The effects of sulfidation and oxidation during metamorphism on compositionally varied rocks adjacent to the Bleikvassli Zn–Pb–(Cu) deposit, Nordland, Norway

Petrographic, electron microprobe, and bulk-rock geochemical analyses indicate that the distribution and composition of ferromagnesian silicates (biotite, garnet, and staurolite) in and adjacent to the metamorphosed Bleikvassli Zn–Pb–(Cu) volcanogenic massive sulfide deposit, Norway, are dependent upon the competing effects of f O₂–f S₂ and host-rock composition. The enrichment in magnesium content of these silicates within the orebody and at distances of as much as 5–10 m away is due to the increased f O₂ and f S₂ conditions imposed on the silicates in zones subject to minor hydrothermal alteration during regional metamorphism. Alternatively, within pelitic country rocks at distances >5–10 m from ore, the host-rock chemistry controls the composition of metamorphic silicate minerals. Also, country rocks within a few meters of ore are distinguished by the common presence of zinc-bearing staurolite (up to 9 wt% ZnO) coexisting with biotite ± garnet. Rocks in the Bleikvassli deposit were hydrothermally enriched in zinc and fluorine prior to metamorphism. The fluorine resides mainly in biotite, which is an additional contributing factor to the magnesium enrichment of that mineral due to Fe²⁺–F avoidance. Our inference that the sulfidation–oxidation halo around the Bleikvassli ore deposit is only meters in width contrasts with the view of Maiga (1983), who proposed the effects of sulfidation could be identified at distances >159 m from ore. It is evident that the delineation of a sulfidation–oxidation halo bordering a metamorphosed massive sulfide deposit must be done carefully in order to discriminate between the effects due to variations in primary rock composition versus those resulting from a sulfur and oxygen fugacity gradient between the massive sulfides and the sulfur-poor country rocks. Received: 1 March 1998 / Accepted: 3 May 2000     

Amphibole compositions and metamorphic history of the Rand Schist and the greenschist unit of the Catalina Schist,Southern California

The Catalina Schist and Rand Schist are two high P/T terranes in southern California. The Catalina Schist is correlated with the Franciscan Complex and occurs in the continental borderland. It consists of a blueschist-facies melange tectonically overlain by a greenschist unit, which, in turn, is overthrust by an amphibolite unit. The greenschist unit itself is inversely zoned from epidote-amphibolite fades at the top through greenschist facies in the center to transitional blueschist-greenschist facies at the base. The Rand Schist is part of the eugeoclinal Pelona-Orocopia Schist terrane, which lies interior to the present continental margin, structurally beneath Precambrian to Mesozoic sialic basement. The Rand Schist is inversely zoned from epidote-amphibolite facies to transitional blueschist-greenschist facies, similar to the greenschist unit of the Catalina Schist.Two trends in amphibole composition, one from actinolite to hornblende in greenschists and epidote amphibolites (calcic series) and the other from actinolite through winchite to crossite in glaucophanic greenschists (sodic-calcic series), are present in both the Rand Schist and the greenschist unit of the Catalina Schist. The transition from actinolite to hornblende in the calcic series is defined by increases in tschermakite, edenite, and glaucophane substitution. Amphiboles of the sodic-calcic series differ mainly in the degree of glaucophanic substitution. The similarity of amphibole trends in the two terranes indicates that they were metamorphosed at approximately the same pressures and temperatures, and is evidence that the Rand Schist originated in a subduction zone, despite its present intracontinental setting.Most glaucophanic greenschists in the Rand and Catalina Schists contain both a sodic and a calcic member of the sodic-calcic series. Textural relations indicate that calcic members generally developed after the sodic ones. This implies that sodic amphibole formerly may have been present in many of the structurally higher greenschists and epidote amphibolites. Preservation of the inverted zonations, as well as microstructural evidence for the syntectonic development of calcic and sodic-calcic amphiboles, suggest that glaucophanic greenschists, greenschists, and epidote amphibolites all formed during underthrusting (subduction). This contrasts with many orogenic belts, where replacement of blueschists by greenschists to amphibolites is attributed to thermal reequilibration during erosional unroofing.     

