Coexisting Amphiboles

Chemical analyses for the following amphibole pairs are presented:anthophyllite—tremolite (or actinolite, or hornblende),cummingtonite (or grunerite)–actinolite (or hornblende),cummingtonite (or grunerite)–anthophyllite (or gedrite),and manganoan cummingtonitemagnesioriebeckite. Nineteen analyses of such pairs are quoted from the literature,and thirty-seven additional pairs have been newly analyzed byelectron probe techniques. Quantitative microprobe determinationsof Si, Al, Fe, Mn, Mg, Ca, and Na were made on polished thin-sections,using naturally occurring, analyzed, homogeneous amphibolesas standards. The literature analyses and the electron probeanalyses for metamorphic, two-amphibole assemblages are givenfor amphiboles in physical contact, which show no textural evidenceof one amphibole being a reaction or alteration product of theother. The chemical data for some of the volcanic, two—amphiboleassemblages were obtained from occurrences that probably donot represent equilibrium pairs. The chemical data are used to determine the extent of the miscibilitygaps between the various amphibole series and the fractionationof the major elements between the two amphiboles of a pair.Anthophyllite and members of the cummingtonite-grunerite seriesgenerally have a larger Fe(total)/Mg ratio than the coexistingcalcic amphibole. The maximum CaO, Al₂O₃and Na₂O contents ofcummingtonite in metamorphic cummingtonite—hornblendepairs are 19 and 32, 02 weight per cent, respectively. Themaximum CaO, A1₂O₃, and Na₂O contents of cummingtonite in metamorphiccummingtonite-hornblende pairs are 19, 32, and 02 weightper cent, respectively. Larger CaO and Al₂O₃ values reportedin the literature were found to be too high because of admixtureof actinolite or hornblende in the analyzed separates. Smallamounts of MnO tend to concentrate preferentially in anthophylliteor cummingtonite of anthophyllite-hornblende and cummingtonite-hornblendepairs. Anthophyllite-cummingtonite pairs may show very similarFe(total)/Mg ratios and differ slightly in Al₂O₃ content only.     

Chemical Petrology of Grenville Schists near Fernleigh, Ontario

A suite of mica schists from the staurolite zone was studiedin detail. Phase rule considerations and distribution relationsindicate that chemical equilibrium was attained within the samplevolume. Iron-magnesium ratios of the silicates vary greatly,and correlate with rock ferrous-ferric ratio, as does the oxidemineral mode. Rock oxidation state varies locally, and is probablydependent on the composition of the original sediment. Distribution coefficients for Fe, Mg, and Mn among garnet, biotote,and staurolite show no vaiation attributable to temperature.Partition of Fe and Mg between staurolite and biotite is regular,but non-ideal. The staurolite structure permits only limited(15–35 percent) substitution of Mg for Fe.     

Potential microbial bioinvasions via ships' ballast water, sediment, and biofilm

A prominent vector of aquatic invasive species to coastal regions is the discharge of water, sediments, and biofilm from ships' ballast-water tanks. During eight years of studying ships arriving to the lower Chesapeake Bay, we developed an understanding of the mechanisms by which invasive microorganisms might arrive to the region via ships. Within a given ship, habitats included ballast water, unpumpable water and sediment (collectively known as residuals), and biofilms formed on internal surfaces of ballast-water tanks. We sampled 69 vessels arriving from foreign and domestic ports, largely from Western Europe, the Mediterranean region, and the US East and Gulf coasts. All habitats contained bacteria and viruses. By extrapolating the measured concentration of a microbial metric to the estimated volume of ballast water, biofilm, or residual sediment and water within an average vessel, we calculated the potential total number of microorganisms contained by each habitat, thus creating a hierarchy of risk of delivery. The estimated concentration of microorganisms was greatest in ballast water>sediment and water residuals>biofilms. From these results, it is clear microorganisms may be transported within ships in a variety of ways. Using temperature tolerance as a measure of survivability and the temperature difference between ballast-water samples and the water into which the ballast water was discharged, we estimated 56% of microorganisms could survive in the lower Bay. Extrapolated delivery and survival of microorganisms to the Port of Hampton Roads in lower Chesapeake Bay shows on the order of 10(20) microorganisms (6.8 x 10(19) viruses and 3.9 x 10(18) bacteria cells) are discharged annually to the region.     

Oxygen-isotope record of Late-Glacial climatic change in western Ireland

Oxygen-isotope profiles for the Late-Glacial carbonate sediments from Red Bog and adjacent Lough Gur in County Limerick in western Ireland are readily correlated with the classical hiozones delineated on pollen diagrams for the same cores. The estimated summer temperatures of the Bølling/Allerød were as high as those in the early Holocene and are correlated with increasing Milankovitch summer insolation. This warm phase was abruptly terminated in the Younger Dryas cold episode by a depletion of 4% in δ¹⁸O, suggesting a summer atmospheric temperature decrease of about 12°C, comparable to that inferred from fossil beetle data. The Younger Dryas phase is attributed to a major cooling of the sea-surface temperature by a postulated discharge of icebergs similar to that of the Heinrich events, for the icebergs were much more effective than simple meltwater in cooling the sea surface and thus the climate over Europe. Shorter-term cool phases (Older Dryas. Gerzensee. Preboreal oscillation) are also recognized.     

