Evidence of hydration of the peridotite mantle wedge recorded in low-CaO olivines from Los Tuxtlas Volcanic Field, Veracruz, Mexico

Abstract

Los Tuxtlas Volcanic Field (LTVF) is an isolated volcanic center located between the Trans-Mexican Volcanic Belt and the Central America Volcanic Arc. Its volcanic products include alkaline and sub-alkaline rocks with variable subduction signatures, some of them considered as high-MgO rocks and some also include ultramafic xenoliths. Most suites show MgO contents ranging from 11 to 16 wt%, reflecting peridotite-derived melts with a primitive character (Mg# ≥ 66) that suffered crystal settling in magma batches during its ascent from the mantle to surface. Parental magma compositions were reconstructed using the significant forsterite compositions (Fo88 to Fo83) to estimate magmatic temperatures that range from 1095 to 1372 °C. Reconstructed parental magmas and low-CaO magmatic olivines allowed us to determine two main groups with water content between 4 and 6% and 8–9%. These magmatic parameters are consistent with those inferred by thermal models, supporting that magmatic products are derived from the melting of a mantle-wedge peridotite at relatively high temperature and wet conditions. Isotopic compositions point to the existence of a high-μ signature (HIMU; μ = 238U/204Pb) in the mantle source similar to that identified at a regional scale from northern Mexico to South America. Low-CaO olivines and their use as a proxy for magmatic H2O content provide evidence of hydration of the mantle wedge by fluids derived from the slab. Hydrous slab-derived contribution induces a higher extent of melting and formation of wet magmas with high LILE/HFSE ratios. Relatively high-NiO olivine compositions record slight metasomatism of the mantle wedge by a small amount of silicic-rich slab components and the contribution of larger proportions of peridotite mantle to the melt over the slightly metasomatized mantle domains. In general, most of the magmatic products record stages of melting and metasomatism triggered and induced by fluids and melts of the slab into an enriched mantle peridotite wedge.