Origin and evolution of geothermal fluids from Las Tres Virgenes and Cerro Prieto fields, Mexico - Co-genetic volcanic activity and paleoclimatic constraints

摘要

Major and trace elements, noble gases, and stable (delta D, delta O-18) and cosmogenic (H-3, C-14) isotopes were measured from geothermal fluids in two adjacent geothermal areas in NW-Mexico, Las Tres Virgenes (LTV) and Cerro Prieto (CP). The goal is to trace the origin of reservoir fluids and to place paleoclimate and structural-volcanic constraints in the region. Measured He-3/He-4 (R) ratios normalized to the atmospheric value (R-a = 1.386 x 10(-6)) vary between 2.73 and 4.77 and are compatible with mixing between a mantle component varying between 42 and 77% of mantle helium and a crustal, radiogenic He component with contributions varying between 23% and 58%. Apparent U-Th/4(H)e ages for CP fluids (0.7-7 Ma) suggest the presence of a sustained He-4 flux from a granitic basement or from mixing with connate brines, deposited during the Colorado River delta formation (1.5-3 Ma). Radiogenic in situ 4He production age modeling at LTV, combined with the presence of radiogenic carbon (1.89 +/- 0.11 pmC - 35.61 +/- 0.28 pmC) and the absence of tritium strongly suggest the Quaternary infiltration of meteoric water into the LTV geothermal reservoir, ranging between 4 and 31 ka BP. The present geochemical heterogeneity of LTV fluids can be reconstructed by mixing Late Pleistocene - Early Holocene meteoric water (58-75%) with a fossil seawater component (25-42%), as evidenced by Br/Cl and stable isotope trends. CP geothermal water is composed of infiltrated Colorado River water with a minor impact by halite dissolution, whereas a vapor-dominated sample is composed of Colorado River water and vapor from deeper levels. dD values for the LTV meteoric end-member, which are 20 parts per thousand -44 parts per thousand depleted with respect to present-day precipitation, as well as calculated annual paleotemperatures 6.9-13.6 degrees C lower than present average temperatures in Baja California point to the presence of humid and cooler climatic conditions in the Baja California peninsula during the final stage of the Last Glacial Pluvial period. Quaternary recharge of the LTV geothermal reservoir is related to elevated precipitation rates during cooler-humid climate intervals in the Late Pleistocene and Early Holocene. The probable replacement of connate water or pore fluids by infiltrating surface water might have been triggered by enhanced fracture and fault permeability through contemporaneous tectonicevolcanic activity in the Las Tres Virgenes region. Fast hydrothermal alteration processes caused a secondary, positive delta O-18-shift from 4 parts per thousand to 6 parts per thousand for LTV and from 2 parts per thousand to 4 parts per thousand for CP geothermal fluids since the Late Glacial infiltration. (C) 2015 Elsevier Ltd. All rights reserved.