The origin of garnet in the anorthosite-charnockite suite of the Adirondacks

Detailed analysis of textural and chemical criteria in rocks of the anorthosite-charnockite suite of the Adirondack Highlands suggests that development of garnet in silica-saturated rocks of the suite occurs according to the reaction: $$\begin{gathered} {\text{Anorthite}} {\text{Orthopyroxene}} {\text{Quartz}} \hfill \\ {\text{2CaAl}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{8}} + (6 - \alpha )({\text{Fe,Mg}}){\text{SiO}}_{\text{3}} + \alpha {\text{Fe - Oxide + (}}\alpha {\text{ - 2)SiO}}_{\text{2}} \hfill \\ {\text{Garnet}} {\text{Clinopyroxene}} \hfill \\ = {\text{Ca(Fe,Mg)}}_{\text{5}} {\text{Al}}_{\text{4}} {\text{Si}}_{\text{6}} {\text{O}}_{{\text{24}}} + {\text{Ca(Fe,Mg)Si}}_{\text{2}} {\text{O}}_{\text{6}} \hfill \\ \end{gathered} $$ , where α is a function of the distribution of Fe and Mg between the several coexisting ferromagnesian phases. Depending upon the relative amounts of Fe and Mg present, quartz may be either a reactant or a product. Using an aluminum-fixed reference frame, this reaction can be restated in terms of a set of balanced partial reactions describing the processes occurring in spatially separated domains within the rock. The fact that garnet invariably replaces plagioclase as opposed to the other reactant phases indicates that the aluminum-fixed model is valid as a first approximation. This reaction is univariant and produces unzoned garnet. It differs from a similar equation proposed by de Waard (1965) for the origin of garnet in Adirondack metabasic rocks, i.e. 6 Orthopyroxene+2 Anorthite = Clinopyroxene+Garnet+2 Quartz, the principle difference being that iron oxides (ilmenite and/or magnetite) are essential reactant phases in the present reactions. The product assemblage (garnet+clinopyroxene+plagioclase ± orthopyroxene ± quartz) is characteristic of the clinopyroxene-almandine subfacies of the granulite facies.     

Mineralogical constraints on the petrogenesis of trachytic inclusions,Carpenter Ridge Tuff,Central San Juan volcanic field,Colorado

Although bulk-rock normative analyses of the trachytic inclusions from the Carpenter Ridge Tuff yield abundant quartz and minor corundum, a portion of the phenocryst assemblage is indicative of an alkaline parentage. Sanidine and biotite contain up to 8 and 5 wt% BaO respectively. In addition, both amphibole and clinopyroxene compositions are compatible with having crystallized from a mildly silica-undersaturated magma. Amphibole is magnesiohastingsite with 3 wt% TiO₂ and less than 0.3 mole fraction vacancies in the A site. Clinopyroxene compositions straddle the calcic augite-salite boundary. Chrondite-normalized REE patterns are similar for both inclusions and rhyolites. The inclusions are slightly poorer in REE and have a positive Eu anomaly versus the negative anomaly of the rhyolites. The similarity in REE patterns would seem to indicate that the two rock types are genetically related with the positive Eu anomalies resulting from feldspar accumulation. However, this possibility is denied by the antithetic alkaline and subalkaline phenocryst assemblages of the two rock types. We suggest that the best explanation for these discrepancies is that a mildly silica-undersaturated magma was the parent for the phenocrysts. This magma intruded the Carpenter Ridge chamber, and because the crystallization temperatures of both magma overlapped, the alkaline magma mixed with the ambient rhyolite to form a hybrid. This hybrid consisted of a portion of the phenocryst assemblage from the alkaline magma but the bulk-rock chemistry depended upon the proportions of the endmember liquids. The abundance of normative quartz, the minor normative corundum, and the similarity of REE patterns indicates that the inclusions are mixtures dominated by the rhyolitic component. Additional processes such as liquid-state diffusion, crystal accumulation, and alkali loss may have contributed to obscure the compositions of the initial liquids involved in the inferred mixing process.     

