REINVIGORATING STAGNANT SILICIC MAGMA SYSTEMS: VOLCANIC AND PLUTONIC VIEWS

Mounting evidence supports substantial longevity of many, probably most, silicic magmatic systems. Both direct and circumstantial evidence suggest that longevity is associated with repeated replenishment. We present examples from our experience with active and ancient volcanoes and plutons, including exposed systems from plutonic to volcanic levels, that lead us to infer that these systems fluctuate in response to periodic replenishment, especially by mafic magma, and that these replenishments are likely eruption triggers:

--Field evidence in many cases spectacularly documents modest to abundant mafic recharging within granitic plutons and in erupted rhyolites, and, more subtly, silicic recharging (Colorado River extensional corridor, Miocene, NV-CA-AZ; Iceland, Miocene to present day)

--Zircon zoning (morphology and elemental compositions) reveals major fluctuations in temperature and melt composition, with common resorption and resumed growth, in host melts (Mount St. Helens; Colorado River extensional corridor; Iceland)

--Zircon ages indicate punctuated growth episodes (resolvable on tens of k.y scale by U-Th [<350 ka: Mount St. Helens, Iceland] to 100’s of k.y. (older Icelandic and Colorado River intrusions])

--Textures – especially in erupted products –document dramatic swings in temperature (e.g. high-T zircon rims, reaction rims and resorption on other ‘evolved’ phenocrysts [quartz, sphene, sanidine]) and collection of unrelated phenocrysts (e.g. olivine-sanidine-quartz)(Colorado River)

Our experience with silicic systems spanning subduction zone, oceanic, and intracontinental settings and ranging from small, persistent eruptive centers to supereruptions suggests that thermal rejuvenation plays a vital role in the dynamic behavior of many and perhaps most silicic systems. It is further consistent with the hypothesis that such systems are long-lived but that their normal state is as melt-poor, immobile, uneruptible bodies that only sporadically spring to life. An intriguing question persists: during their stagnant stages, do systems retain some melt (remain magmatically alive), or die (fall beneath their solidi), before reinvigoration by replenishment? (Do magmatic systems emulate Snow White or Lazarus?)