Crustal structure of Lanzarote and magma ascent path for the Timanfaya 1730 to 1736 eruption recorded by mineralogy and fluid inclusions of lower crustal xenoliths

Abstract

The Timanfaya eruption on Lanzarote (1730 to 1736) was among the largest historic eruptions on any ocean island, and understanding their eruption dynamics can help to enhance hazard mitigation measures. Gabbroic xenoliths with MORB-type mineral compositions, interpreted as fragments of the Jurassic ocean crust, were transported to the surface during the eruption. The xenoliths have to variable extents been affected by heating, causing formation of intergranular reaction rims, and also partial melting reflected by melt-filled veins. Two-pyroxene thermobarometry using the abundant orthopyroxene exsolution lamellae in clinopyroxene give average formation temperatures of 865 °C. CO₂-dominated fluid inclusions in plagioclase, olivine and clinopyroxene show two density modes, the main mode at 0.62 ± 0.11 g/cm³ with a skewed distribution towards higher densities up to 0.80 g/cm³, and a less prominent mode at 0.28 ± 0.10 g/cm³, respectively. The primary fluid inclusions are contained in the main mode. Coexistence of secondary CO₂-dominated fluid inclusions with melt inclusions demonstrate late-stage magmatic temperatures in the xenoliths. We suggest that the xenoliths originated as magma chamber wall rocks, and were entrained in the Timanfaya magmas at pressures of about 315 ± 95 MPa. Heating of the xenoliths to magmatic temperatures during limited magma stagnation in the lower crust led to isobaric re-equilibration of the primary fluid inclusions. Some xenoliths record an additional transient stagnation level at 100 ± 45 MPa, which corresponds to the shallowest level of syn-eruption magma stagnation. This ascent history is strikingly similar to the events recorded during the 2021 La Palma eruption, and to those of mafic magmas on ocean islands worldwide, suggesting that such lower crustal magma stagnation beneath ocean islands is the rule rather than the exception.