures.     

Disc structure around strongly magnetic accretors: a full disc solution with turbulent diffusivity

The problem of the effect of a strongly magnetic star on a surrounding accretion disc is considered. For stellar rotation periods greater than a critical value, a numerical solution is found for a steady disc with turbulent magnetic diffusion, including electron scattering opacity and radiation pressure. Inside the corotation radius, the extraction of disc angular momentum by magnetic coupling to the star becomes strong and this leads to enhanced viscous stress and dissipation. The resulting elevated temperature causes electron scattering opacity and radiation pressure to become significant further from the star than in the absence of its magnetic field. The disc ends as its height increases rapidly due to the large central pressure, its density decreases and magnetically induced viscous instability occurs.     

Measurement technique. Very long baseline interferometry

In the last decade there was a large increase in the number of VLBI antennas around the world, both for radioastronomy and geodesy. This increase is connected to the dramatic improvements in their results, in particular for geodesy. In Europe a network of six antennas, spanning from Sweden to Spain and Italy, one of the world's most densely spaced VLBI networks of fixed antenna, has been operating since 1990. This network was enlarged during 1994 with the Crimea and Ny-Alesund antennas. More are under construction in Russia. The first five years of activity have produced more than 20 purely European experiments of very good quality allowing the measurements of the horizontal velocities of Italian stations with errors at the mm yr⁻¹ level. The very latest results from Bologna solutions are presented. The future evolution of the technique at the stations and at the correlator is also presented.     

P—T—t—deformation—fluid characteristics of lode gold deposits: evidence from alteration systematics

