Geochemistry of noble gases and CO2 in mantle xenoliths and arc lavas from central Mexico

The Ventura Espiritu Santo Volcanic Field (VESVF) and the Sierra Chichinautzin (SCN) are two monogenetic volcanic fields originated in different tectonic environments in the central portion of Mexico (continental rift and subduction). The VESVF is located 35 km NE of the city of San Luis Potosí in the south of the Basin and Range extensional province. This volcanic field was formed by the eruption of alkaline magmas of mafic composition transporting mantle xenoliths described as spinel lherzolites and pyroxenites (Luhr et al., 1989; Aranda -Gómez and Luhr, 1996). The SCN is a Quaternary volcanic field located in the Trans-Mexican Volcanic Belt (TMVB) between two Quaternary arc-volcanoes (Popocatepetl and Nevado de Toluca[AR1] ). Some authors believe that its origin has been related to the subduction of the Cocos plate beneath the North American plate (Marquez et al., 1999; Meriggi et al., 2008); however, the basalts present in the SCN are geochemically similar to OIBs.

New isotopic data of noble gases and CO₂ in fluid inclusions from the VESVF and SCN are presented in this work, since these two areas offer a great opportunity to study the local lithospheric mantle features and related processes (e.g., metasomatism, partial melting) occurring beneath Mexico. Twelve fresh xenoliths from the VESVF and two aliquots of olivine phenocrysts of andesites from SCN were selected. Based on the petrographic analysis, it was determined that the set of xenoliths exhibit same paragenesis: Ol> Opx>> Cpx> Spinel; all samples are plagioclase-free and are classified as spinel-lherzolites and harzburgites. Both the boundaries and the fractures of the crystals develop veins composed of yellowish glass and tiny crystals of carbonates. Lavas from SCVF were previously described as olivine andesites mainly aphanitic and porphyritic with few (<10%) phenocrysts of olivine and orthopyroxene (Marquez et al., 1999; Straub et al., 2011).

The mantle xenoliths and the olivine phenocrysts have comparable Rc/Ra values (where Rc/Ra is the ³He/⁴He corrected for air contamination and normalized to air He). We find Rc/Ra compositions of 6.9-7.7 and 7.2-7.3, respectively, which are within the MORB-like upper-mantle range (Graham, 2002). The highest CO₂ concentrations are observed in olivine phenocrysts from SCN (9.2·10^-7 mol/g and 1.3·10^-6 mol/g), while the xenoliths cover a wide range of concentrations with values as high as 3.9·10^-7 mol/g in Cpx. The isotopic composition of CO₂ (d¹³C vs PDB) in the olivine phenocrysts is around -6.2‰ with CO₂/³He ratios of 3.3·10⁹, which are comparable to MORB-like range (-8‰<d¹³C<-4‰); the mantle xenoliths in contrast, although displaying similar CO₂/³He ratios (2.8·10⁹), exhibit more positive d¹³C signature between -1.0 and -2.7%. We propose that these differences testify for isotopic heterogeneity in the mantle beneath the two areas, with and reflect mantle metasomatism underneath VESVF driven by interaction with carbonate rich-melts (likely consequence of carbonate recycling during the subduction process), as also evidenced by the petrographic analysis.