An experimental marine ecosystem response to crude oil and corexit 9527: Part 1—Fate of chemically dispersed crude oil

The fate of Prudhoe Bay crude oil. labelled with n(1−¹⁴C)-hexadecane and dispersed with Corexit 9527, was studied for 24 days in a polyethylene bag enclosure of sea water by time-series observations of the alkane composition of the crude oil, oil fluorescence, ¹⁴C-labelled hexadecane in the particulate phase, bacterial biomass, amounts of sedimented material and parameters of temperature, salinity, particulate organic carbon and nitrogen, and nutrients. By the seventh day, convective and diffusive mixing, important mechanisms for the dispersion of oil, resulted in a fairly homogeneous distribution of oil throughout the enclosed water column. Rapid bacterial biodegradation removed the n-alkane fraction initially, while oil-Corexit dispersion suppressed phytoplankton growth. After 7 days, with the recovery of phytoplankton growth, much of the aged oil sedimented with sinking of diatoms.     

Nutrient and Plankton Dynamics in the NE and NW Gyres of the Subarctic Pacific Ocean

The western subarctic gyre (WSG) and the eastern Alaska Grye (AG) on each side of the subarctic North Pacific, have many similarities. In both gyres, macronutrients are generally high and chl is low, and hence both gyres are High Nitrate, Low Chlorophyll (HNLC) regions. Despite the general similarities between these two gyres, there are many important differences. The time series station established at Stn KNOT on the southwest edge of the WSG and two in situ mesoscale iron enrichment experiments at each of the gyres has provided more information on iron concentrations, the dual role of iron and silicate limitation and seasonal cycles in the gyres. There is more seasonality in many parameters at Stn KNOT than at Stn P. There is an increase in Chl and primary productivity at Stn KNOT in May followed by increased iron limitation in summer. Low DIC:NO₃ ratios and high Si:NO₃ ratios in the WSG, indicate lower calcification and higher diatom production than at Stn P. The sources of iron for these areas are still not clear, but horizontal transport of iron rich coastal water and vertical transport could be important sources at certain times of the year in addition to dust input. Satellite images show that chl-rich coastal waters occasionally extend to the vicinity of Stn KNOT and therefore Stn KNOT may not always represent conditions in the main part of the WSG. This review focuses mainly on a comparison of Stn KNOT and Stn P, two time series stations on the edge of two very large gyres. At present, we have a limited understanding of the temporal and spatial variability within each of these large gyres. This revised version was published online in July 2006 with corrections to the Cover Date.     

Introduction to ‘The 1997–8 El Niño Atlas of oceanographic conditions along the west coast of North America (23°N–50°N)’

Hydrographic data collected along the West Coast of North America between January 1997 and January 1999 have been compiled into a web-based Atlas of the 1997–8 El Niño event. This paper discusses the organization of the Atlas, describes the data that were incorporated into the Atlas and explains how vertical and horizontal distributions for the different properties were constructed. Examples of figures from the Atlas are shown.     

The effect of glucose on the productivity of seawater: An experimental approach using controlled aquatic ecosystems

Small quantities (1 to 5 mg litre) of glucose were added to 60-ton enclosed water columns together with inorganic nutrients. Heterotrophic production by bacteria in the glucose-enriched water columns depressed photosynthetic production by algae. This effect was believed to have been caused by heterotrophic growth out-competing the autotrophic organisms for a limited supply of nitrate. Secondary and tertiary production in the glucose-enriched water columns was higher than under conditions of predominantly autotrophic production.     

Structure of Haida Eddies and Their Transport of Nutrient from Coastal Margins into the NE Pacific Ocean

Anticyclonic mesoscale eddies form near shore each winter in the Gulf of Alaska. One site near the Queen Charlotte Islands is shown to produce eddies that transport from 3000 to 6000 km³ of coastal water up to 1000 km westward. Eddies carry shelf nutrients either into the high nutrient, low chlorophyll waters of the NE Pacific, or in a more southerly direction into seasonally nitrate depleted waters. A large eddy sampled in summer 1998 was found to have elevated particulate levels on its perimeter. Nitrate supplied to the euphotic zone by this eddy during its natal summer is estimated to be three times greater than the usual seasonal nutrient transport in the Gulf of Alaska. This revised version was published online in August 2006 with corrections to the Cover Date.     

