An analytical and experimental study of zoning in plagioclase

A detailed electron microprobe study has been conducted on natural and experimentally grown zoned plagioclase feldspars. Discontinuous, sector, and oscillatory chemical zoning are observed superimposed on continuous normal or reverse zoning trends. The relative accuracy of 3 element (Na, Ca, K) microprobe traverses was found statistically to be 2 mole percent. Comparison of microprobe data on natural zoned plagioclase with zoned plagioclase grown in controlled experiments has shown that it may be possible to distinguish zonal development resulting from physio-chemical changes to the bulk magma from zoning related to local kinetic control on the growth of individual crystals.     

Grain boundary processes and development of metamorphic plagioclase

Plagioclase from a progressively metamorphosed granodiorite changes as the metamorphic grade increases. Lower grade plagioclase are chemically inhomogeneous, with zoned rims containing distinct compositional levels of An_0?3, An₁₇, and An₂₅. As grade increases the plagioclase becomes more chemically homogeneous with An_0?3 rims dominating. Microcline inclusions are controlled by internal defects at lower grades and grain boundaries at higher grades. Myrmekite rims are developed at the highest grade. Rims are dependent on surface energy factors and occur at triple points, high angle lattice misfits and other high energy surfaces. At low grades, rims form at plagioclase-plagioclase contacts and at higher grades, at plagioclase-microcline contacts. These changes are due to impurity segregation and grain boundary migration, and an increase of the letter process at higher grades.     

Tidal Habitats Support Large Numbers of Invasive Blue Catfish in a Chesapeake Bay Subestuary

The introduction of a non-native freshwater fish, blue catfish Ictalurus furcatus, in tributaries of Chesapeake Bay resulted in the establishment of fisheries and in the expansion of the population into brackish habitats. Blue catfish are an invasive species in the Chesapeake Bay region, and efforts are underway to limit their impacts on native communities. Key characteristics of the population (population size, survival rates) are unknown, but such knowledge is useful in understanding the impact of blue catfish in estuarine systems. We estimated population size and survival rates of blue catfish in tidal habitats of the James River subestuary. We tagged 34,252 blue catfish during July–August 2012 and 2013; information from live recaptures (n = 1177) and dead recoveries (n = 279) were used to estimate annual survival rates and population size using Barker’s Model in a Robust Design and allowing for heterogeneity in detection probabilities. The blue catfish population in the 12-km study area was estimated to be 1.6 million fish in 2013 (95% confidence interval [CI] adjusted for overdispersion: 926,307–2,914,208 fish). Annual apparent survival rate estimates were low: 0.16 (95% CI 0.10–0.24) in 2012–2013 and 0.44 (95% CI 0.31–0.58) in 2013–2014 and represent losses from the population through mortality, permanent emigration, or both. The tagged fish included individuals that were large enough to exhibit piscivory and represented size classes that are likely to colonize estuarine habitats. The large population size that we estimated was unexpected for a freshwater fish in tidal habitats and highlights the need to effectively manage such species.     

Geochemical and mineralogical evidence for Sahara and Sahel dust additions to Quaternary soils on Lanzarote,eastern Canary Islands,Spain

Africa is the most important source of dust in the world today, and dust storms are frequent on the nearby Canary Islands. Previous workers have inferred that the Sahara is the most important source of dust to Canary Islands soils, with little contribution from the Sahel region. Soils overlying a late Quaternary basalt flow on Lanzarote, Canary Islands, contain, in addition to volcanic minerals, quartz and mica, exotic to the island’s bedrock. Kaolinite in the soils also likely has an exotic origin. Trace‐element geochemistry shows that the soils are derived from varying proportions of locally derived basalt and African dust. Major‐element geochemistry, clay mineralogy and interpretation of satellite imagery suggest that dust additions to the Canary Islands come not only from the Sahara Desert, but also from the Sahel region.     

Tourmaline mineralization in the Barberton greenstone belt,South Africa: early Archean metasomatism by evaporite-derived boron

Tourmaline-rich rocks are common in the lowgrade, interior portions of the Barberton greenstone belt of South Africa, where shallow-marine sediments and underlying altered basaltic and komatiitic lavas contain up to 50% tourmaline. The presence of tourmaline-bearing rip-up clasts, intraformational tourmalinite pebbles, and tourmaline-coated grains indicates that boron mineralization was a low-temperature, surficial process. The association of these lithologies with stromatolites, evaporites, and shallow-water sedimentary structures and the virtual absence of tourmaline in correlative deep-water facies rocks in the greenstone bels strengthens this model.Five tourmaline-bearing lithologic groups (basalts, komatiites, evaporite-bearing sediments, stromatolitic sediments, and quartz veins) are distinguished based on field, petrographic, and geochemical criteria. Individual tourmaline crystals within these lithologies show internal chemical and textural variations that reflect continued growth through intervals of change in bulk-rock and fluid composition accompanying one or more metasomatic events. Large single-crystal variations exist in Fe/Mg, Al/Fe, and alkali-site vacancies. A wide range in tourmaline composition exists in rocks altered from similar protoliths, but tourmalines in sediments and lavas have similar compositional variations. Boron-isotope analysis of the tourmalines suggest that the boron enrichment in these rocks has a major marine evaporitic component. Sediments with gypsum pseudomorphs and lavas altered at low temperatures by shallow-level brines have the highest ¹¹B values (+2.2 to-1.9); lower ¹¹B values of late quartz veins (-3.7 to-5.7) reflect intermediate temperature, hydrothermal remobilization of evaporitic boron. The ¹¹B values of tourmaline-rich stromatolitic sediments (-9.8 and-10.5) are consistent with two-stage boron enrichment, in which earlier marine evaporitic boron was hydrothermally remobilized and vented in shallow-marine or subaerial sites, mineralizing algal stromatolites. The stromatolite-forming algae preferentially may have lived near the sites of hydrothermal discharge in Archean times.     

Similarity forms for ground-source surface-layer diffusion

Past work on analyzing ground-source diffusion data in terms of surface-layer similarity theory is reviewed; these analyses assume that _z /L orh/L is a function of u ^* x/L (where h = Q/ dy). It is argued that an alternative scaling, h ^*/L versus x/L, is nearly as universal in that it is very weakly influenced by surface roughness, except for a modest influence in the free convective case (h ^* = Q/u ^* dy); the advantage of this scaling is that it eliminates the need to reassess as vertical diffusion progresses. The Prairie Grass data set is adjusted for the difference in source and sampling heights, and is plotted with this scaling. Simple analytic equations are suggested that fit the resultant data plots for stable and unstable conditions, and suggestions are made towards practical application of these results.On assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.