Prograde and retrograde equilibria in garnet-cordierite gneisses in south-central Massachusetts

Partial electron microprobe analyses of garnet, biotite and cordierite in sillimanite-K feldspar gneisses of the Brimfield Formation in south-central Massachusetts indicate that the compositions of these minerals are not constant in a thin section. The FeO/MgO mol ratio of biotite is sensitive to the nature of other FeO-MgO minerals occurring in close proximity. The most iron-rich biotites are those that do not contact either cordierite or garnet. The most iron-poor biotites occur as inclusions in garnet. Biotites in direct contact with either cordierite or garnet have intermediate FeO/MgO ratios. The bulk of a given grain of garnet or cordierite is homogeneous in composition. Chemical zoning is absent. All grains of garnet and cordierite in a thin section are constant in composition. However, where garnet and cordierite abut biotite, the FeO/MgO ratio of the garnet rim is increased and that of cordierite is decreased. The FeO/MgO ratios of garnet, cordierite and biotite bare a regular relation to each other indicating a possible equilibrium state. However the distribution coefficient defined by the compositions of minerals in direct contact are greater than those defined by the compositions of the interiors of garnet and cordierite matched with the compositions of biotites removed from these phases. This pattern is believed to be the result of two thermal events. The first event produced the mineral assemblages and widespread equilibrium was obtained. A subsequent retrograde event left the mineralogy intact but caused cation exchange reactions at immediate contacts between garnet, cordierite and biotite. The physical conditions of the first event are estimated at P=5–6 kb, T=700–750° C. The retrograde event occurred at lower temperatures and very low activities of H₂O since no muscovite is developed at microcline-sillimanite contacts.     

The variation of tetrahedral bond lengths in sodic plagioclase feldspars

Multiple linear regression analysis has been applied to the geometric and chemical variables in sodic plagioclases in order to determine their relative effects on individual T-O bond lengths in the Al_1+xSi_3?xO₈ tetrahedral framework. Using data from crystal structure analyses of low and high albite, An₁₆ and An₂₈, and assuming that low albite is completely ordered, 1 $$\begin{gathered} {\text{T}} - {\text{O = 1}}{\text{.568}} + {\text{[(0}}{\text{.122) x (Al content of the T site)]}} \hfill \\ {\text{ }} - {\text{[(0}}{\text{.037) x (}}\Delta {\text{{\rm A}l}}_{{\text{br}}} )] + [0.063){\text{ x }}(\Sigma {\text{[}}q{\text{/(Na,Ca}} - {\text{O)}}^{\text{2}} ])] \hfill \\ {\text{ }} + {\text{[(0}}{\text{.029) x (}} - {\text{1/cosT}} - {\text{O}} - {\text{T)]}} \hfill \\ \end{gathered}$$ where the Al content of a particular tetrahedral (T) site can be estimated from empirically-derived determinative curves, where Δ Al_br is a linkage factor to account for the Al content of adjacent tetrahedral sites, where the formal charge on the (Na_1?xCa_x) atom is q=1+x, and where T-O-T is the inter-tetrahedral angle involving the T-O bond. For sodic plagioclases it is essential to know only the anorthite content and the 2Θ₁₃₁-2Θ_1¯31 spacing (CuK _α radiation) in order to determine the independent variables in this equation and thus to evaluate the individual T-O distances. The 64 individual T-O distances predicted for the four sodic plagioclases by this equation agree well with the observed T-O bond lengths (σ=0.004 Å; r=0.994), and the method has been used by way of example to rationalize the T-O bond lengths in analcime (cf. Ferraris, Jones and Yerkess, 1972).     

