T95 Experimental Investigations into Hydrothermal Systems: Implications for Mass Transfer in the Earth’s Crust

Monday, 23 October 2006
1:30 PM - 5:30 PM

EXPERIMENTAL EVIDENCE OF NON-REDOX TRANSFORMATION BETWEEN MAGNETITE AND HEMATITE UNDER H2-RICH HYDROTHERMAL CONDITIONS

Paper number: T95-13
Program: Technical Sessions
Time: 4:45 PM – 5:00 PM

Authors

Abstract

Magnetite (Fe3O4) and hematite (Fe2O3) are used to constrain the redox conditions of fluid (or melt) from their presence or absence in rocks. Magnetite-rich Banded Iron Formations (BIFs) and secondary hematite-rich iron ore formations have been linked to the oxygenation of the ocean and atmosphere, assuming redox transformation of magnetite – hematite. However, magnetite – hematite transformation may proceed though a non-redox (i.e., acid-base) reaction(1): Fe3O4 + 2H+ ↔ Fe2O3 + Fe2+ + H2O. Magnetite is transformed to hematite as a result of leaching Fe2+ from magnetite while hematite is transformed to magnetite as a result of incorporating Fe2+ into hematite. We have examined the reactions (forward and reverse) using a hydrothermal cell with hydrogen electrodes for pH measurements and a hydrogen-permeable membrane for pH2 measurements The experiments were conducted at 100 - 250°C under highly reducing conditions (pH2 = 0.5 – 50 bar) and mildly acid conditions (pH = 4 – 6). After the system reached a steady state, iron concentration in the withdrawn sample solutions was measured. The residual solid was analyzed using XRD, SEM and HRTEM.

Our experiments have demonstrated that euhedral crystals of hexagonal dipyramidal hematite grow rapidly by reaction between magnetite and acid solutions at T ≤ 200 ºC, suggesting dissolution and reprecipitation of Fe3+ under highly reducing conditions. Chemical composition of the experimental solution at the given temperatures are independent of the H2 pressure and remain constant over weeks, controlled by reaction (1). Reaction (1) is reversible, although the reverse reaction is more sluggish than the forward reaction. However, at T = 250ºC, reductive dissolution of magnetite (2) (i.e., redox reaction) controls the chemical compositions of the solution after 4 days: 1/3Fe3O4 + 2H+ + 1/3H2 → Fe2+ + 4/3H2O. No hematite was found in the residual solid from experiments at the temperature. The results of our studies suggest magnetite and hematite act as an acid-base buffer, rather than a redox buffer in low temperature environments. Because magnetite – hematite transformation does not require a redox reaction, secondary hematite-rich iron ore developed from BIFs may have formed by subsurface reaction between magnetite and acidic hydrothermal solutions at T ≤ 200 ºC.