Deep Magma Storage Revealed by Multi-Method Elemental Mapping of Clinopyroxene Megacrysts at Stromboli Volcano

Abstract

The magmatic architecture and physicochemical processes inside volcanoes influence the style and timescale of eruptions. A long-standing challenge in volcanology is to establish the rates and depths of magma storage and the events that trigger eruption. Magma feeder systems are remarkably crystal-rich, and the growth stratigraphy of minerals sampled by erupted magmas can reveal a wealth of information on pre-eruptive processes. Here we combine detailed textural and chemical data acquired on large (>5 mm), euhedral augite megacrysts from Roman era activity (Pizzo scoria cone, 2.4-1.8 ka) at Stromboli (Italy) to investigate the plumbing system prior to the onset of current steady-state activity. Our dataset includes novel laser ablation time-of-flight mass spectrometry (LA-ICP-TOFMS) maps, which rapidly visualise multi-element zoning patterns across entire megacryst sections. The clinopyroxene data are complemented with geochemical constraints on mineral and melt inclusions, and adhering glassy tephra. Megacrysts are sector and oscillatory zoned in trace elements, yet their major element compositions are relatively uniform and in equilibrium with shoshonite-buffered melts. Mild sector zoning documents dynamic crystallisation under conditions of low undercooling during magma residence and growth. Clinopyroxene-melt thermobarometric and hygrometric calibrations, integrated with thermodynamically derived equilibrium equations, accurately track the P-T-H 2 O path of magmas. The refined models return restricted crystallisation depths that are deeper than those reported previously for historical and current eruptions, but consistent with deep clinopyroxene-dominated crystallisation (≥10 km), resembling other water-rich alkaline mafic systems. Megacryst cores are overgrown by oscillatory zoned mantles recording continuous input of magma that failed to trigger eruption. Crystal rims are characterised by a mild increase in compatible transition metals Cr and Ni, and depletion in incompatible elements, indicative of pre-eruptive mafic replenishment and magma mixing. The volcanic system appears to have been dominated by protracted periods of replenishment, convection, Ubide et al. Mapping Clinopyroxene Megacrysts at Stromboli and crystal residence, punctuated by rapid megacryst evacuation and eruption upon arrival of more mafic magma (days-weeks). Since the inception of current steady-state activity, eruption-triggering melts have become appreciably more mafic, suggesting that intrusion of primitive magma may be a key driver of the steady-state regime.

Shallow conduit dynamics fuel the unexpected paroxysms of Stromboli volcano during the summer 2019

“…Noteworthy, the two zoning types distribute with equal percentage in both deposits of July 3 and August 28, 2019, that is of 71-72% for Type 1 and 28-29% for Type 2. The range of plagioclase compositions, together with the close association of scoria and pumice in the same clast, supports the assumption that both paroxysmal eruptions were the products of syn-eruptive mingling of two magmas characterized by distinct physio-chemical conditions [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] .…”

“…According to previous petrological studies and field observations 28,29,[32][33][34][35][36] , large-scale paroxysms at Stromboli are characterized by the emission of a nearly degassed and highly-porphyric (HP) magma residing in the volcanic conduit at shallow levels (2-4 km below the summit) that interacts with gas-rich, low-porphyric (LP) magma coming from the deep levels of the volcano plumbing system (7-10 km below the summit). During paroxysmal eruptions, the deep-seated LP magma can rise under dominant closed-system conditions emitting highly vesicular basaltic pumices in addition to a shoshonitic crystal-rich scoria that is typically ejected during the ordinary activity 28,29,32,36 .…”

“…In the case of Stromboli, two divergent points of view (and relative intermediate/mixed models) have been so far proposed for explaining the source of overpressure needed to trigger an explosive event. Some authors stress the role of the injection and fast ascent of deep volatile-rich melts, the so-called low porphyric (LP) magmas, and fluids in pressurizing the shallow crystal-rich and degassed part of the conduit system occupied by the so-called high porphyric (HP) magmas 25,28,[32][33][34][35][36] . Other authors suggest that shallow processes, such as shallow conduit clogging, shallow gas pressurization, change in magmastatic load following the extrusion of lava, determine the volcano explosivity and that the ascent of deep LP magma is a mere passive consequence of these processes [37][38][39][40] .…”

Strain Signals Before and During Paroxysmal Activity at Stromboli Volcano, Italy

“…Since the magma plumbing system of the Stromboli volcano is widely studied from geophysical, vulcanological, and geochemical perspectives, it has been recognized that the feeding system can be depicted as a vertical column (see, e.g., Métrich et al, 2010), connecting the open vents on the surface with a shallow ponding zone located at 2–4 km bsl (Bonaccorso et al, 2012). The deeper reservoir is located at 7–10 km depth (Morelli et al, 1975; Ubide et al, 2019): It is now widely recognized that these deep, low porphyricity magma reservoirs play an important role in triggering paroxysmal activity. These eruptions are triggered by a larger supply of deep magma or gas‐magma as demonstrated from the petrological analysis of products of the 1930 and XVI century eruptions (Bertagnini et al, 2011).…”

Time-window into the transcrustal plumbing system dynamics of Dominica (Lesser Antilles)

“…Compositional plateaus in zoned opx crystals can be identified, with EPMA, as zones of at least 10 µm length where the enstatite content (En content; ) is constant with a variability of ± 0.5%. If the same compositional plateau is found in a significant amount of crystals, these plateaus have been produced by significative changes in the magmatic conditions 7 , 52 and can be considered as ME (Supplementary Figure S15 ). An En increase (reverse-zoned opx) implies a temperature increase, with, or without, magma mixing with a hotter and more basic magma.…”