Quartz-grain shape: A record of the source of fine-grained sediments

The traditional petrographic means of determining the source of sediments, namely, heavy mineral analysis, is not easily practiced on fine-grained sediments. As a result, we have little information about the sources of muds, the most common sediment in the stratigraphic record.The shapes of quartz grains are also a record of their source. There have been a number of studies that have applied shape analysis to the problem of identifying the sources of sand. In this report, we extend this sourcing approach to fine-grained sediments in a study of the sources of silt on the South Texas Shelf.     

Enhanced Acoustic Backscatter Due to High Abundance of Sand Dollars, Dendraster excentricus

Detailed acoustic surveys of benthic sediments were conducted in July 1995 and September 1998 in the vicinity of Humboldt Bay, California. During these surveys, a band of enhanced acoustic backscatter was observed offshore from the bay entrance, approximately parallel to the isobaths, in water depths ranging from 16-24 m. In order to assess the cause of the increase in backscatter levels, a more comprehensive study was conducted in August and September 1999 using 100 kHz side-scan sonar, bottom grab sampling and underwater video recording. New observations indicated that a dense population of sand dollars ( Dendraster excentricus ) coincided with the enhanced backscatter band. Compared to the two previous acoustic studies, the central section of the band expanded westward by 180 m and the southern section of the band shifted eastward by 160 m, possibly resulting from a change in the biological or physical factors which influence the location and breadth of sand dollars. The relationship between high sand dollar abundance and enhanced acoustic backscatter was further verified in the nearshore region off Samoa Beach California, where a dense, banded population of sand dollars was previously observed. Video footage confirmed the presence of a band of sand dollars, also nominally parallel to the isobaths, in water depths of 8-15 m. A band of enhanced backscatter coincided with the dense sand dollar population. The identification of dense aggregations of sand dollars through enhanced acoustic backscatter could lead to the use of acoustic techniques to study sand dollar distributions and abundance.     

Water ice cloud formation on Mars is more difficult than presumed: Laboratory studies of ice nucleation on surrogate materials

The role of water ice clouds in the martian water cycle and climate depends on cloud properties such as particle size and number distribution. These properties, in turn, depend on heterogeneous nucleation parameters which are poorly understood. Here we report laboratory experiments performed under martian temperature and water partial pressure conditions (158–185 K, 9 × 10⁻⁷–1 × 10⁻⁴ Torr H₂O) to determine the critical saturation ratio for ice onset, S_crit, as a function of temperature and dust composition. Using infrared spectroscopy to monitor ice nucleation and growth, we find a significant barrier to ice formation, with a pronounced temperature dependence. Even on clay minerals which show uptake of non-crystalline water before ice nucleation, we find a saturation ratio of 2.5 or more (RH_ice > 250%) is needed to begin ice growth at temperatures near 160 K. These results could lead to changes of four orders of magnitude in the nucleation rate relative to the presumptions used currently in Mars microphysical models, which commonly set the contact parameter, m, to a single value of 0.95. Our results range from m = 0.84 to m = 0.98. For ice nucleation on Arizona Test Dust, the temperature dependence is described by m = 0.0046 * T_nucl + 0.1085, while m = 0.0055 * T_nucl + 0.0003 on a smectite-rich clay sample. Our findings suggest that cloud formation will be more difficult than previously thought, potentially leading to areas of increased near-surface humidity but generally drier conditions in the atmosphere of Mars, overall.     

Petrogenesis of boninites in the Ordovician Ballantrae Complex ophiolite, southwestern Scotland

Primitive lava and hyaloclastite with unusual, highly refractory compositions, form part of the Early Ordovician Balcreuchan Group within the ophiolitic Ballantrae Complex, southwestern Scotland. They are identified as likely high-Ca boninites on the basis of new XRF and INAA results and are the first unambiguous boninites to be discovered in the British Isles. The boninites are interbedded with low-Ti tholeiitic lavas with which they share some distinctive geochemical characteristics suggestive of a close petrogenetic relationship. The low-Ti tholeiite lavas have been interpreted as island-arc tholeiites but they also resemble back-arc basin basalts. The newly discovered boninites confirm an intra-oceanic environment of eruption; their distinctive features include relatively high SiO₂, MgO, Cr and Ni but low Al₂O₃ and HFSE abundances, U-shaped REE patterns, low Ti/Zr and high Zr/Hf ratios. Bulk geochemical trends are indicative of low-temperature, seawater-dominated alteration of the lavas but these alteration conditions apparently had little effect on the distribution of critical diagnostic elements such as Zr, Ti, Sc, Ta and the mid-heavy rare earths. We suggest that the Ballantrae boninites and low-Ti tholeiites represent different batch melts derived from a common, depleted mantle source region variably modified compositionally (i.e., made “streaky”) by fluids and/or melts during slab interaction (subduction metasomatism). A contribution from slab-derived pelagic sediments and/or a carbonatite melt is necessary to account for the fractionated, non-chondritic Zr/Hf ratios in the boninites. In view of the close compositional similarity of the Ballantrae lavas to Cenozoic boninite suites, we believe that these interpretations may have wider application to the petrogenesis of boninites in general.     

