Wj. Lee et Pj. Wyllie, The system CaO-MgO-SiO2-CO2 at 1 GPa, metasomatic wehrlites, and primary carbonatite magmas, CONTR MIN P, 138(3), 2000, pp. 214-228
New experimental data in CaO-MgO-SiO2-CO2 at 1GPa define the vapor-saturate
d silicate-carbonate liquidus field boundary involving primary minerals cal
cite, forsterite and diopside. The eutectic reaction for melting of model c
alcite (1% MC)-wehrlite at 1 GPa is at 1100 degrees C, with liquid composit
ion (by weight) 72% CaCO3 (CC), 9% MgCO3 (MC), and 18% CaMgSi2O6 (Di). Thes
e data combined with previous results permit construction of the isotherm-c
ontoured vapor-saturated liquidus surface for the calcite/dolomite field, a
nd part of the adjacent forsterite and diopside fields. Nearly pure calcite
crystals in mantle xenoliths cannot represent equilibrium liquids. We rece
ntly determined the complete vapor-saturated liquidus surface between carbo
nates and model peridotites at 2.7 GPa; the peritectic reaction for dolomit
e (25% MC)-wehrlite at 2.7 GPa occurs at 1300 degrees C, with liquid compos
ition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these tw
o surfaces provide the road-map for interpretation of magmatic processes in
various peridotite-CO2 systems at depths between the Moho and about 100 km
. Relationships among kimberlites, melilitites, carbonatites and the liquid
us phase boundaries are discussed. Experimental data for carbonatite liquid
protected by metasomatic wehrlite have been reported. The liquid trends di
rectly from dolomitic towards CaCO3 with decreasing pressure. The 1.5 GPa l
iquid contains 87% CC and 4% Di, much lower in silicate components than our
phase boundary. However, the liquids contain approximately the same CaCO3
(90 +/- 1 wt%) in terms of only carbonate components. For CO2-bearing mantl
e, all magmas at depth must pass through initial dolomitic compositions. Ri
sing dolomitic carbonatite melt will vesiculate and may erupt as primary ma
gmas through cracks from about similar to 70 km. If it percolates through m
etasomatic wehrlite from 70 km toward the Moho at 35-40 km, primary calcic
siliceous carbonatite magma can be generated with silicate content at least
11-18% (70-40 km) on the silicate-carbonate boundary.