Osumilite–melt interactions in ultrahigh temperature granulites: phase equilibria modelling and implications for the P–T–t evolution of the Eastern Ghats Province,India     

F.J. Korhonen  M. Brown  C. Clark  S. Bhattacharya 《Journal of Metamorphic Geology》2013,31(8):881-907

The exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological interpretations. One of the petrological disputes in residual high Mg–Al granulites concerns the origin of fine‐grained mineral intergrowths comprising cordierite + K‐feldspar ± quartz ± biotite ± sillimanite ± plagioclase. These intergrowths wrap around porphyroblast phases and are interpreted to have formed by the breakdown of primary osumilite in the presence of melt trapped in the equilibration volume by the melt percolation threshold. The pressure (P)–temperature (T) evolution of four samples from three localities across the central Eastern Ghats Province is constrained using phase equilibria modelling in the chemical system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO). Results of the modelling are integrated with published geochronological results for these samples to show that the central Eastern Ghats Province followed a common P–T–t history. This history is characterized by peak UHT metamorphic conditions of 945–955 °C and 7.8–8.2 kbar followed by a slight increase in pressure and close‐to‐isobaric cooling to the conditions of the elevated solidus at 940–900 °C and 8.5–8.3 kbar. In common with other localities from the Eastern Ghats Province, the early development of cordierite before osumilite and the peak to immediate post‐peak retrograde reaction between osumilite and melt to produce the intergrowth features requires that the prograde evolution was one of contemporaneous increasing pressure with increasing temperature. This counter‐clockwise (CCW) evolution is evaluated for one sample using inverse phase equilibria modelling along a schematic P–T path of 150 °C kbar^?1 starting from the low P–T end of the prograde P–T path as constrained by the phase equilibria modelling. The inverse modelling is executed by step‐wise down temperature reintegration of sufficient melt into the residual bulk chemical composition at the P–T point of the 1 mol.% melt isopleth at each step, representing the melt remaining on grain boundaries after each prograde drainage event, to reach the melt connectivity transition (MCT) of 7 mol.%. The procedure is repeated until a plausible protolith composition is recovered. The result demonstrates that clastic sedimentary rocks that followed a CCW P–T evolution could have produced the observed mineral assemblages and microstructures preserved in the central Eastern Ghats Province. This study also highlights the role of melt during UHT metamorphism, particularly its importance to both chemical and physical processes along the prograde and retrograde segments of the P–T path. These processes include: (i) an increase in diffusive length scales during the late prograde to peak evolution, creating equilibration volumes larger than a standard thin section; (ii) the development of retrograde mineral assemblages, which is facilitated if some melt is retained post‐peak; (iii) the presence of melt as a weakening mechanism and the advection of heat by melt, allowing the crust to thicken; and (iv) the effect of melt loss, which makes the deep crust both denser and stronger, and reduces heat production at depth, limiting crustal thickening and facilitating the transition to close‐to‐isobaric cooling.  相似文献   

Continental impact on marine boundary layer coarse particles over the Atlantic Ocean between Europe and Antarctica     

Jarkko V. Niemi  Heikki Tervahattu  Aki Virkkula  Risto Hillamo  Kimmo Teinil  Ismo K. Koponen  Markku Kulmala 《Atmospheric Research》2005,75(4):109

Aerosol samples were collected in the Atlantic marine boundary layer between the English Channel and Antarctica during November–December 1999. The composition of coarse (aerodynamic diameter 1–3 μm) individual aerosol particles was studied using the SEM/EDX method. The major particle types observed were fresh sea salt, sea-salt particles reacted partly or totally with sulphuric acid or nitric acid, Mg-sulphate, Ca-sulphate, mixed aluminosilicates and sea salt, aluminosilicates, Ca-rich particles and Fe-rich particles. The relative fractions of sea-salt particles with moderate or strong Cl depletion were high near the coasts of Europe (65–74%) and Northern Africa (44–87%), low far from the coast of Western Africa (10–20%) and very low in remote sea areas between Africa and Antarctica (1%). The Cl depletion was strongest when air masses arrived from the direction of anthropogenic pollution sources. The fractions of Mg-sulphate particles were high (18–25%) in 2 samples near Europe. The Mg-sulphate particles were probably formed as a result of fractional recrystallization of sea-salt particles in which Cl was substituted by sulphate. It remained unclear whether these particles were formed in the atmosphere or during and after sampling. The relative fractions of particles from continental sources were quite low (10–15%) near Europe, very high (25–78%) near the coast of Northwestern Africa and very low in the remote sea areas (0–2%). Most of the continental particles were aluminosilicates and some of them were internally mixed with sea salt. Near the coast of Northwestern Africa, the main source of aluminosilicates was Saharan dust, and near the Gulf of Guinea, emissions from biomass burning were also mixed with aluminosilicates and sea salt.  相似文献   

The Finnish seismic array     

S.E. Pirhonen  S. Nurminen  U. Luosto  H. Korhonen 《Physics of the Earth and Planetary Interiors》1979,18(2):121-126

This paper deals with the Finnish seismic array, which in its present state consists of two tripartite arrays, HESA and JYSA. The first of these arrays, situated near Helsinki, has been in continuous operation since 1964 and uses analog recording by frequency-modulated telemetry. The JYSA array is sited in central Finland near the town of Jyväskylä. This array consists of three substations equipped with vertical-component SP seismometers. The signals are transmitted by digital FM telemetry in the UHF band and recorded at Helsinki on magnetic tape. Events are detected visually from monitoring drums. The digital data of the events detected are handled by a microprocessor with a graphic display and fed to a Burroughs 6700 analysing computer.Comparison with the NORSAR array indicates that at frequencies below 2 Hz the absolute noise level at JYSA is about 6 dB lower, but above 2 Hz the noise level for single instruments at the two arrays is about the same. As regards detection capability, the incremental threshold is about the same for JYSA as for Hagfors and 0.5 magnitude units higher than for NORSAR, but 0.5 magnitude units lower for JYSA than for the WWSSN station NUR in southern Finland.  相似文献   

