Chemical zoning in wolframite from San Cristobal,Peru

Fifty wolframite crystals from San Cristobal (Peru) were analysed with the electron microprobe for Fe, Mn, and W. Detailed studies of several samples reveal complex compositional zoning within individual crystals. One sample contains two crystals with contrasting zoning: one crystal has a high-Fe core and an Fe-poor rim, whereas the other has an Fe-poor core and a high-Fe rim. This suggests that these two crystals formed at different moments and that the wolframite composition did not change monotonically with time. The full range of wolframite compositions measured is from 28 to 97 mol% ferberite (FeWO₄). Although a range of 59 mol% was determined for one sample, the average range for all fifty samples is 10 mol%. Samples with average compositions in the range of 84–92 mol% ferberite have relatively small compositional variations, whereas those with average compositions in the range of 61–84 mol% are quite variable. On a vertical longitudinal section of the Main Vein of San Cristobal it appears that the low ferberite values correspond spatially with high tungsten grades. The iron content of the wolframite goes through a minimum across its depositional interval. This may be due to a change in the Fe/Mn ratio of the mineralizing solution with either distance or time.     

Radar observations of the icy Galilean satellites

We report 12.6-cm-wavelength radar observations of Europa, Ganymede, and Callisto made at the Arecibo Observatory in November 1977 and February 1979. When combined with previous observations, our results establish firmly the distinguishing radar properties of these satellites: (i) high geometric albedos, α; (ii) circular polarization ratios, μ_C, which anomalously exceed unity; (iii) linear polarization ratios, μ_L, which are approximately 0.5; and (iv) diffuse scattering which varies as cosⁿθ, where θ is angle of incidence and 1 ? n ? 2. We tabulate weighted-mean values of α, μ_C, μ_L, and n derived from observations between 1975 and 1979. The values of μ_C for Ganymede and Europa are nearly identical and significantly larger than that for Callisto. The values of n for Ganymede and Callisto are nearly identical and significantly smaller than that for Europa. Although significant albedo and/or polarization features are common in the radar spectra, the fractional rms fluctuation in disk-integrated properties is only ～10%. No time variation in the radar properties has been evident during 1976–1979.     

Characterisation and origin of New Zealand nephrite jade using its strontium isotopic signature

Nephrite jade occurs in three terranes (Dun Mountain–Maitai, Caples and Torlesse) in New Zealand, where it is associated with ultramafic and ophiolitic rocks in narrow metasomatic reaction zones at the margins of serpentinite (having harzburgite/gabbro/dolerite precursors) with silicic metasediments and metavolcanics. True nephrite fabrics are developed only locally where marginal shearing is intense, and late in the metamorphic history. ⁸⁷Sr/⁸⁶Sr values of these nephrites do not display the primitive values of their gabbro/dolerite precursor component i.e. 0.7030–0.7035, as expected if formed during serpentinisation. Rather, the nephrites have more evolved ⁸⁷Sr/⁸⁶Sr values inherited from the metasediment component at a later stage, and which fall within particular terrane groups: Dun Mountain–Maitai 0.7045–0.7060, Caples 0.7058–0.7075 and Torlesse 0.7085–0.7110. Rb–Sr ages and initial ⁸⁷Sr/⁸⁶Sr ratios of the metasediment component from in situ nephrite localities, when compared with their counterparts throughout the host terrane, show that nephrite Sr isotopic compositions are characteristic of the host terrane.     

The estuarine chemistry and isotope systematics of U in the Amazon and Fly Rivers

Natural concentrations of ²³⁸U and δ²³⁴U values were determined in estuarine surface waters and pore waters of the Amazon and Fly (Papua New Guinea) Rivers to investigate U transport phenomena across river-dominated land–sea margins. Discharge from large, tropical rivers is a major source of dissolved and solid materials transported to the oceans, and are important in defining not only oceanic mass budgets, but also terrestrial weathering rates.On the Amazon shelf, salinity-property plots of dissolved organic carbon, pH and total suspended matter revealed two vastly contrasting water masses that were energetically mixed. In this mixing zone, the distribution of uranium was highly non-conservative and exhibited extensive removal from the water column. Uranium removal was most pronounced within a salinity range of 0–16.6, and likely the result of scavenging and flocculation reactions with inorganic (i.e., Fe/Mn oxides) and organic colloids/particles. Removal of uranium may also be closely coupled to exchange and resuspension processes at the sediment/water interface. An inner-shelf pore water profile indicated the following diagenetic processes: extensive (1 m) zones of Fe(III)—and, to a lesser degree, Mn(IV)—reduction in the absence of significant S(II) concentrations appeared to facilitate the formation of various authigenic minerals (e.g., siderite, rhodocrosite and uraninite). The pore water dissolved ²³⁸U profile co-varied closely with Mn(II). Isotopic variations as evidenced in δ²³⁴U pore waters values from this site revealed information on the origin and history of particulate uranium. Only after a depth of about 1 m did the δ²³⁴U value approach unity (secular equilibrium), denoting a residual lattice bound uranium complex that is likely an upper-drainage basin weathering product. This suggests that the enriched δ²³⁴U values represent a riverine surface complexation product that is actively involved in Mn–Fe diagenetic cycles and surface complexation reactions.In the Fly River estuary, ²³⁸U appears to exhibit a reasonably conservative distribution as a function of salinity. The absence of observed U removal does not necessarily imply non-reactivity, but instead may record an integration of concurrent U removal and release processes. There is not a linear correlation between δ²³⁴U vs. 1/²³⁸U that would imply simple two component mixing. It is likely that resuspension of bottom sediments, prolonged residence times in the lower reaches of the Fly River, and energetic particle–colloid interactions contribute to the observed estuarine U distribution. The supply of uranium discharged from humid, tropical river systems to the sea appears to be foremost influenced by particle/water interactions that are ultimately governed by the particular physiographic and hydrologic characteristics of an estuary.     

