Forsterite-diopside-spinel-liquid equilibria in the system CaO−MgO−Al2O3−SiO2 at 20 kbar and petrological applications |
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| Authors: | Biswajit Mukhopadhyay |
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| Institution: | (1) Department of Geosciences, The University of Texas at Dallas, 75083 Richardson, TX, USA;(2) Present address: Department of Geological Sciences, Southern Methodist University, 75275 Dallas, TX, USA |
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| Abstract: | Liquidus phase relationships in the CaAlAl–SiO6–Mg2SiO4–CaMgSi2O6–CaAlSi2O8 portion of the simplified basalt tetrahedron in the CaO–MgO–SiO2–Al2O3 system have been experimentally determined at 20 kbar pressure. The fo+di
ss+sp+li univariant curve, that pierces the fo-di-an join and meets the fo+di
ss+ enss+sp+li invariant point in the basalt tetrahedron, extends all the way to and pierces the di-fo-CaTs join, the limit of the simplified basalt tetrahedron toward the silica undersaturated portion.An algebraic method, relying on compositions of two successive liquids on a univariant curve and those of the crystalline phases in equilibrium with the respective liquids, is developed to identify the type of reaction that takes place along an isobarically univariant curve and to detect whether there is a temperature maximum on that curve. Use of this method for the di
ss+fo+sp+li univariant equilibria shows that a temperature maximum exists on this curve at the composition Fo11Di56An3CaTs30, very close to and slighthly to the SiO2-rich side of the fo-di-CaTs join. The temperature along the univariant curve continuously decreases from the temperature maximum (1500°C) to the invariant point (1475°C) where the univariant curve is terminated by the appearance of e
ss as a member of the equilibrium assemblage. Along this part of the curve, a reaction relationship occurs according to the equation fo+li=di
ss+ sp. Compositions of di
ss in equilibrium with the liquids from the temperature maximum to the fo+di
ss+enss+ sp+li invariant point range from Di66En9CaTs25 to Di36En40CaTs24. Because of the reaction relationship of forsterite with liquid, fractional crystallization of a model alkalic basaltic liquid would cause liquids to move off the fo-di
ss-sp-li univariant curve onto the sp-di
ss divariant surface. Crystallization of di
ss and sp would then lead to silica enrichment of residual liquids. Thus at pressures below 30 kbar, at which pressure the Al2O3–CaSiO3–MgSiO3 plane becomes a new thermal divide cutting through both the tholeiitic and alkalic volumes, alkalic liquids will fractionate toward tholeiitic compositions without crossing a thermal divide. This relationship would be expected to persist at pressures down to about 4 kbar where a maximum on the fo-di-an-li boundary line causes a thermal divide near the fo-di-an plane. Strongly SiO2-undersaturated liquids (e.g. nephelinites, basanites), on the other hand, cannot be derived from SiO2-undersaturated basalts (e.g. alkali olivine basalt) by fractional crystallization at 20 kbar. We also found that no gt primary phase volume cuts the wo-en-Al2O3 join at 20 kbar pressure. The wehrlite, the olivine clinopyroxenite, and the Al-augite group lherzolite xenoliths, containing highly aluminous clinopyroxenes (enriched in Ca-Tschermak), can be interpreted as crystal cumulates from alkalic basalts in the light of this experimental study. This is consistent with the mode of origin of these xenoliths proposed from petrographic, mineralogic, and geochemical studies.Abbreviations and notations
di
CaMgSi2O6
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fo
Mg2SiO4
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an
CaAl2Si2O8
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CaTs
CaAlAlSiO6
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sp
MgAl2O4
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en
MgSiO3
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wo
CaSiO3
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gt
Ca3Al2Si3O12–Mg3Al2Si3O12
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qz
SiO2
-
li
Liquid
-
gl
glass
-
ss
Solid Solution
- A
An mxn matrix
- X
A column vector
- kbar
kilobar |
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