Petrological Insights into Shifts in Eruptive Styles at Volcán Llaima (Chile)

Tephra and lava pairs from two summit eruptions (ad 2008 and 1957) and a flank fissure eruption (∼ ad 1850) are compared in terms of textures, phenocryst contents, and mineral zoning patterns to shed light on processes responsible for the shifts in eruption style during typical eruptive episodes at Volcán Llaima (Andean Southern Volcanic Zone, Chile). The mineralogy and whole-rock compositions of tephra and lavas are similar within eruptive episodes, suggesting a common magma reservoir for Strombolian paroxysms and lava effusion. The zoning profiles and textures of plagioclase record successive and discrete intrusions of volatile-rich mafic magma accompanied by mixing of these recharge magmas with the resident basaltic-andesitic crystal mushes that are commonly present at shallow levels in the Llaima system. Each recharge event destabilizes the plagioclase in equilibrium with the resident crystal mush melt and stabilizes relatively An-rich plagioclase, as is recorded by the numerous resorption zones. Lavas typically have ∼15-20 vol. % more phenocrysts than the tephra. Differences in plagioclase and olivine textures and zoning, combined with different phenocryst contents, indicate that a greater volume fraction of recharge magma is present in the explosively erupted magma than in subsequent effusively erupted magma. We propose that Strombolian paroxysms at Volcán Llaima are triggered by interactions with large volume fractions of recharge magma, which decrease the bulk viscosity and increase the volatile contents of the erupted magmas, leading to the conditions required for the fragmentation of basaltic-andesite. Lava effusion ensues from reduced interactions with the recharge magma, after it has partially degassed and crystallized, thereby impeding rapid ascent. This process could be operating at other steady-state basaltic volcanoes, wherein shallow reservoirs are periodically refilled by fresh, volatile-rich magma

Crystal and melt inclusion timescales reveal the evolution of magma migration before eruption

Volatile element concentrations measured in melt inclusions are a key tool used to understand magma migration and degassing, although their original values may be affected by different re-equilibration processes. Additionally, the inclusion-bearing crystals can have a wide range of origins and ages, further complicating the interpretation of magmatic processes. To clarify some of these issues, here we combined olivine diffusion chronometry and melt inclusion data from the 2008 eruption of Llaima volcano (Chile). We found that magma intrusion occurred about 4 years before the eruption at a minimum depth of approximately 8 km. Magma migration and reaction became shallower with time, and about 6 months before the eruption magma reached 3–4 km depth. This can be linked to reported seismicity and ash emissions. Although some ambiguities of interpretation still remain, crystal zoning and melt inclusion studies allow a more complete understanding of magma ascent, degassing, and volcano monitoring data.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Magmatic crystal records in time, space, and process, causatively linked with volcanic unrest

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 compositional – stratification 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

Petrological insights into shifts in eruptive styles at Volcàn Llaima (Chile)

ISSN:0022-3530ISSN:1460-241

Investigating the role of volatile sources in the 1991 eruption of Mount Pinatubo (Philippines)

International audienc

How do olivines record magmatic events? Insights from major and trace element zoning

Reconciling the diverse records of magmatic events preserved by multiple crystals and minerals in the same sample is often challenging. In the case of basaltic–andesites from Volcán Llaima (Chile), Mg zoning in olivine is always simpler than Ca zoning in plagioclase. A model that explains a number of chemical patterns is that Llaima magmas stall in the upper crust, where they undergo decompression crystallization and form crystal-mush bodies. Frequent magma inputs from deeper reservoirs provide the potential for remobilization and eruption. The records of multiple recharge events in Llaima plagioclase versus an apparent maximum of one such event in coexisting olivine are addressed by using trace element zoning in olivine phenocrysts. We have integrated elements that (1) respond to changes in magma composition due to recharge or mixing (Mg, Fe, Ni, Mn, ±Ca), with (2) elements that are incorporated during rapid, disequilibrium crystal growth (P, Ti, Sc, V, Al). A more complex history is obtained when these elements are evaluated considering their partition coefficients, diffusivities, and crystal growth rates. The olivine archive can then be reconciled with the plagioclase archive of magma reservoir processes. Olivine (and plagioclase) phenocrysts may experience up to three or more recharge events between nucleation and eruption. Diffusion modeling of major and trace element zoning in two dimensions using a new lattice Boltzmann model suggests that recharge events occur on the order of months to a couple of years prior to eruption, whereas crystal residence times are more likely to be on the order of a few years to decades.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Pahoehoe lavas at arc volcanoes with >50% crystals. How and why?