The role of tectonism in sequence development and facies distribution of Upper Oligocene cool-water carbonates: Coromandel Peninsula, New Zealand

The Torehina Formation is part of a cool‐water carbonate succession of Oligocene age in New Zealand that crops out on the Coromandel Peninsula, North Island. It contains two major transgressive sequences that record successive onlap of a once emergent landmass. The first sequence records marine flooding of non‐marine to marginal marine fan delta/estuarine facies, followed by deepening upward and formation of a low‐energy, deep (100+ m) muddy carbonate ramp. The capping sequence boundary is characterized by differential uplift and varies considerably in its character over a small (9 km²) area, varying from a burrowed glauconitic firmground to an erosional hardground to an undulatory marine contact to a palaeokarst with < 25 m relief. Sequence 2 sediments, which overlie the palaeokarst with minor (< 10°) angular unconformity, are clayey, marine (offshore) siltstones, whereas open‐marine limestones of equivalent age overlie the other boundary types with no apparent angular discordance. The siliciclastics could either represent lowstand channel deposits or may define interbank deposits contemporary with adjacent carbonates. Palaeogeographic restriction of palaeokarst and sequence 2 siliciclastics identifies a structural corridor oriented strike‐parallel to the adjacent Harauki Graben, which began to develop by this time. Palaeogeographical differences in the character of the basal limestone facies of sequence 2 also occur. These differences identify variation in accommodation during initial stages of deposition imposed by previous differential movement of fault blocks. As a result, relatively warm‐water (20 °C) Amphistegina‐bearing limestones in one area contrast with co‐existing deeper water, silty foraminiferal (benthic > planktic), echinoderm and bivalve limestones in another. This variation disappears upsection, which suggests that initial bathymetric differences were eliminated with renewed rise in sea level, yielding deeper water inner‐shelf sediment facies followed by the accumulation of still deeper, but higher energy, outer‐shelf bivalve and bryozoan facies. The sequence architecture of the Torehina Formation is controlled by tectonism, both long‐term subsidence and short‐term differential uplift. This arose as a result of increasing tectonic activity throughout proto‐New Zealand during the Late Oligocene. In such a system, local and regional variation in tectonism among adjacent basins can impose subtle to marked differences in the timing of sequence boundaries and the character of basin‐fill patterns.     

Formation of metal in the CH chondrites ALH 85085 and PCA 91467

Siderophile element distributions within individual metal grains in two CH chondrites, Allan Hills 85085 and Pecora Escarpment 91467, were measured by laser ablation inductively coupled plasma mass spectrometry. Those metal grains that are zoned in Ni were also found to be zoned in other refractory siderophile elements, such as Ru, but not in Pd, which is not refractory but is highly siderophile. This pattern is consistent with an origin by condensation from a gas of approximately solar composition, but not with an origin by redox processes or fractional crystallization. The unzoned metal grains in CH chondrites were found to be frequently depleted in Ru but not in Pd, consistent with later stage condensation from a solar gas after removal of the zoned metal. Gold is inversely correlated with Ni in the unzoned metal grains, and mean Au abundances in zoned metal are always low. Both zoned and unzoned metal in CH chondrites could plausibly be produced from a thermostatically regulated nebula, followed by rapid removal of the zoned metal, and slower removal of the unzoned metal, both at temperatures near or above the condensation temperature of Au (∼1250 K). This is also consistent with the isolation temperatures inferred from silicate grains in CH chondrites by previous workers based on their volatile element inventories. The volatile siderophile Cu is enriched in the rims relative to the interiors of both zoned and unzoned grains, and is interpreted as the product of diffusion during low-grade thermal processing. The similarity of Cu distributions, and degree of kamacite/taenite exsolution, between zoned and unzoned metal in CH chondrites suggests that the two populations of metal experienced modest thermal metamorphism after they were brought together in the same environment, probably on the CH parent body. Fragmentation and size-sorting of the metal must have post-dated the Cu zoning, and may have occurred in a regolith on the CH parent body. The compositions of CH metal, like that of metal from QUE 94411 and HH 237, are consistent with a nebular origin, and may be the most primitive nebular materials (as distinct from presolar grains) sampled by chondrites.