Partitioning of platinum-group elements and Au in the Fe−Ni−Cu−S system: experiments on the fractional crystallization of sulfide melt

Partitioning of platinum-group elements (PGE) between sulfide liquid and monosulfide solid solution (mss) has been investigated by crystallizingmss from Fe–Ni–Cu sulfide liquid at 1,000–1,040° C, using bulk compositions and PGE contents typical of magmatic sulfides associated with mafic and ultramafic systems. Products were analyzedin situ for PGE and Au using SIMS. Sulfide liquid compositions were more Ni- and Cu-rich than coexistingmss. Liquid/mss partition coefficients are: Os-0.23±0.04, Ir-0.28±0.11, Ru-0.24±0.05, Rh-0.33±0.06, Pt-4.8±0.7, Pd-4.8±1.9, Au-11.4. Partitioning of PGE is independent of PGE concentration and Ni content in the composition range investigated. Additionally, Henry's law appears to be obeyed up to minor-element contents in the sulfide liquid andmss. Osmium, Ir, Ru, and Rh are compatible elements in the anhydrous Fe–Ni–Cu–S system, whereas Pt, Pd and Au are incompatible elements. These affinities correspond to the partitions of PGE between massive and Cu-rich magmatic sulfides. However, the detailed precious-metal compositions of the Cu-rich sulfides of mafic rock systems, disseminated ores of komatiites and Cu-rich assemblage of droplet ore from the Noril'sk-Talnakh deposits are not consistent with those expected for pristine fractionated sulfide liquids.     

Energy cost-accounting of Louisiana fishery production

A quantitative relationship between gross primary production and secondary production of aquatic upper level consumers was derived for the inshore portion of the Louisiana coastal zone. We estimate that 1,700 units of gross primary production are required to produce one unit of upper level consumers. Lower and mid-level consumers are estimated to require lesser amounts, 300 and 600 units, respectively. Harvested fishery groups in the inshore ecosystem represent a total average of about 23,000 metric tons (dry wt) or about 1.2×10¹¹ Kcal (5.0×10¹¹ KJ) annually. This harvest “costs” the system about 7×10¹³ Kcal (3.3×10¹⁴ KJ) of gross primary production, or an equivalent of 4×10¹⁴ Kcal (1.7×10¹⁵KJ) of fossil fuel.     

The ages of fractures in the Eye-Dashwa pluton,Atikokan, Canada

Ages of fractures in the Eye-Dashwa granite pluton north of Atikokan, Ontario, are estimated from analyses of their filling materials (pegmatite, aplite, hornblendeporphyry, epidote, chlorite, gypsum, diabase, iron oxides, carbonate and clays). The analyses include radiometric age determinations of filling materials, comparison of crystallization temperatures of filling materials with the expected temperature of the pluton through time, and paragenetic relations derived from textures. The major fractures (primarily epidote and chlorite-filled) were produced by penetrative deformation during cooling immediately after intrusion of the pluton. From 2.6 Ga to 2.4 Ga they transected the pluton and largely defined the present overall fracture network. Less severe penetrative deformation occurred about 1.1 Ga, when a few diabase dikes and small gypsum fractures developed. A second source of fracturing appears to have existed through most of geological time, and it reopened segments of old fractures within about 400 m of the present erosional surface. It is unknown whether the near-surface, or supergene, fractures formed more or less continuously through time or in distinct episodes, such as during glacial periods. Open fractures, which are most common in the near-surface layer, tend to transmit groundwater, but fracture sealing due to the accumulation of filling materials seems to occur with time. Underground excavations such as nuclear fuel waste disposal vaults, would be less susceptible to the incursion of groundwater, if they were located below the near-surface layer.