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

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 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.

Ba, Sr, and Rb feldspar/melt partitioning in recent eruptions from Teide-Pico Viejo volcanic complex, Tenerife: New insights into pre-eruptive processes

“…They have been registered in the online System for Earth Science Registration database (http:// www.geosamples.org), where all the information related to the samples can be accessed (UTM coordinates, geological units following the units described by Ablay and Martí, 2000). Four are phonolites: ODG-4A, 10.58052/IEODG000B (Montaña Rajada, 2-4 ka; Ablay et al, 1995;Brown et al, 2022); ODG-13A, 10.58052/IEODG000K (Teide L or Lavas Negras, 1.0 ka; Brown et al, 2022), ODG-37A, 10.58052/IEODG001B (Arenas Blancas, 4 ka; Brown et al, 2022) and ODG-39A, 10.58052/ IEODG001D (Teide J2, between 2 and 2.6 ka; Carracedo et al, 2003;Carracedo et al, 2007;Martí et al, 2008). Two are phonotephrites:…”

“…Groundmass (87.5%) is glassy with a significant amount of feldspar microlites and vesicles (9.2%). Lavas Negras is the last eruption of Teide volcano at ~1 ka (Carracedo et al, 2007;Brown et al, 2022). The sample was collected from a thick blocky lava flow between Teide and Pico Viejo and is characterised by a porphyritic texture in a dark-brownish glassy groundmass.…”

“…For the melt composition, we calculate different D values using both the compositions of the whole rock and glasses. In addition, we also calculate the D for the feldspar/groundmass composition using analyses obtained by Brown et al (2022) and Ramos et al (in prep). For samples in which groundmass analyses were not available, estimated compositions have been obtained by mass balance using the compositions of the whole rock, the compositions of the main mineral phases, and their modal percentages.…”

“…To assess the calculated coefficients, the distribution coefficients of Ca, Ba, and Sr were used to construct "Onuma" curves of D vs ionic radius (Onuma et al, 1968) and the corresponding value of the divalent cation site elastic modulus, E 2+ , was calculated (Blundy and Wood, 1994; Figure 11). Acceptable values for this parameter for divalent cations in alkali feldspar, derived from feldspar/melt partitioning experiments (Long, 1978;Icenhower and London, 1996;Fabbrizio et al, 2009), lie in the range 120 ± 40 GPa (Brown et al, 2022). In Figure 12, calculated D values for Sr, Ba, and Rb are compared with phenocryst/melt results for all pairs (Figures 12A, C, E), and for those that pass the E 2+ = 120 ± 40 GPa filter (Figures 12B, D, F).…”

“…Detailed relationships between crystals and their enclosing melt phase in volcanic rocks provide one avenue to understanding Earth evolution and the dynamics of volcanic feeding systems (Blundy and Wood, 2003;Ramos and Tepley, 2008). Moreover, they are necessary for accurate calculation of magmatic timescales using diffusion chronometry (e.g., Costa et al, 2003), and for dating of single crystals (Ramos et al, 2019;Brown et al, 2022).…”