The depth, thermal, physical and chemical status of magma storage plumbing systems under active volcanoes may significantly change with time. The crystal mush under persistently active basaltic volcanoes can be completely disrupted, via cannibalization processes, in less than a decade with profound implications for intensity, frequency and type of eruptions. Determining the rates and causes of significant changes in the architecture of magmatic systems is of paramount importance to develop detailed time-integrated models that successfully outline plumbing system evolution over societally relevant timescales. Focusing on crystal cargos and magmatic xenoliths, we investigate the decadal to centennial temporal evolution of magma-mush dynamics at Mt. Etna volcano, a template for persistently active basaltic volcanoes. This project will be the first time that a synergistic approach combining temporal, chemical and petrological evolution and volcanological data over a period of 8 centuries of eruptive activity is undertaken. Results will provide a needed, novel perspective on basaltic magma-mush dynamics at depth, below the ascent gas exsolution level. Through this approach, we will 1) evaluate the state of the crystal mush, 2) obtain microstructural proxies of cooling rate and fluid dynamical regime, 3) determine the extent and dynamics of mush disruption/formation over time, and 4) quantify the magnitude of micro-chemical and -isotopic variation, (dis)equilibria between phases and temporal trends potentially related to mush evolution and/or mafic recharge.
In this proposal, the state of the mush encompass both the rock-to-mush and the mush-to-magma transitions. Crystals can be stored in mush layers for decades to thousands of year. However, mafic mushes are transient, ~ 100 years or slightly longer, before triggering events. Mush destabilization causes crystal disruption through a combination of reactive flow, partial melting, melt percolation and cannibalization, rapid ascent and a rejuvenation of the system. Therefore, long-lived mafic systems raise questions regarding 1) the variations in the rates and locations of the magma supply and their impact on mush evolution in space and time and, 2) the key mechanisms and conditions modulating the magma-mush dynamics and the resulting eruptive behavior.
This project investigates the crystal cargo in cognate xenoliths and volcanics (i.e., entrained mush crystals) erupted over 8 centuries of activity at Mt. Etna as a template for a decadal- to centennial time-integrated, polybaric-polythermal and chemical framework describing magma-mush dynamics, magma pathways and the implication for eruptive behavior at basaltic volcanoes. Traditional investigation of a single eruption that only provide a snapshot of the state of the magma plumbing system, must give way to a wider view of the evolution of the plumbing system over longer, decadal to centennial timescales. This will provide the framework to recognize patterns that aid interpretation of current and future eruptive activity, building on knowledge of past activity. Evidence of the evolution of the plumbing system lies in the crystal cargo, particularly when combining volcanic rocks with their plutonic counterparts represented by cognate xenoliths. Volcanic rocks provide a snapshot into the pre-eruptive state of the magma body, and their crystal cargo carries information on pre-eruptive processes and conditions within mush zones. Glomerocrysts and antecrysts, often in disequilibrium with their host magma, are actually part of the crystal mush (i.e., entrained mush crystals). Cognate xenoliths provide a time-integrated history of the crystal mush, rock-to mush and magma-to-mush conversion, and magma storage zones, often spanning several thousands to million years.
The 1974 flank eruption marked the transition between two long-term deep magmatic components accessed by cognate xenolith bearing flank eruptions, including the magma composition characteristic of the 14th to the 20th centuries, and the younger 21st composition possibly indicating a renewal of the plumbing system. Sr isotopic disequilibrium in pyroxene is evidence of cannibalism of pre-existing crystal mush(es), generated in a variable dynamic regime (i.e., closed- vs. open-system) of changing isotopic composition of the mantle source. Periods of constant Sr isotopic composition alongside a lack of crystal-melt disequilibrium (i.e., pre-1974, 1995-1999, 2006-2011) have been attributed to a less dynamic regime with lower magma supply rate and more uniform composition. However, it is equally possible that chemically homogenous magmas and sustained mushes are associated with high magma flux. We observe that periods of constant Sr isotopic composition also correspond to a predominance of summit eruptions as opposed to flank eruptions characterized by higher Sr isotopic variability and crystal-melt disequilibrium. Eruption frequency, style, and discharge rates appear modulated by changes in the magma storage architecture, thus offering an ideal case study to investigate the role of evolving magmatic plumbing systems on the eruptive behavior.
We will provide new insights on the crystal cargo from key eruptions from the 14th to the 21st century eruptive period, using volcanics and cognate xenoliths (1763, 2001, 2002-2003, 2013 and 2019) as samples representing mush, rock-to-mush and mush-to-magma transitions. The selected samples include the major 1970s compositional change. They also represent a key snapshot of the steady-state behavior of Etna and allow us to determine the age of cognate xenoliths, their physical nature at the time of mush disruption, and relationships to their host magma relative to both clinopyroxene-dominated to plagioclase-dominated crystal mushes.