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Immiscibility synthesis as an indication of cooling rates of basalts

1991; Elsevier BV; Volume: 45; Issue: 3-4 Linguagem: Inglês

10.1016/0377-0273(91)90064-7

1872-6097

Béatrice Martin, Ikuo Kushiro,

Geology and Paleoclimatology Research

Silicate liquid immiscibility of iron-rich and silica-rich compositions have been experimentally reproduced at various cooling rates to evaluate the possible effect of cooling rate on the size of the globules. The experiments were performed at constant cooling rates from 1 to 32°C/hr under atmospheric pressure and controlled oxygen fugacity. A tholeiitic basalt which occurs as a 36 cm thick dike in the Izu Peninsula (Japan) was used as starting material. The sample was melted at the liquidus temperature (1190 ± 5°C) and cooled to 1000°C. Two immiscible liquids occur in the charges, as do in the dike. In the experiments, the size of the globules is strongly related to the cooling rate; the diameter of the globules decreases exponentially with increasing cooling rate and the number of globules decreases with decreasing cooling rate. These facts together with the typical coalescence shape suggests a growth of globules by coalescence rather than a decrease of the nucleation rate. Compositional variations of the liquid just prior to splitting within each charge do not affect those of the two immiscible liquids. However, the compositional differences between the two liquids slightly increase with decreasing cooling rate. Using the cooling rate vs size relationships, the size of globules in basalts allows a cooling rate estimate. In the Izu dike, the 10 largest silica-rich globules range from 2.3 μm to 0.8 μm in the center and the margin, suggesting cooling rates of about 15°C/hr and greater than 32°C/hr respectively, at temperatures near 1000°C. Other estimates from natural samples show a good agreement between deduced cooling rates and observed textures.