โ Scribed by David H. Eggler
No coin nor oath required. For personal study only.
Phase relations have been determined for a Paricutin Volcano andesite at pressures to 10 kilobars and for I-I20 contents in the melt of 2 to 10 weight percent. Runs were made under H20-saturated and undersaturated conditions. In undersaturated runs a H20-CO ~ fluid phase was always present. Fugacity of H20 in melt, which is directly related to H~O content in the melt, was calculated from thermodynamic data. Plagioclase was found to be the liquidus phase when H20 contents in melt were less than about two percent. With more H20 , orthopyroxene, in some cases joined by olivine, assumes the liquidus. Clinopyroxene crystallizes near the liquidus only :for H~O contents greater than five percent. The upper temperature stability limit of hornblende is about 950 ~ C, well below the other silicate liquidi except at H~O-saturated conditions above 5 kb.
The geometry of undersaturated liquidi and experimental phase compositions may be compared to the mode and phase compositions of the natural rock. From this comparison, megaphenocrysts of the natural rock are interpreted to have crystallized from a lava which had a water content of 2.2-4-0.5 percent and a temperature of 1110 4-40~ Mass-balance calculations on experimental and natural phase assemblages show that the Paricutin series could not have formed by fractionation at pressures less than 10 kb; rather, it was probably derived by partial melting of subducted basaltic oceanic floor.
๐ SIMILAR VOLUMES