The effect of oxygen fugacity and temperature on solubilities of nickel, cobalt and molybdenum in silicate melts

Holzheid, A.; Borisov, A.; Palme, H.

Авторы: Holzheid A., Borisov A., Palme H.
Журнал: Geochimica et Cosmochimica Acta
Том: 58
Год издания: 1994
Издательство: Pergamon Press Ltd.
Местоположение издательства: United Kingdom
Первая страница: 1975
Последняя страница: 1981
Аннотация: Solubilities of Ni, Co, and Mo in silicate melts of anorthite-diopside eutectic composition were determined at reducing conditions, with fO2, values ranging from 1O-8.6 to 10-2.6 atm and at temperatures of around 1400°C. In a log (solubility ) vs. log (fO2) diagram, Ni and Co data plot along straight lines with slopes of 0.45 for Ni and 0.52 for Co, approximately corresponding to a valence state in the melt of +2 for both metals. No indications for the presence of metallic Ni and/or Co were found, even at the lowest fO2 (IW-3) reached with gas mixing. This result is in agreement with independent experimental data for Ni solubilities in silicate melts by DINGWELL et al. ( 1994) but is in conflict with recent experimental results by two other groups (COLSON, 1992; EHLERS et al., 1992). The experimental data for Mo suggest a change in valence slightly below the IW-1 buffer, from +6 to +4. This is the first reliable determination of the valence of Mo in silicate melts. No effect of the composition of gas mixtures used for fixing the fO2, was found; i.e., CO2-CO-N2 and C02-H2-N2 mixtures defining the same fO2, gave the same results. Temperature dependences for Ni, Co, and Mo solubilities were derived from experiments at temperatures ranging from 1349 to 1438°C. Solubilities of all three metals decrease with increasing temperature at constant oxygen fugacities. Metal/silicate partition coefficients were calculated from solubilities. Although Ni and Co metal/ silicate partition coefficients decrease with increasing temperature at fo, appropriate for core formation, temperature extrapolated Ni-metal/silicate partition coefficients are much higher than corresponding Co-metal/silicate partition coefficients. Thus, the nearly chondritic Ni/Co ratio of the mantle is difficult to reconcile with a global mantle-core equilibrium. In addition, although the metal/silicate partition coefficient of MO decreases with temperature at fo, values appropriate for core formation, the decrease is less steep than that of Ni and Co leading to a Ni/Mo concentration ratio in a hypothetical magma ocean ten times above the Ni/Mo ratio characteristic of the Earth’s mantle.
Добавил в систему: Борисов Александр Александрович

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The effect of oxygen fugacity and temperature on solubilities of nickel, cobalt and molybdenum in silicate meltsстатья

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