Constraining pre-eruptive magma conditions and unrest timescales during the Monte Nuovo eruption (1538 ad; Campi Flegrei, Southern Italy): integrating textural and CSD results from experimental and natural trachy-phonolites

Abstract

We present crystallization experiments representing a broad range of growth conditions of alkali feldspar and sodalite in a trachy-phonolite magma composition during later stages of evolution. Our results include (i) textural data and mineral assemblages of synthetic samples; (ii) feldspar nucleation kinetics and growth rate estimates; and (iii) textural data, mineral abundances, and crystal size distribution measurements on samples representative of the Monte Nuovo eruption (1538 ad), the last eruption of Campi Flegrei, Southern Italy. Experiments reproduced the texture and feldspar content of natural products indicating that kinetic data can provide insights into processes within the volcanic system shortly before and during this small-magnitude eruption and, particularly, about magma ascent timescale. We suggest that the groundmass crystallization of Monte Nuovo magma started between 4 and 7 km depth (∼100–200 MPa) at a temperature between 825 and 840 °C (close to the liquidus of alkali feldspar). The crystallization kinetics of alkali feldspar and the absence of sodalite in most of the natural samples indicate that magma ascent rate increased in the shallow part of the conduit from about 3 km depth to the quenching level (possibly fragmentation point; ∼30 MPa), during the first phases of the eruption. The crystallization time of the magma requires that it ascended from pre-eruptive storage to the quenching level in several hours to a few days. We also observe that a small decrease in pressure can induce a dramatic increase in crystallinity, with associated rheological changes, leading to changes in the eruption style, and such changes could occur on timescales of hours to several days. The products from the later phases of the Monte Nuovo eruption are more crystalline and contain sodalite in response to the decrease in magma ascent rate, which in turn allowed for more degassing during ascent, resulting in more time spent at very shallow depths.