Effects of magma storage and ascent on the kinetics of crystal growth (original) (raw)
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Lithos, 2010
A systematic study of textural and compositional zoning (An% and FeO variation) in plagioclase phenocrysts of historic (pre-1971) and recent (post-1971) lavas at Mount Etna was made through back-scattered electron (BSE) images and electron microprobe analyses (EMP). The textures considered include oscillatory zoning and several types of dissolution, resorption and growth textures at the phenocryst cores and/or rims. Two patterns of oscillation were recognized from the combined An–FeO variation: 1) Low Amplitude–High Frequency (LAHF) and 2) High Amplitude–Low Frequency (HALF). The first pattern is interpreted here as due to kinetic effects at the plagioclase/melt interface which developed during crystallization in closed reservoirs. The second, which sometimes involves thin dissolution surfaces marked by irregular edges, angular unconformities and complex dissolution–regrowth patterns, might imply crystallization in a more dynamic regime, probably driven by chemical and physical gradients of the system (e.g., convection in a steadily degassing open-conduit). Dissolution and resorption textures at the core vary from patchy (exclusive to plagioclases within pre-1971 lavas) to strongly sieved, and can be related to increasing rates of decompression under H2O-undersaturated conditions. Thick sieve-textured envelopes at the phenocryst rims, generally coupled with marked An–FeO increase, result from mixing with more primitive and volatile-rich magmas. In the same crystals from recent activity, An and, to a lesser extent, FeO increase, consistent with the mixing of H2O-rich magmas similar in their mafic character to the resident magma (cryptic mixing). Two types of growth textures were also recognized at the crystal rims: 1) stripes of regularly-shaped melt inclusions and 2) swallow-tailed, skeletal crystals. In the first instance, the concordant An–FeO decrease suggests crystallization caused by fast ascent-related decompression accompanied by volatile loss. In the second, An decrease at effectively constant FeO contents may indicate crystallization at a high level of undercooling from already degassed magma, followed by rapid quenching; such a feature might be acquired during syn- or post-eruptive conditions.Although textures found in historic lavas are rather similar to those in the recent ones, some differences occur, such as lack of crystals with patchy cores in recent products and lower An contents in crystals of historic ones. The available data allowed us to obtain information on the dynamics of the feeding system, highlighting their possible modifications over time. In particular, historically erupted magmas, generally acknowledged to be volatile-poor, may have ascended through the deep portions of the plumbing system under H2O-undersaturated conditions at lower rates than the recent ones, recognized as more volatile-rich. Eruption triggering mechanisms from closed reservoirs in the shallow portions of the feeding system are similar for both historic and recent events, and may be generally favoured by a recharging phase of more primitive, undegassed magma or by a few episodes of important fracture opening (e.g., in response to an earthquake swarm).
Earth-Science Reviews, 2014
Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well as in most lavas erupted worldwide. Its stability field is strongly dependent on the physico-chemical conditions of the melt and, consequently, it can be used as a tool to record the processes occurring within the feeding system. With this aim, a detailed textural and compositional study of plagioclase was performed on the products emitted during the eruptions. Four distinct textures were recognized at the crystal cores: (1) clear and rounded (An 73-85 ), (2) dusty and rounded (An 73-85 ), (3) sieved (An 82-88 ) and (4) patchy (An 60-81 ), while two distinct textures are commonly observed at the crystal rim: (1) dusty (An 73-90 ) and (2) with melt inclusion alignments (An 70-76 ). Moreover all plagioclases present a thin (10-20 μm) outermost less calcic (An 53-76 ) rim. For each crystal a complex evolutionary path was reconstructed, and several growth and resorption episodes were identified. The fO 2 was estimated using Plag-Cpx/liquid equilibrium in order to calculate the Fe +3 /Fe 2+ ratio in the melt and, in turn, to reconstruct the primitive magma composition by adding a wehrlitic assemblage to the least evolved lava of the four eruptive episodes. MELTS modeling was then developed using this primary magma composition, as well as a trachybasaltic lava. Calculations were performed at variable pressures (400-50 MPa, step of 0.50 MPa) and H 2 O contents (3.5-0 wt.%, step 0.5 wt.%) in order to estimate the crystallization temperature of olivine, clinopyroxene, plagioclase and spinel, decreasing T from the liquidus down to 1000°C at steps of 20°C. P-T and water contents were also determined using geothermobarometers and plagioclase-melt hygrometers respectively, aiming at verifying the parameters used in the MELTS modeling. At this point plagioclase textural features and compositions were related to specific P-T-fO 2 -H 2 O conditions. Plagioclase stability models indicate that: (1) H 2 O strongly influences the plagioclase-melt equilibrium allowing the crystallizations of more calcic compositions only at shallow levels; (2) patchy cores form at high pressure (up to 350 MPa) and low water content (b 1.7 wt.%); (3) clear dissolved cores form at lower pressure (150 MPa) and higher water content (1.5-2.8 wt.%); (4) dusty rims form at even lower pressure straddling the H 2 O-saturation curve and, (5) melt alignments form during degassing. According to experimental works each of these textures can be related to a different process within the feeding system, such as multiple magma inputs (patchy core), volatile addition or increase in T (clear core), mixing (dusty rims) and rapid decompression and degassing (melt inclusion alignment at rims). These inferences were successfully compared with the eruptive evolution of each event as deduced from direct observations, and geophysical and petrological data. The overall picture shows that plagioclase crystallizes under polybaric conditions in a vertically extended and continuous feeding system in which at least two magma crystallization levels were identified. Plagioclase stability also indicates that a large variability in water content characterizes the magma within the feeding system.