International audienceRheological studies of particle-melt suspensions assert that magmas with >50% crystals are generally incapable of flow due to rapid increase in effective viscosities at intermediate crystal contents. Nonetheless, pahoehoe lavas of evolved basaltic composition with >50% crystals occur at a number of arc volcanoes. As these were remarkably fluid during emplacement, we suggest that this was due to eruption as a suspension of melt and crystals in a high fraction of gas that originates by remobilization of crystal mush as a consequence of vigorous recharge and gas sparging. The 1780-90 eruption of Volcán Llaima (38.7° S, Chilean Andes) began with pahoehoe flows and then switched to a'a (45-55% crystals, dominantly plagioclase), leading to extremely thick late lavas carrying surface rafts of early pahoehoe. 1780-90 is one of six large to very large Llaima eruptions since 1640. Many smaller eruptions occurred during the 30-90 year periods between voluminous events. The 3.5 year average repose period between eruptions of all sizes since 1850 is consistent with a high frequency of magma recharge. Some large eruptions are compositionally heterogeneous, and whole-rock chemical heterogeneity correlates with the diversity of crystal core compositions and zoning patterns. Olivine core compositions in 1780 pahoehoe span the same narrow range as those in the unusually homogeneous 1751 lava, but many of the 1780 olivines have near-rim reversely zoned shoulders. Olivine populations in more compositionally variable 1780-90 a'a lavas are diverse. 1780 pahoehoe is remobilized 1751 magma which was followed immediately by a'a formed by mixing of degassed recharge magma with multiple crystal mush bodies. Rheologically stiff mush bodies initially trap gas at shallow depths, but sparging into bubbly interstitial melt leads to inflation, convection, rupturing, and intrusion, thereby triggering a very short-lived fountaining or violent Stombolian phase, and fountain-fed flows

Mafic magma replenishment, unrest and eruption in a caldera setting: insights from the 2006 eruption of Rabaul (Papua New Guinea)

<p>Understanding the magmatic processes that drive unrest at silicic calderas remains a major goal in Volcanology. Rabaul in Papua New Guinea is an exceptional location because after two decades of unrest and a peak in seismicity and deformation in 1983–85, eruptive activity began in 1994 and is still ongoing. A particularly large sub-Plinian eruption occurred from Tavurvur in October 2006. Whole-rock compositions are andesitic and reflect mixing/mingling between basaltic and dacitic magmas from the same system. The magmas that fed the 2006 eruption were stored at about 930°C, with 1–3 wt% H₂O, 25–520 ppm CO₂, and 50–2500 ppm SO₂ in the melt. Melt inclusions hosted in pyroxene, and plagioclase phenocrysts record fractional crystallization at ≤200 MPa under relatively dry and poorly oxidizing conditions. Magma mixing/mingling is expressed as heterogeneous glass compositions, strongly zoned phenocrysts, and mafic crystal aggregates. A textural maturation from fine, acicular to large, blocky crystal clots implies different relative ages of formation. Modelling the chemical zoning of plagioclase shows that mafic–silicic interactions started a couple of decades prior to the 2006 eruption and continued until days to weeks prior to eruption. Basaltic replenishments have been driving unrest and eruption at the Rabaul caldera since the 1970s. </p

Combined Petrological and Numerical Modeling Approach to Address Highly Crystalline Magma Remobilization Prior to Eruption at Volcán Llaima (Chile, 38.7°S)

International audienceVolcán Llaima (Chile, 38.7° S) has an eruptive behavior that can be termed hyperactive Strombolian, with over 50 eruptions since 1640. It appears to have been erupting on similarly short repose time for at least 1000 years. If Llaima is locked into a highly repetitive eruptive mode because the processes operating in its sub-volcanic plumbing system are well established and control the eruptive behavior, it has the potential to be a predictable volcano. We aim at defining these processes by integrating petrologic insights with numerical modeling results. Llaima has produced fundamentally similar magmas in terms of whole-rock compositions (evolved basalt to basaltic andesite), mineralogy (plagioclase, olivine, and minor clinopyroxene), and crystallinity (30-55 vol% crystals) during the growth of the late Holocene edifice. Whole-rock compositions and matrix glasses span narrow ranges for a given eruption (51-54 wt% SiO2), whereas olivine crystals record wide ranges of core compositions (Fo84-67) indicative of multiple sources. An olivine-hosted melt inclusion study, focused on four historic tephra deposits, shows that magmas are stored in multiple batches, at shallow depths (<1.5 km according to calculated H2O-CO2-saturation pressures), in highly crystalline states. Magmas have mixed thoroughly enough to minimize variability at the whole-rock scale, and even at the matrix glass scale (rapid chemical diffusion in melts), but olivine phenocrysts did not have sufficient time to equilibrate before eruption. Time lags between recharge-related magma mixing and eruption were obtained by diffusion modeling of reversely and normally zoned rims on olivine crystals. These models yield incubation times on the order of a few months to a year. Rapid time scales linking magma recharge and eruption are in agreement with the high frequency of minor to intermediate scale eruptions at Llaima. A 2D, multiphase flow, finite volume model is used to explore in greater detail the dynamics of mixing and remobilization of crystal-rich mush bodies. We consider the injection of a slightly warmer and less crystalline magma into highly crystalline mush. Preliminary results suggest that the time scales of magma mixing and remobilization depend strongly on the vertical dimension of the mush body and the ratio of mush to recharged magma