Experimentally determined activity-composition relations for Ca-rich scapolite in the system CaAl2Si2O8-NaAlSi3O8-CaCO3 at 7 kbar

Equilibrium compositions of coexisting Ca-rich carbonate scapolite and plagioclase in the presence of calcite have been investigated experimentally at 7.0 kbar in the temperature range 775-850 °C. The CaO-AI203-Si02-C02 univariant reaction is 3CaAl2Si20s + CaC03 = Ca4A16Si6024C~03 (1) anorthite (An) calcite meionite (Me) in plagioclase in scapolite with NaSi(CaAl)_1 substitution in both plagioclase and scapolite in the Na20-CaO-AI203Si02-C02 divariant assemblage. The experiments were carried out in a 3/4in. diameter NaCI pressure cell with a hematite + magnetite buffer and a hydrous oxalic acid flux. Starting materials were a natural Cland S-free mizzonite (Me63), Baker reagent precipitated calcite, and synthetic meionite and plagioclases of composition AnlOOand Ann. Changes of compositions of the reactants were determined by microprobe analysis. The experimental results confirm the inferences based on obsc:~rvations in natural assemblages made by earlier workers that Na substitution greatly stabilizes scapolite relative to plagioclase. The marked stabilization of mizzonite to lower tenlperatures is largely a consequence of the large amounts of atomic mixing possible in the scapolite structure. An activity model for the meionite component in terms of its rnole fraction, XMe, which involves complete disordering of Ca and Na and, independently, Si and AI, has been derived from the experimental brackets. = (1 + XMe)6(3XMe)6(3XMe+ 1)4 [ -13450 v (1 _ v )2 ] aMe 410 exp T . /iMe AMe. This formula may lose validity in a lower temperature range if scapolite develops greater atomic ordering near the mizzonite composition. The disordered scapolite model is used to calculate apparent C02 activities for recrystallization of some Grenville granulites studied by Moecher and Essene (1991). We find close agreement with their predicted C02 activities based on scapolite + garnet + plagioclase + quartz equilibrium at 800 °C and 10 kbar, despite the different scapolite activity models used in the two sets of calculations. The simplest explanation of this coincidence is that their empirical scapolite model and the present experi]mentally based model are both valid, although the latter is restricted to high temperatures (above 750°C). INTRODUCTION somatic settings (e.g., the Idaho Batholith aureole; Hietanen, 1967), whtereas scapolite from high-grade metamorphic calc-silicates and mafic gneisses has a composition close~r to the meionite and sulfate meionite end-members, with many scapolite grains having close to two-thirds of the meionite component. This particular composition range is termed mizzonite (Shaw, 1960; Evans et aI., 1969; Moecher and Essene, 1991). Carbonate scapolite is unique in being a refractory CO2bearing phase that can coexist stably with common siliThe mineral scapolite can be chemically represented as a solid solution of four end-members: a calcium carbonate end-member, meionite, Ca4Al6Si6024C03 (Me); a sodium chloride end-member, marialite, Na4Al3Si9024CI (Ma); a hypothetical end-member Na3CaAl3Si9024C03 (Na-Me); and a calcium sulfate end-member, sulfate meionite, Ca4A16Si6024S04 (S04-Me). Scapolite rich in the marialite component occurs in high-temperature meta0003-004X/9 5/0708-0744$02.00 744 BAKER AND NEWTON: ACTIVITY RELATIONS FOR Ca-RICH SCAPOLITE cate minerals at elevated temperature conditions. Moecher and Essene (1990, 1991) showed that the reaction meionite quartz in scapolite anorthite in plagioclase in fluid grossular in garnet is useful for calculating C02 activities driving recrystallization of high-grade, deep-crustal metamorphic rocks containing the common assemblage scapolite + plagioclase + garnet + quartz, providing a new approach to debate about the role of C02 in the formation of granulite facies terranes (Newton et a! ., 1980b; Valley et a! ., 1990; Frost and Frost, 1987; Waters, 1988). JHowever, calculations involving carbonate scapolite require a knowledge of the activity-composition relations along the join from meionite to sodium meionite. The stability of the scapolite end-menlber meionite at crustal pressures is limited, at low temperatures, by Reaction 1. Goldsmith and Newton (1977) reported broad reversals of this univariant reaction, and these experiments suggested that meionite is stable relative to anorthite and calcite above 875 °C, almost independently of pressure. Huckenholz and Seiberl (1989) presented a second experimental study of this equilibrium and suggested markedly lower temperature limits for nleionite stability and a steep negative dPldT slope for th~~reaction. More recent experiments (Baker and Newton, 1994) favor the results of Goldsmith and Newton (1977) and require a steep positive dPldT slope for the univariant reaction. The incorporation of Na into meionite through a plagioclase substitution, NaSi(CaAI)_l' expands the range of conditions over which scapolite is stable:. Goldsmith and Newton (1977) found that carbonate scapolite ranging from pure meionite to compositions near Meso (or Eq Anso) were stable at temperatures above 850 °C. Orville (1975) and Ellis (1978) experimentally determined the stability of mizzonite with respect to plagioclase plus calcite, and both authors found that mizzonite is stable to temperatures at least 100 °C lower than pure meionite. Ellis (1978) found a relatively wide range~of solid solution from MeS3 to Me83 at 750 °C and 4 kbar, whereas Orville (1975) found a much narrower range of solid solution at identical conditions. Observations froml natural calcareous assemblages formed at temperatures of 600 °C or below indicate that mizzonite, Me67, cot~xisting with plagioclase of composition An44-48or An94__9s, is the lowesttemperature Na-free scapolite (Oterdoom and Gunter, 1983; Moecher and Essene, 1990). Estimates of mixing properties along the join from meionite to sodium meionite on the basis of paragenetic analysis have been made by Oterdoom and Gunter (1983) and Moecher and Essene (1990). However, no reversed experimental data describing the compositions of coexisting scapolite and plagioclase have been available here745 tofore. The data of Goldsmith and Newton (1977) indicate that the scapolite-plagioclase compositional tie lines are quite temperature dependent, as indicated also by the natural parageneses. They found that the sulfate to carbonate ratio of the synthetic scapolites has a marked effect on the tie lines. However, they were unable to reverse the coexisting compositions in their high-temperature dry experiments. It is the object of the present work to attempt to secure reversals of the temperature dependence of the composition of coexisting carbonate scapolite and plagioclase in the presence of calcite in order to define experimentally the activity-composition relations of calcic carbonate scapolite. EXPERIMENTAL METHODS