Experimental Data Bearing on Liquid Immiscibility, Crystal Fractionation, and the Origin of Calciocarbonatites and Natrocarbonatites

Abstract

Controversies about the role of carbonate-rich melts in processes ranging from mantle metasomatism to carbonatite eruptions involve debates about the following issues. (1) Are carbonatite magmas primarily from the mantle, or differentiates from silicate parents in the crust? (2) Are parental carbonatite magmas calcic, dolomitic, or sodic? (3) Are carbonatite magmas derived in the crust by liquid immiscibility or crystal fractionation? Experimental data provide the following constraints: primary carbonatite magmas formed in mantle lherzolite at depths greater than ∼70 km are dolomitic; melts richer in calcite can be formed only in wehrlites at shallower depths; immiscible carbonatite magmas formed in the crust have variable Ca/Na, with no more than 80% dissolved CaCO_3, and commonly less; dolomitic melts from fertile peridotite, and more calcic melts from immiscibility, can fractionate to yield sodic melts. A test experiment confirms previous results in the join NaAlSiO_4-NaAlSi_3O_8-CaCO_3(Ne-Ab-CC)-H_2O (at 0.1 GPa and below 960° C), illustrating that fractional crystallization of nepheline-normative silicate-CO_2 liquid can yield a residual carbonatitic liquid, precipitating calcite and becoming enriched in Na_2CO_3; the fractionation path passes below the silicate-carbonate liquid miscibility gap that exists at higher temperatures. The experiment with 10% H_2O (near Ne_(35)CC_(65) by weight) at 833° C identifies the crystallization reaction: 1 liquid + 0.2 melilite(sodic) + 0.2 CO_2 = 0.5 cancrinite + 0.6 calcite + 0.1 H_2O. The liquid quenches to a fine-grained assemblage, including laths of nyerereite, equant grains of cancrinite, and calcite, mostly precipitated in quench overgrowth rims enclosing cancrinite that border the round, primary calcite crystals. The liquid composition was determined from scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analyses over areas up to 2,500 μm^2, modal counts, and mass balance calculations; the maximum H_2O content was assigned using published solubility data, yielding a liquid composition: 26% CaO, 10% Na_2O, 14% Al_2O_3, 15% SiO_2, 23% CO_2, and 11% H_2O, equivalent to 36% nepheline, 43% calcite, and 9% nyerereite with H_2O and about 1% Al_2O_3, slightly enriched in Na_2CO_3 compared with the starting material. Deductions from available experimental studies indicate that fractional crystallization of a hydrous CO_2-bearing nepheline-normative magma at moderate temperatures may produce continuous liquid compositions from silicate to carbonate rich, precipitating a series of mineral assemblages analogous to rocks at the Oka Complex. Alkali-enriched carbonatite melts capable of precipitating cumulate sovites can be generated in several ways at crustal depths, but there is as yet no experimental evidence for petrological processes capable of forming nearly pure CaCO_3 liquids at feasible temperatures and pressures. Previous claims for such liquids through immiscibility are now revised. The proposed existence of calciocarbonate parental magmas requires substantiation by a viable process.