Years of magma intrusion primed Kīlauea Volcano (Hawai'i) for the 2018 eruption: evidence from olivine diffusion chronometry and monitoring data

International audienceThe mechanisms that led to the exceptionally large Kīlauea 2018 eruption are still poorly understood and actively debated. External processes such as rainfall events or flank sliding have been proposed to play a triggering role. Here, we present field, geophysical, and petrological observations to show that internal changes within the magmatic plumbing system most likely led to the eruption. Chemical zoning in olivine crystals records the intrusion of primitive magma that is concurrent with deep seismicity and inflation at the volcano's summit. Magma replenishment and pressurization of the summit reservoirs already started around 2014 and accelerated towards the eruption. Kīlauea volcano was therefore primed to experience a shift in eruptive activity in 2018. This pressure increase associated with reservoir replenishment may have been sufficient to overcome a previously blocked conduit. These findings imply that precursory signs of years of protracted magma intrusion and pressurization of the system may be recognizable in the future, which could lead to improved hazards mitigation

Genesis of Carbonatite at Oldoinyo Lengai (Tanzania) from Olivine Nephelinite: Protracted Melt Evolution and Reactive Porous Flow in Deep Crustal Mushes

International audienceAbstract Carbonatites, carbon-rich magmatic rocks, are thought to form by low-degree partial melting of a relatively carbon-poor mantle followed by protracted differentiation and immiscibility. However, the nature of parental magmas and the characteristics of the early stages of differentiation that shape the subsequent crystal and liquid lines of descent remain poorly constrained. To provide new constraints, deep crustal cumulative xenoliths from Oldoinyo Lengai (East African Rift), the only active volcano erupting carbonatite magmas, were studied. We use major and volatile elements in primitive olivine-hosted melt inclusions, as well as major and trace elements in crystals, to reconstruct the conditions of formation and evolution of cumulates (pressure, temperature, composition). Xenoliths are composed of olivine, diopside, phlogopite, amphibole and accessory minerals. One remarkable feature is the presence of diopside and phlogopite oikocrysts enclosing roundish olivine chadacrysts. Melt inclusions do not have vapor bubble and have major element compositions resembling olivine nephelinite (7–10 wt % MgO after corrections for post-entrapment crystallization). The absence of vapor bubbles implies that the concentrations of volatile components (i.e. CO2, H2O, S) were not compromised by well-known post-entrapment volatile loss into the vapor bubble. Based on the melt inclusion study by SIMS, the volatile concentrations in olivine nephelinite magmas (early stage of differentiation) at Oldoinyo Lengai were 20–130 ppm S, 390–4500 ppm F, 50–540 ppm Cl, up to 6074 ppm CO2 and up to 1.5 wt % H2O. According to the calculated CO2-H2O saturation pressures and geophysical data, xenoliths from Embalulu Oltatwa document a mushy reservoir in the lower crust. Primitive olivine nephelinite melt inclusions have higher H2O contents than olivine nephelinite lavas from other further South volcanoes from the North Tanzanian Divergence (0.2–0.5 wt % H2O), suggesting that the lithospheric mantle source beneath the Oldoinyo Lengai is more hydrated than the mantle beneath the rest of North Tanzanian Divergence. We present a model in which resorption features observed in olivine chadacrysts, together with the LREE enrichments in olivine grains, are the consequences of reactive porous flows in a deep crustal mushy reservoir. We provide constraints on the major, trace and volatile element composition of the parental magmas of carbonatite series and demonstrate with Rhyolite-MELTS models that phonolites and related natrocarbonatites from Oldoinyo Lengai can be produced by protracted differentiation of olivine nephelinite melts

Phosphorus and aluminum zoning in olivine : contrasting behavior of two nominally incompatible trace elements

