24. MELTING RELATIONS AND VISCOSITY OF AN ABYSSAL OLIVINE THOLEIITE

Melting experiments at 1 atmosphere on an olivine tholeiite, Sample 395A-8-1, 50 cm, suggest that the magma erupted at or near 1215 °C. On the basis of the experiments at high pressures up to 12 kbar, it appears that olivine tholeiite magma could be equilibrated with Plagioclase lherzolite at a depth of about 25 km or be formed by fractional crystallization from more magnesian magma at depths shallower than 25 km. It also appears that the process of fractional crystallization is most likely at a depth of 25 km if pyroxenes take an important part in such a process. Viscosity and density measurements of melt of the sample at high pressure suggest that owing to low viscosity, olivine and pyroxene sink rapidly in the melt, whereas Plagioclase floats. The phenocrysts of porphyritic basalt, therefore, do not always represent the first liquidus phase. Some plagioclase-phyric tholeiite may be derived from olivine tholeiite magma by floating of Plagioclase phenocrysts at high pressures. INTRODUCTION Petrography and geochemistry of oceanic basalts have been studies by many investigators (e.g., Bryan et al, 1976). Experimental data on such rocks are relatively scarce, however. It is important to know the melting relations of oceanic basalt at 1 atmosphere and at high pressures, in order to understand the genesis of oceanic basalt magmas. Viscosity and density data on oceanic basalt magma at high pressures are also necessary to understand processes of fractionation at high pressures. For these reasons, melting experiments and density and viscosity measurements have been conducted on melt of a sample collected near the Mid-Atlantic Ridge during Leg 45. Sample 395A-8-1, 50 cm, was recovered at a depth of about 140 meters beneath the ocean floor, and belongs to type A2 aphyric tholeiite. This sample is glassy olivine tholeute, probably from the chilled margin of a pillow basalt, containing very small amounts of olivine microphenocryst (0.3 mm across, Fo85) and platy skeletal crystals of olivine (0.1 mm long, Fo83) set in the glassy matrix. The modal amount of olivine microphenocrysts is less than 1 per cent. The microphenocryst also shows skeletal habit, indicating that the microphenocrysts grew rapidly probably just before or at the time of eruption of magma. It may thus be concluded that this sample represents the liquid composition. The chemical composition and CIPW norm of the sample are listed in Table 1. This sample is one of the Now at Geological Institute, University of Tokyo, Tokyo, Japan. most magnesian rocks (Mg/Mg + Fe*=0.57) among the aphyric olivine tholeiite recovered on Leg 45, and also has high Ni content (178 ppm), low K2O content (0.13 wt%), and low ignition loss (Bougault et al., this volume). It therefore appears to be the least fractionated and least altered olivine tholeiite recovered during Leg 45. MELTING EXPERIMENTS The 1-atmosphere melting experiments were performed at p θ 2 = I 0 8 bar, where p θ 2 was controlled by changing the mixing ratio of CO2 and H2 gases. Powdered samples were pressed into pellets and a fragment of the pellet tied with fine Pt wire was fused at 1300°C and ρ θ 2 = I08 bar for 15 minutes to make a glass sphere. These glass spheres, held on fine Pt wire, were used through runs in order to minimize iron loss. Runs were quenched by dropping the material into a metal cup cooled with ice from outside. The high-pressure melting experiments were carried out in graphite capsules in a solid-media apparatus (Boyd and England, 1960) using the piston-out method. The powdered samples were dried at 110°C for several days and heated in air in graphite capsules at about 1000 °C for a minute just before the experiment to eliminate water. The oxygen fugacity of each run was within the stability field of wustite (Thompson and Kushiro, 1972). The results of melting experiments are listed in Tables 2A and 2B, and plotted in Figure 1. Olivine is the liquidus phase at pressures below 8 kbar. Above this pressure, Ca-rich clinopyroxene appears on the liqui-