Structurally hosted lode gold-bearing quartz vein systems in metamorphic terranes possess many characteristics in common, spatially and through time; they constitute a single class of epigenetic precious metal deposit, formed during accretionary tectonics or delamination. The ore and alteration paragenesis encode numerous intensive and extensive variables that constrain the pressure—temperature—time—deformation—fluid (P—T—t—d—f) evolution of the host terrane and hence the origin of the deposits. The majority of lode gold deposits formed proximal to regional translithospheric terrane—boundary structures that acted as vertically extensive hydrothermal plumbing systems; the structures record variably thrust, and transpressional—transtensional displacements. Major mining camps are sited near deflections, strike slip or thrust duplexes, or dilational jogs on the major structures. In detail most deposits are sited in second or third order splays, or fault intersections, that define domains of low mean stress and correspondingly high fluid fluxes. Accordingly, the mineralization and associated alteration is most intense in these flanking domains. The largest lode gold mining camps are in terranes at greenschist facies; they possess greenschist facies hydrothermal alteration assemblages developed in cyclic ductile to brittle deformation that reflects interseismic—coseismic cycles. Interseismic episodes involve the development of ductile S—C shear zone fabrics that lead to strain softening. Pressure solution and dislocation glide microstructures signify low differential stress, slow strain rates of ≤ 10⁻¹³ s⁻¹, relatively high confining stress, and suprahydrostatic fluid pressures. Seismic episodes are induced by buildup of fluid pressures to supralithostatic levels that induce hydraulic fracturing with enhanced hydraulic conductivity, accompanied by massive fluid flow that in turn generates mineralized quartz veins. Hydrothermal cementing of ductile fabrics creates ‘hardening’, lowers hydraulic conductivity, and hence promotes fault valve behaviour. Repeated interseismic (fault valve closed), coseismic (valve open) cycles results in banded and/or progressively deformed veins. Alteration during both interseismic and coseismic episodes typically involves the hydrolysis of metamorphic feldspars and Fe, Mg, Ca-silicates to a muscovite/paragonite—chlorite ± albite/K-feldspars assemblage; carbonization of the metamorphic minerals to Ca, Fe, Mg-carbonates; and sulphidation of Fe-silicates and oxides to sulphides. Geochemically this is expressed as additions of K, Rb, Ba, Cs, and the volatiles H₂O, CO₂, CH₄, H₂S in envelopes of meter to kilometer scale. K/Rb and K/Ba ratios are close to average crustal values, potentially ruling out late stage magmatic fluids where K/Rb and K/Ba are respectively lower and higher than crustal values. Smaller deposits are present in subgreenschist, and amphibolite to granulite facies terranes. The former are characterized by subgreenschist facies alteration assemblages, vein stockworks, brittle fracturing and cataclastic microstructures, whereas the latter feature amphibolite to granulite facies alteration assemblages, ductile shear zones, ductilely deformed veins, and microstructures indicative of dislocation climb during interseismic episodes. Hence the lode gold deposits constitute a crustal continuum of deposits from subgreenschist to granulite facies, that all formed synkinematically in broad thermal and rheological equilibrium with their host terranes. These characteristics, combined with the low variance of alteration assemblages in the higher temperature deposits, rules out those being metamorphosed counterparts of greenschist facies deposits. Deposits at all grades have a comparable metal inventory with high concentrations of Au and Ag, where Au/Ag averages 5, with enrichments of a suite of rare metals and semi-metals (As, Sb, ± Se, Te, Bi, W, Mo and B), but low enrichments of the base (Cu, Pb, Zn, Cd) and other transition (Cr, Ni, Co, V, PGE, Sc) metals relative to average crust. The hydrothermal ore-forming fluids were dilute, aqueous, carbonic fluids, with salinities generally ≤ 3 wt.% NaCl equivalent, and X_{(CO2 ± CH4)} 10–24 wt.%. They possess low Cl but relatively high S, possibly reflecting the fact that metamorphic fluids are generated in crust with ∼ 200 ppm Cl, but ∼ 1 wt.%S. Primary fluid inclusions are: (1) H₂OCO₂, (2) CO₂-rich with variable CH₄ and small amounts of H₂O, and (3) 2-phase H₂O (liquid-vapor) inclusions. Inclusion types 2 and 3 represent immiscibility of the type 1 original ore fluid. Immiscibility was triggered by fluid pressure drop during the coseismic events and possibly by shock nucleation, leading to highly variably homogenization temperatures in an isothermal system. A thermodynamic evaluation of alteration assemblages constrains the ore fluid pH to 5–6; redox controlled by the HSO₄/H₂S and CO₂/CH₄ buffers; and X_CO2 that varies. The higher temperature deposits formed under marginally more oxidizing conditions. Stable isotope systematics of the ore and gangue minerals yields temperatures of 200–420°C, consistent with the crustal spectrum of the deposits, very high fluid rock ratios, and disequilibrium of the externally derived ore fluids with wall rocks. The ore fluid δD and δ¹⁸O overlap the metamorphic and magmatic ranges, but the total dataset for all deposits is consistent only with dominantly metamorphic fluids. Carbon isotope compositions of carbonates span −11 to +2% and show provinciality: this is consistent with variable proportions of reduced C (low δ¹³C) and oxidized C (higher δ¹³C) in the source regions contributing CO₂ and CH₄ to the ore fluids. In some instances, C appears to have been derived dominantly from proximal to the deposits, as in the Meguma terrane (δ¹³C ∼ − 22%). Sulphur isotope compositions range from 0 to +9‰, and are consistent with magmatic S, dissolution or desulphidation of magmatic sulphides, or average crustal sulphides. ³⁴S-depleted sulphides occur in ore bodies such as Hemlo where fluid immiscibility led to loss of H₂S and consequent fluid oxidation. Gold is probably transported as an Au(HS)⁻₂ complex. Relatively high S but low Cl in the hydrothermal fluid may explain the high Au slow base metal characteristic of the deposits. Gold precipitated in ore bodies due to loss of S from the ore fluid by sulphidation of wall rock, or immiscibility of H₂S; and by oxidation or reduction of the fluid, or by chemisorption, or some combination of these processes. Most lode gold deposits have been brittly reactivated during uplift of host terranes, with secondary brines or meteoric water advecting through the structures. These secondary fluids may remobilize gold, generate retrograde stable isotope shifts, reset mineral geochronometers, and leave trails of secondary fluid inclusions. Data on disturbed minerals has led to invalid models for lode gold deposits. The sum of alteration data leads to a model for lode gold deposits involving a clockwise P—T—t evolution and synkinematic and synmetamorphic mineralization of the ‘deep later’ type. During terrane accretion oceanic crust and sediments are subcreted beneath the terrane boundary. Thermal equilibration generates metamorphic fluids that advect up the terrane structure, at lithostatic fluid pressure, into the seismogenic zone where the majority of deposits form. Thus many lode gold deposits are on intrinsic part of the development of subduction—accretion complexes of the high-T, low-P type.     

Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in zircon: application to porphyry copper deposits of northern Chile

Major- and trace-element compositions of zircons and whole rocks from 14 barren and seven ore-bearing calc-alkaline intrusions from the Chuquicamata-El Abra porphyry copper belt of northern Chile have been measured in situ by excimer laser ablation (ELA) ICP-MS. These data permit the Ce(IV)/Ce(III) ratio within zircon to be calculated using a lattice-strain model for mineral-melt partitioning of Ce(IV) and Ce(III). Zircon Ce(IV)/Ce(III) and Eu_N/Eu_N* ratios, and by inference magmatic oxidation states, generally increase from older, mafic to younger, felsic units. Within this sequence, porphyry copper mineralization is directly associated only with intrusions with zircon Ce(IV)/Ce(III)>300 and Eu_N/Eu_N*>0.4. Such trends can be understood in terms of interdependent relations between oxygen fugacity, sulfur speciation and solubility, and chalcophile element partitioning in silicate magmas. Because zircon occurs in most calc-alkaline intrusions and is resistant to subsolidus alteration, zircon Ce(IV)/Ce(III) ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic-hydrothermal Cu-Au mineralization. The approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks.