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Moore, J.G. and Evans, B.W. (1967). The role of olivine in the crystallization of the prehistoric Makaopuhi tholeiitic lava lake, Hawaii. Contributions to Mineralogy and Petrology 15(3): 202-223. |
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On eruption, the tholeiitic basalt lava of the prehistoric Makaopuhi lake contained nearly seven percent euhedral olivine phenocrysts of approximately Fa14 composition. In the center of the 225 foot vertical section of the lake, the lava became more than 90 percent solid at 1000 degrees C after about 30 years. At the surface the lava was quenched to air temperature, whereas, at the bottom, quenching to 800 degrees C was followed by a 40 year period before the temperature reached 700 degrees C. The olivine phenocrysts settled at an average rate of about 4 x 10 (super -6) cm/sec to form a zone that contains 21 percent olivine 75 feet above the base. Sinking of olivines continued until some time after the beginning of the crystallization of augite and plagioclase. Thin rims of iron-rich olivine (up to Fa55) surrounding the phenocrysts, and a second generation of fine-grained olivines (Fa20-Fa48) restricted to the uppermost 20 feet indicate local extensions of the period of crystallization of olivine. During crystallization of the groundmass and later subsolidus cooling in the range 1000 degrees C to at least as low as 800 degrees C, the olivine phenocrysts were converted to Fa30-40 by interdiffusion of Fe, Mg, Ni, and Mn. Homogenization of Mg-rich cores and Fe-rich margins and equilibration of olivine composition with the groundmass phases was progressively less well achieved toward the top of the lake. Reaction rims around the olivines are composed primarily of Ca-rich pyroxene. Pigeonite crystallized alongside augite except in the uppermost 5 feet where there is abundant groundmass olivine. Poikilitic hypersthene grew at the expense of preexisting ferromagnesian minerals in the cumulate zone. |
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Keywords
basalts; composition; crystallization; differentiation; East, Pacific Ocean Islands; Hawaii; Hawaii County Hawaii; Hawaii, Island; igneous rocks; Kilauea; magmas; major-element analyses;, Makaopuhi lava lake; Makaopuhi lava lake tholeiite; nesosilicates;, Oceania; olivine; olivine group; olivine role; orthosilicates;, petrology; Polynesia; relation to crystallization history; role of, olivine; silicates; tholeiitic; United States; volcanic rocks, 05, Igneous and metamorphic petrology |
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