GLOMEROCRYST FORMATION IN A BASALTIC LAVA FLOW FROM KALAPANA, HAWAII

This paper deals with groupings of crystals (glomerocrysts) in pahoehoe basaltic lava flow which have been examined by backscattered electron microscopy and x-ray microanalysis. Each glomerocyst consists of distinct crystals of a high calcium pyroxene and a mid-range plagioclase. The pyroxenes are euhedral and form a central nucleus from which swallowtail plagioclase extend into the surrounding matrix. The work is part of an ongoing study of crystallization processes in lavas. The rocks we have examined and reported on previously (Lowther & Brunstad 1998, 1999) are pophyritic with the glomerocrysts and phenocrysts of olivine and swallowtail plagioclase. The matrix in the upper part is glass and this grades downward into a ground mass consisting mainly of plumose pyroxenes and Fe/Ti oxides. Chemically the pyroxene ranges 52 – 54% silica, 17-19.5% CaO, 16-18% MgO, and 6-7% FeO which identifies the mineral as close to Augite. The plagioclase ranges between An 69 and An 71. Glass chemistry ranges 51-52% silica, 10-11.5% CaO, 6-7.5% MgO, 10.5-11.5% FeO. The Mg# of the pyroxenes are 81-83 and of the glass matrix 48-52 . We believe that the glomerocrysts were present in the lava before it was emplaced and that they formed in the liquid as it flowed after it erupted. The magma already contained olivine with chromite inclusions. As it erupted and began to cool the swallowtail (quench textured) plagioclase crystallized. Near the surface wherever several of the plagioclase laths (sticks) have clumped close enough together they induced the formation and growth of euhedral pyroxene crystals. These crystal clumps (the glomerocrysts) were then stirred by turbulent flow (maybe by entering a lava tube) and became distributed throughout the lava bed before it slowed and began to solidify .