Pahoehoe lavas at arc volcanoes with >50% crystals. How and why?
Abstract
Rheological studies of particle-melt suspensions assert that magmas with >50% crystals are generally incapable of flow due to rapid increase in effective viscosities at intermediate crystal contents. Nonetheless, pahoehoe lavas of evolved basaltic composition with >50% crystals occur at a number of arc volcanoes. As these were remarkably fluid during emplacement, we suggest that this was due to eruption as a suspension of melt and crystals in a high fraction of gas that originates by remobilization of crystal mush as a consequence of vigorous recharge and gas sparging. The 1780-90 eruption of Volcán Llaima (38.7° S, Chilean Andes) began with pahoehoe flows and then switched to a'a (45-55% crystals, dominantly plagioclase), leading to extremely thick late lavas carrying surface rafts of early pahoehoe. 1780-90 is one of six large to very large Llaima eruptions since 1640. Many smaller eruptions occurred during the 30-90 year periods between voluminous events. The 3.5 year average repose period between eruptions of all sizes since 1850 is consistent with a high frequency of magma recharge. Some large eruptions are compositionally heterogeneous, and whole-rock chemical heterogeneity correlates with the diversity of crystal core compositions and zoning patterns. Olivine core compositions in 1780 pahoehoe span the same narrow range as those in the unusually homogeneous 1751 lava, but many of the 1780 olivines have near-rim reversely zoned shoulders. Olivine populations in more compositionally variable 1780-90 a'a lavas are diverse. 1780 pahoehoe is remobilized 1751 magma which was followed immediately by a'a formed by mixing of degassed recharge magma with multiple crystal mush bodies. Rheologically stiff mush bodies initially trap gas at shallow depths, but sparging into bubbly interstitial melt leads to inflation, convection, rupturing, and intrusion, thereby triggering a very short-lived fountaining or violent Stombolian phase, and fountain-fed flows.