Unleashing alkali feldspar: Ra/Th ages and chemical and isotopic constraints on Holocene phonolite magmatism, Canary Islands

Abstract

Abstract Accurately dating phenocrysts in Holocene volcanic rocks poses many challenges but is critical to placing magmatic processes that occur prior to eruption into a temporal frame-work. We dated alkali feldspar (i.e., orthoclase Or10 to Or46) crystals in four young phonolites from the Teide–Pico Viejo volcanic complex, Tenerife (Spain), using (226Ra)/(230Th) isotopes. Partition coefficients of Ra (DRa) and DRa/DBa of feldspars were predicted using an approach based on the lattice strain model, which yielded crystallization ages that overlap or predate known eruption ages for the Lavas Negras (ca. 1 ka), Montaña Blanca (ca. 2 ka), Arenas Blancas (ca. 2–4 ka), and Teide H (ca. 6 ka) phonolites. Crystallization of feldspar may occur up to the time of eruption, with >8 ka crystals also present, possibly suggesting extended magma differentiation times. However, feldspars yielding finite (226Ra)/(230Th) ages are mostly in equilibrium with the groundmass, unlike >8 ka crystals, which were therefore identified as antecrysts/xenocrysts. The 87Sr/86Sr ratios of feldspars indicate that crystallization predated late-stage assimilation, affecting 87Sr/86Sr ratios of some melts. The (226Ra)/(230Th) ages also constrain the tempo of phonolite magma evolution on Tenerife. Integration of (226Ra)/(230Th) ages with feldspar major elements, trace elements, and isotopes provides a powerful means for investigating crystallization histories using a dominant mineral that controls the overall magmatic evolution of phonolites on thousand-year time scales.