Boron isotope variations in a single monogenetic cone: La Poruña (21°53′S, 68°30′W), Central Andes, Chile

La Poruña is a monogenetic volcano located within the Altiplano-Puna Volcanic Complex (21°-24°S) in the Central Andean subduction zone. Since crustal contamination of Andean magmas is ubiquitous, and because extensive geochemical data exist for La Poruña, we employ this volcano as a case study to examine the behavior of boron isotopes during crustal assimilation. We present whole-rock boron concentration and ¹¹B/¹⁰B ratios (as δ¹¹B values) for La Poruña lava samples that were prepared as nano-particulate pressed pellets. La Poruña B contents range from 14 to 20 μg/g and δ¹¹B values range from −1.39 ± 0.54 ‰ (2σ) to +0.94 ± 0.30 ‰ (2σ), which overlap with the range of available whole-rock data for Central Andean lavas. Moreover, La Poruña δ¹¹B values correlate negatively with ⁸⁷Sr/⁸⁶Sr ratios from the same samples. Since ⁸⁷Sr/⁸⁶Sr is a proxy for crustal contamination at La Poruña, the data lead us to suggest that La Poruña magmas assimilated a low-δ¹¹B, high ⁸⁷Sr/⁸⁶Sr component such as Andean continental crust. Mixing models based on B and Sr isotopes support a broadly two-step magma evolution for La Poruña. In step 1, mantle-derived primary melts interacted with boron-rich slab-derived fluids with high δ¹¹B values, which yielded subduction-modified parental magmas with ca. 3 μg/g B and relatively high δ¹¹B values. In step 2, the high δ¹¹B parental magmas ascended through the crust where they assimilated up to 20% crustal material, which further modified their δ¹¹B values and ⁸⁷Sr/⁸⁶Sr ratios. In comparison to available regional values for B and δ¹¹B, it appears that La Poruña and nearby volcanic centers shared a similar source and magmatic history, whereas volcanoes south of 23°S differ. We stress, however, that deconvolving the roles of various subduction and crustal inputs in the Central Andes would require further studies on individual volcanoes along the arc.