Sr and O Isotope Systematics in the Central Andes: Resolving Crustal and Mantle Contributions to the Evolution of the Altiplano Puna Magma Body

The combination of Sr and O isotopes allow for the determination of crustal homogenization and hybridization during magma accumulation. We present oxygen isotope values of mineral separates, and Sr isotope ratios of whole rock and in situ plagioclase from <1 Ma andesitic to dacitic composition lava flows from four volcanoes in the Central Andes with geographic relationship to the Altiplano Puna Magma Body (APMB): Uturuncu (1.1 Ma to 250 ka), Ollagüe (1.2 Ma to 130 ka), Aucanquilcha (1.04 Ma to 240 ka) and Lascar (49 ka to 0 ka). Sr isotope ratios were determined in situ using both laser ablation and standard cation exchange methods of plagioclase phenocryst and on whole rock powders. Oxygen isotope analyses were determined by laser fluorination of mineral separates. Mineral separate O isotope data has been re-calculated to account for mineral-melt ¹⁸O/¹⁶O-fractionation at various SiO₂ contents to better constrain magma source variations and magma isotope heterogeneity with time and space. Variation in δ¹⁸O_WR values for each lava suite is relatively limited (Lascar - δ¹⁸O_WR =7.4-9.0‰; Aucanquilcha- δ¹⁸O_WR =8.2-9.2‰; Ollagüe- δ¹⁸O_WR =7.9-9.4‰) with the exception of Uturuncu which displays a large O isotopic value range (δ¹⁸O_WR =8.7-11.4‰). Whole-rock ⁸⁷Sr/⁸⁶Sr ratios show a correlation of the lowest ratios along the arc front and generally increase towards the center of the APMB. Lascar (⁸⁷Sr/⁸⁶Sr= 0.7057-0.7067) and Aucanquilcha (⁸⁷Sr/⁸⁶Sr= 0.7058-0.7068) rocks exhibit the most homogeneous Sr isotope ratios and δ¹⁸O_PL values, while Uturuncu exhibits the largest range of ratios (⁸⁷Sr/⁸⁶Sr= 0.7101-0.7165). Furthermore, in situ ⁸⁷Sr/⁸⁶Sr ratios of Uturuncu plagioclase phenocrysts exhibit more variation within a single crystal (⁸⁷Sr/⁸⁶Sr= 0.7098-0.7165) than the observed at all four volcanoes. Uturuncu magmas contain the highest variability, both in plagioclase mineral separates and whole rock values, and therefore must contain high ¹⁸O crustal material. This hypothesis is further supported by the textures and isotopic variation within plagioclase phenocrysts suggesting repeated crustal contamination followed by mixing occurring in the shallow crustal reservoir. Conversely, the arc front source is more homogeneous and hybridized, and arc-front magmas assimilate similar composition crustal material.