RESUMO
RATIONALE: Knowledge of the accuracy and precision for oxygen (δ18 O values) and hydrogen (δ2 H values) stable isotope analyses of geothermal fluid samples is important to understand geothermal reservoir processes, such as partial boiling-condensation and encroachment of cold and reinjected waters. The challenging aspects of the analytical techniques for this specific matrix include memory effects and higher scatter of delta values with increasing total dissolved solids (TDS) concentrations, deterioration of Pt-catalysts by dissolved/gaseous H2 S for hydrogen isotope equilibration measurements and isotope salt effects that offset isotope ratios determined by gas equilibration techniques. METHODS: An inter-laboratory comparison exercise for the determination of the δ18 O and δ2 H values of nine geothermal fluid samples was conducted among eleven laboratories from eight countries (CeMIEGeo2017). The delta values were measured by dual inlet isotope ratio mass spectrometry (DI-IRMS), continuous flow IRMS (CF-IRMS) and/or laser absorption spectroscopy (LAS). Moreover, five of these laboratories analyzed an additional sample set at least one month after the analysis period of the first set. Statistical evaluation of all the results was performed to obtain the expected isotope ratios of each sample, which were then subsequently used in deep reservoir fluid composition calculations. RESULTS: The overall analytical precisions of the measurements were ± 0.2 for δ18 O values and ± 2.0 for δ2 H values within the 95% confidence interval. CONCLUSIONS: The measured and calculated δ18 O and δ2 H values of water sampled at the weir box, separator and wellhead of geothermal wells suggest the existence of hydrogen and oxygen isotope-exchange equilibrium between the liquid and vapor phases at all sampling points in the well. Thus, both procedures for calculating the isotopic compositions of the deep geothermal reservoir fluid - using either the analytical data of the liquid phase at the weir box together with those of vapor at the separator or the analytical data of liquid and vapor phases at the separator -are equally valid.
RESUMO
The prevailing tectonic setting in the Gulf California suggests the presence of an undetermined number of short spreading centres with associated hydrothermal systems. However, to date, active seafloor spreading phenomena have been documented in only three of the eight tectonically active basins. Here we report heat flow values as high as 15,436 mW m(-2) in two of the northernmost basins of the Gulf of California, providing evidence of intense hydrothermal activity associated with the transition from continental rifting to seafloor spreading. The mean heat flow for the Wagner and Consag basins area is 1,875 mW m(-2), more than 15 times higher than the mean value for oceanic crust (105.4 mW m(-2)). Additional evidence for vigorous hydrothermal circulation and a shallow heat source includes intense gas discharge (CO(2) and CH(4)), widespread low pH (average 7.7), locally high (222)Rn concentrations in the bottom water and a high extent of organic matter maturation in the sediments.
