Masters Theses

Date of Award

12-1981

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Lawrence A. Taylor

Committee Members

Otto Kopp, Harry McSween

Abstract

Compositions of coexisting phases from the ilmenite-hematite and ulvöspinel-magnetite solid solution series have commonly been used to determine the temperature and fO2 of formation for many igneous and metamorphic rocks (Buddington-Lindsley geothermometer/oxybarometer). Temperatures determined by this method for some igneous rocks are lower than might be expected for magmatic crystallization. These low temperatures are probably the result of slow cooling rates which allow the Fe-Ti oxides to reequilibrate. The reequilibration process is one in which titano-magnetites are oxidized and ilmenites are reduced upon cooling along a buffer curve or in the presence of a fluid of constant composition. Textural studies indicate that the reduction of the ilmenite-hematite phase may be the slower reaction and, consequently, the rate-controlling step for the reequilibration process.

The kinetics of the reduction reactions in the two-phase region between the joins for the solid solution series were studied by thermogravimetric analysis using CO-CO2 gas mixtures to control the fO2. The fO2 was measured by a solid ceramic electrolyte cell. During experiments conducted near the Ni/NiO and QFM buffers, annealed ilmenite(ss)-rich synthetic charges were reduced isothermally by rapidly lowering the fO2 approximately 1.25 log units. The temperature range for the experiments was 900°C to 1200°C with total gas flow rates of 0.35 and 1.0 cm/sec. Kinetic data were collected by measuring the weight change during each experiment as a function of time. A textural study combined the results from these experiments with those obtained from three oxidation and other reduction experiments conducted in the temperature range of 900°C to 1200°C with &Dlta;fO2 varying from 0.75 to 3.0 log units. Additionally, a cooling-rate (0.5°C/hr) experiment was conducted near the QFM buffer from 1150°C to 985°C starting with an ilmenite-saturated spinel phase.

At 1200°C and 0.35 cm/sec flow rate, the reaction mechanism was an interfacial process where mass transport of the reacting gases is seen as the rate-controlling step. In all other isothermal experiments, diffusion of reactants through a solid product layer was the controlling mechanism. The activation energy for this diffusion process is 59 ± 6 kcal/mole. A comparison of diffusion coefficients with those in the literature indicates that significant enhancement of diffusion occurred along grain boundaries.

The growth of magnetite lamellae that formed in the ilmenite host during the reduction experiments increased with the reaction rate until, at the highest diffusivities, they became so coarse that only composite (granule) intergrowths were present in the final products. Similar results were obtained in the three oxidation experiments. A distinct gradation of ilmenite intergrowths, oriented along crystallographic planes of titano-magnetite products from the reduction experiments, occurred in the outer 100-200 µm of the charges. These textures appear to be directly related to the initial driving force of reaction and may be the result of diffusional processes. The cooling-rate experiment produced a completely reequilibrated assemblage with external granule "exsolution" textures.

These preliminary kinetic experiments indicate that extreme caution must be exercised when applying this ilmenite/titano-magnetite geothermometer/oxybarometer in nature. It may only truly "quench" in the parameters of formation in the case of certain hypabyssal and extrusive occurrences. It is of dubious significance for metamorphic situations.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS