Bubble-enhanced basanite–tephrite mixing in the early stages of the Cumbre Vieja 2021 eruption, La Palma, Canary Islands

Syneruptive magma mixing is widespread in volcanic eruptions, affecting explosivity and composition of products, but its evidence in basaltic systems is usually cryptic. Here we report direct evidence of mixing between basanitic and tephritic magmas in the first days of the 2021 Tajogaite eruption of Cumbre Vieja, La Palma. Groundmass glass in tephritic tephra from the fifth day of the eruption is locally inhomogeneous, showing micron-scale filamentary structures of Si-poor and Fe-, Mg-rich melt, forming complex filaments attached to bubbles. Their compositional distribution attests the presence of primitive basanitic magma, with compositions similar to late-erupted melts, interacting with an evolved tephritic melt during the first week of the event. From filament morphology, we suggest their generation by dragging and folding of basanitic melt during bubble migration through melt interfaces. Semi-quantitative diffusion modelling indicates that the filamentary structures are short-lived, dissipating in timescales of tens of seconds. In combination with thermobarometric constraints, we suggest a mixing onset by sub-Moho remobilization of a tephritic reservoir by basanite input, followed by turbulent ascent of a mingled magma. In the shallow conduit or lava fountain, bubble nucleation and migration triggered further mingling of the distinct melt-phases. This phenomenon might have enhanced the explosive behaviour of the eruption in such period, where violent strombolian explosions were common

238U-230Th-226Ra Disequilibria Constraints on the Magmatic Evolution of the Cumbre Vieja Volcanics on La Palma, Canary Islands

A suite of 48 samples, including both historical and prehistoric lavas and some plutonic rocks, have been analysed from the Cumbre Vieja rift, La Palma, Canary Islands. Additionally, mineral–melt partition coefficients have been measured for clinopyroxene, plagioclase, amphibole, titanite and apatite in selected rocks. The lavas range from basanite to phonolite (SiO2 = 41·2–57·5 wt % and MgO = 10–0·8 wt %) in composition and form coherent, curvilinear major and trace element arrays in variation diagrams, irrespective of eruption age. The mafic lavas have typical ocean island incompatible trace element patterns and Sr, Nd and Pb isotope compositions show little variation but have a HIMU-type character. Generation of the parental magmas is inferred to have involved ∼4% dynamic melting of a garnet lherzolite source that may have previously been metasomatized by melts derived from a recycled mafic component containing residual phlogopite. The major process of differentiation to phonotephrite involved fractional crystallization of basanitic magmas that evolved along the same liquid line of descent under similar pressure–temperature conditions. Numerical simulations using the MELTS algorithm suggest that this occurred across a temperature interval from c. 1320 to 950°C at 400 MPa and an oxygen fugacity equivalent to quartz–fayalite–magnetite (QFM), with an initial H2O content of 0·3 wt %. The later stages of differentiation (<5 wt % MgO) were dominated by mixing with partial melts of young syenites formed from earlier magma batches. All of the lavas are characterized by 230Th and 226Ra excesses and (230Th/238U) decreases with decreasing Nb/U and increasing SiO2, with no accompanying change in (226Ra/230Th). To explain the observations, we propose a model in which there was a significant role for amphibole, and more importantly accessory titanite, in decre'asing Nb/U, Ce/Pb and Th/U ratios and increasing or buffering (226Ra/230Th) ratios during the later stages of differentiation and magma mixing. These processes all occurred over a few millennia in small magma batches that were repeatedly emplaced within the mid-crust of the Cumbre Vieja rift system prior to rapid transport to the surface

Volcanic and geochemical evolution of the Teno massif, Tenerife, Canary Islands: some repercussions of giant landslides on ocean island magmatism

Large-scale, catastrophic mass wasting is a major process contributing to the dismantling of oceanic intraplate volcanoes. Recent studies, however, have highlighted a possible feedback relationship between flank collapse, or incipient instability, and subsequent episodes of structural rearrangement and/or renewed volcano growth. The Teno massif, located in northwestern Tenerife (Canary Islands), is a deeply eroded Miocene shield volcano that was built in four major eruptive phases punctuated by two lateral collapses, each removing >20–25 km3 of the volcano's north flank. In this paper, we use detailed field observations and petrological and geochemical data to evaluate possible links between large-scale landslides and subsequent volcanism/magmatism during Teno's evolution. Inspection of key stratigraphic sequences reveals that steep angular unconformities, relics of paleolandslide scars, are marked by polymict breccias. Near their base, these deposits typically include abundant juvenile pyroclastic material, otherwise scarce in the region. While some of Teno's most evolved, low-density magmas were produced just before flank collapses, early postlandslide lava sequences are characterized by anomalously high proportions of dense ankaramite flows, extremely rich in clinopyroxene and olivine crystals. A detailed sampling profile shows transitions from low-Mg # lavas relatively rich in SiO2 to lavas with low silica content and comparatively high Mg # after both landslides. Long-term variations in Zr/Nb, normative nepheline, and La/Lu are coupled but do not show a systematic correlation with stratigraphic boundaries. We propose that whereas loading of the growing precollapse volcano promoted magma stagnation and differentiation, the successive giant landslides modified the shallow volcano-tectonic stress field at Teno, resulting in widespread pyroclastic eruptions and shallow magma reservoir drainage. This rapid unloading of several tens of km3 of near-surface rocks appears to have upset magma differentiation processes, while facilitating the remobilization and tapping of denser ankaramite magmas that were stored in the uppermost mantle. Degrees of mantle melting coincidently reached a maximum in the short time interval between the two landslides and declined shortly after, probably reflecting intrinsic plume processes rather than a collapse-induced influence on mantle melting. Our study of Teno volcano bears implications for other oceanic volcanoes where short-term compositional variations may also directly relate to major flank collapse events

Impact of sample pretreatment on the measured element concentrations in the bivalve Arctica islandica

Correlating metal to calcium (Me/Ca) ratios of marine biogenic carbonates, such as bivalve shells, to environmental parameters has led to contradictory results. Biogenic carbonates represent complex composites of organic and inorganic phases. Some elements are incorporated preferentially into organic phases, and others are incorporated into inorganic phases. Chemical sample pretreatment to remove the organic matrix prior to trace element analysis may increase the applicability of the investigated proxy relationship, though its efficiency and side effects remain questionable. We treated inorganic calcium carbonate and bivalve shell powder (Arctica islandica) with eight different chemical treatments including H2O2, NaOH, NaOCl, and acetone and analyzed the effects on (1) Me/Ca ratios (Sr/Ca, Mg/Ca, Ba/Ca, and Mn/Ca), (2) organic matter (≈N) content, and (3) mineralogical composition of the calcium carbonate. The different treatments (1) cause element and treatment specific changes of Me/Ca ratios, (2) vary in their efficiency to remove organic matter, and (3) can even alter the phase composition of the calcium carbonate (e.g., formation of Ca(OH)2 during NaOH treatment). Among all examined treatments there were none without any side effects. In addition, certain Me/Ca changes we observed upon chemical treatment contradict our expectations that lattice-bound elements (Sr and Ba) should not be affected, whereas non-lattice-bound elements (Mg and Mn) should decrease upon removal of the organic matrix. For instance, we observe that NaOCl treatment did not alter Sr/Ca ratios but caused unexpected changes of the Mg/Ca ratios. The latter demonstrates that the buildup of complex biogenic composites like the shell of Arctica islandica are still poorly understood

Total vapour and liquid water content from a water vapour radiometer at the Geodetic Observatory Wettzell during the CONT-17 VLBI campaign

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