THE REDOX STATES OF BASIC AND SILICIC MAGMAS - A REFLECTION OF THEIR SOURCE REGIONS

被引:584
作者
CARMICHAEL, ISE
机构
[1] Department of Geology and Geophysics, University of California, Berkeley, 94720, CA
关键词
D O I
10.1007/BF00306429
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
At present the best estimates of the oxygen fugacity of spinel-lherzolites that could be the source material of basic magmas is about five log units below the Ni-NiO buffer to one above it. However partially glassy basic lavas, ranging from MORBs to minettes, all with olivine on their liquidus, cover a wider range, and may have oxygen fugacities that extend to four log units above NNO. Surprisingly the range of oxygen fugacities observed in silicic lavas and ashflows with quartz phenocrysts is smaller, despite a crustal dominated evolution. The peralkaline silicic lava type pantellerite is the most reduced, equivalent to MORBs, whereas the large volume ashflows with phenocrysts of hornblende and/or sphene are the most oxidised. As the concentration of water in the basic lavas is correlated with increase in their redox state, it would seem that water could be the agent of this increase. That this is unlikely is seen in the behavior of silicic ashflows and lavas, particularly those of Yellowstone. Here the silicic magmas of the last 2 Ma contain about 2 wt% FeO(total), and typically phenocrysts of fayalite, quartz and Fe-Ti oxides. Despite extensive exchange of the 18O of the magma with meteoric water after caldera collapse (Hildreth et al. 1984), there is no displacement of the redox equilibria. Thus the thermal dissociation of molecular H2O to H2, and its subsequent diffusive loss to cause oxidation must have been minimal. This could only be so if the activity of water was small, as it would be if H2O reacted with the silicate liquid to form OH groups (Stolper 1982). The conclusion is that silicic magmas with small amounts of iron and large amounts of water do not have their redox states reset, which in turn presumably reflect their generation. By analogy basic magmas with large amounts of iron and far less water are even less likely to have their redox equilibria distrubed, so that their oxygen fugacities will also reflect their source regions. The effect of pressure on the ferric-ferrous equilibrium in basic magmas can be calculated from experimental measurements of the partial molar volumes of FeO and Fe2O3, and their pressure derivatives partial-differential-equation-V/partial-differential-equation-P, in silicate liquids. The effect of pressure is found to be about the same on the liquid as it is for various solid oxygen buffers. Accordingly there should be mantle source regions covering the same range of oxygen fugacity as that found in basic lavas, but so far samples of spinel-lherzolite of equivalent oxygen fugacity to minettes or other potassic lavas have not been found. Whether or not the redox state of phlogopite-pyroxenites is equivalent to these potassic lavas cannot be established without experiment.
引用
收藏
页码:129 / 141
页数:13
相关论文
共 62 条
[41]  
LUHR JF, 1981, CONTRIB MINERAL PETR, V71, P434
[42]   INFLUENCE OF DEGASSING ON OXIDATION-STATES OF BASALTIC MAGMAS [J].
MATHEZ, EA .
NATURE, 1984, 310 (5976) :371-375
[43]   THE NATURE AND DISTRIBUTION OF CARBON IN SUBMARINE BASALTS AND PERIODOTITE NODULES [J].
MATHEZ, EA ;
DELANEY, JR .
EARTH AND PLANETARY SCIENCE LETTERS, 1981, 56 (DEC) :217-232
[44]   SULFUR SOLUBILITY AND MAGMATIC SULFIDES IN SUBMARINE BASALT GLASS [J].
MATHEZ, EA .
JOURNAL OF GEOPHYSICAL RESEARCH, 1976, 81 (23) :4269-4276
[45]   MAGNETITE ACTIVITIES ACROSS THE MGAL2O4-FE3O4 SPINEL JOIN, WITH APPLICATION TO THERMOBAROMETRIC ESTIMATES OF UPPER MANTLE OXYGEN FUGACITY [J].
MATTIOLI, GS ;
WOOD, BJ .
CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1988, 98 (02) :148-162
[46]   UPPER MANTLE OXYGEN FUGACITY AND ITS RELATIONSHIP TO METASOMATISM [J].
MATTIOLI, GS ;
BAKER, MB ;
RUTTER, MJ ;
STOLPER, EM .
JOURNAL OF GEOLOGY, 1989, 97 (05) :521-536
[47]   THE PARTIAL MOLAR VOLUME OF FE2O3 IN MULTICOMPONENT SILICATE LIQUIDS AND THE PRESSURE-DEPENDENCE OF OXYGEN FUGACITY IN MAGMAS [J].
MO, X ;
CARMICHAEL, ISE ;
RIVERS, M ;
STEBBINS, J .
MINERALOGICAL MAGAZINE, 1982, 45 :237-245
[48]   ESTIMATE OF JUVENILE SULFUR CONTENT OF BASALT [J].
MOORE, JG ;
FABBI, BP .
CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1971, 33 (02) :118-&
[49]  
NELSON SA, 1980, GEOL SOC AM BULL, V91, P2290, DOI DOI 10.1130/0016-7606(1980)91<639:GAP0VC>2.0.C0
[50]  
2