Experimental Testing of the Garnet-Olivine Geothermometer at Extreme Redox Conditions

Sonja Aulbach & Gerhard Peter Brey (brey@em.uni-frankfurt.de)

Institute of Mineralogy, University of Frankfurt, Senckenberganlage 28, D-60054 Frankfurt, Germany

The garnet-olivine geothermometer by O'Neill and Wood (1979) based on the exchange of Fe²⁺ and Mg is widely and successfully used in mantle petrology. An inherent state of uncertainty, however, is the oxidation state of iron both in the natural samples applied to and in the original experiments. The magnitude of the possible error was evaluated in some recent papers by determining the Fe³⁺ content in garnet by Mössbauer spectrometry. Experimental testing was lacking so far. We have carried out experiments at 5 GPa and 1200 and 1350°C at reducing and oxidizing conditions in the system FMAS and reversed the Fe³⁺ content in garnet by using a pyrope-almandine garnet free of Fe³⁺ in one starting material and an Fe³⁺-bearing pyrope-almandine-skiagite garnet in a second together with enstatite, olivine and magnetite (for oxidizing conditions) and metallic iron (for reducing conditions). Capsules were made from Ag, Mo or metallic Fe welded into Pt. Run products were grt, opx, sp and olivine with about Fo₈₀ at oxidizing conditions. At reducing conditions with metallic iron and Fe-capsules all orthopyroxene was reacted out and olivine with about Fo₆₂ formed. All phases were analysed with the electron microprobe. Temperatures calculated with the O'Neill and Wood geothermometer yielded for the 1200°C experiments 1290 and 1205°C for the two runs in the Fe-capsules. The two oxidizing runs in Ag-capsules gave 1010 and 1005°C for all iron calculated as Fe²⁺ and 1340 and 1310°C for Fe³⁺ calculated from the structural formula of garnet.

O'Neill HSt.C & Wood BJ, Contrib. Mineral. Petrol., 70, 59-70, (1979).

Study of the Formation Conditions of the Gold-Quartz-Faded Ore Mineralization (East Sayan, Russia) by Thermobarogeochemical and Autoradiographical Methods

Evgenia Ayryants (zhmodik@uiggm.nsc.ru), Sergey Zhmodik (zhmodik@uiggm.nsc.ru) & Nadejda Gibsher

Geological Institute, pr. Koptuga 3, Novosibirsk, 630090, Russia

In the East Sayan exists a group of deposits with the gold-quartz-faded ore mineralization, which be situated to the contact of carbonate rocks of cornstonical rock mass (PR2) with granitogneisses. Orebodies are present of quartz veins, which are located in limestones. Mineral composition of ore is unusual for the given region, since practically are absent sulfides Fe. The distribution gold is extremely irregularly. Methods of autoradiography were installed that the grains gold be situated to places of clustered sulfides. The autoradiographical data is indicative of probable entering Au in the structure of galena. There is definite type of the quartz for which distinctive diffusion Thermobarogeochemical study (methods of homogenization, decrepitation, of gas-fluid inclusions and gas chromatography) shown that the forming mineralithation occurred in several stages. The first - forming a early quartz at temperatures 320°-360°C. The second - a process of collecting recrystallization at the temperature 220°-270°C. With reduction of temperature fluid saturation was increased, but itself fluid was enriched by carbon dioxide. The sections with diffusion distribution of gold correspond to this type of the quartz. The quartz, which has felt local stressful deformation, has quite a number of including with the organic material. The last - a formation of post-ore quartz at the temperature 180°-210°C. Using the methods thermobarogeochemistry and autoradiography has allowed to determine different forms of distribution gold: native, mineral, diffused and membranaceous. The sedimentation gold and ore minerals occurred at the temperature 220-270°C. Composition of fluid was changed aside increase fluid saturation and part of CO₂ to moment deposits. Presence of the diffused form of existence gold in quartz was evidence about carrying gold with silictous acid. The following deformation was given rise to redistribution gold with formation of difussion form and probably grains gold.

Electrical Conductivity and Pressure Dependence of Trigonal-to-Cubic Phase Transition in Lithium Sodium Sulphate

Nikolai Bagdassarov (nickbagd@geophysik.uni-frankfurt.de)¹, Hans-Christof Freiheit (freihei@nwz.uni-muenster.de)² & Andrew Putnis (putnis@nwz.uni-muenster.de)²

¹ Institut für Meteorologie und Geophysik, Universität Frankfurt, Feldbergstrasse 47, 60323 Frankfurt, Germany

² Institut für Mineralogie, Universität Münster, Correnstraße 24, 48149 Münster, Germany

The pressure and temperature dependence of electrical impedance as a function of frequency (from 0.1 Hz to 100 kHz) has been measured in crystalline LiNaSO₄ over temperature range 400-800°C and pressures 5, 10 and 20 kbars in a piston-cylinder apparatus. The cell for electrical impedance measurements represents a co-axial cylindrical capacitor with a geometric factor 5.5-7 cm. LiNaSO₄ undergoes a displacive first order phase transition (e.g. Freiheit et al., 1998) from trigonal structure with a space group P31c to a body-centred cubic structure bcc with a high ionic conductivity > 0.1 S/cm at 10 kHz. The bulk resistance has been estimated at each temperature from Argand plots. According to electrical impedance drop at 5 kbar the phase transition occurred at 584°C (±1), at 10 kbar at 641°C and at 20 kbar at 764°C. The temperature of the phase transition has been estimated from plots of log[(sigma) * T] vs. 1/T. By cooling there is small hysteresis of phase transition temperature, the hysteresis disappears with the pressure increase. The slope dT/dP estimated from these experiments is 12.5°C/kbar and corresponds to Clausius-Clayperon slope, S ~26.4 J/K/mol and V ~ 3.3 cm³/mol, of this phase transition. In the extrinsic region the electrical conductivity has an activation energy which varies from 1.48 eV at 5 kbar to 1.35 eV at 20 kbar, in the intrinsic region the activation energy (E_a) is ca. 2.4 - 2.30 eV. In the high conductive bcc-phase E_a is ca. 0.33-0.4 eV. E_a in bcc-phase does not depend on pressure and corresponds to the value at 1 bar (Gundusharma et al., 1986; Lundén, 1988). With the pressure increase temperature intervals of extrinsic and intrinsic conductivities start to overlap, which results in a non-Arrhenian temperature dependence. The frequency dependence of the electrical conductivity at low frequencies (< 500 Hz) may be approximated as (sigma)= (sigma)₀+const*(omega)ⁿ, where (omega) is frequency and n is experimental exponent. With the temperature increase from extrinsic region to fast ionic phase n increases from 0 to ~0.5. In the fast ionic bcc n does not change with temperature.

Gundusharma UM, MacLean C, Secco EA, Solid State Commun., 57, 479-481, (1986).

Lundén A, Solid State Commun, 65, 1237-1240, (1988).

Freiheit H-Ch, Kroll H, Putnis A, Zeitschrift für Kristall, 213, 575-584, (1998).