Phosphorus zoning in olivine is receiving considerable attention for its capacity to preserve key information about rates and mechanisms of crystal growth. Its concentration can vary significantly over sub-micron spatial scales and form intricate, snowflake-like patterns that are generally attributed to fast crystal growth. Ostensibly similar aluminum enrichment patterns have also been observed, suggesting comparable incorporation and partitioning behavior for both elements. We perform 1-atm crystallization experiments on a primitive Kīlauea basalt to examine the formation of P and Al zoning as a function of undercooling − ΔT (− ΔT = Tliquidus − Tcrystallization) during olivine growth. After 24 h spent at Tinitial = 1290 °C (10 °C above olivine stability), charges are rapidly cooled to final temperatures Tfinal = 1220–1270 °C, corresponding to undercoolings − ΔT = 10–60 °C (with Tliquidus = 1280 °C). Compositional X-ray maps of experimental olivine reveal that only a small undercooling (≤ 25 °C) is required to produce the fine-scale enrichments in P and Al associated with skeletal growth. Concentration profiles indicate that despite qualitatively similar enrichment patterns in olivine, P and Al have contrasting apparent crystal/melt mass distribution coefficients of KPol/melt = 0.01‒1 and KPol/melt = 0.002‒0.006. Phosphorus can be enriched by a factor > 40-fold in the same crystal, whereas Al enrichment never exceed factors of 2. Glass in the vicinity of synthetic and natural olivine is usually enriched in Al, but, within analytical uncertainty, not in P. Thus, we find no direct evidence for a compositional boundary layer enriched in P that would suffice to produce P enrichments in natural and synthetic olivine. Numerical models combining growth and diffusion resolve the conditions at which Al-rich boundary layers produce the observed enrichment patterns in olivine. In contrast, the same models fail to reproduce the observed P enrichments, consistent with our observation that P-rich boundary layers are insignificant. If instead, P olivine/melt partitioning is made to depend on growth rate, models adequately reproduce our observations of 40-fold enrichment without boundary layer formation. We surmise that near-partitionless behavior (KPol/melt close to 1) of P is related to the olivine lattice being perhaps less stiff in accommodating P during rapid crystallization, and/or to enhanced formation of vacancy defects during fast growth. Our results confirm that P is a robust marker of initial rapid growth, but reveal that the undercooling necessary to induce these enrichments is not particularly large. The near-ubiquitous process of magma mixing under volcanoes, for instance, is likely sufficient to induce low-to-moderate degrees of undercooling required for skeletal growth.National Research Foundation (NRF)Accepted versionThis work was funded by National Science Foun- dation Grant EAR-17225321 to TS and by a National Research Foundation Investigatorship Award (Grant number NRF-NRFI2017-06) to FC. The authors acknowledge Benoît Welsch, Francois Faure, Caroline Bouvet-de-Maisonneuve, and Mike Garcia, for the conversations that stimulated some of the ideas presented in this work. Reviews by Bruce Watson and Youxue Zhang helped improve the clarity of the manuscript. We also thank the editor Gordon Moore for his timely handling of the manuscript. This is SOEST contribution 10796

Trace elements in olivine fingerprint the source of 2018 magmas and shed light on explosive-effusive eruption cycles at Kīlauea Volcano

International audienceUnderstanding magma genesis and the evolution of intensive parameters (temperature, pressure, composition, degree of melting) in the mantle source of highly active volcanic systems is crucial for interpreting magma supply changes over time and recognizing cyclic behavior to anticipate future volcanic behavior. Major and trace elements in olivine are commonly used to study variations in mantle lithologies and melting conditions (e.g., temperature, pressure, oxygen fugacity) affecting the mantle over time. Here, we track the temporal evolution of primary melts through the most recent cycle of explosive and effusive eruptions at Kīlauea (Hawai'i), which spans the last ∼500 years. We report major and trace elements in olivine from the last explosive period (∼1500 - early 1820's Keanakāko'i Tephra) and the most recent decade of the current effusive period (2018 LERZ, 2015-2018 Pu'u'ō'ō, 2008-2018 lava lake and 2020 eruption in Halema'uma'u). Scandium concentrations in olivine allow characterizing changes in mantle source between 1500 and 2018, and suggest that the recent (2015-2018) magma feeding the Pu'u'ō'ō cone did not significantly interact with the magma that erupted in the LERZ in 2018. The evolution of olivine and melt compositions over the past 500 years is not easily reconcilable with variations in mantle potential temperature, pressure of mantle melt pooling and storage, or oxygen fugacity. Instead, Sc, Mn, and Co concentrations and Ni/Mg ratio in high forsterite (Fo >87) olivine advocate for an increase in the proportion of clinopyroxene in the mantle source associated with a slightly higher degree of partial melting from 1500 to 2018. Changes in primitive melt compositions and degrees of mantle melting may well modulate magma supply to the crust and formation-replenishment of steady or ephemeral summit reservoirs, and thereby control transitions between explosive and effusive periods at Kīlauea. Analyzing trace elements in olivine at Kīlauea and elsewhere could therefore provide important clues on subtle changes occurring at the mantle level that might herald changes in volcanic behavior