Isotopic and chemical evolution of the post-caldera rhyolitic system at Long Valley, California

Abstract

Post-caldera rhyolites of the Long Valley magmatic system are chemically less evolved than pre-caldera rhyolites or the initial phases of the Bishop Tuff and record temporal variations in composition which imply open-system magma differentiation involving magma replenishment. All post-caldera rhyolites lie on a well-defined Pb–Sr isotope mixing line between the ∼700 ka Resurgent Dome and the recent Inyo lava domes, precluding a simple cogenetic origin. Coherent temporal trends in Pb and Sr isotopes provide compelling evidence for the near-continuous addition of magma into a silicic magma chamber that was residual after Bishop Tuff eruption. Nd isotope ratios do not record such consistent variations, arguing against their use as a proxy for major additions of new magma and hence as a monitor of potential volcanic hazard. The lowering of 87Sr/86Sr with time demonstrates that there was little crustal interaction and that the Nd isotope composition of the added magmas was variable. The rhyolites of Mammoth Knolls are the most differentiated and define an Rb–Sr isochron of 277±124 ka. These data are consistent with stratification of the magma chamber and subsequent isolation of the upper, more evolved, sections at ∼275 ka. Post 400 ka, rhyolites become chemically more varied, supporting evidence of stratification. The high- and low-silica rhyolites record distinct temporal Sr isotope evolution, implying that the low-silica rhyolites sampled the bulk of the magma chamber whereas the high-silica rhyolites represent the upper isolated, parts of the magma chamber, where they resided for >30 and >300 kyr. After the eruption of the Bishop Tuff, the Long Valley magma chamber was well mixed for the first ∼400 kyr of its evolution and produced rhyolitic magmas at an average of ∼0.0001 km3/year. Upon stratification post 400 ka, tens of km3 of chemically more evolved magma were rapidly produced. Magma addition to the system was at a constant rate and more frequent than eruptions.