Phase Equilibrium Studies Bearing on the Limestone-Assimilation Hypothesis

Phase relationships in parts of the join NaAlSi₃O₈-CaCO₃-Ca(OH)₂-H₂O at 1-kb pressure, 600° C to 1100° C, illustrate the effect of limestone assimilation on feldspathic magmas. Addition of 20 weight percent of CaO-rich material to a vapor-saturated albite liquid introduces the successive phase fields plagioclase + L + V, wollastonite + L + V, and plagioclase + wollastonite + L + V: addition of 20 to 25 percent produces a small field just above the solidus for plagioclase + wollastonite + nepheline + L + V, with nepheline present in trace amounts; crystallization of the liquid yields little additional nepheline: 25 percent or more of CaO-rich material causes complete crystallization because the solidus temperature increases abruptly, then more nepheline develops and melilite, rankinite, and larnite are stabilized in a series of subsolidus phase fields. Thus, about 20 weight percent of limestone must be assimilated before a feldspathic melt becomes sufficiently desilicated to yield nepheline. Even if enough superheat is available to permit solution of so much limestone, assimilation causes crystallization before desilication proceeds very far, and the formation of feldspathoids in quantity results only from subsolidus reactions. Limestone assimilation releases CO2, which also tends to induce crystallization of a hydrated magma. A magma would be forced to crystallize as syntexis proceeds, and the local formation of alkalic silica-undersaturated rocks in reaction zones at limestone contacts with silica-saturated igneous magmas is to be expected. The development of large volumes of alkalic, silica-undersaturated magma by limestone syntexis appears to be unlikely, however. Production of alkalic magma by sialic-rock assimilation by carbonatite magma is similarly unlikely.