Phase relations of osumilite and dehydration melting in pelitic rocks: a simple thermodynamic model for the KFMASH system

 Thermodynamic modelling of (1) osumilite solid solutions and (2) dehydration melting in pelitic compositions within the KFMASH system is quite successful in reproducing the invariant and univariant reactions determined in experimental studies. Even though rather preliminary, such melt thermodynamic models may be very useful in interpolating and extrapolating the limited information available from a small number of experimental runs. These methods allow the compositions of all phases to be monitored as a function of pressure, temperature and equilibrium phase assemblage for any desired bulk composition. Locating the higher variance phase fields (e.g. quadrivariant, quinivariant) is often difficult or impossible by inspection, but is made relatively easy using thermodynamic software such as thermocalc. In the KFMASH system the calculated partition of Fe and Mg between osumilite, garnet, cordierite, orthopyroxene and biotite are shown to be in good agreement with experimental and natural data and allow reliable calculation of mineral compositions coexisting with quartz-saturated and H₂O-undersaturated melts for a variety of bulk compositions. These phase diagram calculations allow quite tight limits to be placed on the pressure, temperature and water activity conditions which accompanied metamorphism of natural osumilite occurrences in Nain, Namaqualand, and Rogaland. At fixed bulk composition, the initial melting of pelites by dehydration of biotite can occur via univariant, divariant or trivariant equilibria depending upon pressure of metamorphism. Of particular interest is that, for low pressures or more magnesian bulk compositions, fluid-absent melting begins by generating liquid from the high-variance assemblage biotite+cordierite+K-feldspar+ quartz. This type of modelling allows investigation, at least qualitatively, of the fine scale details of melt production as a function of changes in pressure, temperature and bulk composition. Received: 29 November 1995 / Accepted: 22 April 1996     

Feasibility and cost of creating an iron-phosphate coating on pyrrhotite to prevent oxidation

 Acid mine drainage (AMD) occurs when sulfide minerals are exposed to an oxidizing environment. Most of the methods for preventing AMD are either short-term or high cost solutions. Coating with iron phosphate is a new technology for the abatement of AMD. It involves treating the sulfide with a coating solution composed of H₂O₂, KH₂PO₄, and sodium acetate as a buffer agent. The H₂O₂ oxidizes the sulfide surface and produces Fe³⁺ so that iron phosphate precipitates as a coating on the sulfide surface. Experiments performed under laboratory conditions prove that an iron phosphate coating can be established on pyrrhotite surfaces with optimal concentrations of the coating solution in the range of: 0.2M/0.01M H₂O₂, 0.2M KH₂PO₄, and 0.2M sodium acetate NaAc, depending on the experimental scale. Iron phosphate coating may be a long-term solution to the problem of AMD. The method would be easy to implement; the reagent cost, however, is not low enough, although it is lower than the conventional treatment with lime. Received: 30 March 1995 · Accepted: 6 September 1995     

A new, instantaneous aeolian sand trap design for field use

A new aeolian sediment trap is described which can give up to 1 Hz measurement frequency in field conditions. The trap adopts a circular, horizontal trap design with a load cell connection to give continuous, unobtrusive trap measurement of sediment flux. Simultaneous velocity recording is carried out using an anemometer. Trap construction costs are approximately £200. Initial results in field conditions using a direct comparison of wind velocity data, sampled at an equivalent frequency, have given a first-order relationship between sediment flux and velocity. The trap enables simultaneous sampling of wind velocity and sediment flux at a sufficiently short interval to enable investigation of sediment transport dynamics under a variety of field conditions.     

A note on thermomechanical anisotropy of clays

Deep clays exhibit a pronounced strain anisotropy both during mechanical loading as well as during heating and cooling at constant stress in drained isotropic conditions. During mechanical loading vertical strain is larger than the horizontal one. During heating the vertical strain is larger than the horizontal one within the elastic range; the opposite is observed in the elasto-plastic range. The above described response can be interpreted adopting a consistent rotational, kinematic hardening thermo-elasto-plastic constitutive law.     

Thermomechanical properties of deep argillaceous formations

A difficulty in the interpretation of mechanical and thermo-mechanical tests on specimens drawn from large argillaceous formations is the strong inhomogeneity of void ratio, clay minerals and carbonates content.

In this paper a relationship is developed to link strength and the maximum preconsolidation stress to the initial void ratio and carbonate content. Compressibility is also correlated to carbonates.

Thermal strains in drained and undrained conditions for a Spanish, a belgian and an Italian natural clay are compared. In the elastic state strains are comparable, while in the plastic range thermal strains are highest for the Belgian clay, lower in the Spanish cemented clay and lowest in the Italian clay, very stiff and cemented.     

Stratigraphy of the upper cretaceous and lower tertiary strata in the Tethyan Himalayas of Tibet (Tingri area,China)

The 1500-m-thick marine strata of the Tethys Himalaya of the Zhepure Mountain (Tingri, Tibet) comprise the Upper Albian to Eocene and represent the sedimentary development of the passive northern continental margin of the Indian plate. Investigations of foraminifera have led to a detailed biozonation which is compared with the west Tethyan record. Five stratigraphic units can be distinguished: The Gamba group (Upper Albian - Lower Santonian) represents the development from a basin and slope to an outer-shelf environment. In the following Zhepure Shanbei formation (Lower Santonian - Middle Maastrichtian), outer-shelf deposits continue. Pebbles in the top layers point to beginning redeposition on a continental slope. Intensified redeposition continues within the Zhepure Shanpo formation (Middle Maastrichtian - Lower Paleocene). The series is capped by sandstones of the Jidula formation (Danian) deposited from a seaward prograding delta plain. The overall succession of these units represents a sea-level high at the Cenomanian/Turonian boundary followed, from the Turonian to Danian, by an overall shallowing-upward megasequence. This is followed by a final transgression — regression cycle during the Paleocene and Eocene, documented in the Zhepure Shan formation (?Upper Danian - Lutetian) and by Upper Eocene continental deposits. The section represents the narrowing and closure of the Tethys as a result of the convergence between northward-drifting India and Eurasia. The plate collision started in the Lower Maastrichtian and caused rapid changes in sedimentation patterns affected by tectonic subsidence and uplift. Stronger subsidence and deposition took place from the Middle Maastrichtian to the Lower Paleocene. The final closure of remnant Tethys in the Tingri area took place in the Lutetian.     

The Silurian of Gotland (Sweden): facies interpretation based on stable isotopes in brachiopod shells

The Silurian of Gotland, Sweden, consists of 440 m of carbonate deposits. Repeatedly, uniform sequences of micritic limestones and marls are interrupted by complex-structured reefs and by adjacent platform sediments. Generally, the alteration of facies is interpreted as the result of sea-level fluctuations caused by a gradual regression with superimposed minor transgressive pulses. The purpose of this study is a facies interpretation based on both field observations and stable isotope measurements of brachiopod shells. Approximately 700 samples from stratigraphically arranged localities in different facies areas have been investigated. The carbon and oxygen isotopes show principally parallel curves and a close relationship to the stratigraphic sequence. Lower values occur in periods dominated by deposition of marly sequences. Higher values are observed in periods dominated by reefs and extended carbonate platforms. The oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input, rather than to paleotemperature. Carbon isotope ratios are believed to have been connected to global changes in the burial of organic carbon in black shales during periods of euxinic deep water conditions. Consequently, the facies succession on Gotland results from global paleoclimatic conditions. Changes in terrigenous input due to different rates of weathering and freshwater runoff, rather than sea-level fluctuations, control the carbonate formation of the Silurian on Gotland.