Some high-pressure pyroxenes

Microprobe analyses of Ca-rich pyroxenes crystallized in the melting ranges of a magnesian alkali basalt, a transitional basalt, an olivine tholeiite, a tholeiitic andesite, and an augite leucitite at pressures between 8 and 45 kb show complex variation. Ca-poor pyroxene precipitated only from the alkali basalt at pressures between 14 and 18 kb. Pyroxene falling near the Di-Hed join in the pyroxene quadilateral formed at all pressures and temperatures from the leucitite, whereas 'Ca-rich' pyroxene crystallizing from the other four compositions was Ca-poor augite to sub-calcic augite. The liquidus Ca-rich pyroxenes all show rising A1 and Na and falling Ti with increasing pressure and temperature. Other elements show complex behaviour; all but the leucitite pyroxenes tend to make temporary excursions of solid solution towards Ca-poor pyroxene at intermediate pressures, returning to more Ca-rich compositions at high pressures. At sub-liquidus temperatures Na and Ti consistently rise with falling T at constant P and also with rising P at constant T in these pyroxenes. The behaviour of the other elements in these circumstances depends on the nature of the coexisting phases. Fe/Mg distribution between Ca-rich pyroxene and liquid, in the form