Dynamics of rotationally fissioned asteroids: Source of observed small asteroid systems

We present a model of near-Earth asteroid (NEA) rotational fission and ensuing dynamics that describes the creation of synchronous binaries and all other observed NEA systems including: doubly synchronous binaries, high-e binaries, ternary systems, and contact binaries. Our model only presupposes the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, “rubble pile” asteroid geophysics, and gravitational interactions. The YORP effect torques a “rubble pile” asteroid until the asteroid reaches its fission spin limit and the components enter orbit about each other (Scheeres, D.J. [2007]. Icarus 189, 370-385). Non-spherical gravitational potentials couple the spin states to the orbit state and chaotically drive the system towards the observed asteroid classes along two evolutionary tracks primarily distinguished by mass ratio. Related to this is a new binary process termed secondary fission - the secondary asteroid of the binary system is rotationally accelerated via gravitational torques until it fissions, thus creating a chaotic ternary system. The initially chaotic binary can be stabilized to create a synchronous binary by components of the fissioned secondary asteroid impacting the primary asteroid, solar gravitational perturbations, and mutual body tides. These results emphasize the importance of the initial component size distribution and configuration within the parent asteroid. NEAs may go through multiple binary cycles and many YORP-induced rotational fissions during their approximately 10 Myr lifetime in the inner Solar System. Rotational fission and the ensuing dynamics are responsible for all NEA systems including the most commonly observed synchronous binaries.     

Influences of agricultural landuse and seasonal changes in abiotic conditions on invertebrate colonisation of riparian leaf detritus in intermittent streams

The structure and function of agricultural stream reaches with sparse riparian and floodplain vegetation differ from those of forested reaches, but may be ‘reset’ as these streams flow through reaches with forested riparian zones. We investigated whether invertebrate colonisation of River Red Gum (Eucalyptus camaldulensis) leaf packs in lowland intermittent streams was influenced by the adjacent reach-scale landuse (cleared farmland or forested reserve) within an agricultural catchment in Victoria, Australia. Further, we examined the influence of seasonal changes in hydrology and associated changes in abiotic conditions on the colonisation of leaves by repeating experiments over two summers and one spring. Across these experiments, there were no consistent differences in the structure of communities that colonised leaves in farmland and reserve reaches. In both seasons, most leaf colonists were collectors and few were shredders in both farmland and reserve reaches. Relative abundances of gastropod grazers were much higher in summer than in spring. The structure of invertebrate communities colonising leaves in the different reaches converged over time when streams flowed in spring, but diverged over time as the streams dried and abiotic conditions within disconnected pools became increasingly harsh in summer. Thus, patterns of leaf pack colonisation were influenced by the regional climate causing large seasonal changes in hydrology, but not by reach-scale landuse. The large-scale disturbances of agricultural landuse across the catchment and a supra-seasonal drought probably contributed to low diversities of invertebrate communities in the streams.     

The major ion, δCa, δCa, and δMg geochemistry of granite weathering at pH = 1 and T = 25 °C: power-law processes and the relative reactivity of minerals

We dissolved Boulder Creek Granodiorite in a plug flow reactor for 5794 h at pH = 1 and T = 25 °C. The primary purpose of the experiment was to identify controls on dissolved δ^44/40Ca, δ^44/42Ca, and δ^26/24Mg values during granite weathering. Herein, we also examine the origin of Ca and Mg isotopic variability among minerals composing the Boulder Creek Granodiorite, and we constrain fundamental characteristics of granite weathering important for quantifying the elemental and isotopic geochemistry of the reactor output. Nine Ca-bearing minerals display an 8.80‰ range of δ^44/40Ca values and a 0.51‰ range of δ^44/42Ca values. Three Mg-bearing minerals display a 1.53‰ range of δ^26/24Mg values. These ranges expressed at the mineralogical scale are higher than the ranges thus far reported for bulk igneous rocks. Most of the δ^44/40Ca variability reflects ⁴⁰Ca enrichment in K-feldspar, and to a lesser extent, biotite, due to the radioactive decay of ⁴⁰K over the 1.7 Ga age of the rock, whereas the entire range of δ^44/42Ca values reflects mass-dependent isotope fractionation during igneous differentiation and crystallization. The range of δ^26/24Mg values may represent either fractionation during the chloritization of biotite or interaction of the Boulder Creek Granodiorite with Mg-rich metamorphic fluids having low δ^26/24Mg values.The elemental and isotopic composition of the reactor output varied substantially during the experiment. We synthesize the mineralogical and fluid data using coupled mass-conservation equations solved at non-steady-state. Model calculations reveal an intricate balance between increasing specific surface area and decreasing mineral concentrations. While surface area normalized dissolution rate constants were time-invariant, specific surface area increased as a power-law function of time through positive feedbacks between mechanical disaggregation, chemical dissolution, and mineral depletion. Variations in dissolved δ^44/40Ca, δ^44/42Ca, and δ^26/24Mg values reflect conservative mixing rather than fractionation. Apatite and calcite initially control δ^44/40Ca and δ^44/42Ca values, followed by biotite, titanite, epidote, hornblende, and plagioclase. The release of radiogenic ⁴⁰Ca clearly defines the period where biotite dissolution dominates. The brucite layer of chlorite initially controls δ^26/24Mg values, followed by biotite, the TOT layer of chlorite, and hornblende. Through direct isotopic tracking, these results demonstrate that trace minerals, such as apatite and calcite in the case of Ca and brucite in the case of Mg, dominate elemental release during the incipient stages of granite weathering. The results further show that biotite dissolution dominates the middle stages of granite weathering and that plagioclase dissolution only becomes important during relatively late stages. The Ca and Mg isotope variations associated with these stages are distinct and potentially resolvable in soil mineral weathering studies.     

