TY - JOUR

T1 - Effect of oxygen fugacity and water on phase equilibria of a hydrous tholeiitic basalt

AU - Feig, Sandrin T.

AU - Koepke, Jürgen

AU - Snow, Jonathan E.

N1 - Funding Information: ODP is sponsored by the US National Science Fundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. The funding for this research was provided by a grant from the Deutsche Forschungsgemeinschaft (KO 1723/4-2). Jonathan E. Snow was supported by a Heisenberg Fellowship from the Deutsche Forschungsgemeinschaft.

PY - 2010/10

Y1 - 2010/10

N2 - The influence of oxygen fugacity and water on phase equilibria and the link between redox conditions and water activity were investigated experimentally using a primitive tholeiitic basalt composition relevant to the ocean crust. The crystallization experiments were performed in internally heated pressure vessels at 200 MPa in the temperature range 940-1,220°C. The oxygen fugacity was measured using the H2-membrane technique. To study the effect of oxygen fugacity, three sets of experiments with different hydrogen fugacities were performed, showing systematic effects on the phase relations and compositions. In each experimental series, the water content of the system was varied from nominally dry to water-saturated conditions, causing a range of oxygen fugacities varying by ~3 log units per series. The range in oxygen fugacity investigated spans ~7 log units. Systematic effects of oxygen fugacity on the stability and composition of the mafic silicate phases, Cr-spinel and Fe-Ti oxides, under varying water contents were recorded. The Mg# of the melt, and therefore also the Mg# of olivine and clinopyroxene, changed systematically as a function of oxygen fugacity. An example of the link between oxygen fugacity and water activity under hydrogen-buffered conditions is the change in the crystallization sequence (olivine and Cr-spinel) due to a change in the oxygen fugacity caused by an increase in the water activity. The stability of magnetite is restricted to highly oxidizing conditions. The absence of magnetite in most of the experiments allows the determination of differentiation trends as a function of oxygen fugacity and water content, demonstrating that in an oxide-free crystallization sequence, water systematically affects the differentiation trend, while oxygen fugacity seems to have a negligible effect.

AB - The influence of oxygen fugacity and water on phase equilibria and the link between redox conditions and water activity were investigated experimentally using a primitive tholeiitic basalt composition relevant to the ocean crust. The crystallization experiments were performed in internally heated pressure vessels at 200 MPa in the temperature range 940-1,220°C. The oxygen fugacity was measured using the H2-membrane technique. To study the effect of oxygen fugacity, three sets of experiments with different hydrogen fugacities were performed, showing systematic effects on the phase relations and compositions. In each experimental series, the water content of the system was varied from nominally dry to water-saturated conditions, causing a range of oxygen fugacities varying by ~3 log units per series. The range in oxygen fugacity investigated spans ~7 log units. Systematic effects of oxygen fugacity on the stability and composition of the mafic silicate phases, Cr-spinel and Fe-Ti oxides, under varying water contents were recorded. The Mg# of the melt, and therefore also the Mg# of olivine and clinopyroxene, changed systematically as a function of oxygen fugacity. An example of the link between oxygen fugacity and water activity under hydrogen-buffered conditions is the change in the crystallization sequence (olivine and Cr-spinel) due to a change in the oxygen fugacity caused by an increase in the water activity. The stability of magnetite is restricted to highly oxidizing conditions. The absence of magnetite in most of the experiments allows the determination of differentiation trends as a function of oxygen fugacity and water content, demonstrating that in an oxide-free crystallization sequence, water systematically affects the differentiation trend, while oxygen fugacity seems to have a negligible effect.

KW - Basalt

KW - Crystallization experiments

KW - Differentiation

KW - Oxygen fugacity

KW - Phase equilibria

KW - Water

UR - http://www.scopus.com/inward/record.url?scp=77956410258&partnerID=8YFLogxK

U2 - 10.1007/s00410-010-0493-3

DO - 10.1007/s00410-010-0493-3

M3 - Article

AN - SCOPUS:77956410258

VL - 160

SP - 551

EP - 568

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 4

ER -