Mantle-derived xenoliths from the Cumbre Vieja 2021 lava flows: insights on the composition of the lithosphere beneath La Palma (Canary Islands)

Small, cm-sized ultramafic xenoliths have been reported from the opening phase of the 2021 eruption at Cumbre Vieja, where clinopyroxene aggregates, sometimes amphibole, olivine and/or magnetite-bearing (Pankhurst et al., 2021), likely represent early fractionation products and/or relics of mush-like systems located beneath the volcanic edifice.

Detailed sampling of the lavas produced during the intermediate-late eruptive phase (November 9^th) revealed the existence, in the massive portion of the flows, of a 1 cm sized dunitic xenolith with protogranular to partly recrystallized texture. The internal portion of the xenolith is composed of Fo_88-89 olivine (0.33-0.34 wt% NiO), Ti-Al-poor clinopyroxene (Mg# = 87-92; Al₂O₃ <1.7 wt%; TiO₂ <0.5 wt%), Cr-rich spinel and rare Mg-rich orthopyroxene (Mg# = 88-91; Al₂O₃ from 0.4-0.5 to 1.7-1.9 wt%). Textural and chemical data (Fe-Mg distribution) indicate that olivine, orthopyroxene and clinopyroxene are not far from equilibrium. Preliminary calculations show that the equilibrium T recorded by the xenolith ranges from 950 to 1070°C, with good consistency between results obtained from olivine-spinel and orthopyroxene-clinopyroxene pairs. Silica oversaturated interstitial glasses (SiO₂= 67 wt%) were found in the partly recrystallized part of the xenolith.

Part of the coarse-grained xenolith forms a corona of fine-grained and worm-like association of, olivine, orthopyroxene, clinopyroxene and spinel. Both parts are surrounded by a continuous narrow external zone consisting of Ti-magnetite and sub-euhedral greenish Ti-augite, which is in contact with the host basalt. The composition of olivine and orthopyroxene in the corona keeps getting more Fe-rich towards the external zone whereas the clinopyroxene changes gradually from Ti-free to Ti-bearing diopside.

The small xenolith recovered from the November 9^th lava flow is apparently a mantle-derived xenolith similar to those from the Duraznero 1949 eruption described by Klügel (1998), and those from the San Antonio 1677 eruption described by Neumann & Wulff-Pedersen, 1997.  On the way to surface, the mantle xenolith likely reacted with basaltic melts to form the first corona. These processes presumably took place in depths between 0.10-0.12 GPa as can be inferred by the presence of silica oversaturated glasses (Neumann & Wulff-Pedersen, 1997). The external zone probably formed as the result of a late-stage stagnation of the host magma at sub-crustal depths, as suggested by the compositional similarity between the clinopyroxene-spinel assemblage and the phenocrysts in the matrix.

               

Klüger, A. (1998). Reactions between mantle xenoliths and host magma beneath La Palma (Canary Islands: constraints on magma ascent rates and crustal reservoirs. Contrib Mineral Petrol 131:238-257

Neumann, E.-R. and E. Wulff-Pedersen (1997). The Origin of Highly Silicic Glass in Mantle Xenoliths from the Canary Island. Journal of Petrology 32: 1515-1539

Pankhurst, M. J., et al., (2021). Petrology of the opening eruptive phase of the 2021 Cumbre Vieja eruption, La Palma, Canary Islands. Volcanica: 5(1), 1–10