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Journal of Petrology Volume 41 Number 1 Pages 3-20 2000
© Oxford University Press 2000
Liquidus Phase Relations in the System CaO–MgO–Al2O3–SiO2 at 2·0 GPa: Applications to Basalt Fractionation, Eclogites, and Igneous Sapphirine
1DEPARTMENT OF EARTH SCIENCES, NATIONAL TAIWAN NORMAL UNIVERSITY, TAIPEI 117, TAIWAN
2MAGMALOGY LABORATORY, DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF TEXAS AT DALLAS, RICHARDSON, TX 75083-0688, USA
To model magmatic crystallization processes for mafic to intermediate compositions at high pressure, liquidus phase relations in the forsterite–anorthite–diopside–silica (FADS) tetrahedron within the CaO–MgO–Al2O3–SiO2 system have been determined at 2·0 GPa. Compositions of five liquidus invariant points have been determined and the approximate compositions of five others have been inferred. These involve primary phase volumes for forsterite (fo), enstatite (en), diopside (di), high quartz (qz), spinel (sp), sapphirine (sa), garnet (gt), anorthite (an), and corundum (cor). The determined (with wt % coefficients) and inferred reactions (without coefficients) that define each isobaric invariant point are as follows:
23 en + 68 di + 9 sp = 84 liq + 16 fo
37 di + 63 sa = 47 liq + 40 sp + 13 en
100 gt = 21 liq + 27 sa + 55 en + 18 di
1 di + 59 en + 41 an = 43 liq + 57 gt
18 di + 21 qz + 15 en + 47 an = 100 liq
di + an + gt = liq + sa
an + gt = liq + sa + en
sa + an + di = liq + sp
sa + an = liq + cor + sp
di + cor = liq + an + sp.
These phase relations provide a diverse range of constraints on igneous processes at pressures near 2 GPa. They show that fractional crystallization of a model basalt gives a residual liquid strongly enriched in SiO2, strongly depleted in MgO, and mildly enriched in Al2O3. Such a trend is consistent with the calc-alkaline fractionation trend observed at subduction zones, but is in disagreement with suggestions that fractionation of tholeiitic basalt in this pressure range yields an alkalic basalt. Both trends may occur for natural basalts depending on the Na2O content of the parental magma. Also, the data show that the minimum pressure for the formation of cumulate eclogites and garnet pyroxenites is about 1·8–1·9 GPa. The lower limit of pressure at which sapphirine can crystallize from a liquid in the FADS tetrahedron is estimated to be 1·1–1·5 GPa and the upper limit is >3 GPa. Sapphirine crystallizes from magmas intermediate in composition between basalt and andesite. Probable igneous sapphirine in mafic associations is rare, but it occurs as part of a pyroxenite xenolith from Delegate, Australia, that we suggest is a cumulate assemblage and in a sapphirine norite at Wilson Lake, Labrador, Canada.
KEY WORDS: basalt; eclogite; sapphirine; fractional crystallization
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