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.
Highlights
The aim of the studyAlkali feldspar is an abundant phase in evolved alkaline rocks and is widespread in Campania Province magmas (e.g., Piochi et al 2005)
To constrain the conditions and the timescales of magmatic processes, we focus on the crystallization kinetics of alkali feldspar in trachytic melts and new crystal size distribution (CSD) data for groundmass feldspars in the natural samples
Acicular, and curved-shape feldspar crystals are visible in the run products (Fig. 5). They were not included in the data that were used to estimate the kinetics of crystallization
Summary
The aim of the studyAlkali feldspar is an abundant phase in evolved alkaline rocks (phonolites, trachytes) and is widespread in Campania Province magmas (e.g., Piochi et al 2005). Its occurrence as microlites in pumice and scoria provides potential information on the timescale of magma migration within the crust, from the magma chamber to the fragmentation level (Marsh 1988; Cashman and Marsh 1988; Marsh 1998) This information may be unraveled by studying the size distribution of microlites pre- and syn-eruptively crystallized and quenched in the matrix of juvenile magma fragments. Bull Volcanol (2016) 78: 72 possible to link textural observations on rocks with experimentally derived data for rates of crystal growth (YL) and crystal number density (Na) for specific mineral phases and the undercooling (ΔT) values of the parental melt This approach makes it possible to obtain information on magmatic processes and their timescales using textural observation and growth rates of crystals (Cashman and Marsh 1988; Brugger and Hammer 2010a; Eberl et al 2002)
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