Incorporation Of Air Into The Campanian Ignimbrite Pyroclastic Density Current

Abstract:

Knowing the temperature of emplacement of an ignimbrite can tell us how much cooling air it incorporated during eruption and transport. Currents that incorporate cool matter (air, water, cold clasts) cool more than those that do not. Lithic fragments record the maximum temperature they reached, up to their maximum unblocking temperature. Studies of large ignimbrites (e.g. Cerro Galan Ignimbrite) emplaced by dense currents show they do not cool very much, with emplacement temperatures often above 580 ^oC. Smaller currents, such as those from Vesuvius and Colima, lose significant heat in the eruption column, and then lose some, but less, heat as they travel laterally. The amount of atmosphere incorporated by large dilute currents is not known. The ~40 ka Campanian Ignimbrite (CI) erupted from the Campi Flegrei caldera near Naples, Italy, and extends to ~75 km from the caldera. The CI was emplaced from a density-stratified current with a dilute transport system and a denser depositional system that overtopped 1600-m-high ridges, with the depositional system re-forming on the far side. Modeling of dilute currents shows that they can pass over obstacles ~1.5 times their thickness without losing momentum, which implies the CI current was >1 km thick. Much of that dilute current was gas, but how much was atmospheric? Partial thermal demagnetization of lithic clasts allows the identification of the temperature of emplacement. We sampled lithic fragments from the CI in 13 locations from proximal to distal along several azimuths. The current passed over 30-35 km of sea to get to two sites. Partial thermal demagnetization of 10 specimens from each site show that they were heated and deposited above 580 ^oC, the unblocking temperature of magnetite, implying the temperature of emplacement was at or above this temperature. The CI is poor in lithic clasts (<1% in most places) and evidence of non-magmatic water in the outflow sheet is absent. We suggest the CI current was a large, continuous, and broadly distributed current and the body of the current had little contact with the atmosphere, except at the dilute and likely gradational top of the flow, so incorporated little air. This implies that the great expansion of the CI current was accomplished with magmatic gases or perhaps superheated hydrothermal fluids.