Alkaline centres of the Gardar province, South Greenland, show evidence for subsolidus alteration of the consolidated rocks by associated metasomatic fluids. Oxygen isotopic values of metasomatised and unmetasomatised samples alike lie in the range 4-7%, and indicate that metasomatic fluids were juvenile in origin, evolved from the silicate magmas themselves after final crystallisation. The fluid catalysed catastrophic coarsening in alkali feldspars at temperatures below 450<SUP>o</SUP>C, and at lower temperatures, also brought about changes in their cathodoluminescence (CL) colours, altering blue luminescing feldspar to a red-luminescent variant. Ratios of blue to red-luminescing alkali feldspar have demonstrated that up to 96% of the volume of some syenitic rocks have undergone reequilibration with the metasomatic fluid. F-contents of biotites from across the Gardar have been examined to determine fluorine contents of the metasomatic fluids using the method of Munoz (1984). Undersaturated magmas were associated with more fluorine-rich fluids than those associated with oversaturated magmas. The most F-rich (Late Igdlerfigsalik) contained approximately four times as much available fluorine as the least F-rich fluid (Younger Giant Dyke Complex). Compositions of fluids associated with centres across the province can be related to the compositions of their original magmas. Oversaturated magmas are associated with CO<SUB>2</SUB>-bearing, relatively C- and F-poor fluids with pHs at, or near to, neutrality. Redox potentials are relatively oxidising. Undersaturated miaskitic magmas are associated wih CO<SUB>2</SUB>-bearing relatively C- and F-rich fluids: pHs are significantly alkaline and redox potentials are more reducing than in oversaturated magmas. Agpaitic magmatism is associated with CH<SUB>4</SUB>-bearing, highly reduced, F-rich fluids and pHs are more alkaline than in miaskitic centres. Low redox potentials are initially brought about by the crystallisation of magmatic aegirine, which removes iron as Fe<SUP>3</SUP>+ against the prevailing redox potential in the magma. This process causes progressively reduced redox potentials in the remaining magma and its associated volatiles. Since the oxidation state of the evolved fluid depends on the amount of aegirine crystallised, fluids become more reducing with increasing silica-undersaturation, and are most reducing in fluids from agpaitic magmas. pH and redox potential are related by the equation: CH<SUB>4 +</SUB> 8 OH<SUP>- +</SUP> 6 H<SUB>2</SUB>O + 8e<SUP>-</SUP> Zr- and Hf-mobility are observed in some fluids if the activity of F is high enough, due to the formation of polyfluoro-zirconate(IV) and -hafnate(IV) complexes. RE-mobility is observed from all of the fluids studied since the activity of either CO<SUB>2</SUB> or F is sufficient in all cases.

A number of experimental studies have shown that the first melt of carbonated mantle peridotite is carbonatitic in character. This first melt will contain a sizeable portion of the mantle budget for incompatible trace elements and, owing to its physical properties, it is likely to be very mobile. If no further melting occurs, the carbonatitic melt will migrate through the mantle, scavenging incompatible elements, before freezing in the lithosphere. Alternatively, melting can progress, resulting in the incorporation of this precursor melt in more voluminous silicate melt. In any case, carbonate melt - silicate mantle equilibria are likely to influence the trace-element characteristics of material produced. One way of constraining the geochemical characteristics of mantle-derived carbonate melt is by investigation of the partitioning behaviour between coexisting carbonate and silicate liquids for trace elements of interest. This has been achieved by a series of experimental runs under a range of temperatures and pressures (0.2-2.0 GPa, 1100-1400<SUP>o</SUP>C), involving both natural rock compositions (phonolites, nephelinites and melilitites), and synthetic charges in the system SiO<SUB>2</SUB>-Al<SUB>2</SUB>O<SUB>3</SUB>-CaO-Na<SUB>2</SUB>O-CO<SUB>2</SUB>. Whilst showing that carbonate and silicate melts are likely to be completely miscible under mantle conditions, these experiments have also shown that a wide range of Ca-Mg-Na-Fe carbonate liquids can be experimentally produced by liquid immiscibility at crustal pressures. This range incorporates the vast majority of carbonatite compositions seen at the Earth's surface. Coexisting carbonate and silicate liquids in the experimental charges produced were characterised by electron probe microanalysis. Melt compositions were found to be accurately reproducible, and subject to small but consistent variations with pressure and temperature. The charges were studied optically to ensure that the coexisting liquids were in equilibrium, to investigate quenching processes occurring in the liquids on cooling, and to analyse the two-liquid textures obtained.

