Crystallization of tholeiitic basalt in Alae Lava Lake, Hawaii

Abstract

The eruption of Kilauea Volcano August 21–23, 1963, left 600,000 cubic meters of basaltic lava in a lava lake as much as 15 meters deep in Alae pit crater. Field studies of the lake began August 27 and include repeated core drilling, measurements of temperature in the crust and melt, and precise level surveys of the lake surface. The last interstitial melt in the lake solidified late in September 1964; by mid August 1965 the maximum temperature was 690°C at a depth of 11.5 meters. Pumice air-quenched from about 1140°C contains only 5 percent crystals — clinopyroxene, cuhedral olivine ( Fo 80 ), and a trace of plagioclase, ( An 70 ). Drill cores taken from the zone of crystallization in the lake show that olivine continued crystallizing to about 1070°C; below that it reacts with the melt, becoming corroded and mantled by pyroxene and plagioclase. Below 1070°C, pyroxene and plagioclase crystallized at a constant ratio. Ilmenite first appeared at about 1070°C and was joined by magnetite at about 1050°C; both increased rapidly in abundance to 1000°C. Apatite first appeared as minute needles in interstitial glass at 1000°C. Both the abundance and index of refraction of glass quenched from melt decreased nearly linearly with falling temperature. At 1070°C the quenched lava contains about 65 percent dark-brown glass with an index of 1.61; at 980°C it contains about 8 percent colorless glass with an index of 1.49. Below 980°C, the percentage of glass remained constant. Progressive crystallization forced exsolution of gases from the melt fraction; these formed vesicles and angular pores, causing expansion of the crystallizing lava and lifting the surface of the central part of the lake an average of 19.5 cm. The solidified basalt underwent pneumatolitic alteration, including deposition of cristobalite at 800°C, reddish alteration of olivine at 700°C, tarnishing of ilmenite at 550°C, deposition of anhydrite at 250°C, and deposition of native sulfur at 100°C. Ferric-ferrous ratios suggest that oxidation with maximum intensity between 550°C and 610°C moved downward in the crust as it cooled; this was followed by reduction at a temperature of about 100°C. The crystallized basalt is a homogeneous fine-grained rock containing on the average 48.3 percent by volume intergranular pyroxene (augite > pigeonite), 34.2 percent plagioclase laths (An 60 70 ), 7.9 percent interstitial glass, 6.9 percent opaques (ilmenite > magnetite), 2.7 percent olivine (Fo 70 80 ), and a trace of apatite. Chemical analyses of 18 samples, ranging from initially quenched pumice to lava cored more than a year after the eruption from the center and from near the base of the lake, show little variation from silica-saturated tholeiitic basalt containing 50.4 percent SiO 2 , 2.4 percent Na 2 O, and 0.54 percent K 2 O. Apparently there was no significant crystal settling and no appreciable vapor-phase transport of these components during the year of crystallization. However, seven samples of interstitial liquid that had been filter-pressed into gash fractures and drill holes from partly crystalline mush near the base of the crust show large differences from the bulk composition of the solidified crust—lower MgO, CaO, and Al 2 O 3 ; and higher total iron, TiO 2 , Na 2 O, K 2 O, P 2 O 5 , and F, and, in most samples, SiO 2 . The minor elements Ba, Ga, Li, Y, and Yb and possibly Cu tend to be enriched in the filter-pressed liquids, and Cr and possibly Ni tend to be depleted.