MAGMA MIXING BETWEEN RHYOLITIC AND BASALTIC MAGMAS IN THE BRUNEAU-JARBIDGE ERUPTIVE CENTER, SNAKE RIVER PLAIN (USA): AN EXPERIMENTAL STUDY

Volcanic and magmatic activities in the Snake River Plain are characterised by a rhyolitic-basaltic bimodal geochemical character. The Bruneau-Jarbidge eruptive center (BJEC is a 95 x 55 km structural basin formed ca.12 to 8 Ma ago) concists of multiple eruptions of rhyolitic pyroclastic and lava flows. The rhyolitic units are interlayered with a series of basaltic lava flows. The BJEC is an example in which basalt underplating induced partial melting of the crust and where mantle and crustal derived melts experienced mutual interaction.

In this work we study experimentally the physical and chemical interaction of basaltic and rhyolitic magmas to evaluate whether this process can explain the variation in major and trace elements in the BJEC rhyolitc units.

Following other geochemical studies, the Mary’s Creek basalt and the Cougar Point Tuff rhyolite were chosen as end-members for mixing experiments. Petrographic analyses indicate that the basalt contains phenocrysts of plagioclase, clinopyroxene, and olivine in a glassy groundmass with micro-crystals of plagioclase. The rhyolite contains phenocrysts of plagioclase, quartz, sanidine, elongated vesicles, in a glassy groundmass.

Static and chaotic mixing experiments were performed using completely molten end-members in a newly developed experimental apparatus working at constant temperature (1400°C) and under controlled flow fields. Viscosity measurements of each sample has been performed using concentric cylinder. Samples resulting from the mixing experiments have been analyzed for major elements, and compared to natural data.

Experimental results indicate that efficient physical and chemical mixing between the end-members can occur (despite the high viscosity ratio, of the order of 10³). This interaction produced a strong modulation of compositional variability leading to the a partial extinction of the basalt and to significant variations in the rhyolite with the prodution of a continuous geochemical mixing trend. Inter-elemental plots display a clear similarity to the compositional variability of natural samples indicating that magma mixing may have played a role to the compositional variability in the SRP magmatism.