Geocheminar fall-2014

The potential for cataclysmic volcanic hazards depends on the longevity and volume of shallow-crustal magmatic reservoirs, which are controlled by the crystal content of the reservoir and the flux of magma into the system. Published measurements constraining the timescales of crystal growth and upper crustal liquid residence have been determined exclusively for the snapshots of magma evolution recorded by volcanic rocks. However, to understand why certain magmas erupt and others do not, comprehensive datasets from both volcanic and plutonic rocks are required. Here I determine crystal growth rates in a plutonic system through high-precision U-Pb geochronology on zircon inclusions within cores and rims of 6-8 cm K-feldspar “megacrysts” in a ~7 million year old upper crustal intrusion in Elba, Italy. I determine K-feldspar growth rates of 0.2 to 1.1 µm/yr, which when combined with petrologic observations and phase equilibrium modeling show that the transition from eruptible magma to immobile granitic mush occurred in 10-40 kyr. Such a short post-emplacement eruption window substantiates evidence for brief upper crustal magma residence in systems capable of eruptions similar to those documented in historical records.