High pressure metamorphism in the Scourian of NW Scotland: Evidence from garnet granulites

Ultramafic and mafic granulites from Archaean gneisses in N.W. Scotland (the Scourian) show evidence of two periods of granulite facies mineral growth. The first produced a high pressure clinopyroxene +garnet±plagioclase assemblage at an estimatedP-T of 12–15 kb and 1,000° C. Uplift of the complex caused partial breakdown of the garnet by reaction with clinopyroxene to produce orthopyroxene +plagioclase ±spinel±amphibole symplectites, at an estimatedP-T of 10–14 kb and 800°–900° C. Garnet stability is shown to depend on both whole-rock Fe/Mg ratios and onP-T conditions. The pressures imply crustal thicknesses in the Archaean of least 35–45 km.     

Extinct I129 in C3 chondrites

Eight C3 chondrites were examined by the I¹²⁹Xe¹²⁹ dating method, to see whether their IXe “ages” (better, initial $\text{I}{}^{129}\text{I}{}^{127}$ratios ≡ R₀) correlate with any other properties. The R₀'s range from 1.60 × 10^?4 to 1.09 × 10^?4, corresponding to IXe ages from 2.0 Myr before to 6.7 Myr after Murchison magnetite. Three C3O's (Lancé, Felix, Ornans) have essentially indistinguishable R₀'s of (1.41 ± 0.13) to (1.17 ± 0.10) × 10^?4; the fourth C3O, Warrenton, is undatable owing to homogenization of radiogenic and trapped Xe.Four C3V's show a distinct spread: Vigarano and Grosnaja are highest [R₀ = (1.60 ± 0.07) and (1.57 ± 0.14) × 10^?4], Mokoia is intermediate, and Kaba is lowest [R₀ = (1.38 ± 0.06) and (1.09 ± 0.10) × 10^?4]. Literature values for Allende place it near Kaba. These R₀'s correlate inversely with 4 other properties: I-, Br-, and Cd-content, and olivine composition, both percent mean deviation (PMD) and proportion of iron-poor olivine grains (≤2% fayalite).It is difficult to accept the ～9 Myr spread in R₀ as a true age, reflecting either nebular or parent-body processes. This time span is more than an order of magnitude longer than the lifetime of the solar nebula inferred from astronomical evidence. Nor does the degree of thermal metamorphism, which is slight for C3's anyway, correlate with R₀. A more plausible interpretation is that the variations in R₀ reflect mainly isotopic heterogeneity of iodine. The simplest model that accounts for the correlations with R₀ involves mixing of two iodine components in the solar nebula, associated with gas and grains, respectively. The second, of lower $\text{I}{}^{129}\text{I}{}^{127}$ ratio, predominated at later times and thus became enriched in late-formed meteorites, along with other volatiles such as Cd and Br. The low Fe content and large PMD of olivine may reflect either less metamorphism owing to shallow location in the parent body, or greater reduction of Fe²⁺ during chondrule formation.     

Quantitative P-T paths from zoned minerals: Theory and tectonic applications

An analytical approach to the analysis of zoning profiles in minerals is presented that simultaneously accounts for all of the possible continuous reactions that may be operative in a given assemblage. The method involves deriving a system of simultaneous linear differential equations consisting of a Gibbs-Duhem equation for each phase, a set of linearly independent stoichiometric relations among the chemical potentials of phase components in the assemblage, and a set of equations describing the total differential of the slope of the tangent plane to the Gibbs free energy surface of solid solution phases. The variables are the differentials of T, P, chemical potentials of all phase components, and independent compositional terms of solid solution phases. The required input data are entropies, volumes, the compositions of coexisting phases at a reference P and T, and an expression for the curvature of the Gibbs functions for solid solution phases. Results derived are slopes of isopleths (dP/dT, dX/dT or dX/dP) which can be used to contour P-T diagrams with mineral composition.To interpret mineral zoning, T and P can be expressed as functions of n independent composition parameters, where n is the variance of the mineral assemblage. The total differentials of P and T are differential equations that can be solved by finite difference techniques using the derivatives obtained from the analytical formulation of phase equilibria.Results calculated from Zone I and Zone IV garnets of Tracy et al. (1976) indicate that Zone I garnets grew while T increased (T+72° C) and P decreased sharply (P–3 kb). Zone IV garnets zoned in response to decreasing T (T–17° C) and P (P–1 kb). A P-T path calculated for a zoned garnet from the Greinerschiefer series, western Tauern Window, Austria, also indicates growth during decompression (–3kb) and heating (T+15° C). A P-T path calculated for the Wissahickon schist (Crawford and Mark 1982) indicates growth during cooling and compression (T–25 C, P+2.2 kb). The calculated P-T paths differ according to structural environment and can be used to relate mineral growth to tectonic processes.     

