EPITACTIC OVERGROWTHS AND INTERGROWTHS OF CLINOPYROXENE ON ORTHO-PYROXENE - IMPLICATIONS FOR PATHS OF CRYSTALLIZATION, 1881 LAVA FLOW, MAUNA-LOA VOLCANO, HAWAII

The 1881 tholeiite lava flow on Mauna Loa Volcano, Hawaii, contains ep itactic overgrowths and intergrowths of clinopyroxene on orthopyroxene . The clinopyroxene overgrowths form only on faces parallel to the opt ic axial plane of orthopyroxene, (100). The lava flow contains microph enocrysts of olivine and pyroxene in a fine-grained groundmass of plag ioclase, three pyroxenes, opaque minerals, and dark glass. The average composition of orthopyroxene is En(85+/-3). The epitactic overgrowths consist of clinopyroxene whose composition ranges from pigeonite to a ugite. The pyroxenes forming the epitactic overgrowths can be distingu ished from one another by their relief against orthopyroxene in sectio ns cut normal to (100). The olivine microphenocrysts are normally zone d, with a range in composition from Fo(82) to Fo(73) Plagioclase is a groundmass phase in the 1881 lava flow and displays normal zoning. Cry stallization paths predicted by thermodynamic modelling (MELTS) reprod uce the mineral compositions found. The crystallization sequence requi red by the epitactic relationship is orthopyroxene followed by pigeoni te or augite. The skeletal nature of the olivine is consistent with it s saturation later than orthopyroxene. At pressures greater than 0.18 GPa, melts with the composition of the rock never become saturated wit h olivine. If, however, a fractionation path is followed at a pressure greater than 0.18 GPa, then the sequence of pyroxenes found in the ep itactic overgrowths would be developed. If, after reaching clinopyroxe ne saturation, the magma is transported to shallower depths where pres sures are less than approximately 0.1 GPa, then the melt would become saturated in olivine.