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  • Springer Science + Business Media  (4)
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  • 1
    Publication Date: 2010-10-01
    Description: 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 H_2-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. ©2010 Springer-Verlag
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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  • 2
    Publication Date: 2004-01-01
    Description: We performed hydrous partial melting experiments at shallow pressures (0.2 GPa) under slightly oxidizing conditions (NNO oxygen buffer) on oceanic cumulate gabbros drilled by ODP (Ocean Drilling Program) cruises to evaluate whether the partial melting of oceanic gabbro can generate SiO_2-rich melts with compositions typical of oceanic plagiogranites. The experimental melts of the low-temperature runs broadly overlap those of natural plagiogranites. At 940 °C, the normalized SiO_2 contents of the experimental melts of all systems range between 60 and 61 wt%, and at 900 °C between 63 and 68 wt%. These liquids are characterized by low TiO_2 and FeOtot contents, similar to those of natural plagiogranites from the plutonic section of the oceanic crust, but in contrast to Fe and Ti-rich low-temperature experimental melts obtained in MORB systems at ~950 °C. The ~1,500-m-long drilled gabbroic section of ODP Hole 735B (Legs 118 and 176) at the Southwest Indian Ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high-temperature shearing. The compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins, while the compositions of the crystals of low-temperature runs correspond to those of minerals from high-temperature microscopic veins occurring in the gabbroic section of the Hole 735B. This suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water-rich fluid phase. If the temperature estimations for high-temperature shear zones are correct (up to 1,000 °C), and a water-rich fluid phase is present, the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust. ©2003 Springer-Verlag
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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  • 3
    Publication Date: 2007-01-01
    Description: Small amounts of felsic, evolved plutonic rocks, often called oceanic plagiogranites, always occur as veins or small stocks within the gabbroic section of the oceanic crust. Four major models are under debate to explain the formation of these rocks: (1) late-stage differentiation of a parental MORB melt, (2) partial melting of gabbroic rocks, (3) immiscibility in an evolved tholeiitic liquid, and (4) assimilation and partial melting of previously altered dikes. Recent experimental data in hydrous MORB-type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust. Experiments show that TiO_2 is a key parameter for the discrimination between different processes: TiO_2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low TiO_2 contents of the protolith, due to the depleted nature of typical cumulate gabbros formed in the oceanic crust. On the other hand, TiO_2 is relatively high in those melts generated by MORB differentiation or liquid immiscibility. Since the TiO_2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple MORB differentiation or immiscibility, these rocks may be regarded as products of anatexis. This may indicate that partial melting processes triggered by water-rich fluids are more common in the deep oceanic crust than believed up to now. At slow-spreading ridges, seawater may be transported via high-temperature shear zones deeply into the crust and thus made available for melting processes. ©2006 Springer-Verlag
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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  • 4
    Publication Date: 2006-11-01
    Description: To investigate the effect of water on phase relations and compositions in a basaltic system, we performed crystallization experiments at pressures of 100, 200 and 500 MPa in a temperature range of 940 to 1,220°C using four different water contents. Depending on the water activity, the oxygen fugacity varied between 1 and 4 log units above the quartz-magnetite-fayalite buffer. Addition of water to the dry system shifts the solidus  〉 250°C to lower temperatures and increases the amount of melt drastically. For instance, at 1,100°C and 200 MPa, the melt fraction increases from 12.5 wt% at a water content of 1.6 wt% to 96.3% at a water content of 5 wt% in the melt. The compositions of the experimental phases also show a strong effect of water. Plagioclase is shifted to higher anorthite contents by the addition of water. Olivine and clinopyroxene show generally higher MgO/FeO ratios with added water, which could also be related to the increasing oxygen fugacity with water. Moreover, water affects the partitioning of certain elements between minerals and melts, e.g., the Ca partitioning between olivine and melt. Plagioclase shows a characteristic change in the order of crystallization with water that may help to explain the formation of wehrlites intruding the lower oceanic crust (e.g., in Oman, Macquarie Island). At 100 MPa, plagioclase crystallizes before clinopyroxene at all water contents. At pressures 〉 100 MPa, plagioclase crystallizes before clinopyroxene at low water contents (e.g. 〈 3 wt%), but after clinopyroxene at H_2O in the melt 〉 3 wt%. This change in crystallization order indicates that a paragenesis typical for wehrlites (olivine–clinopyroxene–without plagioclase) is stabilized at low pressures typical of the oceanic crust only at high water contents. This opens the possibility that typical wehrlites in the oceanic crust can be formed by the fractionation and accumulation of olivine and clinopyroxene at 1,060°C and 〉 100 MPa in a primitive tholeiitic basaltic system containing more than 3 wt% water. The comparison of the experimental results with evolution trends calculated by the thermodynamic models “MELTS” and “Comagmat” shows that neither model predicts the experimental phase relations with sufficient accuracy. ©2006 Springer-Verlag
    Print ISSN: 0010-7999
    Electronic ISSN: 1432-0967
    Topics: Geosciences
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