Kimberlites, Carbonatites, Peridotites and Silicate-carbonate Liquid Immiscibility Explained in Parts of the System CaO-(Na_2O+K_2O)-(MgO+FeO)-(SiO_2+Al_2O_3)-CO_2

Abstract

New experimental data between 1.0 and 2.7 GPa in the system CaO-MgO-SiO_2-CO_2 and in selected joins through the more complex system CaO-(Na_2O+K_2O)-(MgO+FeO)-(SiO_2+Al_2O_3)-CO_2 have advanced understanding of: (1) the possible relationships among lherzolite and wehrlite, carbonate-rich liquids, and kimberlite and melilitite liquids, and (2) the effect of alkalis in promoting silicate-carbonate liquid immiscibility. Experiments indicate that immiscible carbonatite magmas contain no more than 80 % CaCO_3; silicate-derived liquids with 90-99 % CaCO_3 cannot be found in phase diagrams; conclusion - the calcite ocelli in mantle xenoliths must represent rounded crystals and not quenched immiscible liquids. Near-solidus magmas generated in carbonated peridotites are carbonatitic. The dolomitic liquid composition (magnesiocarbonatite) determined in model systems (lherzolite, harzburgite) for depths greater than ~70 km (confirmed by analyses in whole-rock experiments) remains dolomitic to at least 7 GPa, even when subsolidus carbonate has changed to magnesite. At depths shallower than ~70 km, only wehrlite can be carbonated and yield carbonatite magmas. Wid1 decreasing pressure from ~70 km to ~35 km, the near-solidus liquid coexisting with wehrlite-CO_2 changes from ~60 % CaCO_3(CC)-30 % MgCO_3(MC)-10 % CaMgSi_2O_6(Di) to ~73 % CC-9 % MC-18 % Di. If magnesiocarbonatite liquid rising above the ~70 km level metasomatizes lherzolite to form protective wehrlite, and if the liquid continues to rise and react with wehrlite, it becomes calciocarbonatite (with 10- 15 % silicate) after rising through perhaps 15-20 km. The carbonatite magmas generated from CO_2-bearing peridotites are the low-temperature part of a continuum of small-volume partial melts which follow long, vapor-absent liquidus paths toward the volatile-free peridotite assemblages. We have mapped fields of liquid compositions on vapor-absent paths from parent CO_2-bearing lherzolites and harzburgites at depths greater than 70 km, and for wehrlite at depths from 70 km to 35 km. Melilitites (lower Mg/Ca) overlap the high-temperature, low-pressure part of the wehrlite field, not lherzolites. Average kimberlites (higher Mg/Ca) are in the high-temperature pai1 of the harzburgite host field to at least 100 km depth, but clustered around the lherzolite host field by 200 km depth.

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