Factors controlling net ecosystem metabolism in U.S. estuaries

High frequency dissolved oxygen data were analyzed to calculate primary production, respiration and net ecosystem metabolism (NEM) from 42 sites within 22 National Estuarine Research Reserves (NERR), 1995–2000. NERR sites are characterized by a variety of dominant plant communities including phytoplankton, salt marsh, seagrass, macroalgae, freshwater macrophyte, and mangrove, and are representative of the coastal bioregions of the United States. As expected from the wide diversity of sites, metabolic rates were temporally and spatially variable with the highest production and respiration occurring during the summer in Southeastern estuaries. Sites within different regions exhibited consistent seasonal trends in production, respiration, and NEM. Temperature was the most important environmental factor explaining within-site variation in metabolic rates; nutrient concentrations were the second most important factor. All but three of the 42 sites were heterotrophic (respiration was greater than production) on an annual basis. Habitat adjacent to the monitoring site, estuarine area, and salinity explained 58% of the variation in NEM. Open water sites or sites adjacent to mangroves or in marsh creeks were heterotrophic, while sites in or adjacent to submerged aquatic vegetation (eelgrass or macroalgal beds) were either autotrophic or near balance. Estuarine area was also a significant factor explaining variability in NEM; larger systems were closer to balance than smaller systems that trended toward heterotrophy. Freshwater sites were more heterotrophic than saline sites. Nutrient loading explained 68% of the variation in NEM among some of the sites. When these estimates were compared to the literature, metabolic rates from the NERR sites were much larger, often two to five times greater than rates from other estuarine and coastal systems. One explanation is that these small, generally shallow sites located near shore may have greater allochthonous organic inputs as well as significant benthic primary production than the large, deeper systems represented by the literature.     

Extent, timing, and climatic implications of glacier advances Mount Rainier, Washington, U.S.A., at the pleistocene/holocene transition

The North Atlantic Younger Dryas climatic reversal did not cause a glacier advance on Mount Rainier. The glaciers on Mount Rainier seem to have advanced in response to regional or local shifts in climate. However, the Younger Dryas climatic reversal may have affected the Mount Rainier area, causing a cold, but dry, climate unfavorable to glacier advances. Glaciers in the vicinity of Mount Rainier advanced twice during late glacial/early Holocene time. Radiocarbon dates obtained from lake sediments adjacent to the corresponding moraines are concordant, indicating that the ages for the advances are closely limiting. The first advance occurred before 11,300 ¹⁴C yr BP (13,200 cal yr BP). During the North Atlantic Younger Dryas event, between 11,000 and 10,000 ¹⁴C yr BP (12,900 and 11,600 cal yr BP), glaciers retreated on Mount Rainier, probably due to a lack of available moisture, but conditions may have remained cold. The onset of warmer conditions on Mount Rainier occurred around 10,000 ¹⁴C yr BP (11,600 cal yr BP). Organic sedimentation lasted for at least 700 years before glaciers readvanced between 9800 and 8950 ¹⁴C yr BP (10,900 and 9950 cal yr BP).     

Benthic metabolism and nutrient cycling in Boston Harbor, Massachusetts

To gain insight into the importance of the benthos in carbon and nutrient budgets of Boston Harbor and surrounding bays, we measured sediment-water exchanges of oxygen, total carbon dioxide (DIC), nitrogen (ammonium, nitrate+nitrite, urea, N₂O), silicate, and phosphorus at several stations in different sedimentary environments just prior to and subsequent to cessation of sewage sludge disposal in the harbor. The ratio of the average annual DIC release to O₂ uptake at three primary stations ranged from 0.84 to 1.99. Annual average DIC:DIN flux ratios were consistently greater than predicted from the Redfield ratio, suggesting substantial losses of mineralized N. The pattern was less clear for P: some stations showed evidence that the sediments were a sink for P while others appeared to be a net source to the water column over the study period. In general, temporal and spatial patterns of respiration, nutrient fluxes, and flux ratios were not consistently related to measures of sediment oxidation-reduction status such as Eh or dissolved sulfide. Sediments from Boston Harbor metabolize a relatively high percentage (46%) of the organic matter inputs from phytoplankton production and allochthonous inputs when compared to most estuarine systems. Nutrient regeneration from the benthos is equivalent to 40% of the N, 29% of the P, and more than 60% of the Si demand of the phytoplankton. However, the role of the benthos in supporting primary production at the present time may be minor as nutrient inputs from sewage and other sources exceed benthic fluxes of N and P by 10-fold and Si by 4-fold. Our estimates of denitrification from DIC:DIN fluxes suggests that about 45% of the N mineralized in the sediments is denitrified, which accounts for about 17% of the N inputs from land.     

Spatial and seasonal patterns in sediment nitrogen remineralization and ammonium concentrations in San Francisco Bay,California

Nitrogen remineralization and extractable ammonium concentrations were measured in sediments from several locations in North and South San Francisco bays. In South Bay, remineralization rates decreased with depth in sediment and were highest in the spring following the seasonal phytoplankton bloom. At the channel stations, peak remineralization lagged peak water-column phytoplankton biomass (as measured by chlorophylla) by a month. Remineralization rates were generally higher in South Bay than North Bay. The lower remineralization rates in North Bay may be a result of anomalously low phytoplankton production and thus reduced deposition to the sediments, as well as low reiverine organic inputs to the upper estuary in recent years. Remineralization rates were positively correlated to carbon and nitrogen content of the sediments. In general, ammonium profiles in South Bay sediments showed no increase in deeper (4–8 cm) sediments. In North Bay, ammonium concentrations were greatest at stations with highest remineralization rates, and, in contrast to South Bay, extractable ammonium increased in deeper sediment. Differences in ammonium pools between North Bay and South Bay may be a result of increased irrigation by deep-dwelling macrofauna, which are more abundant in South Bay.