Complex formation in the ternary system Mg(II)-CO2-H2O

The carbonato and hydrogencarbonato complexes of Mg²⁺ were investigated at 25 and 50° in solutions of the constant ClO₄^? molality (3 M) consisting preponderantly of NaClO₄. The experimental data could be explained assuming the following equilibria: Mg²⁺ + CO_2B + H₂O ag MgHCO⁺₃ + H⁺, $\text{log}{}^1\text{β}{}_1\text{= ?7.64}{}_4\text{± 0.01}{}_7\text{(25°), ?7.46}{}_2\text{± 0.01}{}_1\text{(50°)}$, Mg²⁺ + 2 CO_2g + 2 H₂Oag Mg(HCO₃)⁰₂ ± 2 H⁺, $\text{log}{}^1\text{β}{}_2\text{= ?15.00 ± 0.14 (25°)}$, ?15.37 ± 0.39 (50°), Mg²⁺ + CO_2g + H₂Oag MgCO⁰₃ + 2 H⁺, $\text{log}{}^1\text{k}{}_1\text{= ?15.64 ± 0.06 (25°)}$,?15.23 ± 0.02 (50°), with the assumption γ_MgCO3⁰ = γ_Mg(HCO3)⁰2, ΔG⁰(I = 0) for the reaction MgCO⁰₃ + CO_2g + H₂O = Mg(HCO₃)⁰₂ was estimated to be ?3.9₁ ± 0.8₆ and 0.6 ± 2.4 kJ/mol at 25 and 50°C, respectively. The abundance of carbonate linked Mg(II) species in fresh water systems is discussed.     

Magma fractionation systems beneath the mid-Atlantic ridge at 36–37 ° N

Variation of major and trace elements in drilled basalts from the Mid-Atlantic Ridge (DSDP Leg 37) reflects distinct cycles of low pressure fractionation operating independently within a complex network of magma storage reservoirs beneath the crustal spreading axis. Low pressure phase relations are determined by parental magma composition, which varies from An-rich (An/Di > ca. 1.4) to Di-rich (An/Di < ca. 1.4). High An/Di magmas probably formed under slightly hydrous conditions in the mantle. They have low LIL element contents, low P/Y and high Mg/(Mg + Fe) ratios. Zr, P and Y abundance and inter-element ratios are highly diagnostic of primary magma type, and are used to quantify fractional crystallization models.Low pressure fractionation hypotheses were tested by least-squares modelling of whole-rock and phenocryst chemistry, which indicated removal or addition of phenocryst assemblages: ol; pl; ol + pl; ol + pl + cpx; pl + cpx, (± sp). Accumulation of plagioclase or olivine is an important mechanism for generating highly porphyritic rocks. A rare 3-phase (ol + pl + cpx) cumulate resulted from cotectic fractionation of a low An/Di magma type. Olivine and plagioclase cumulates appear to be related to high An/Di magmas. Olivine accumulation has been monitored by comparison of olivine/bulk rock partitioning of Fe and Mg to experimental measurements of the equilibrium K_D value. A single extensive sub-axial magma chamber could not account for the observed chemical variation and would probably be dynamically unstable.     

Rank of coal beds of the Narragansett basin,Massachusetts and Rhode Island

Coal of the Narragansett basin generally has been considered to be anthracite and/or meta-anthracite. However, no single reliable method has been used to distinguish these two ranks in this basin. Three methods — chemical, X-ray, and petrographic — have been used with some degree of success on coal of the Narragansett basin, but too often the results are in conflict. Chemical methods have been limited by inadequate sampling on a coal-bed-by-coal-bed basis and by a lack of analyses made according to (American Society for Testing and Materials, 1974) standard specifications.In addition, when corrections are made by using the Parr formulas, as required by the ASTM (1974) procedures, the generally high to very high ash content of coal from the Narragansett basin causes the fixed-carbon content to appear higher than it actually is. X-ray methods using the degree of graphitization as a measure of rank are not reliable because some of the graphite is related to shearing and brecciation associated with folding and faulting. Petrographic methods using reflectance on vitrinite give results that are generally consistent with results from chemical determinations. However, it is not clear whether the mean maximum reflectance or mean bireflectance is a better indicator of similar rank of such high-rank coals that have been structurally deformed.Coal from the Cranston Mine, RI, is probably meta-anthracite and coal from the Portsmouth Mine is probably anthracite. These ranks are based on chemical,X-ray, and petrographic data and are supported by associated metamorphic mineral assemblages that indicate that the Cranston Mine is in a higher metamorphic zone than the zone containing the Porthmouth Mine. Interpretation of the rank of Mansfield, MA, coal on the basis of extant chemical data is difficult because it is an impure coal with an ash content of 33 to 50%. Reflectance data indicate that the Mansfield, Foxborough, and Plainville coals in the northern part of the Narragansett basin are meta-anthracite but this is in disagreement with the rank suggested by the low degree of metamorphism of the associated rocks.     

