Journal of Petrology Advance Access originally published online on October 1, 2004
Journal of Petrology 2005 46(1):135-167; doi:10.1093/petrology/egh066
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Journal of Petrology vol. 46 issue 1 © Oxford University Press 2004; all rights reserved
An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa
INSTITUT FÜR MINERALOGIE, UNIVERSITÄT HANNOVER, WELFENGARTEN 1, D-30167 HANNOVER, GERMANY
Crystallization experiments were performed at 200 MPa in the temperature range 1150–950°C at oxygen fugacities corresponding to the quartz–fayalite–magnetite (QFM) and MnO–Mn3O4 buffers to assess the role of water and fO2 on phase relations and differentiation trends in mid-ocean ridge basalt (MORB) systems. Starting from a primitive (MgO 9·8 wt %) and an evolved MORB (MgO 6·49 wt %), crystallization paths with four different water contents (0·35–4·7 wt % H2O) have been investigated. In primitive MORB, olivine is the liquidus phase followed by plagioclase + clinopyroxene. Amphibole is present only at water-saturated conditions below 1000°C, but not all fluid-saturated runs contain amphibole. Magnetite and orthopyroxene are not stable at low fO2 (QFM buffer). Residual liquids obtained at low fO2 show a tholeiitic differentiation trend. The crystallization of magnetite at high fO2 (MnO–Mn3O4 buffer) results in a decrease of melt FeO*/MgO ratio, causing a calc-alkaline differentiation trend. Because the magnetite crystallization temperature is nearly independent of the H2O content, in contrast to silicate minerals, the calc-alkaline differentiation trend is more pronounced at high water contents. Residual melts at 950°C in a primitive MORB system have compositions approaching those of oceanic plagiogranites in terms of SiO2 and K2O, but have Ca/Na ratios and FeO* contents that are too high compared with the natural rocks, implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites.
KEY WORDS: differentiation; MORB; oxygen fugacity; water activity; oceanic plagiogranite
* Corresponding author. Present address: Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 24, D-48149 Münster, Germany. Telephone: +49 (0)251 83 33049. Fax: +49 (0)251 83 38397. E-mail: jberndt{at}uni-muenster.de
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