A Preliminary Study on Skarn-Related Calc-silicate Rocks Associated with the Batu Hijau Porphyry Copper-Gold Deposit, Sumbawa Island, Indonesia

Batu Hijau is a world-class gold-rich porphyry copper deposit, situated in Sumbawa Island, Indonesia. Deep drilling indicates that several intervals of calc-silicate rock were intersected, where they are apparently interbedded with volcaniclastic rocks. The calc-silicate rocks occur at the contact with copper-gold-bearing tonalite porphyries. The rocks are fine-grained and granular with green, reddish-brown and white layers. The green layers consist mostly of fine-grained clinopyroxene (diopside and hedenbergite) and the reddish-brown layers consist mostly of garnet (andradite), whereas the white layers are commonly composed of calcite and zeolite (chabazite). The calc-silicate rocks were formed by contact metasomatism of andesitic volcaniclastic rocks, as it is calcic in composition. Paragenesis study reveals at least two stages of calc-silicate mineralization. Stage 1 (prograde) is characterized by the presence of garnet (andradite), clinopyroxene (diopside and hedenbergite), anorthite and quartz at 340–360 C (high salinity 35–45 NaCl wt percentage eqn.). Stage 2 (retrograde) is characterized by chlorite and rare epidote at 280–300 C (low salinity 1–10 NaCl wt% eqn.). Late calcite ± quartz veinlets and calcite + chabazite veins/veinlets may also be related to this stage and cross cut the oldest mineral assemblages. Mineralization (magnetite, chalcopyrite and pyrite) may occur during the retrograde stage. Clinopyroxene and garnet were modified by Fe-rich hydrothermal fluid (oxidizing condition) indicated by increase of Fe from core to rim of both the cogenetic minerals. The presence of the calc-silicate rocks associated with massive magnetite-chalcopyrite-pyrite assemblage indicates the occurrence of calcic-exoskarn surrounding the Batu Hijau porphyry copper-gold deposit.     

Origins and genesis of loess deposits in central Spain,as indicated by heavy mineral compositions and grain‐size variability

The formation of loess deposits strongly depends on the availability of deflatable dust‐sized material. Identification of source areas and mechanisms of dust production is essential for an appropriate characterization of related palaeoenvironmental conditions. So far, little research has been done on loess deposits in the Mediterranean region and information about mechanisms of dust production is very rare. In this context, it is not clear from where the loess deposits from the upper Tagus Basin in central Spain originated. The main objective of this study was to find out whether these loess deposits were formed primarily by mountain processes and fluvial comminution, or whether they have their origin in the weathering of Tertiary marls in the centre of the Madrid Basin. A further concern was to link the determined source areas with loess formation mechanisms and concomitant environmental contexts. Following a comprehensive approach using heavy minerals and grain‐size data, together with information on stratigraphic features and geomorphic positions, different local loess sources could be identified. This study shows that during the last 35 kyr a major proportion of the Tagus loess deposits was deflated from river floodplains, while the mountain region of the Iberian Range was identified as a significant source of respective floodplain sediments. Based on heavy mineral compositions and dating results, it was found that during Heinrich Stadial 3, sediment supply from the Iberian Range strongly increased, suggesting an environmental shift from initially warmer towards colder temperatures in late Marine Isotope Stage 3. Furthermore, it was found that grain‐size patterns clearly indicate maximum wind strengths during Heinrich Stadial 3, followed by Heinrich Stadial 2 and Heinrich Stadial 1. These results demonstrate that the formation of dust‐sized particles in this part of the Mediterranean operated similar to many other places in temperate and continental regions, and that weathering processes linked to hot and dry environments were only of minor importance.