Gulf of Mexico hydrocarbon seep communities—IX. Sterol biosynthesis of seep mussels and its implications for host-symbiont association

The sterol analysis of six hydrocarbon seep mussels (mytilid II and mytilid Ib) from the Alaminos Canyon in the Gulf of Mexico is reported. The sterol composition of the mussel-symbiotic bacteria complex reflects a preponderant synthesis of 4α-methyl sterols (seep mytilid II), and a predominant biosynthesis of 4-desmethyl sterols with some amounts of 4α-methyl sterols (seep mytilid Ib). This suggests a methane-based symbiotic relationship between the mussels and methanotrophic bacteria. It also suggests that the biosynthesis of sterols in the mussel-bacteria complex is completed to the level of cholest-5-en-3ß-ol (mytilid II) or 5α-cholestan-3ß-ol (mytilid Ib).     

Using data from natural environments to improve models of uranium speciation in groundwaters

It is essential that computer-based models used in the safety assessment of radioactive waste repositories accurately represent the processes occurring in real field systems. Confidence in long-term predictions of radionuclide migration will then depend upon the completeness of data available, particularly those obtained from the disposal site, and correct implementation of the model. The study of natural geochemical systems provides information on the adequacy of the underlying “generic” database and enhances our understanding of the transport mechanisms which form the basis of performance assessment. This paper concentrates on speciation-solubility modelling and describes four natural occurrences of uranium, each of which displays a different facet of uranium migration behaviour. The attributes of each site and the means by which uranium is immobilised are described. Retardation is highly species specific and this is illustrated through the use of site data in equilibrium speciation and coupled chemical transport calculations. Oxidation of U(IV) to U(VI) species promotes leaching of uranium ore at all the locations studied, emphasising the need to ensure that reducing conditions persist in a repository dominated by its actinide inventory.     

A 50-year record of NO<Subscript>x</Subscript> and SO<Subscript>2</Subscript> sources in precipitation in the Northern Rocky Mountains,USA

Ice-core samples from Upper Fremont Glacier (UFG), Wyoming, were used as proxy records for the chemical composition of atmospheric deposition. Results of analysis of the ice-core samples for stable isotopes of nitrogen (δ¹⁵N, ) and sulfur (δ³⁴S, ), as well as and deposition rates from the late-1940s thru the early-1990s, were used to enhance and extend existing National Atmospheric Deposition Program/National Trends Network (NADP/NTN) data in western Wyoming. The most enriched δ³⁴S value in the UFG ice-core samples coincided with snow deposited during the 1980 eruption of Mt. St. Helens, Washington. The remaining δ³⁴S values were similar to the isotopic composition of coal from southern Wyoming. The δ¹⁵N values in ice-core samples representing a similar period of snow deposition were negative, ranging from -5.9 to -3.2 ‰ and all fall within the δ¹⁵N values expected from vehicle emissions. Ice-core nitrate and sulfate deposition data reflect the sharply increasing U.S. emissions data from 1950 to the mid-1970s.     

New rebbachisaurid (Dinosauria: Sauropoda) material from the Wessex Formation (Barremian, Early Cretaceous), Isle of Wight, United Kingdom

Rebbachisauridae is a poorly understood clade of diplodocoid sauropod dinosaurs, currently known only from the Cretaceous of Africa, Europe and South America. European representatives are particularly rare and fragmentary. Here, we report an anterior caudal vertebra from the Barremian (Early Cretaceous) Wessex Formation of the Isle of Wight, off the southern coast of England. This specimen possesses several features known only in rebbachisaurids and shares two synapomorphies with the Afro-European taxa Demandasaurus darwini and Nigersaurus taqueti, both pertaining to the morphology of the neural spine. These features are the development of triangular lateral processes and the presence of an elliptical fossa on the lateral surface, bounded by the lateral lamina and postspinal rugosity. The Isle of Wight specimen also shares several features solely with Demandasaurus, indicating a close relationship with the Spanish taxon. These include the presence of a hyposphenal ridge, as well as an anteriorly excavated caudal rib that is restricted almost entirely to the neural arch. However, it differs from Demandasaurus in a number of ways, including the lack of excavation on the posterior surface of the caudal rib, the orientation of the neural spine, and the composition and morphology of the lateral lamina. In addition, the Isle of Wight vertebra possesses one potential autapomorphy: bifurcation of the elliptical fossa on the neural spine. However, because of the fragmentary nature of the material, a new name is not erected. Along with Demandasaurus and Histriasaurus boscarollii, this caudal vertebra indicates the presence of at least three European rebbachisaurid taxa and provides new anatomical information on this enigmatic clade of sauropod dinosaurs.