H. F. Wang

Thermobarometry, diffusion modelling and cooling rates of crustal garnet peridotites: Two examples from the Moldanubian zone of the Bohemian Massif

Abstract Different origins for crustal garnet peridotites are illustrated by two Variscan bodies in adjacent, tectonically juxtaposed terranes in the Moldanubicum of the Bohemian Massif (eastern Moravia). The Nove Dvory garnet peridotite contains lenses of eclogite and is allochthonous and allofacial with respect to surrounding Gfohl gneiss. The Mohelno spinel-garnet peridotite is devoid of eclogite and has concordant contacts with surrounding felsic granulite. The bulk of the Mohelno body is spinel peridotite, and garnet occurs only along its margins, where it has formed at the expense of spinel. Thermobarometry of mineral cores from garnet-bearing assemblages yields values for Nove Dvory from 1195°C, 54,6 kbar to 970°C, 41.8 kbar and for Mohelno from 1265°C, 27.9 kbar to 1070°C, 23.4 kbar. The compositional zoning profiles in garnets at both localities are matched closely by diffusion modelling in which bilinear cooling rates are utilized. At Nove Dvory initial cooling rates of 50–200°C/Ma are followed by slower rates of 10–50°C/Ma. In contrast, at Mohelno an extremely rapid initial cooling rate of 1°C/year was followed by one of 500°C/Ma. An idealized tectonic model is proposed for the emplacement of the Mohelno peridotite to account for its cooling history. The predicted garnet zoning profiles that result from conductive cooling agree closely with the observed zoning profiles. Both peridotites were derived from the upper mantle. The Nove Dvory body was emplaced from a deep fault zone into the crust as garnet peridotite, and the Mohelno body was emplaced as a hot slab of spinel peridotite, in which garnet formed only along contacts as the body cooled into the garnet stability field.

A thermal-tectonic model for high-pressure rocks in the Basal Gneiss Complex of western Norway

Abstract The Basal Gneiss Complex (BGC) of western Norway is a segment of continental crust that was subjected to eclogite facies metamorphism during the Caledonian Orogeny and then was overprinted by amphibolite facies conditions. Numerical methods have been used to construct a model for thermal evolution of the BGC. The calculated temperature-depth-time ( Tzt ) paths for the BGC are in good agreement with the sequence of mineral assemblages that occur in metamorphosed mafic and ultramafic rocks. However, the thermal model indicates that retrograde zoning of minerals in the garnet-bearing assemblages of eclogite and ultramafite may have developed metastably at pressures below 10 kbar. Numerical modeling of iron diffusion in garnet grains adjacent to olivine inclusions was used to calculate zoning profiles based upon the thermal model. The calculated zoning profiles have shorter decay distances than the observed zoning profiles, which may be due to uncertainties in the diffusion coefficients. Also, discrepancies occur between the Tzt paths and specific PT values calculated for eclogite and garnet ultramafite due either to the preservation of pre-Caledonian mineral assemblages or to utilization of geobarometers and thermometers which are based on different elements in different phases (such as A1 in orthopyroxene and FeMg in olivine and garnet).

An Experimental Investigation of Distributed Acoustic Sensing (DAS) on Lake Ice

ABSTRACT A vibration-sensitive, Distributed Acoustic Sensor (DAS) array, using fiber-optic cables, was deployed in a triangularly shaped geometry on the frozen surface of Lake Mendota in Madison, Wisconsin, USA. The purpose of the array and testing program was to analyze the DAS response and to utilize the high spatial density of the distributed array for system response characterization in a well-constrained, small, surface array. A geophone array was also deployed to provide a reference system. The design of the array allowed us to assess the response of DAS with respect to distance from the seismic sources, the degradation of the response with length of the cable, the directivity of the fiber response with respect of the direction of the particle motion, and the quality of the signal with respect to cable type. The DAS array was examined for different cable constructions and orientations relative to the source propagation direction. Tight-buffered and loose-tube fiber-optic cable constructions were use...

The influence of pore pressure and confining pressure on dynamic elastic properties of Berea sandstone; discussion and reply

The authors present their results as if Berea sandstone were an elastic material; that is, velocities are given as functions of confining and pore pressure. In fact, most rocks are inelastic and velocities depend on the history of the confining and pore pressure, and not just on the present values. Some measurements of hysteresis were reported by Gardner et al. (1965). The confining pressure was cycled between two pressures Pc(1) and Pc(2) for a fixed pore pressure Pp, following a fixed schedule of pressure changes, until repeatable values of velocity were obtained. (At any intermediate pressure the velocity measured for increasing pressure was different from the value for decreasing pressure, giving rise to a hysteresis cycle). When the same schedule of pressure changes for the differential pressure Pc-Pp was followed by holding Pc fixed and varying Pp, the measured velocities followed the same hysteresis curve within the limits of experimental accuracy. In brief, when hysteresis was taken into account, ...