Dynamics of Development of Orthomagmatic and Mixing Fluid Systems

Aleksey Zhmodik (zhmodik@uiggm.nsc.ru) & Viktor Sharapov (vik@uiggm.nsc.ru)

Institute of Geology, pr. Koptuga, 3, Novosibirsk, Russia

A physical self-consistent model of dynamics of separation of volatiles from basic melt after filling out an intrusion chamber in the system "magma center ­> conduit ­> magma chamber" is constructed. A combined numerical criterion was proposed allowing one to divide these regimes for different initial and boundary conditions of magma cooling. The COMAGMAT-3.0 program and An-Di-H₂O diagram were used as phase equilibrium models needed to calculate the volatiles separation process. The minimum water content in basic magma at which ore components are removed by magmatic gas from cooling intrusive bodies at depths 2-4 km is estimated at 0.25-0.2 wt.%. It is has been found that, when the initial content of water in basic magma is 0.35-1.5 wt.%, orthomagmatic fluid system form in the above intrusion space of a magmatic shallow body. They have a zone of magmatic gas filtration at the chamber exocontact, extends for 300-2500 m depending on the depth of the roof of the chamber, initial content of water in melt, and permeability of igneous and host rocks. In fractured crust, a filtration zone of liquid condensates of magmatic gas can be three or four times longer or shorter, compared with the above case, depending on the permeabilities of various rock horizons beneath the bottom of rift valley. This research was supported by grants from Russian Foundation of Basic Research (grant N 98-05-64005 and grant N 99-05-64599).

Ir in the Sulfides Fe, Cu, Pb, Zn: Results Hydrothermal Synthesis with Application Radioisotope ¹⁹²Ir

Sergey Zhmodik (zhmodik@uiggm.nsc.ru) & Gennadiy Shvedenkov

UIGGM, pr. Koptuga, 3, 630090, Russia

In the latest year there is a data about high concentrations of PGE in the low temperature formations, such as black schist, hydrothermal deposits of gold, oceanic hydrothermal ores. Experiments on hydrothermal synthesis of Ir-bearing sulfides-Fe, -Cu, -Pb, -Zn has been realized (steel autoclaves with Ti-ampoules, P=500 bar, T=400°C, gradient 14°C). The concentrations of Ir were from 11 ppm up to 74 ppm in the initial mixture (iridium chloride). Ir, beforehand "marked" by radioisotopes ¹⁹²Ir within the reaction ¹⁹⁰Ir(n,(gamma))¹⁹²Ir. Spatial distribution of Ir in the synthesized minerals determined with the method of beta-autoradiography. As a result of experiments were received the polycrystal and crystals of pyrite, pyrrhotite, chalcopyrite, bornite, chalcosine, marmatite, kleiophan and galena. Established that Ir does not embed into the structure nor one of the synthesized minerals, but concentrates on the surfaces of growing crystals and polycrystal component, on the border a crystal-solution. Very dispersed including-concentrators of Ir (probably sulfides Ir) are established in some cases. Marmatit formed first and does not be kept an iridium in polycrystal components of sphalerite. Cleiophan forms small crystals and fills space between crystals of marmatit. Ir is accumulated in intergranular space of marmatit exactly. Ir concentrates on borders of small crystals of cleiophan. The most high concentrations of Ir there is in porous colloform pyrite in the form of microinclusion and disperse form. Ir is accumulated in the remaining products in crystallization process. Ir not discovered in working solution after the experiment. Data of autoradiography show that coefficient of distribution between sulfide minerals and solution probably much less then 1. Distribution of an Ir between growing crystals of sulfides and solution corresponds to the adsorption equilibrium. Data also be indicative of that findings of coefficients of distribution of Ir, tinned on the base of the bulk analysis, can not correspond reality.

Experimental Study of Partitioning of Trace Elements between Carbonate Minerals and Natural Waters

Shaojun Zhong (szhong@ggl.ulaval.ca)

Department of Geology & Geological Engineering, Laval University, Quebec, Canada

Carbonate minerals (calcite, aragonite, dolomite, etc.) are important components of soils and sediments and are commonly found in rock veins. They contain a variety of trace elements in their crystal structures that may have encoded important geological, hydrological or paleoenvironmental information. For example, concentrations of trace elements such as Mg, Sr, Cd, Zn, REEs in marine carbonate minerals have been used as proxies in paleo-oceanographic studies. It is therefore important to determine the trace element partition coefficients between carbonate minerals and various natural waters and to obtain quantitatively the influences of various geophysical and geochemical factors on the partitioning. For these purposes, a simple experimental system capable of maintaining a steady-state condition for the carbonate mineral precipitation and the trace element partitioning reactions was developed. It has been used to determine the partition coefficients of a number of trace elements (Mg, Sr, Cd, Zn, As, Pb, Cu, REEs) in calcite precipitated from natural waters under well defined conditions.

Silica Speciation in Aqueous Fluids at High Pressures and Temperatures

Nikolay Zotov (nikolay.zotov@uni-bayreuth.de) & Hans Keppler (hans.keppler@uni-bayreuth.de)

Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

The speciation of silica dissolved in water in equilibrium with solid quartz was experimentally studied up to 900°C and 15 kbar using an externally heated Bassett-type diamond-anvil cell and micro-Raman spectroscopy. Dissolved silica species were identified by comparing the measured spectra with calculated normal mode frequencies and Raman intensities (Zotov et al., 2000). Below 600°C and 5-8 kbar pressure, depending on the fluid density, mainly orthosilicate acid (H₄SiO₄) monomers exist. At higher temperatures and pressures pyrosilicic acid dimers (H₆Si₂O₇) become increasingly abundant due to polymerization. The equilibrium constant logK of the polymerization reaction 2H₄SiO₄ = H₆Si₂O₇ + H₂O is 0.33 at P=5 kbar and T=1000 K and increases with increasing temperature. The enthalpy of the polymerization reaction at P=5 kbar is 15.7±0.2 kJ/mol and decreases with increasing pressure. The change of the molar volume of the solute V_sol = V_H6Si2O7 - 2V_H4SiO4 in the pressure range 5-15 kbar is practically independent of pressure (about -46±2 cm³/mol) but increases slightly with decreasing temperature. The equilibrium constant of the polymerization reaction increases strongly with pressure at constant temperature, which means that the polymerization is enhanced mainly by pressure, while the effect of temperature at a constant pressure is much smaller.

Zotov N & Keppler H, American Mineralogist, 85, in press, (2000).