Metamorphism and Structure of Penfold Creek Area, near Quesnel Lake, British Columbia

The Quesnel Lake area lies within the Omineca Crystalline Belt,and is underlain by the northern extremity of the Shuswap MetamorphicComplex. Closely spaced and steeply dipping isograds mark themargins of the metamorphic belt. In the Penfold Creek area onlyone and one-half miles separate the biotite and sillimaniteisograds. Related to this sharp increase in metamorphic gradethere is a marked change in the fold style. In the chloritezone are similar folds, showing a strong axial-plane cleavage,and tight refolded isoclines dominate in the sillimanite zone.Three periods of deformation and two periods of prograde metamorphismhave been recognized, with the first metamorphic period beingassociated with Phase 2 deformation and the second being post-Phase2. Regression analysis of possible mineral reactions in these rocksshows that they formed under conditions of a gradient in temperatureand possibly H₂O activity, and that most of the assemblages,although containing many phases, have at least two degrees offreedom. Thermodynamic analysis of mineral equilibria indicatesthat equilibrium was closely approached between solid solutionsinvolving garnet, plagioclase, biotite, white mica, and staurolitein the presence of Al₂SiO₅, quartz, and H₂O. The conditionsof metamorphism of rocks containing Al₂SiO₅ are estimated tobe 7000 ± 1500 bars, 680 ± 30 °C, activityof H₂O = 0.80 (approx.). Partial melting did not occur becauseof the reduced activity of H₂O. ^*Present address: Overseas Division, Institute of Geological Sciences, 5 Princes Gate, London SW7, England.     

The electrification of flowing gases by mechanical abrasion of mineral surfaces

In the past, a number of workers have shown that both charged and neutral particles are released from inorganic materials during fracture and abrasion. In this work we show that charge can be transported by electrification of flowing gases over a surface undergoing abrasion. These results are relevant to possible mechanisms of a number of observed electrical phenomena associated with fault motion and earthquake activity. Received: 14 July 1997 / Revised, accepted: 2 January 1998     

Evaluation of one-dimensional in situ leaching processes

A methodology to characterizing processes of in situ leaching is developed to investigate various parameters that may affect the recovery of a valuable mineral and assure a successful application of the in situ leaching technology. The governing equations of in situ leaching processes for the one-dimensional case are solved both analytically and numerically for both the consumption rate of a lixiviant and the production rate of a target mineral. A numerical simulator, which is developed to evaluate coupled effects among leaching kinetics, solution flow and transport of the dissolved mineral species in saturated ore deposits, is validated against the steady-state solutions and applied to investigate the transient effects of various parameters on the mineral recovery. Results from the evaluation indicate that there exists an optimal flow velocity range of leach solution for the effective leaching of a particular ore deposit. The determination of this optimal leaching velocity may become a key to the design of a real in situ leach mine. Results of the parameter sensitivity study illustrate the relative importance of other parameters such as porosity, ore grade, and reaction rate. Copyright © 1999 John Wiley & Sons, Ltd.     

Geochronology of zircon megacrysts from nepheline-bearing gneisses as constraints on tectonic setting: implications for resetting of the U-Pb and Lu-Hf isotopic systems

Nepheline-bearing gneisses from the 75 km² Tambani body in the Mozambique Belt of southern Malawi, are miaskitic biotite-nepheline monzodiorites, reflecting an absence of K-feldspar, alkali amphiboles or pyroxenes, and contain euhedral zircon megacrysts up to 5 cm across. The zircons contain U = 1–1,860 ppm, Th = 0–2,170 ppm and Y = 400–1,100 ppm, and very low concentrations of all other measured trace elements except Hf (HfO₂ = 0.53–0.92 wt. %). Cathodoluminescence images reveal oscillatory sector growth zoning and no evidence for xenocrystic cores, indicating that the zircons represent primary magmatic crystallization products that have survived amphibolite grade metamorphism. U-Pb isotopic analyses (by TIMS) yield an upper intercept age of 730 ± 4 Ma (MSWD = 1.7), which we interpret as the time of magmatic crystallization of the zircons. This is coincident with 11 SHRIMP spot analyses, which yield a mean age of 729 ± 7 Ma (MSWD = 0.37). Metamorphism, at 522 ± 17 Ma as suggested by monazite, caused partial Pb-loss during local recrystallization of zircon. Lu-Hf isotopic data for three whole-rock samples of nepheline-bearing gneiss are collinear with those for zircon megacrysts, and correspond to an age of 584 ± 17 Ma (MSWD = 0.37. We interpret the Lu-Hf array to represent a mixing line defined by the Hf isotopic signature of primary zircon and that of the rock-forming minerals reset during metamorphic (re-)crystallization; hence the 584 Ma age is likely geologically meaningless. Given the well-defined association of nepheline syenites (and phonolitic volcanic equivalents) with continental rifting, we suggest that the Tambani body represents a magmatic product formed at 730 Ma during the break-up of the Rodinia supercontinent. The 522 Ma age is akin to other Pan-African metamorphic ages that record collisional suturing events during the final assembly of Gondwana. Zircon-bearing nepheline gneisses thus preserve a record of intra-continental rifting and of continental collision in southern Malawi.     

