How cold was the Peperino Albano (Colli Albani, central Italy) phreatomagmatic ignimbrite? A paleomagnetic study to estimate the emplacement temperatures.

Recent developments in the paleomagnetic estimation of emplacement temperature of volcanic rocks have shown that such technique can be successfully applied also to relatively cold pyroclastic flow deposits.The 23 ka old Peperino Albano ignimbrite is a phreatomagmatic pyroclastic flow deposit, that shows lateral and vertical facies transition from valley pond to veneer deposits. The presence of unburnt plants at the base of the ignimbrite suggests that the temperature of the pyroclastic flow was relatively cold, lower than the temperature of ignition of wood.We carried out a paleomagnetic study of thermal remanent magnetization (TRM) of the lithic clasts of the Peperino Albano ignimbrite, to quantitatively determine the range of its emplacement temperatures across the different facies shown by the ignimbrite. Emplacement temperatures estimated for the Peperino Ignimbrite range between 200°C and 440°C. No significant differences in the temperature range have been obtained between valley pond and veneer facies, indicating that the particle temperature of emplacement is mostly related to cooling during the magma-water interaction rather than during the transport. The temperatures defined in the intracrater facies are generally lower than the other two facies, probably because the lithic clasts had not reached thermal equilibrium with their surroundings. This could be interpreted as due to the large size of the sampled clasts in the intracrater facies, and to a low temperature at the vent, that did not heat up the whole volume of the individual clast.The results confirm the reliability of the paleomagnetic approach in defining the thermal history of Peperino Albano ignimbrite.