Crystal Archives of Magmatic Processes

Phenocrysts in volcanic rocks are recorders of magmatic processes that have occurred at depth before and during a volcanic eruption. Our petrological investigations of stratigraphically controlled tephrite and phonotephrite samples from the latest eruption of Fogo (Cape Verde Islands) aimed to reconstructing magma storage and transport. The dates of sample emplacement have been determined using satellite instrument – derived high resolution thermal infrared maps. All samples are strongly phyric and commonly contain complexly zoned clinopyroxene crystals and cumulate fragments. Clinopyroxenes from all samples exhibit 10–50 μm wide rim zones, inferred to have grown in a few days to weeks during the ongoing eruption as a consequence of H₂O loss from the melt. Clinopyroxene-melt thermobarometry using tephrite groundmass compositions suggests that the rims formed at upper mantle pressures of around 600 MPa (21 km depth). This level is interpreted to reflect temporary reduction in magma ascent velocity by near-isobaric movement through a complex storage system. Previously, the tephrite magma had accumulated at a deeper level, possibly between 700 and 900 MPa as indicated by clinopyroxene cores (Mata et al., 2017). The cause for H₂O loss initiating rim growth could be degassing after rise of the magma from the deeper level, or CO₂ flushing by a carbonic fluid phase released at depth. Corresponding data from phonotephrites indicate last equilibration at around 440 MPa (16 km); the phonotephrite magma is inferred to be a residuum from an earlier magmatic event that was entrained by advancing tephrite. Microthermometry of CO₂-dominated fluid inclusions in tephrite clinopyroxenes results in pressures of around 330 MPa (12 km), indicating another short pause in magma ascent in the lowermost crust. Rim zonations of olivine phenocrysts indicate that after leaving this final stalling zone, the magma ascended to the surface in less than half a day. In strong contrast to these petrological equilibration depths, seismic events precursory to the eruption were located at < 5 km below sea level, with only two exceptions at 17 and 21 km depth consistent with our barometry. Our results enhance the understanding of this potentially dangerous volcano, which helps to interpret future pre-eruptive unrest.

Our current knowledge of the genesis, ascent, storage, and eruption of mafic magmas is intimately linked with olivine, its primary crystal cargo. Recent claims that phenocryst-size crystals can grow rapidly and non-concentrically are challenging our perception of what olivine zoning represents (e.g., controlled by growth and/or diffusion), and whether accurate magma crystallization and diffusion timescales can be retrieved. A series of cooling experiments using a dry basalt was carried out in order to quantify the kinetics of olivine growth as a function of the degree of undercooling, and to characterize morphological changes occurring with time. After a 24 h equilibration step at 1290°C (10°C above the olivine liquidus), experiments were rapidly cooled, experiments were rapidly cooled to final temperatures T_f = 1270, 1255, 1240, or 1220°C (imposing undercoolings of −ΔT = 10, 25, 40, and 60°C, respectively). Growth rates estimated via 3D microtomography renderings of experimental crystals attain 10^–7 m/s, and are found to be almost an order of magnitude higher than those calculated using 2D sections of the same experiments. We show that mm-sized crystals similar to those found in natural Kīlauea samples can be produced after a few hours under moderate undercooling conditions (25–60°C). Growth rates decrease faintly with time, accompanying transitions between skeletal/hopper and more polyhedral morphologies. Growth rates generally increase until −ΔT = 40°C, and decrease slightly at −ΔT = 60°C as rates of nucleation likely increase. The −ΔT = 40°C vicinity may therefore represent a thermal “sweet spot” for the formation of phenocrysts in dry basalt. Olivine overgrowths on crystals that survived initial dissolution grow slower than homogeneously nucleated crystals, illustrating how new and old crystals in natural magmas likely respond differently to a thermal perturbation. We suggest that the main growth direction of natural olivine (a- or c-axis) may be a sensitive function of undercooling and the presence of a pre-existing growth substrate. Olivine grows faster along the a-axis under moderate to high undercooling conditions, while preferred development along the c-axis likely occurs under lower undercooling conditions and/or as rims grow around existing crystals. The early history of skeletal olivine crystals is controlled by diffusion in the melt (diffusion-controlled growth regime), while their long-term compositional zoning history is mainly controlled by diffusive re-equilibration.