Influence of intermittent estuary outflow on coastal sediments of adjacent sandy beaches

Outflows from estuaries potentially contribute to the productivity of adjacent coastal waters, although most previous work has been on estuaries with considerable river discharge. We investigated the influence of estuary outflow on aspects of coastal sediments adjacent to two seasonally intermittent estuaries, the Curdies and Anglesea Rivers, in southwest Victoria, Australia. For each estuary, we measured sediment organic matter, microphytobenthic chlorophyll a and microbial utilization of carbon sources at three locations associated with each estuary: (1) inside estuary mouth, (2) estuary swash and (3) control swash (an open beach distant from any estuarine influences). Sampling occurred one week before and at one and nine weeks after both an artificial mouth opening and a separate natural flood at both estuaries. Significant temporal changes were detected for all three variables at the estuary mouth and estuary swash but the direction of change was inconsistent across the two estuaries and between the artificial mouth opening and natural flood. Organic matter in both estuaries showed no difference after the artificial mouth openings. Only Anglesea showed an increase in organic matter in the estuary mouth and estuary swash after the floods. Microphytobenthic chlorophyll a concentrations were highest when the estuary mouths were closed. Concentrations decreased at all locations at Curdies after the mouth was artificially opened. The estuary mouth at Anglesea sustained high chlorophyll concentrations and the estuary swash increased one week post artificial opening. The flood event resulted in an increase in chlorophyll a at the estuary mouth and swash at both estuaries, one week post flood. At Curdies, the microbial utilization of different carbon sources changed after both mouth events; estuary mouth and estuary swash showed similar patterns at one and nine weeks post opening. At Anglesea, the bacteria utilized different carbon sources between locations and the only significant interaction between location and time was post flood with change in carbon sources utilized by bacteria in the estuary mouth and estuary swash for one and nine weeks post flood. The southern coastline of Australia is characterized by estuaries with small catchments. This study highlights the spatial and temporal variability in the effects of the output of relatively small, intermittent estuaries on coastal sediment of adjacent beaches, particularly during prolonged periods of drought.     

Orbits of Saturn's F ring and its shepherding satellites

The shape and orientation of Saturn's F ring and the orbits of its two shepherding satellites have been determined from Voyager images. The data and processing are described, and orbital parameter estimates and associated uncertainties are presented. In addition, evidence that suggests that the F-ring braids are formed very near the conjunctions of the shepherding satellites is presented.     

CLAIRE: First light for a gamma-ray lens

The objective of the R&D project CLAIRE was to prove the principle of a gamma-ray lens for nuclear astrophysics. CLAIRE's Laue diffraction lens has a diameter of 45 cm and a focal length of 277 cm; 556 germanium-silicon crystals are tuned to focus 170 keV photons onto a 1.5 cm diameter focal spot. Laboratory measurements of the individual crystals and the entire lens have been used to validate a numerical model that we use to estimate the lens performance for a source at infinity. During a stratospheric balloon flight on 2001 June 14, CLAIRE was directed at the Crab nebula by a pointing system able to stabilize the lens to within a few arcseconds of the target. In 72 min of valid pointing time, 33 photons from the Crab were detected in the 3 keV bandpass of the lens: CLAIRE's first light! The performance of CLAIRE's gamma-ray lens, namely the peak reflectivity for a polychromatic source (9±1%), has been confirmed by ground data obtained on a 205 meter long test range. CLAIRE's measured performance validates the principle of a Laue lens for nuclear astrophysics, opening the way for a space-borne gamma-ray lens telescope that will achieve one to two orders of magnitude improvement in sensitivity over present technologies.