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Field-based measurement of xenolith size, shape, and number density in volcanic deposits: Example from Lanzarote (Canary Islands) and implications for magma ascent

Marc-Antoine Longpré1,2, Benjamin Black3, Franco Cortese1,2, Holly Valgardson3, and Claire Forrest1

  • Affiliations: 1School of Earth and Environmental Sciences, Queens College, City University of New York, New York, NY, USA; 2Earth and Environmental Sciences, The Graduate Center, City University of New York, New York, NY, USA; 3Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ, USA

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 72

  • Programme No: 1.2.17

  • Theme 1 > Session 2

Abstract

Mantle xenoliths constrain the ascent rates of their carrier magmas, as magmas must ascend faster than xenoliths sink. While Stokes Law dictates that xenolith size controls settling velocity, quantitative data on xenolith sizes in volcanic deposits are scarce. Here, we present field-based measurements of xenolith size, shape, and number density from the five-phase 1730--1736 CE Timanfaya eruption and the smaller 1824 eruption on Lanzarote to assess magma ascent conditions and eruption styles over time. On outcrops, xenolith sizes (equivalent circle radius up to 7.5 cm) and number densities (up to ~20 xenoliths m-2) were highest during Phase 1 of the eruption, decreased in subsequent phases, and increased in the final episode. The largest xenoliths, with equivalent circle radius of 9.0 cm, are found as bombs at the Phase 1 Pico Partido cinder cone. Xenoliths show mean aspect ratios (long/short axes) of 1.4--1.9. The 1824 Volcán Nuevo del Fuego eruption also produced large xenoliths, but pahoehoe flow cross-sections reveal xenoliths are heterogeneously distributed within specific flow lobes over time. Despite similar maximum xenolith sizes and inferred magma ascent rates (~0.1--0.2 m/s), Pico Partido exhibited explosive activity, while Volcán Nuevo del Fuego was dominantly effusive, highlighting an interesting case of decoupling between ascent rate and eruptive style. Our dataset of >960 xenoliths provides testable constraints for independent estimates of magma ascent rate at Timanfaya and general ascent models for mantle xenolith-bearing magmas.