Multi-technique approach to characterise the effects of cryopreservation on larval development of the Pacific oyster (Crassostrea gigas)

Cryopreservation experiments were conducted on D-stage larvae of the Pacific oyster (Crassostrea gigas) to investigate the effects of two cryoprotectant solutions and three cooling rates on larval development from 1 to 22 days post-fertilisation. Cryoprotectant solutions were made up to final concentrations (after 1:1 dilution with larvae) of 10% ethylene glycol, 1% polyvinylpyrrolidone and either 0.2 or 0.4 M trehalose. Three cooling rates (0.5, 1 and 2 °C min^?1 between ?10 and ?35 °C post-holding) were tested in an orthogonal design with the two cryoprotectants. Results indicate that control larvae out-performed all cryopreservation treatments for survival, feeding consumption and shell length parameters. However, larvae exposed to 0.4 M trehalose did considerably better than those exposed to 0.2 M trehalose, regardless of cooling rate conditions. Scanning electron and light microscopy observations were used to assess larval morphology and organogenesis, indicating that treatments with surviving larvae were morphologically and developmentally similar to control larvae.     

Ordinary (mesostasis) and not‐so‐ordinary (symplectites) late‐stage assemblages in howardites

Abstract– Two categories of symplectites have been observed in howardites: three‐phase, composed of vermicular intergrowths of ferroan augite, fayalitic olivine, and silica, and two‐phase, composed of vermicular intergrowths of orthopyroxene and troilite. Three‐phase symplectites have been previously shown to represent the breakdown products of metastable pyroxene. In howardites, they appear to be genetically related to gabbroic eucrites. In some cases and under yet‐to‐be specified conditions, ferroan clinopyroxene in gabbroic eucrites may undergo only localized decomposition resulting in oriented exsolution‐like features. Breakdown phases in those cases are fayalitic olivine, silica, and—depending on the MgO content of the system—orthopyroxene. As opposed to three‐phase symplectites, two‐phase symplectites are most likely of diogenitic origin. They probably formed via impact‐induced localized melting of diogenitic orthopyroxene in the presence of troilite (grain boundary melting). Three‐phase symplectites in howardites occasionally contain accessory amounts of ilmenite, troilite, and/or kamacite and are exclusively associated with medium‐grained FeO‐rich pyroxene, silica, and plagioclase. All minerals involved are late‐stage crystallites or mesostasis phases. In general, highly evolved eucritic lithologies constitute only a minor fraction of howardites. However, considering that three‐phase symplectites are generated in a low‐pressure, i.e., near‐surface, environment, FeO‐ and CaO‐rich eucritic rocks may be exposed locally on Vesta’s surface. This, in turn, is highly relevant to the ongoing DAWN mission.     

High fO2 During Sillimanite Zone Metamorphism of Part of the Barrovian Type Locality, Glen Clova, Scotland

The redox state of sillimanite zone (650–700°C, 5–6kbar) metasediments of the Barrovian type area, Scotland, wasinvestigated using estimates of metamorphic oxygen fugacity(f_O2), sulfur fugacity (f_S2), and fluid chemistry based on newdeterminations of mineral and rock compositions from 33 samples.A total of 94% of the samples lack graphite, contain both ilmenite–hematitesolid solutions (RHOMOX) and magnetite, and had metamorphicf_O2 about 2 log₁₀ units above the quartz–fayalite–magnetite(QFM) buffer. The regional variation in metamorphic f_O2 forthese rocks was minimal, about ±0·3 log₁₀ units,reflecting either a protolith that was homogeneous with respectto redox state, or an initially variable protolith whose redoxstate was homogenized by metamorphic fluid–rock interaction.RHOMOX inclusions in garnet porphyroblasts that become richerin ilmenite from the interior to the edge of the host porphyroblastsuggest that at least some syn-metamorphic reduction of rockoccurred. Significant variations in bulk-rock oxidation ratio(OR) that are probably inherited from sedimentary protolithsare found from one layer to the next; OR ranges mostly between     

The Two-Phase Region and Excess Mixing Properties of Paragonite-Muscovite Crystalline Solutions

The chemical compositions of synthetic paragonite-muscovitepairs were obtained by electron probe microanalysis of run productsprepared hydrothermally at 300, 400, 500, and 600 ?C and 2.07kbar. The microcrystalline run products were dispersed on polishedberyllium rods, and Na, K, and Si were determined simultaneously.Compositions were determined from K/Si and Na/Si ratios referredto standards. Tentative excess thermodynamic mixing properties of paragonite-muscovitecrystalline solutions, based on the two-phase composition dataand on X-ray diffraction data, are represented by the followingthird-order Margules formulation: which leads to a critical temperature of 833 ?C and a criticalcomposition of 39.0 mole per cent Mu at 2.07 kbar (the criticalphase is probably metastable with respect to alkali feldsparand corundum). These results are quantitatively in agreementwith Iiyama's (1964) ion-exchange data. From relationships amongthe quantities (arctanh s)/s, In r², and 1/T we obtain criticalconditions which are in good agreement with the above (T_c =829 ?C, N_2c = 0.396). The critical curve obtained from the aboveMargules parameters is given by: T_c(?C) = 768.8 + 31.00P(kbar). The above results are complicated by polymorphism and by possiblelack of complete equilibrium between the two-mica synthesisproducts and possible substitution of hydronium for alkalies.We emphasize, therefore, that the phase diagrams and derivedmixing properties should be applied with caution to naturalmuscovite- and paragonite-bearing assemblages.