Liquidus phase relationships in the CaAlAl–SiO
6–Mg
2SiO
4–CaMgSi
2O
6–CaAlSi
2O
8 portion of the simplified basalt tetrahedron in the CaO–MgO–SiO
2–Al
2O
3 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+ en
ss+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 Fo
11Di
56An
3CaTs
30, very close to and slighthly to the SiO
2-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+en
ss+ sp+li invariant point range from Di
66En
9CaTs
25 to Di
36En
40CaTs
24. 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 Al
2O
3–CaSiO
3–MgSiO
3 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 SiO
2-undersaturated liquids (e.g. nephelinites, basanites), on the other hand, cannot be derived from SiO
2-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-Al
2O
3 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
CaMgSi
2O
6
-
fo
Mg
2SiO
4
-
an
CaAl
2Si
2O
8
-
CaTs
CaAlAlSiO
6
-
sp
MgAl
2O
4
-
en
MgSiO
3
-
wo
CaSiO
3
-
gt
Ca
3Al
2Si
3O
12–Mg
3Al
2Si
3O
12
-
qz
SiO
2
-
li
Liquid
-
gl
glass
-
ss
Solid Solution
- A
An
mxn matrix
- X
A column vector
- kbar
kilobar
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