No description available.

Using solid-media apparatus, synthetic trace-element-doped peridotitic starting materials were equilibrated between 1.8 GPa and 2.8 GPa, and 970°C and 1150°C under water-saturated conditions at an fO<SUB>2</SUB> equal to the Ni/NiO buffer. Melt compositions were calculated from analyses of melt inclusions trapped in an olivine crystal included in the capsule. However, analysis by secondary ion mass spectrometry (SIMS) showed that many of these experiments had been contaminated by boron from the surrounding boron-nitride sleeve, apparently via holes which formed and subsequently sealed. Water had not always been lost from contaminated capsules, and hence their failure had not previously been suspected. Comparison with uncontaminated experiments shows that boron fluxes melting and thus invalidates the results from the contaminated runs. This may be a general experimental problem, affecting past studies. Some experiments performed at 1.8 GPa were not contaminated and showed that with increased temperature, the solidus is encountered, followed by the amph-out, ilm-out, ru-out, cpx-out and opx-out curves, consistent with previous studies. Calculated melt compositions were tested by reversal, i.e. equilibration of the melt composition under similar conditions to the original experiment. A forward experiment conducted at 1.8 GPa, 1020°C produced ol, opx, cpx, amph, ru, ilm, sp, and basaltic melt (53.6% SiO<SUB>2</SUB>, 22.7% Al<SUB>2</SUB>O<SUB>3</SUB>). However the phases on the liquidus of the calculated melt composition were opx, cpx, gt, amph, and were more aluminous than the original phases. In reversal experiments performed without a separate olivine crystal, an <I>extra</I> 20 wt% forsterite component was necessary to stabilise olivine. The calculated melt is therefore deficient in olivine component and is more aluminous than the true melt.

No description available.

Heavy metals (Fe, Mn, Pb, Zn, Cu, Cd, Co and Ni) and radionuclides <SUP>210</SUP>Pb, <SUP>134</SUP>Cs and <SUP>137</SUP>Cs were studied in the sediments of Scottish freshwater lochs (acidified Round Loch of Glenhead, oligotrophic/mesotrophic Loch Lomond, eutrophic Loch Leven and Balgavies Loch and Loch Coire nan Arr in the remote north-west) to compare metal behaviour and associations in the different systems. Metal concentrations were determined in acid digested and sequentially extracted sediment and sediment porewaters. Radionuclide concentrations were measured to investigate sedimentation processes. Redox cycling of Fe and Mn was important in all the lochs. In Balgavies Loch, porewater data for Fe and Mn were related to redox conditions at the time of sampling, whereas the solid phase concentrations were still changing in response to recent water column stratification. The association of Fe with organic matter was additionally important in surface sediment of Round Loch and Loch Coire nan Arr. Mn behaviour in Round Loch was also influenced by surface water acidification via post-depositional leaching of Mn from the sediment and/or a pH-related decrease in the efficiency of Mn sedimentation, resulting in increasing Mn concentration with depth. Post-depositional mobility of Pb was not evident, despite the association of Pb with Fe oxides/hydroxides in sediments from all the lochs. This was supported by low, or non-detectable, porewater Pb concentrations and little change with depth in the relative concentrations of Pb in each fraction. Zn was generally more labile than Pb and, in Loch Coire nan Arr and Balgavies Loch, redox related release of Zn from the sediment and short-range mobility was apparent. In Round Loch, Zn concentrations, decreasing sharply towards the sediment surface, were explained by historical trends in pollutant Zn deposition to the loch, as well as a pH-related decrease in the efficiency of Zn deposition to the sediment.

No description available.

