TY - JOUR

T1 - An experimental investigation of the influence of water and oxygen fugacity on differentiation of MORB at 200 MPa

AU - Berndt, Jasper

AU - Koepke, Jürgen

AU - Holtz, François

N1 - Funding Information: We thank Otto Dietrich for preparing the samples, and Bettina Aichinger and Willi Hurkuck for technical assistance. Harald Behrens and Marcus Nowak are thanked for many helpful discussions. Reviews by Othmar Mu€ntener, Fidel Costa, Tom Sisson and Leonid Danyushevsky have significantly improved the manuscript. The authors also thank Colin W. Devey for editorial handling. This study was supported by Deutsche Forschungsgemeinschaft grant (project no. KO 1723/1). Copyright: Copyright 2008 Elsevier B.V., All rights reserved.

PY - 2004/10/1

Y1 - 2004/10/1

N2 - 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-Mn 3O4 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.

AB - 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-Mn 3O4 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.

KW - Differentiation

KW - MORB

KW - Oceanic plagiogranite

KW - Oxygen fugacity

KW - Water activity

UR - http://www.scopus.com/inward/record.url?scp=12444262267&partnerID=8YFLogxK

U2 - 10.1093/petrology/egh066

DO - 10.1093/petrology/egh066

M3 - Review article

AN - SCOPUS:12444262267

VL - 46

SP - 135

EP - 167

JO - Journal of Petrology

JF - Journal of Petrology

SN - 0022-3530

IS - 1

ER -