Surface Stokes drift in the Baltic Sea based on modelled wave spectra     

Laura Tuomi  Olga Vähä-Piikkiö  Pekka Alenius  Jan-Victor Björkqvist  Kimmo K. Kahma 《Ocean Dynamics》2018,68(1):17-33

The Stokes drift is an important component in the surface drift. We used the wave model WAM to evaluate the mean values and exceedance probabilities of the surface Stokes drift in the Baltic Sea. As there is no direct way to verify the accuracy of the modelled Stokes drift, we compared the bulk parameters calculated by the wave model against buoy measurements to ensure the quality of the wave hindcast. Furthermore, we evaluated the surface Stokes drift from measured wave spectra to assess the accuracy of the modelled surface Stokes drift. The importance of the Stokes drift as a component of the total surface drift was evaluated by calculating the hindcast mean values and percentiles of the surface Stokes drift. The mean values were between 0.08 and 0.10 ms^?1 in the open sea areas, thus being of the same order of magnitude as the mean wind shear currents. The highest values of the surface Stokes drift were slightly larger than 0.6 ms^?1. The comparison of modelled Stokes drift values to estimates obtained from measured spectra suggests that the mean values are well represented by the model. However, the higher modelled values are most likely slightly too large because the wave energy was overestimated during high wind situations in some of the sub-basins, such as the Gulf of Finland. A comparison to a drifter experiment showed that use of the Stokes drift improves the estimate of both the drift speed and the direction in the Gulf of Finland. Parameterised methods to evaluate the Stokes drift that are used, e.g. in currently available Baltic Sea drift models, overestimate the smaller values (under 0.3 ms^?1) and underestimate the larger values of the Stokes drift compared to the values calculated by the wave model. The modelled surface Stokes drift direction mostly followed the forcing wind direction. This was the case even in the Gulf of Finland, where the direction of the wind and the waves can differ considerably.  相似文献   

Changes in frost, snow and Baltic sea ice by the end of the twenty-first century based on climate model projections for Europe     

Kirsti Jylhä  Stefan Fronzek  Heikki Tuomenvirta  Timothy R. Carter  Kimmo Ruosteenoja 《Climatic change》2008,86(3-4):441-462

Changes in indices related to frost and snow in Europe by the end of the twenty-first century were analyzed based on experiments performed with seven regional climate models (RCMs). All the RCMs regionalized information from the same general circulation model (GCM), applying the IPCC-SRES A2 radiative forcing scenario. In addition, some simulations used SRES B2 radiative forcing and/or boundary conditions provided by an alternative GCM. Ice cover over the Baltic Sea was examined using a statistical model that related the annual maximum extent of ice to wintertime coastal temperatures. Fewer days with frost and snow, shorter frost seasons, a smaller liquid water equivalent of snow, and milder sea ice conditions were produced by all model simulations, irrespective of the forcing scenario and the driving GCM. The projected changes have implications across a diverse range of human activities. Details of the projections were subject to differences in RCM design, deviations between the boundary conditions of the driving GCMs, uncertainties in future emissions and random effects due to internal climate variability. A larger number of GCMs as drivers of the RCMs would most likely have resulted in somewhat wider ranges in the frost, snow and sea ice estimates than those presented in this paper.  相似文献   

Climate change mitigation strategy under an uncertain Solar Radiation Management possibility     

Tommi Ekholm  Hannele Korhonen 《Climatic change》2016,139(3-4):503-515

  相似文献   

Comparison between radiocarbon and varve dating in Lake Lampellonjärvi, south Finland     

Kimmo Tolonen 《Boreas: An International Journal of Quaternary Research》1980,9(1):11-19

The annual nature of organic laminations in the sediment from a small lake, Lampellonjärvi (61°04'N; 25°04'E), was determined. The core was obtained using an in situ freezing method and the laminations were checked by a detailed microscopical analysis of adhesive tape preparations. A series of six radiocarbon dates were obtained for levels in the core which had previously been dated by means of varve counts to ages between A.D. 182 and AD. 1513. The radiocarbon dates were between 547 and 1525 years older. Two additional ¹⁴C dates from the lower part of the same profile (ca. 1880 B.C. and ca. 3100 B.C.), however, gave expected results. Erosion of old organic terrestrial material due to agricultural activity in the surroundings of the lake was assumed to have been the cause of abnormally old radiocarbon ages. Dates from the pre-agricultural period had a deviation from varve years similar to the difference between tree rings, historical documents and radiocarbon dating recorded in other studies.  相似文献   

Comparison of crustal structure along DSS profiles in SE Fennoscandia   总被引：1，自引：0，他引：1  

H. Korhonen  I. P. Kosminskaya  I. Azbel  N. Sharov  V. Zagorodny  U. Luosto 《Geophysical Journal International》1990,103(1):157-162

  相似文献   

Crust and upper mantle structure along the DSS Baltic profile in SE Finland   总被引：4，自引：0，他引：4  

U. Luosto  T. Tiira  H. Korhonen  I. Azbel  V. Burmin  A. Buyanov  I. Kosminskaya  V. Ionkis  N. Sharov 《Geophysical Journal International》1990,101(1):89-110

  相似文献