Stochastic simulation of patterns using Bayesian pattern modeling

In this paper, a Bayesian framework is introduced for pattern modeling and multiple point statistics simulation. The method presented here is a generalized clustering-based method where the patterns can live on a hyper-plane of very low dimensionality in each cluster. The provided generalizationallows a remarkable increase in variability of the model and a significant reduction in the number of necessary clusters for pattern modeling which leads to more computational efficiency compared with clustering-based methods. The Bayesian model employed here is a nonlinear model which is composed of a mixture of linear models. Therefore, the model is stronger than linear models for data modeling and computationally more effective than nonlinear models. Furthermore, the model allows us to extract features from incomplete patterns and to compare patterns in feature space instead of spatial domain. Due to the lower dimensionality of feature space, comparison in feature space results in more computational efficiency as well. Despite most of the previously employed methods, the feature extraction filters employed here are customized for each training image (TI). This causes the features to be more informative and useful. Using a fully Bayesian model, the method does not require extensive parameter setting and tunes its parameters itself in a principled manner. Extensive experiments on different TIs (either continuous or categorical) show that the proposed method is capable of better reproduction of complex geostatistical patterns compared with other clustering-based methods using a very limited number of clusters.     

Holocene changes on a peatland in northwestern Ontario interpreted from testate amoebae (Protozoa) analysis

Radiocarbon-dating and analyses of fossil testate amoebae (Protozoa) have established changes in soil moisture conditions on the developing surface of a Sphagnum -dominated peatland near Emo in northwestern Ontario.
The distribution and composition of modern testate amoebae communities were studied from peatlands in the region of Ontario and Minnesota as a guide to interpreting fossil assemblages. Although the core spans all of the Holocene, fossil testate amoebae were recovered only from the part post-dating 6500 BP. Earliest testate amoebae assemblages associated with bryophytic and cyperaceous-rich fen peat are dominated by species in the genera Cyclopyxis and Centropyxis . By 5000 BP, Amphitrema Jraaum, Assulina muscorum, Heleopera sphugni and Hyalosphenia subjaoa become important species as Sphagnum-rich peat accumulated at the site. Present-day microtopographic differentiation probably developed during historic time when the site became progressively drier, as indicated by a change of Nebela griseola, N. militaris and Trigonopyxis arcula . Although it is possible to derive quantitative estimates of changing soil moisture conditions from testate amoebae, care should be taken in interpreting results, particularly from non-Sphagnum-rich peats, until more is learned about the distribution and ecology of modern faunas.     

Controlled moraines: origins,characteristics and palaeoglaciological implications

Controlled moraines are supraglacial debris concentrations that become hummocky moraine upon de-icing and possess clear linearity due to the inheritance of the former pattern of debris-rich folia in the parent ice. Linearity is most striking wherever glacier ice cores still exist but it increasingly deteriorates with progressive melt-out. As a result, moraine linearity has a low preservation potential in deglaciated terrains but hummocky moraine tracts previously interpreted as evidence of areal stagnation may instead record receding polythermal glacier margins in which debris-rich ice was concentrated in frozen toe zones. Recent applications of modern glaciological analogues to palaeoglaciological reconstructions have implied that: (a) controlled moraine development can be ascribed to a specific process (e.g. englacial thrusting or supercooling); and (b) controlled moraine preservation potential is good enough to imply the occurrence of the specific process in former glacier snouts (e.g. ancient polythermal or supercooled snouts). These assumptions are tested using case studies of controlled moraine construction in which a wide range of debris entrainment and debris-rich ice thickening mechanisms are seen to produce the same geomorphic features. Polythermal conditions are crucial to the concentration of supraglacial debris and controlled moraines in glacier snouts via processes that are most effective at the glacier–permafrost interface. End moraines lie on a process–form continuum constrained by basal thermal regime. The morphological expression of englacial structures in controlled moraine ridges is most striking while the moraines retain ice cores, but the final deposits/landforms tend to consist of discontinuous transverse ridges with intervening hummocks, preserving only a weak impression of the former englacial structure. These are arranged in arcuate zones of hummocky moraine up to 2 km wide containing ice-walled lake plains and lying down flow of streamlined landforms produced by warm-based ice. A variety of debris entrainment mechanisms can produce the same geomorphic signature. Spatial and temporal variability in process–form relationships will lead to the sequential development of different types of end moraines during the recession of a glacier or ice sheet margin.     

Isotopic constraints on the Easter Seamount Chain source

New isotopic data (Sr, Nd and Pb) for lavas from Easter and Salas y Gomez Islands, and nine seamounts along the length of the Easter Seamount Chain (ESC) to the east of these islands, exhibit a regular behavior that is consistent with two-component mixing between a MORB-like source and more enriched mantle (the Easter Plume source) characterized by relatively radiogenic Pb and unradiogenic Sr. Based on the linearity of mixing trends on the isotope diagrams, the plume source appears to have maintained approximately the same isotopic composition over the 20 to 30 Ma history of the Easter Plume sampled by the seamounts. In current nomenclature, the plume source resembles a mix of predominantly HIMU mantle with a small amount of EMI. The proportions of MORB and plume source mantle sampled by the lavas erupted along the ESC vary in a systematic way, with Easter Island and seamounts to the west containing larger fractions of the MORB component than Salas y Gomez Island and the older seamounts to the east. This variability is probably a reflection of the nearness to the spreading center, and therefore lithospheric thickness, at the time of volcanism Received: 27 April 1998 / Accepted: 11 December 1998     

上部地壳的流体作用与大理岩的低温塑性

本文应用光学显微镜、透射电子显微镜和阴极发光显微镜系统研究了Ｗａｔｅｒｂｅｒｇ断层带（纳米比亚）内粗晶大理岩中发育的构造岩,宏观碎裂结构与微观糜棱装结构是断层构造岩的主要特点。巨大的孪形碎屑与弥漫的细小粒质形成了鲜明的反差,广泛发育的压溶颖合线与多阶段方解石细脉遍布构造带不同部位,ＴＥＭ亚微粒粒度具有简单的峰值分布（～０．３～０．４μｍ）,但动态重结晶颗粒的粒度却具有较大的变化范围（０．１～３．０