Journal of Volcanology and Geothermal Research, 2009
After the 6 month-long effusive event of 2002-2003, a new lava effusion occurred at Stromboli between 27 February and 2 April 2007. Despite the different durations, approximately the same volume of magma was emitted in both eruptions, in the order of 10 7 m 3 . A paroxysmal eruption occurred at the summit craters in both the 2002-2003 and 2007 episodes, during which a significant amount of low porphyritic (LP), volatile-rich magma was erupted. In both cases, the paroxysms did not interrupt the lava emission. Here, we present compositional data, including texture, mineralogy, chemistry and Sr and Nd isotope ratios of bulk-rock, groundmass and separated minerals of lavas erupted in 2007, together with chemistry and Sr and Nd isotope composition of the pumices emitted during the 15 March paroxysm. As a whole, the lavas have the same texture and chemistry that characterize the highly porphyritic (HP) products usually erupted at Stromboli during normal Strombolian activity and effusive events. Compared to the previous HP products, the 2007 lavas show minor but systematic mineralogical and isotopic variations which are consistent with a modest increase of the magma supply rate of the volcano. Compositional A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 2 variations during the entire duration of the event are very modest. Glass chemistry changes in lavas erupted in the second half of March can be explained by the minor mixing between the volatilerich LP magma rising through the shallow magmatic system during the 15 March paroxysm and the degassed residing HP magma. A first conclusion of this study is that there is no compositional evidence supporting major changes in the magma dynamics of the volcano accompanying the effusive activity, as also suggested for the 2002-2003 event.
Crystal size distributions of plagioclase in lavas from the July–August 2001 Mount Etna eruption
Bulletin of Volcanology, 2015
During the 2001 eruption of Mount Etna, two independent vent systems simultaneously erupted two different lavas. The Upper Vents system (UV), opened between 3100 and 2650 m a.s.l., emitted products that are markedly porphyritic and rich in plagioclase, while the Lower Vents system (LV), opened at 2100 and 2550 m a.s.l., emitted products that are sparsely porphyritic with scarce plagioclase. In this study, the crystal size distributions (CSDs) of plagioclase were measured for a series of 14 samples collected from all the main flows of the 2001 eruption. The coefficient of R 2 determination was used to evaluate the goodness of fit of linear models to the CSDs, and the results are represented as a grid of R 2 values by using a numerical code developed ad hoc. R 2 diagrams suggest that the 2001 products can be separated into two main groups with slightly different characteristics: plagioclase CSDs from the UVs can be modeled by three straight lines with different slopes while the plagioclase CSDs from the LVs are largely concave. We have interpreted the CSDs of the UVs as representing three different populations of plagioclases: (i) the large phenocrysts (type I), which started to crystallize at lower cooling rate in a deep reservoir from 13 to 8 months before eruption onset; (ii) the phenocrysts (type II), which crystallized largely during continuous degassing in a shallow reservoir; and (iii) the microlites, which crystallized during magma ascent immediately prior to the eruption. The plagioclase CSD curves for the LVs lava are interpreted to reflect strong and rapid changes in undercooling induced by strong and sudden degassing.
