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 first 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 first 2021 La Palma volcanic eruption effusive and explosive products (see supplementary materials).
Understanding the lithologically varied Canarian archipelago origins and evolution (Fig. 1a), has long attracted international research efforts1,2. La Palma has experienced several magmatic episodes: Basal Complex (~4.0-3.0 Ma); Garafia, Taburiente, Cumbre Nueva and Bejenado volcanoes (~1.7–490 ka); and Cumbre Vieja (~125 ka-present), and is the most active system in historical times: ~1480; 1585; 1646, 1677, 1712, 1949, 1971 and 20213,4,5 indicating a ~230-20 year return period.
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 fire fountains from a fissure. Samples presented here were collected during the first week of activity from initial lava flow (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, eruption6,7. Current initial eruption products contain coarse minerals identifiable 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 confirmed major mineral phases are clinopyroxene, plagioclase, and amphibole. Feldspathoids are notably absent.
Thin sections show the lava is hypocrystaline and porphyritic with ~15% vesicularity (Fig. 1g). Clinopyroxene is the most common coarse mineral (~15-20% vol.) and is present as euhedral-subhedral solitary crystals (1-3 mm diameter) or in mono- and polymineralic (Cpx, Ox ± Ol ± Amp) clusters (up to 7 mm), including possible xenoliths with 120º grain boundaries (Fig. 1gi). Cpx commonly displays concentric and sector zoning, with some embayments and abundant Fe-Ti oxides and apatite inclusions; sieve-textures are rare. Amphibole crystals (~4% vol.) are anhedral-subhedral, 0.5-2.5 mm, and have distinct reaction rims (Fig. 1d). Olivine (~1% vol.) usually forms euhedral-anhedral isolated crystals (0.5-1.5 mm). Fe-Ti oxides (~1% vol.) are subhedral-anhedral and 0.5-1 mm. Groundmass minerals include abundant plagioclase, oxides, clinopyroxene, and olivine. Tephra is mineralogically comparable to the lava but with a fragmented, hypocrystalline texture (Fig. 1g).
QEMSCAN® analysis of 27 mm2 of CAN_LLP_0001 representing ~8 M points measured in <17 hr with a 5 µm pixel size highlights three dominant groups accounting for 90.07% modal mineralogy (Fig. 2a): Ca-Fe-Al silicates (49.54%) and Ca-Mg-Fe silicates (20.94%), interpreted to be groundmass+amphibole and clinopyroxene, respectively, and plagioclase (19.59%). The next most abundant minerals are olivine (2.70%), Ti-Magnetite (2.12%), ilmenite (1.95%) and biotite (1.65%). Plagioclase is present as groundmass crystal laths, whereas olivine, Ti-magnetite and ilmenite are present as both equant and microlitic crystals. Groundmass with textures finer than excitation volume contributes to total Ca-Mg-Fe silicates, hence the elevated value with respect to petrographic analysis.
Whole-rock XRF and ICP-MS analyses show products have restricted, primitive, metaluminous, alkaline whole-rock compositions (Fig. 2b-e); lava (SiO2 44.27-44.59 wt%) is slightly more primitive than tephra (44.82-45.61 wt%). Normalised to NMORB, all rocks show positive anomalies, relative to adjacent elements, in Ba, Th, U, La, Ce and Eu (Fig. 2c). Relative to chondrite, LREE are enriched relative to HREE (LaN/YbN 23.9-26.5), Eu anomalies are absent. All rocks have ~15.5-16% normative nepheline. In both rock types Al2O3, Na2O and K2O, Zr plus large ion lithophile elements correlate positively with differentiation index SiO2, whereas FeOT, MgO, CaO and TiO2 plus Sc and V correlate negatively (Fig. 2d-e).
EPMA confirms clinopyroxene as titanaugite,and demonstrates no clear major element difference exists between cores, corroded crystals and monomineralic clusters. Rims, however, are generally richer in SiO2 and MgO, and poorer in TiO2, Al2O3, and Na2O. Large olivine grains have uniform compositions, Fo78−80, Cr-spinel is present as inclusions. Ti-magnetite (3-5 wt% TiO2) is present both as discrete grains and in the groundmass with slightly higher Ti concentration. Amphibole is kaersutite; no core-rim zoning was detected. Groundmass plagioclase is An58−67, two grains have higher Na, An32−43.
Geochemically, eruptive products plot as basanite-tephrites (Fig. 2b), yet mineralogical observations lead to their classification as alkali basalts8, implying comparatively higher degree mantle melting9. 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.
Major and trace element trends together with petrographic observations indicate limited cpx (~85 %) and titanomagnetite (~15 %) fractionation, interpreted as winnowing1,10, 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 flow 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 flow13 gas charging, and crystallisation. Having now warmed the country rock, parental magma can ascend more efficiently. These models will be addressed by continued petrological eruption tracking.