Deciphering chemical complexity in monogenetic eruptions: Xitle Volcano, Mexico

Xitle Volcano, an ∼1700-year-old scoria cone located at the southern edge of Mexico City and within the central Trans-Mexican Volcanic Belt (TMVB), provides a unique opportunity to investigate the causes of geochemical diversity in monogenetic eruptions. We present the first integrated major-trace element and Sr–Nd–Pb–Os isotopic dataset which, coupled with new stratigraphic mapping and petrologic observations of Xitle's eruptive products, provides constraints on the roles of mantle heterogeneity, subduction-related metasomatism, and crustal assimilation in Xitle magma genesis. The lavas and tephras range in composition from basalt to basaltic andesite and basaltic andesite/basaltic trachyandesite. All samples have arc-like trace element patterns with enrichments in large ion lithophile elements and depletions in high field strength elements, with early-stage magmas exhibiting stronger arc-like signatures, while later magmas trend toward intraplate affinities. Isotopic compositions span 87 Sr/ 86 Sr = 0.70335–0.70398, 143 Nd/ 144 Nd = 0.512790–0.512953, 206 Pb/ 204 Pb = 18.667–18.770, 207 Pb/ 204 Pb 15.570–15.612, and 208 Pb/ 204 Pb = 38.355–38.554. 187 Os/ 188 Os ratios range from 0.1368 to 0.1968, with Os concentrations between 10.7 and 168.4 pg/g. These 187 Os/ 188 Os values are more radiogenic than the primitive upper mantle (∼0.1296) and fall within the range observed for other arc magmas in the TMVB. The samples collectively define three distinct compositional groups, but the compositional variations do not evolve monotonically with time; rather abrupt compositional and isotopic shifts occur over the course of the eruption. Neither closed system fractional crystallization nor assimilation-fractional crystallization of a single parental magma can reproduce the observed geochemical range. Instead, we propose an open-system scenario involving high-temperature fractional crystallization periodically interrupted by recharge events from compositionally and isotopically distinct magmas, supported by MELTS and Magma Chamber Simulator (MCS) modeling. Isotope systematics argue against significant crustal assimilation and point to a heterogeneous, subduction-metasomatized mantle wedge as the primary source of the magmas from Xitle. The mantle source beneath Xitle was variably fluxed by two distinct slab-derived components, both dominated by terrigenous sediment with variable inputs of altered oceanic crust (AOC)-derived fluid/melts, in relative proportions that evolved over time. Our results demonstrate that Xitle's chemical complexity reflects serial addition of subduction fluids, partial melting of a compositionally evolving mantle source, and repeated magma recharge, emphasizing that even small monogenetic systems can record prolonged and multi-stage magmatic histories of mantle derived melts. • Xitle magmas record open-system magma recharge and mixing. • MELTS and MCS modeling support successive recharge by distinct melts. • Isotopic data rule out significant crustal assimilation in Xitle eruptive products. • Mantle metasomatism involved two distinct slab components with variable inputs. • Arc- to intraplate-type signatures reflect the nature (fluid/melt) of the slab flux.