Crystallization Driven by Decompression and Water Loss at Stromboli Volcano (Aeolian Islands, Italy)
Journal of Petrology, 2001
Stromboli, in the Aeolian Archipelago, is famous for its persistent chemistry; volatiles volcanic activity. The 'normal' activity, consisting of rhythmic explosions ejecting crystal-rich scoriae, is periodically interspersed with more energetic explosions during which, in addition to crystalrich scoriae, crystal-poor pumices are also emitted. The scoriae contain >50 vol. % crystals (Plag >65; Cpx >25; Ol >10) INTRODUCTION whereas the pumices display <10 vol. % crystals (Plag >42-50; Stromboli is a volcanic island in the Aeolian Archipelago Cpx >47-30; Ol >11-20). The bulk rocks, mainly ranging (Southern Italy) famous in the volcanological literature for between K-rich basalts and shoshonitic basalts, surprisingly exhibit its persistent state of activity. Recent chronostratigraphic only slight variations in major and trace element contents, and rare studies indicate that this activity started after a period earth element patterns. Systematic studies of melt inclusions (MI) dated between the third and seventh centuries (Rosi and their host minerals were performed on three scoria-pumice pairs et al., 2000). The volcanic cone is broadly elliptical, with erupted together during the violent explosions. The MI cover a a NE elongation, and rises from a depth of >2000 m to compositional range (CaO/Al 2 O 3 = 0•99-0•29) far wider than an elevation of 924 m above sea level (a.s.l.). Eruptions that of the whole rocks (CaO/Al 2 O 3 = 0•69-0•52) and attest occur from three main craters at 750 m a.s.l. in the Sciara to the presence of rather primitive melts not yet identified as erupted del Fuoco, a horseshoe-shaped depression situated on magmas. On the basis of MI analyses, the crystal-poor magmas the NW flank of the cone, produced by gravity collapses
Short eruption window revealed by absolute crystal growth rates in a granitic magma
Nature Geoscience
The potential for cataclysmic volcanic eruptions 1 depends on the volume of magma stored in shallow crustal reservoirs and the amount of time over which magma can accumulate without cooling and crystallizing to form a pluton of solid rock. Magma reservoir volume and longevity are, in turn, controlled by the flux of new magma into the system and the crystal content of the reservoir 2-6 . To understand why some magmas erupt, whereas others solidify in the crust, the timescales for crystal growth and upper-crust magma residence must be determined from both erupted volcanic rocks and intruded plutonic rocks 7 . However, our understanding of these timescales is largely restricted to volcanic rocks only 8-14 and measurements from plutonic rocks are missing. Here we use U-Pb geochronological dating of zircon crystals sampled from a seven-million-year-old upper crustal pluton in Elba, Italy. The zircon crystals were found as inclusions within the cores and rims of 6-8-cm-long potassium-feldspar megacrysts and constrain the rate of megacryst growth to 0.2-1.1 µ µ µm yr −1 . We combine the measured growth rates with petrological observations and phase-equilibrium modelling to show that the transition from eruptible magmas to immobile granitic mush and pluton formation occurred in just 10-40 thousand years. This short time window for a potential eruption implies that some magmas reside in upper crustal reservoirs for only a brief period before eruption.
Magmatic crystal records in time, space, and process, causatively linked with volcanic unrest
Earth and Planetary Science Letters, 2018
How a volcano has behaved throughout its past is a guide to its future behaviour. Detailed knowledge of what preceded eruptions from specific volcanoes, and how this can be recognised in real-time, are pivotal questions of this field. Here, the physical history of the magma that erupted in 2010 from the flank of Eyjafjallajökull volcano, Iceland, is reconstructed in absolute time and space using only chemical records from erupted crystals. The details of this reconstruction include the number of magma bodies, their geometry, their depth, their relative inflation rate and changes to all of the aforementioned through time. Petrology and geodesy (data gathered in real-time) arrive at the same set of conclusions. As such, we report detailed agreement, which demonstrates a causative link between knowledge determined post-eruption via a physical-chemical perspective and knowledge gained syn-eruption from monitoring signals. The composition of olivine crystal cores (∼Fo74-87), and that of the chemical zonation around each core caused by disequilibrium processes, are shown to form systematic patterns at the population scale. Reverse zonation (toward Mg rich) exhibits a constant chemical offset from its crystal core (≤2 mol % Fo), while normal zonation (toward Fe rich) converges to a single composition (∼Fo75). Conventional petrological models-for instance multiple-magma-mixing across a range of crustal depths-can explain the presence of a range of crystal core composition in the erupted rocks, but cannot explain these patterns of crystal disequilibria. Instead, we describe how a single primitive melt produces crystals over a wide range in composition and generates systematic disequilibrium. Cooling causes crystal production from both roof and floor of a horizontal magma geometry. Crystal settling causes asymmetric thermal-and therefore compositionalstratification of the melt due to progressive insulation via development of a crystal mush at the floor, a process we term "Crystal Rain". Crucially, each crystal's record is both a cause and effect of the internal process of simultaneous fractional crystallisation and settling; no external processes or materials are required. We then extract temporal information from our crystals using Fe-Mg interdiffusion modelling, and combine it with the composition and zonation data. The concept of Crystal Rain is applied, and resolves two thin (metres) sills which are staggered in time and depth, and exhibit different inflation rates. Since the approach of integrating crystal chronology within a causative physical framework may be applied to entire volcanic successions, it has potential to yield valuable insights to past, and by inference future, magmatic and volcanic behaviours by deterministic means.
Journal of Volcanology and Geothermal Research, 2018
Here, we present a detailed analysis of a small scale eruption at the type basaltic volcano Mount Etna, Italy, spanning 9 days in May 2016 from both a physical and geochemical perspective. A complexity rarely seen within the short timeframe was present with the style of activity manifesting as outgassing, strombolian explosions to weak fountaining, and lava flows while the eruption migrated between most of the summit craters. Through microprobe analysis of phenocrysts, groundmass glasses and melt inclusions we define geochemical trends by differentiating the eruptive products into two groupsthe explosive tephra produced by lava fountaining, and lava and scoria emitted during Strombolian explosions. We highlight plagioclase and olivine compositions and variations in K 2 O wt.% of glasses as evidence of the eruption of multiple magma bodies.The eruptive sequence was triggered and fed by a batch of magma reaching the surface after limited degassing and crystallisation. Despite its very small mass (about 0.01 % of the total erupted magma), it generated three lava fountains occurring within a four day timespan, producing ash-rich plumes which