Brown, B. S., Ramos, F. C., Wolff, J. A., Dorado, O., & Martí, J. (2022). Unleashing alkali feldspar: Ra/Th ages and chemical and isotopic constraints on Holocene phonolite magmatism, Canary Islands. Geology. https://doi.org/10.1130/G50112.1
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 framework. 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.