THE NEW PETROLOGY

I suppose that one is not far wrong in saying that Petrology is thescience of rocks.But we have not made ourselves very clear by saying it.To the average student of the subject.who must judge from what he is taught,     

Garnet–chloritoid–kyanite assemblages: eclogite facies indicators of subduction constraints in orogenic belts

The assemblage garnet–chloritoid–kyanite is shown to be quite common in high‐pressure eclogite facies metapelites from orogenic belts around the world, and occurs over a narrowly restricted range of temperature ～550–600 °C, between 20 and 25 kbar. This assemblage is favoured particularly by large Al₂O₃:K₂O ratios allowing the development of kyanite in addition to garnet and chloritoid. Additionally, ferric iron and manganese also help stabilize chloritoid in this assemblage. Pseudosections for several bulk compositions illustrate these high‐pressure assemblages, and a new thermodynamic model for white mica to include calcium and ferric iron was required to complete the calculations. It is extraordinary that so many orogenic eclogite facies rocks, both mafic eclogites sensu stricto as well as metapelites with the above assemblage, all yield temperatures within the range of 520–600 °C and peak pressures ～23±3 kbar. Subduction of oceanic crust and its entrained associated sedimentary material must involve the top of the slab, where mafic and pelitic rocks may easily coexist, passing through these P–T conditions, such that rocks, if they proceed to further depths, are generally not returned to the surface. This, together with the tightly constrained range in peak temperatures which such eclogites experience, suggests thermal weakening being a major control on the depths at which crustal material is decoupled from the downgoing slab.     

A new thermodynamic model for clino- and orthoamphiboles in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–O

A recent thermodynamic model for the Na–Ca clinoamphiboles in the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–O (NCFMASHO), is improved, and extended to include cummingtonite–grunerite and the orthoamphiboles, anthophyllite and gedrite. The clinoamphibole model in NCMASH is adopted, but the extension into the FeO- and Fe₂O₃-bearing systems is revised to provide thermodynamic consistency and better agreement with natural assemblage data. The new model involves order–disorder of Fe–Mg between the M2, M13 and M4 sites in the amphibole structure, calibrated using the experimental data on site distributions in cummingtonite–grunerite. In the independent set of end-members used to represent the thermodynamics, grunerite (rather than ferroactinolite) is used for FeO, with two ordered Fe–Mg end-members, and magnesioriebeckite (rather than ferritschermakite) is used for Fe₂O₃. Natural assemblage data for coexisting clinoamphiboles are used to constrain the interaction energies between the various amphibole end-members. For orthamphibole, the assumption is made that the site distributions and the non-ideal formulation is the same as for clinoamphibole. The data set end-members anthophyllite, ferroanthophyllite and gedrite, are used; for the others, they are based on the clinoamphibole end-members, with the necessary adjustments to their enthalpies constrained by natural assemblage data for coexisting clino- and orthoamphiboles. The efficacy of the models is illustrated with P – T grids and various pseudosections, with a particular emphasis on the prediction of mineral assemblages in ferric-bearing systems.     

Meridional zonation of the Barents Sea ecosystem inferred from satellite remote sensing and in situ bio-optical observations

The Barents Sea is a productive, shallow, high-latitude marine ecosystem with complex hydrographic conditions. Zonal hydrographic bands defined by a coastal current. North Atlantic Water, the Polar Front, and the seasonally variable marginal ice edge zone create a meridional zonation of the ecosystem during the spring-summer transition. The features reveal themselves in satellite imagery and by high-resolution (vertical and horizontal) physical-optical-biological sampling.
Surprisingly, the long-term (7-year) mean of Coastal Zone Color Scanner (CZCS) imagery reveals the Barents Sea as an anomalous "blue-water" regime at high latitudes that are otherwise dominated by satellite-observed surface blooms. A combination of satellite imagery and in situ bio-optical analyses indicate that this pattern is caused by strong stratification in summer with surface nutrient depletion. The onset of stratification of the entire region is linked to the extent of the winter ice edge: cold years with extensive sea ice apparently stratify early due to ice melt; warm years stratify later, perhaps due to weaker thermal stratification of the Atlantic waters (e.g. Skjoldal et al. 1987). The apparent "low chlorophyll" indicated by the CZCS 7-year mean is partly due to sampling error whereby the mean is dominated by images taken later in the summer. In fact, massive blooms of subsurface phytoplankton embedded in the pycnocline persist throughout the summer and maintain substantial rates of primary production. Further, these subsurface blooms that are not observed by satellite are responsible for dramatic gradients in the beam (c₁) and spectral diffuse (k) attenuation coefficients. The Barents Sea exemplifies the need to couple satellite observations with spatially and temporally resolved biogeographic ecosystem models in order to estimate the integrated water column primary production, mass flux or spectral light attenuation coefficients.     

The Late Devensian Lateglacial palaeoenvironmental record from Whitrig Bog, SE Scotland. 1. Lithostratigraphy, geochemistry and palaeobotany

This is the first of a series of articles presenting the results of a multi-proxy investigation aimed at reconstructing changes in the ecosystem and climate of Whitrig Bog, SE Scotland, during the last glacial-inter-glacial transition (Devensian Lateglacial, c . 14–10 ka BP). We present here the results from sediment lithology, chemistry, pollen, and plant macrofossil analyses. These data are used to infer the nature of the local catchment soils and both local and regional terrestrial vegetation. The interstadial period ( c . 13–11 ka BP) is characterized by a successional sequence developing from a landscape with bare, poorly developed minerogenic soils supporting a sparse herbaceous flora into open birch woodland with juniper scrub and stable organic soils. At c . 11 ka BP the Younger Dryas climatic cooling event caused an abrupt reversion to an open herbaceous arctic/alpine flora (e.g. macrofossil evidence of Silene furcata and Oxyria digyna ) and high levels of minerogenic erosion into the basin, indicating environmental response to a cold Arctic climate. In addition to this Younger Dryas climatic reversal, two lesser reversion episodes occurred earlier during the interstadial. The more pronounced of the two, late in the intersladial, is characterized by high levels of erosion and a change from birch/juniper woodland to an open herbaceous flora. The older oscillation occurs approximately mid-way through the interstadial sequence and is marked by similar pollen changes, albeit shorter lived and more subtle.