A microprobe study was carried out on zoned clinopyroxene, composed of a pigeonite core mantled by augite, and plagioclase in six tholeiitic basalts dredged from the Puerto Rico Trench (20° 06.8′N, 65° 06.5′W). The pigeonite cores are characterized by high magnesium contents [ Fe Fe + Mg = 0.19 on the average]; their crystallization temperatures based on the three-pyroxene geothermometer are thus estimated to be higher than those of the well-studied tholeiitic pyroxene assemblages (Hakone, Weiselberg, Mull, Skaergaard, and Bushveld). It is suggested from textural and chemical relationships that pyroxene crystallization started first with precipitation of pigeonite followed by simultaneous precipitation of pigeonite and augite and finally by crystallization of augite alone in the groundmass.
ASJC Scopus subject areas
- Geochemistry and Petrology