2CO 3–CaCO 3 at 3?GPa]]>

摘要

It was suggested that alkali–alkaline earth carbonates may have a substantial role in petrological processes relevant to metasomatism and melting of the Earth’s mantle. Because natrite, Na~(2)CO~(3), Na–Ca carbonate (shortite and/or nyerereite), and calcite, CaCO~(3), have been recently reported from xenoliths of shallow mantle (110–115?km) origin, we performed experiments on phase relations in the system Na~(2)CO~(3)–CaCO~(3)at 3?GPa and 800–1300?°C. We found that the system has one intermediate compound, Na~(2)Ca~(3)(CO~(3))~(4), at 800?°C, and two intermediate compounds, Na~(2)Ca(CO~(3))~(2)and Na~(2)Ca~(3)(CO~(3))~(4), at 850?°C. CaCO~(3)crystals recovered from experiments at 950 and 1000?°C are aragonite and calcite, respectively. Maximum solid solution of CaCO~(3)in Na~(2)CO~(3)is 20?mol% at 850?°C. The Na-carbonate–Na~(2)Ca(CO~(3))~(2)eutectic locates near 860?°C and 56?mol% Na~(2)CO~(3). Na~(2)Ca(CO~(3))~(2)melts incongruently near 880?°C to produce Na~(2)Ca~(3)(CO~(3))~(4)and a liquid containing about 51?mol% Na~(2)CO~(3). Na~(2)Ca~(3)(CO~(3))~(4)disappears above 1000?°C via incongruent melting to calcite and a liquid containing about 43?mol% Na~(2)CO~(3). At 1050?°C, the liquid, coexisting with Na-carbonate, contains 87?mol% Na~(2)CO~(3). Na-carbonate remains solid up to 1150?°C and melts at 1200?°C. The Na~(2)CO~(3)content in the liquid coexisting with calcite decreases to 15?mol% as temperature increases to 1300?°C. Considering the present and previous data, a range of the intermediate compounds on the liquidus of the Na~(2)CO~(3)–CaCO~(3)join changes as pressure increases in the following sequence: Na~(2)Ca(CO~(3))~(2)(0.1?GPa)?→?Na~(2)Ca(CO~(3))~(2), Na~(2)Ca~(3)(CO~(3))~(4)(3?GPa)?→?Na~(4)Ca(CO~(3))~(3), Na~(2)Ca~(3)(CO~(3))~(4)(6?GPa). Thus, the Na~(2)Ca(CO~(3))~(2)nyerereite stability field extends to the shallow mantle pressures. Consequently, findings of nyerereite among daughter phases in the melt inclusions in olivine from