Noble gas and CO2 isotopic signatures of the lithospheric mantle underneath Mexico and the Canary Islands: clues from mantle xenoliths and arc lavas

Studying the isotopic composition of fluids trapped in mantle xenoliths opens avenues to understand the origin and cycling of volatiles in the Earth’s upper mantle. In this PhD dissertation, new and in most cases the very first data regarding the isotopic (noble gases and CO2) characterization of the lithospheric mantle portions of three different geodynamic environments are presented: (i) Central and NW Mexico, a continental setting dominated by extension; (ii) the Transmexican Volcanic Belt (TMVB) a subduction setting, and (iii) the Canary Islands, particularly El Hierro and Lanzarote, two oceanic islands formed by mantle plume-derived intraplate volcanism. In total 32 peridotites (including spinel lherzolites, spinel harzburgites, 1 pyroxenite and 1 dunite) and four arc lavas (from the TMVB) were investigated. To characterize the isotopic signature of the Mexican lithospheric mantle, the present work was focused on the analysis of fluid inclusions entrapped in mantle xenoliths found in pyroclastic deposits of the Ventura Espiritu Santo Volcanic Field (VESVF), the Durango Volcanic Field (DVF), the San Quintin Volcanic Field (SQVF) (three Quaternary monogenetic volcanic fields formed in the Basin and Range extensional province). Fluid inclusions in olivine phenocrysts found in arc lavas from the Sierra Chichinautzin Volcanic Field (SCN) (a Quaternary monogenetic field located in the Transmexican Volcanic Belt (TMVB)) were also analyzed. According to the petrographic analysis, all xenoliths exhibit similar mineralogy (Ol> Opx> Cpx >> Sp). The VESVF xenoliths, in particular, bring textural evidence of interstitial glass veins bearing dendritic trails of secondary melt and fluid inclusions related to pervasive mantle metasomatism driven by carbonate-rich silicate melts. Inclusions are composed of silicate glass ± CO2 ± Mg-Ca carbonates ± pyrite as indicated by Raman microspectroscopy. Excluding samples possibly affected by secondary processes, the averages Rc/Ra ratios (3He/4He ratios corrected for atmospheric contamination) measured in Mexican localities are within the MORB-like range: VESVF = 7.39 ± 0.14 Ra (1SD, n = 30), DVF= 8.39 ± 0.24 Ra (1SD, n = 10), SQVF = 7.43 ± 0.19 Ra (1SD, n = 1) and SCN lavas = 7.24 ± 0.33 Ra (1SD, n = 4). This noble gas similarity between the VESVF and SCN samples supports the existence of a homogeneous mantle in central Mexico. The 3He/4He signatures observed in xenoliths suggest that (i) either the mantle He budget was scarcely modified by the Farallon plate subduction, and/or (ii) that any (large) crustal contribution was masked by a later metasomatism/refertilization episode, possibly driven by the upwelling mantle from the asthenosphere and the subsequent Basin and Range extension. The association between glass veins and fluid inclusions in VESVF xenoliths revealed that the metasomatism/ refertilization was driven by a silicate-rich melt which is consistent with a calculated helium residence time in the VESVF mantle (20 to 60 Ma) that overlaps the timing of the above geodynamic events. It is proposed that, after the refertilization event (e.g., over the last ~20 Ma), the lithospheric mantle has evolved in a steady-state, becoming slightly more radiogenic. The relative proximity between the DVF and the VESVF suggests a similar process should have happened beneath Durango, and that the difference in 3He/4He ratios with the VESVF is likely to be associated with different ages of mantle refertilization and He residence times (more recent for the DVF mantle; 4 to 10Ma). The Ar and Ne systematics reflect a mixing between MORB-like upper mantle and atmosphere-derived fluids. The mantle beneath the SQVF and the DVF seems to be more impacted by the interaction with atmospheric fluids, as proved by a systematic decrease in 40Ar/36Ar and 4He/20Ne ratios from central (VESVF) to western Mexico (DVF, SQVF) It is proposed that these atmospheric components were likely air-derived fluids recycled by the Farallon plate subduction. 3He fluxes (0.027 - 0.080 mol/g), 4He production rates (340 - 1000 mol/yr), and mantle CO2 fluxes (3.93 x 107 mol/yr to 1.18 x 108 mol/yr) were also estimated using the helium isotopic values measured in VESVF mantle xenoliths. Finally, DVF and VESVF nodules exhibit CO2/3He ratios comparable to those of the upper mantle (from 3.38 x 108 to 3.82 x 109) but more positive δ13C values (between -1.0 and -4.0‰), supporting the involvement of a recycled crustal carbonate component likely inherited by the Farallon plate subduction. Conversely, the SCN samples exhibit δ13C values within the MORB range (comparable to other values previously reported in fluid inclusions and fumaroles from Popocatépetl, Colima and Ceboruco volcanoes) and unlike the mantle beneath VESVF-DVF, indicate a negligible mantle contamination by subduction-related crustal carbon. The Canary Islands, in the central-eastern Atlantic, are among the most enigmatic Oceanic Island provinces on Earth, as the mantle source feeding their volcanism is spatially heterogeneous and with a multiplicity of involved components. Multi-isotope whole-rock studies have long revealed the presence of a recycled oceanic crust/lithosphere in the mantle. However, noble gas systematics have been more challenging to interpret, and carbon isotope data have remained sparse and incomplete. Our very first fluid inclusion data for El Hierro and Lanzarote nodules indicate carbon isotopic compositions of CO2 (δ13C) range from –2.38 to –1.23‰ in pyroxenes and from –0.19 to +0.96‰ in olivines. These unusually positive δ13C values, well above the typical upper mantle range (–8‰<–4‰), prove, for the first time, the presence of a regional recycled crustal carbon component in mantle beneath the Canary Islands. We interpret this 13C-rich component as inherited from a mantle metasomatism event(s) driven by fluids carrying carbon from subducted altered oceanic crust (AOC) and/or oceanic lithosphere (OL). Regarding noble gas isotopes, El Hierro xenoliths identify a depleted mantle source with MORB-like He signature. The average Rc/Ra ratio (3He/4He normalized to air ratio and corrected for atmospheric contamination) of 7.45±0.26 Ra (2SD, n = 14) overall indicates a marginal role played by past subduction events in modifying the local mantle He budget. Instead, Lanzarote xenoliths point to a more radiogenic mantle with an average of 5.97±0.44 Ra (2SD, n = 13) which we interpret as reflecting the involvement of an EM component. When put in the context of previous 3He/4He measurements in fluid inclusions and surface gases along the Canary archipelago, these results confirm an overall west-to-east decrease of Rc/Ra ratios (from El Hierro to Lanzarote), which may reflect a combination of i) increasing contributions of the African continental lithosphere, ii) the addition of radiogenic 4He during magma migration in the oceanic crust (whose thickness increases eastward) and/or iii) magma ageing. Finally, as proposed for Mexico, the involvement of depleted mantle-like fluids, variably admixed with air-derived components (possibly recycled via paleo-subduction event(s)), is corroborated by Ne-Ar isotopic compositions.