FLUID SOURCES AND STRUCTURAL CONTROLS OF THE PESCADORES FUMAROLE, GULF OF CALIFORNIA EXTENSIONAL PROVINCE, MEXICO
This study presents the first geochemical and isotopic characterization of a fumarole at the southern tip of the Pescadores fault (Baja California, Mexico). The fumarole was activated following the Mw 7.2 El Mayor-Cucapah (EMC) earthquake of April 2010. Gas samples were collected and vent temperature was measured (99.7 ˚C) during two field campaigns in 2022 and 2023. The principal objectives of this study are to investigate the origin of the fumarolic fluids and evaluate their link to the regional tectonic setting and other thermal fluids across the Baja California rift. The fumarole fluids are essentially from crustal origin, steam-dominated and CO2 as the principal dry gas. The relative abundances of N2, He, and Ar are characteristic of geothermal fluids in divergent and intraplate tectonic settings. The origin of helium is crustal-dominated with a mantle-derived contribution of up to ~6%. The δ13CCO2 values and CO2/3He ratios suggest that most of the CO2 was generated by decarbonation of carbonate rocks and thermal degradation of organic-rich sediments, with a minor mantle contribution. Likewise, the δ13CCH4 and δDCH4 values are consistent with a thermogenic origin of methane. The magma aging model reveals lower R/Ra values than those measured in fumarole fluid, which allows us to infer some contribution of mantle-derived fluids ascending through the Pescadores and the intersecting “Unnamed Keystone” faults.