The 1-atmosphere melting patterns of some basaltic volcanic series

Abstract

The results of l-atm melting experiments on suites of lavas from Kilauea (Hawaii), Snake River Plain (Idaho), Reunion and Anjouan (Indian Ocean), and Skye (Scotland) are summarized. The patterns formed by plotting the melting stages of each lava in a series against its iron:magnesium ratio are compared with a simple synthetic model, derived from the system MgO-"FeO"-SiO2, at 1 atm. Although there is close agreement between the magnesian parts of the natural patterns and the synthetic model, this relationship does not extend the fusion behavior of the more iron-rich lavas. The deviations of their 1-atm thermal behavior from both the model and their observed phenocryst mineralogy are ascribed to the effects of load pressure and rising PH2O during the evolution of the magma series. Comparison of the deduced fractionation histories of the natural suites with currently used models for basalt crystallization shows some notable discrepancies, especially with iron- and alkali-free models based on the system CaO-MgO-Al2O3-SiO2 and its subsystems. It is shown that by limiting the basalt composition field to rocks containing a normative plagioclase more calcic than An50 most of the iron-rich lavas forming "anomalous" melting patterns are at the margin of the basalt composition field or slightly outside it. However, if the same criterion is applied equally carefully to the equilibria in published models for basalt crystallization, it appears probable that parts of the low-temperature, low-variance regions of these models are in fact constructed from and relevant to the crystallization behavior of intermediate lavas, not basalts. Available l-atm melting data on magnesian aphanites and natural glasses from widely dispersed areas suggest that the maximum temperatures of eruption of basaltic liquids are 1220° to 1240°C. The basalt-intermediate lava transition takes place at 1160° to 1190°C at 1 atm in the lava suites considered here. Magnetite joins the liquidus phases at approximately 1170° and 1190°C in the alkalic lavas of Anjouan and Skye, respectively. Anomalies attributed to PH2O in the magmas occur in the melting patterns of both tholeiitic and alkalic series up to approximately 1190°C. These thermal data place considerable restrictions on any approach to basalt crystallization that does not include alkalies, iron, and water as components in its model.