This study deals with the mineralogy and geochemistry of some Namurian sediments from the Central Pennine Basin, England. The rocks were collected from the Mam Tor area and a borehole at Tansley near Matlock. The Mam Tor rocks (Ric zone) consist of closely associated turbidite sandstones and shales. In the Tansley Borehole (R-E zones) the Ashover Sandstone overlies a condensed sequence of marine and non-marine shales with a volcanic clay at the base. Petrographic and X-ray, diffraction studies, together with major and trace element analyses, have been carried out on whole rock samples and separated size fractions. The Mam Tor sandstones are greywackes, whereas the Ashover sandstones are arkoses. In the sandstones, kaolinite, chlorite and iron oxides are partly of diagenetic origin. The mineralogy of the rocks is comparatively simple; quartz, kaolinite and illite are the principal minerals, with lesser amounts of feldspar, siderite, pyrite, dolomite, calcite and organic matter. Chemical and mineralogical variations in the Mam Tor rocks are due to textural differences, particularly grain size fractionation. Further information on the association of elements with minerals is provided by correlation analysis and t-tests. Chlorite is shown to be rich in iron and illite is the lMd polytype. It is concluded that these rocks have not reached the stage of deep burial diagenesis. Replacement reactions are important in the sandstones, but not in the shales. The volcanic clay is identified as a K-bentonite and originated by reaction between basalt and seawater. The transition from non-marine to marine shales is accompanied by increases in the contentsof illite, pyrite, calcite and organic matter and a decrease in the kaolinite content. Siderite is confined to the non-marine shales in which phosphate and TiO 2 are relatively concentrated. The marine shales are enriched in Ni, V, Zn, Cu, Sr and Pb, whereas the non-marine shales are higher in Zr. The concentration of Cu, Sr and Pb are the most effective discriminators, using discriminant function analysis, between the two groups of shales. The trace element enrichment is attributed mainly to organic matter and pyrite. The variation in Si02/A1203, quartz/combined silica, T102/A1203 and Na20/K20 ratio are related to grain size, the rate of sedimentation and distance from source. The distribution of diagenetic minerals is attributed to changing physico-chemical conditions including salinity. The change from the pyrite to the siderite stability field is considered to reflect decreasing pH and rising Eh. The lateral variation in facies deposited contemporaneously in different sectors reveals the influence of the Derbyshire Block acting as a submarine ridge. It is shown that the textural and geochemical characteristics of the Mam Tor rocks are similar to those of turbidites. The data of the Tansley Borehole sediments indicate their similarity to those from a shallow water, semi-restricted, basin environment onto which a deltaic sedimentation was encroaching.

During 1990 a 51 metre core was recovered from Lago Grande di Monticchio, a maar lake in S Italy. This provides a high quality laminated sequence extending back into the Late Quaternary which is being examined by a multidisciplinary group. The project here is concerned with sediment geochemistry as a tool for reconstruction of palaeoenvironmental conditions at this locality. The core was analysed at 10cm resolution using techniques such as X-ray fluorescence (XRF). This has produced a highly detailed record of change reflecting either the last 70,000 or 220,000 years. Organic carbon and biogenic silica are believed to represent biogenic productivity and, in particular, nutrient status in the lake system. It is found that the Holocene is marked by very high concentrations of these components in contrast to the minerogenic glacial sediments. Possible earlier interglacials (e.g. the Eemian) contain significantly lower contents of biogenic matter, suggesting that nutrient conditions may have been somewhat poorer during these times. Alternatively, these periods could represent interstadials within the last glacial cycle. C/N ratios and Hydrogen Index values probably reflect both the source and degree of preservation of the organic matter, but should be interpreted cautiously. The δ<SUP>13</SUP>C record for bulk organic matter shows a significant shift to isotopically lighter values during the glacial-Holocene transition, but the profile is difficult to interpret due to the many possible factors involved. Indicators of terrigenous clastic material show that the late-glacial and Holocene sediments have received relatively base-rich clastic material probably derived from contemporary tephra inputs to the catchment. This is marked by ratios such as Na/Al and K/Al which increase in apparent association with increased tephra deposition and Zr/Rb which decreases. The effect of tephra blanketing of the catchment and subsequent release of nutrients is proposed as a cause for the much higher productivity during the Holocene interglacial.

No description available.