Insights from the kinetics of phenocryst nucleation and growth on the timescales between melt generation and eruption of high-SiO2 rhyolites from Long Valley, California

Abstract:

High-SiO₂ (75-78 wt% SiO₂) rhyolite is the most differentiated magma type on Earth and constitutes some of the largest explosive eruptions (>100’s km³) over the last 1 Myr, including those from Long Valley, CA. High-SiO₂ rhyolites are commonly found in regions of continental extension, often in association with basalt (i.e., bimodal volcanism). An outstanding question is why high-SiO₂ rhyolites (near-eutectic melts) fail to segregate and erupt in significant quantities at subduction zones, but do so, often in surprisingly large volumes, in regions of continental extension. The answer to this and related questions is addressed by examining the kinetics of phenocryst nucleation and growth in the Long Valley high-SiO₂ rhyolites, including the ≥600 km³ Bishop Tuff and the preceding ≥ 100 km³ Glass Mountain complex, all of which are relatively cold (≥ 700°C) and hydrous (≤7 wt% H₂O). It is shown, through the results of thermometry, hygrometry, and both phase-equilibrium and decompression experiments, as well as available geochemistry, that their initial segregation and ascent must have occurred in the absence of an H₂O-rich fluid phase, leading the melts to ascend into a super-liquidus conditions until saturation with a fluid phase inevitably occurred. Continued ascent under fluid-saturated conditions caused the rhyolite melts to cross their liquidus without pre-existing nuclei, enabling large undercoolings and the rapid growth of large sparse phenocrysts. The large variations in crystal abundance in Early Bishop pumices, with similar temperatures and melt water contents, is attributed to kinetic delays in nucleation and growth. Phenocrysts of quartz (and sometimes sanidine) display classic diffusion-limited growth textures, consistent with rapid degassing-induced phenocryst growth during transit to the surface. The combined evidence constrains the timescale between melt segregation and eruption to be extremely short, on the order of several weeks or less. In contrast, it is proposed that aplite dikes in arc granitoids fail to erupt if they segregate under H₂O fluid-saturated conditions from a crystallizing magma. During fluid-saturated ascent, the loss of only a small amount of dissolved water from these near-eutectic melts will cause them to immediately crystallize.