Lake shoreline development, frost weathering and rock platform erosion in an alpine periglacial environment, Jotunheimen, southern Norway

Ice-dammed lake Boverbrevatnet existed for 75–125 years in the 'Little Ice Age'. After about A.D. 1826, glacier retreat led to a fall in lake level and to exposure of the former shoreline, which includes well-developed platforms cut in metamorphic bedrock. The rock platforms, up to 5.3 m wide and backed by cliffs up to 1.55 m high, are partially covered by large angular boulders which form pavements. Accurate levelling has permitted correlation of platform fragments, overflow cols and related features of the shoreline, such as benches eroded in moraines, ice-push ridges, a perched delta, vegetation trim-lines, lichen limits and a 'lichen-kill' zone. The evolution of the lake, the chronology of deglaciation and the period of formation of the rock platforms have been dated by lichenometry, supported by ¹⁴C dating, Schmidt hammer 'R'-values and historical data. The morphology of the rock platforms, together with estimates of their rate of erosion ranging from 1.4 to 7.1 cm/year, indicate the importance of frost shattering (frost riving, frost wedging or macrogelivation) at the lake margin under a periglacial climate, while the permanence of such platforms as landscape features suggests their use in the reconstruction of former periglacial environments. A semi-quantitative model is outlined for the development of rock platforms which emphasises deep penetration of the annual freeze-thaw cycle, the movement of unfrozen lake water towards the freezing plane, and the growth of segregation ice in fissures and cracks at the interface between lake ice and bedrock. Ice-push and ice-pull processes are involved primarily as transporting agents in the formation of boulder pavements and in the removal of debris from the platforms. Analogous processes may occur on polar coasts producing coastal rock platforms.     

Alkalic Pyroxenite Xenoliths from the Lashaine Volcano, Northern Tanzania

The ankaramitic scoria and carbonatite tuffs of the Lashainevolcano, northern Tanzania, contain a suite of alkalic pyroxenitexenoliths, in addition to the previously investigated magnesianlherzolite types. The rocks of the pyroxenite suite, which includemica-dunite and iron-rich lherzolite, consist of varying combinationsof olivine (Fo_86–72), sodic diopside, Ti-pargasite, Ti-phlogopite,ilmenite, chromite, and magnetite. The over-all assemblagesare poorer in alumina than those from other alkalic pyroxenitelocalities. Comparison with the products of experimentally investigatedsystems is difficult because of low alumina, and emphasizesthe need for experimental syntheses on rocks of this type.     

Metasomatism and Partial Melting in Upper-Mantle Peridotite Xenoliths from the Lashaine Volcano, Northern Tanzania

A group of chrome-spinel peridotite upper-mantle xenoliths fromthe Lashaine volcano, northern Tanzania, differs from otherxenoliths at this locality in containing glassy melt pockets.Modal, mineral chemical and isotopic evidence indicates that,before the melting that was coincident with the xenolith entrainmentand eruption in the Pleistocene, the sub-Tanzanian mantle lithospherehad a complex history. A major element depletion at     

Suitability of monosaccharides as markers for particle identification in carbonate sediments*†

The identification of the remains of organisms contributing to carbonate sediments by means of scanning electron-microscopy is limited to particles of the 2–20 μm size class. Mineralogy and the content of Mg, Sr and trace elements alone are usually insufficient to solve the problem of identification, especially in the differentiation between algal and coral aragonite. The organic matrix of calcareous organisms consists of stable biopolymers such as polysaccharides and glycoproteins which are intimately associated with the carbonate skeleton. Analysis of these hydrolysed compounds gives rise to characteristic arrays of monosaccharides which provide independent criteria for producer identification. The calcareous green algae Halimeda, Penicillus and Udotea show high xylose and low fucose levels. Xylose and fucose levels are elevated in the red algae Amphiroa but only fucose is prominent in the brown algae Padina. The corals Oculina, Porites, Millipora and Montastrea are relatively rich in fucose and show little or no xylose. In the bivalves Arca, Codakia and in Argopecten mannose may be characteristic. Analysis of artificial and natural sediments demonstrates that coral and algal aragonite can be distinguished on the basis of the total sugar concentration and respective xylose and fucose levels. The applicability of the technique in comparison to geochemical and mineralogical methods has been demonstrated for surface sediments from varying water depths of Harrington Sound, Bermuda.     

Rock glaciers, protalus ramparts and related phenomena, Rondane, Norway: a continuum of large-scale talus-derived landforms

Talus-derived landforms from Rondanc National Park, southern Norway, are described and classified as protalus ramparts, valley-floor and valley-side talus-foot rock glaciers, and a 'push-deformation' moraine. A morphological and developmental continuum of talus and derivative large-scale landforrns is proposed. with simple talus slopes at one end and more complex ridge, lobe and bench forms at the other. The various types of feature probably develop from simple talus slopes via separate developmental routes, rather than as a linear sequence. Lichen size and Schmidt hammer R-values were used to indicate the relative ages of the features. Although all are thought to have originated during the early Holocene, they differ in the presence or extent of recent activity. Hence an age and activity continuum is also suggested, the recency of activity increasing in the direction protalus rampart → rock glacier →'push-deformation' moraine.     

