Experimental magma mixing between rhyolitic and basaltic melts: implications for the Bruneau-Jarbidge magmatism, Snake River Plain (USA)
Volcanic activity in the Snake River Plain (SRP) is bimodal (rhyolitic-basaltic) in geochemical character. The Bruneau-Jarbidge eruptive center (BJEC; southwestern SRP) is a 95 km by 55 km structural basin formed ca.12 to 8 Ma by multiple eruptions of rhyolitic pyroclastic and lava flows, intercalated with a series of basaltic lavas. In the BJEC, underplating of basaltic magmas induced crustal melting and partial assimilation (Leeman et al., 2008). The purpose of this study is to investigate experimentally the physical and chemical interactions between basaltic and rhyolitic magmas and to evaluate whether this process can explain the variation in major and trace element in the BJEC rhyolitic units. The Mary's Creek basalt and the Cougar Point Tuff rhyolite (unit V) were chosen for their end-member signature in the following mixing experiments. First, the temperature dependence of viscosity of each sample has been investigated at superliquidus temperatures (>ca.1200 °C) using a concentric cylinder rheometer. Second, two setups have been used to assess the potential for mixing: 1) three alternating layers of each end-member where mixed (at 1200 to 1600 °C) by density-driven overturn in a Pt80Rh20 crucible; and 2) an offset journal bearing geometry where efficient, controlled chaotic flow fields have been obtained (at 1300, 1350 and 1400°C) by stretching and folding dynamics (e.g., De Campos et al., 2011). Mass fractions of the rhyolitic-to-basaltic melts were 50-50 for the density-driven experiments and 80-20 for the chaotic mixing experiments. Samples resulting from the mixing experiments were analyzed for major and minor elements, and compared to chemical compositional range observed in the BJEC. Experimental results indicate that despite the high viscosity contrast (10^3 Pa s), the end-members show efficient mutual contamination and thus mix on timescale of hours to weeks depending on the surface area of the end-member interface. Inter-elemental plots display a clear similarity to the compositional variability of natural samples indicating that restricted magma mixing may have played a key role in generating the magmas erupted in the SRP.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- 1037 GEOCHEMISTRY / Magma genesis and partial melting;
- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 8445 VOLCANOLOGY / Experimental volcanism