Petrology of the opening eruptive phase of the 2021 Cumbre Vieja eruption, La Palma, Canary Islands


 The first products of the current Cumbre Vieja eruption comprise simultaneous tephra fall from near-continuous, gas-rich eruption plumes and lava flows. From combined field, petrographic and geochemical analyses we identify: low percentage mantle melts with a variably-equilibrated multimineralic crystal-cargo and compositional fractionation by eruptive processes. Hence petrology can untangle complex magmatic and volcanic processes for this eruption, which through further study can assist in active decision making.

magmatic and volcanic processes for this eruption, which through further study can assist in active decision making.

Main
On 19th September 2021, Cumbre Vieja volcano (La Palma, Canary Islands, Fig. 1a) erupted after 50 years of quiescence (day 0). How and why magmatic systems reactivate is still a critical question for monitoring and hazard mitigation efforts during rst response and ongoing volcanic crisis management. To consider this, here we present petrographic, XRD, QEMSCAN®, EPMA, and whole-rock major and trace element geochemistry results for the rst 2021 La Palma volcanic eruption effusive and explosive products (see supplementary materials).
The current eruption began with explosive activity at a new vent that produced lava and near-continuous ash plumes driven by vigorous gas jets, and re fountains from a ssure. Samples presented here were collected during the rst week of activity from initial lava ow (CAN_LLP_0001,2,3,4) and tephra fall (chronologically, increasing in distance from the vent: CAN_TLP_0008, 9, 11) ( Fig. 1b-f).
Petrology provides insight into volcanic plumbing systems as they assemble before, and evolve throughout, eruption 6,7 . Current initial eruption products contain coarse minerals identi able by hand-lens and provide a rapid real-time guide to system evolution. A multimineralic cargo is observed (Fig. 1g), which raises the potential to extract detailed system information and serves as a baseline to track possible trends that may be used to help forecast eruptive behaviour and evolving hazards.
X-ray diffraction analysis con rmed major mineral phases are clinopyroxene, plagioclase, and amphibole. Feldspathoids are notably absent.
Geochemically, eruptive products plot as basanite-tephrites (Fig. 2b), yet mineralogical observations lead to their classi cation as alkali basalts 8 , implying comparatively higher degree mantle melting 9 . Petrography and mineral chemistry illustrate a complex crystal cargo. In addition to euhedral clinopyroxene phenocrysts, rare variably resorbed clinopyroxene is observed. Anhedral olivine is recognised both as ripened-skeletal and also rounded-embayed forms. Amphibole has marked reaction rims and a variably oxidised appearance (Fig. 1gii). We suggest the current eruption is tapping melt-mush magma mingling zones. titanomagnetite (~15 %) fractionation, interpreted as winnowing 1,10 , between lava and tephra with increasing distance from the vent (Fig. 1g and 2b-e).
Olivine abundance is being keenly tracked at the time of writing and appears to be rising, coincident with overall lava production, and ow aspect ratio lowering. End-member interpretations are: earliest eruption products represent older, reactivated magma that is being depleted as newer magma arrives at the vent and now solely drives the eruption. Alternatively, all volcanic products are derived from the same parental magma that traversed colder crust in stages for ~1 week, involving reactive ow 13 gas charging, and crystallisation. Having now warmed the country rock, parental magma can ascend more e ciently. These models will be addressed by continued petrological eruption tracking.

Methods
Lava samples were collected from active ow fronts either warm (CAN_LLP_0001), or for all other samples, hot incandescent, and immediately water quenched; suitable sampling points were identi ed with a thermal camera. Tephra was collected from direct fall deposits. Each sample was viewed using a ZEISS Discovery V20 stereomicroscope for initial assessment.
All samples were powdered in an agate automatic mortar and pestle grinder.
Whole-rock major element determinations were performed by X-ray uorescence, after fusion with lithium tetraborate. Typical precision was better than ±1.5% for an analyte concentration of 10 wt.%. Zirconium was determined by X-ray uorescence on glass beads, with a precision better than ±4% for 100 ppm Zr.
Whole-rock trace element determinations were done by ICP-mass spectrometry (ICP-MS) after HNO3+HF digestion of 0.1000 g of sample powder in a Te on-lined vessel at ~180 °C and 200 psi for 30 min, evaporation to dryness, and subsequent dissolution in 100 ml of 4 vol.% HNO 3 . Instrument measurements were carried out in triplicate with a PE SCIEX ELAN-5000 spectrometer using rhodium as an internal standard. Precision, as determined from standards WSE, BR and AGV run as unknowns, was better than ±2% and ±5% for analyte concentrations of 50 and 5 ppm, respectively.
Whole-rock X-ray diffraction of the agate milled sample produced diffractograms using a PANalytical X'Pert Pro diffractometer (CuKα radiation, 45 kV, 40 mA) equipped with an X'Celerator solid-state linear detector, using a step increment of 0.008∘ 2θ and a total counting time of 10 s per step. Data were processed using HighScore software to identify key mineral peaks, d spacing Å: clinopyroxene 2.55-2.99; plagioclase 3.19-3.21; amphibole 8.35-8.44.
Major element analyses of minerals were obtained by wavelength dispersive analyses with a Cameca SX-100 electron microprobe, using mainly synthetic standards. Accelerating voltage was 20 kV, and beam current was 20 nA.Spot size was 5 microns.
For automated mineralogy, sample CAN_LLP_0001 was mounted in a 30 mm diameter epoxy resin block, polished to a 1 micron nish, carbon-coated to 25 nm, then analysed by a QEMSCAN® 4300 at the University of Exeter (Gottlieb et al., 2000). Sample measurement and data processing used iMeasure v4.2SR1 and iDiscover 4.2SR1 and 4.3 ). The QEMSCAN® settings used 25kV, 5nA, a 1000 X-ray count rate per pixel, a WD of around 22 mm under high vacuum and beam calibration every analyse the samples at an X-ray resolution/pixel spacing of 5 microns and a 1000 micron 2 eld size (x68 magni cation). Figure 1 (a) location of tephra samples, and historic eruptions3,4,5; (b) selected ow extent snapshots and lava sample locations; CAN_LLP_0001 (sampling: c; hand specimen: d); e) CAN_TLP_0001 coarse particles; f) CAN_TLP_0011 stereomicroscope image including ultra-rare restingolite (pending con rmation). g. Ash, tephra and lava photomicrographs.