Evaluation of Unified Model Microphysics in High-resolution NWP Simulations Using Polarimetric Radar Observations

The UK Met Office Unified Model(UM) is employed by many weather forecasting agencies around the globe. This model is designed to run across spatial and time scales and known to produce skillful predictions for large-scale weather systems. However, the model has only recently begun running operationally at horizontal grid spacings of ～1.5 km [e.g.,at the UK Met Office and the Korea Meteorological Administration(KMA)]. As its microphysics scheme was originally designed and tuned for large-scale precipitation systems, we investigate the performance of UM microphysics to determine potential inherent biases or weaknesses. Two rainfall cases from the KMA forecasting system are considered in this study: a Changma(quasi-stationary) front, and Typhoon Sanba(2012). The UM output is compared to polarimetric radar observations in terms of simulated polarimetric radar variables. Results show that the UM generally underpredicts median reflectivity in stratiform rain, producing high reflectivity cores and precipitation gaps between them. This is partially due to the diagnostic rain intercept parameter formulation used in the one-moment microphysics scheme. Model drop size is generally both underand overpredicted compared to observations. UM frozen hydrometeors favor generic ice(crystals and snow) rather than graupel, which is reasonable for Changma and typhoon cases. The model performed best with the typhoon case in terms of simulated precipitation coverage.     

Ultrabasic Xenoliths and Lava from the Lashaine Volcano, Northern Tanzania

The Lashaine tuff-ring consists of carbonatite tuff and glassyscoria of ankaramitic composition. The pyroclastics encloseejected blocks of country-rock metamorphic rocks and a suiteof ultramafic blocks which are divisible into two groups. Thefirst group, characterized by xenomorphic granular textures,contains rocks comprising varying combinations of pyrope garnet,spinel, magnesian olivine and orthopyroxene, chromiferous diopside,and phlogopite. Analyses are given for garnet lherzolite, lherzolite,harzburgite, and wehrlite and their separate phases. The chemistryof the garnet lherzolite and its phases resembles that of garnetperidotite nodules in kimberlite diatremes, and the A1₂O₂ contentand Ca/Ca+Mg ratio of the clinopyroxenes in the lherzolite andwehrlite indicate more affinities with those in mantle-derivedrocks rather than with peridotites derived by accumulation froma basaltic melt. The phlogopite in a mica garnet lherzolite,that otherwise resembles other mantle garnet peridotites, isan unusual variety containing > 9 per cent TiO₂. The othergroup of ultramafic xenoliths, characterized by cumulate andidiomorphic textures, comprises pyroxenite, with or withoutolivine, mica and amphibole, and mica dunite. Analyses are givenfor a mica dunite and its separate phases. The pressure andtemperature of formation of the various rock-types are estimated,and the relationship of the rocks to each other and to the hostlava is discussed. The chemistry of the host lava is discussedin the light of current experimental data and also in relationto the Northern Tanzania volcanic province. The significanceof the presence of mica in the upper mantle is also discussed.     

The Quantitative Mineralogical Composition and Variation of the Lacorne, La Motte, and Preissac Granitic Complex, Quebec, Canada

Variation of quartz percentage, colour index, potash feldspar/plagioclaseratio, feldspar percentage, and specific gravity within theLacorne, La Motte, and Preissac granitic complex is studiedby means of trend surface analyses based on 198 specimens. Markedpatterns of variation emerge; the more clear-cut patterns relateto massifs with mafic- and potash feldspar-rich centres, andquartzose peripheries. The trend components suggest that theLacorne massif is a composite body, and re-evaluation of thefield evidence supports this prediction. These results are usedto demonstrate a quantitative technique and its applicationto geographically distributed data of granitic complexes; aplea is made for the collection of areally distributed quantitativedata of all types.     

Equilibration and reaction in Archaean quartz-sapphirine granulite xenoliths from the Lace kimberlite pipe, South Africa

Ultrahigh-temperature quartz-sapphirine granulite xenoliths in the post-Karoo Lace kimberlite, South Africa, comprise mainly quartz, sapphirine, garnet and sillimanite, with rarer orthopyroxene, antiperthite, corundum and zinc-bearing spinel; constant accessories are rutile, graphite and sulphides. Comparison with assemblages in the experimentally determined FMAS and KFMASH grids indicates initial equilibration at >1040 °C and 9–11  kbar. Corona assemblages involving garnet, sillimanite and minor cordierite developed on a near-isobaric cooling P–T  path as both temperature and, to a lesser extent, pressures decreased. Garnet-orthopyroxene Fe-Mg exchange thermometers record temperatures of only 830–916 °C. These estimates do not indicate the peak metamorphic conditions but instead reflect the importance of post-peak Fe-Mg exchange during cooling. Correction of mineral Fe-Mg compositions for this exhange using a convergence approach of Fitzsimons & Harley (1994 ) leads to retrieved P–T  estimates from garnet-orthopyroxene thermobarometry ( c . 1000 °C and 10.5±0.7  kbar) that are consistent with the petrogenetic grid constraints. U-Pb dating of a single zircon grain gives an age of 2590±83  Ma, interpreted as the age of the metamorphic event. Protolith major and trace element chemistries of the xenoliths differ from sapphirine-quartzites typical of the Napier Complex (Antarctica) but are comparable to less siliceous, high Cr and Ni, sapphirine granulites reported from several ultrahigh temperature granulite terranes.