Internal Differentiation of the Archean Continental Crust: Fluid-Controlled Partial Melting of Granulites and TTG-Amphibolite Associations in Central Finland

Fault bound blocks of granulite and enderbite occur within upperamphibolite-facies migmatitic tonalitic–trondhjemitic–granodioritic(TTG) gneisses of the Iisalmi block of Central Finland. Theseunits record reworking and partial melting of different levelsof the Archean crust during a major tectonothermal event at2·6–2·7 Ga. Anhydrous mineral assemblagesand tonalitic melts in the granulites formed as a result ofhydrous phase breakdown melting reactions involving amphiboleat peak metamorphic conditions of 8–11 kbar and 750–900°C.A nominally fluid-absent melting regime in the granulites issupported by the presence of carbonic fluid inclusions. Thegeochemical signature of light rare earth element (LREE)-depletedmafic granulites can be modelled by 10–30 wt % partialmelting of an amphibolite source rock leaving a garnet-bearingresidue. The degree of melting in intermediate granulites isinferred to be less than 10 wt % and was restricted by the availabilityof quartz. Pressure–temperature estimates for the TTGgneisses are significantly lower than for the granulites at660–770°C and 5–6 kbar. Based on the P–Tconditions, melting of the TTG gneisses is inferred to haveoccurred at the wet solidus in the presence of an H₂O-rich fluid.A hydrous mineralogy, abundant aqueous fluid inclusions andthe absence of carbonic inclusions in the gneisses are in accordancewith a water-fluxed melting regime. Low REE contents and strongpositive Eu anomalies in most leucosomes irrespective of thehost rock composition suggest that the leucosomes are not meltcompositions, but represent plagioclase–quartz assemblagesthat crystallized early from felsic melts. Furthermore, similarplagioclase compositions in leucosomes and adjacent mesosomesare not a ‘migmatite paradox’, as both record equilibrationwith the same melt phase percolating along grain boundaries. KEY WORDS: Archean continental crust; fluid inclusion; granulite; migmatite; partial melting     

Trace element partitioning in the granulite facies

Analyses of trace elements in the mineral phases of granulites provide important information about the trace element distribution in the lower crust. Since granulites are often considered residues of partial melting processes, trace element characteristics of their mineral phases may record mineral/melt equilibria thus giving an opportunity to understand the nature and composition of melts in the lower continental crust. This study provides an extensive set of mineral trace element data obtained by LA-ICP-MS analyses of mafic and intermediate granulites from Central Finland. Mass balance calculations using the analytical data indicate a pronounced contribution of the accessory minerals apatite for the REE and ilmenite for the HFSE. Coherent mineral/mineral ratios between samples point to a close approach to equilibrium except for minerals intergrown with garnet porphyroblasts. Mineral trace element data were used for the formulation of a set of D ^mineral/melt partition coefficients that is applicable for trace element modelling under lower crustal conditions. D ^mineral/melt were derived by the application of predictive models and using observed constant mineral/mineral ratios. The comparison of the calculated D ^mineral/melt with experimental data as well as the relationship between mineral trace element contents and a leucosome with a composition close to an equilibrium melt provides additional constraints on mineral/melt partitioning. The D values derived in this study are broadly similar to magmatic partition coefficients for intermediate melt compositions. They provide a first coherent set of D values for Sc, V, Cr and Ni between clinopyroxene, amphibole, garnet, orthopyroxene, ilmenite and melt. In addition, they emphasize the strong impact that ilmenite exerts on the distribution of Nb and Ta.     

Changes in North Sea macrofauna communities and species distribution between 1986 and 2000

The North Sea Benthos Project 2000 was initiated as a follow-up to the 1986 ICES North Sea Benthos Survey with the major aim to identify changes in the macrofauna species distribution and community structure in the North Sea and their likely causes.The results showed that the large-scale spatial distribution of macrofauna communities in the North Sea hardly changed between 1986 and 2000, with the main divisions at the 50 m and 100 m depth contours. Water temperature and salinity as well as wave exposure, tidal stress and primary production were influential environmental factors on a large (North Sea-wide) spatial scale.The increase in abundance and regional changes in distribution of various species with a southern distribution in the North Sea in 2000 were largely associated with an increase in sea surface temperature, primary production and, thus, food supply. This can be most likely related to the North Sea hydro-climate change in the late 1980s influenced by the variability in the North Atlantic Oscillation (NAO). Only one cold-temperate species decreased in abundance in 2000 at most of the stations. Indications for newly established populations of offshore non-native species were not found.Differences in macrofauna community structure on localised spatial scales were predominantly found north of the 50 m depth contour off the British coast along the Flamborough Head Front towards the Dogger Bank, off the coast of Jutland and at the Frisian Front. These changes were most likely attributed to stronger frontal systems in 2000 caused by the increased inflow of Atlantic water masses in relation to the hydro-climate change in the late 1980s.     

Anthropogenic nutrient load of the Baltic Sea

The discharge of nutrients is investigated in relation to their sources and effects in two case studies. The reduction of 47% in the phosphorus load from Denmark to marine areas between 1989 and 1993 has resulted in significantly lower phosphorus concentrations in most Danish coastal waters, and tendency to decrease can be seen in the Belt Sea and Kattegat as well. No general changes in nitrogen concentrations have been observed. This is due to the fact that more than 80% of the nitrogen load in Danish waters originate from diffuse agricultural sources.In the Pomeranian Bight strong nutrient gradients are generated by the mixing of Odra river water and coastal water. The spreading of the river plume could be exactly observed especially in winter, when biological activity is low. In general, different types of distribution, transport and modification patterns can be described.The annual input of nutrients from the catchment area to the Baltic Sea was estimated to be around 1000 kt N and 46 kt P. As a result, winter concentrations of phosphate and nitrate are characterized by positive overall trends in the surface layer in all subregions of the Baltic Proper for the period 1969 to 1993. These trends stem mainly from the strong increase in the 1970ies and early 1980ies. Thereafter, the concentrations of both nutrients fluctuate strongly around a high level. The drastic decrease in fertilizer consumption since the late 1980ies mainly caused by the great economic changes in the countries of the former East Bloc is not yet significantly reflected in decreasing winter concentrations, but first signs already have been found in the decrease in averaged phosphate concentrations in winter, especially in the Arkona and Bornholm Seas.     

A New Mathematical Formulation for Underground Cut-off Grade Determination in the Presence of Pre-concentration Systems

Natural Resources Research - The mining industry is facing increasingly lower grades, which intensifies the need to reduce costs. The depletion of orebodies close to the surface has pushed the...