Ruth Esther Villanueva-Estrada

High heat flow and ocean acidification at a nascent rift in the northern Gulf of California

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 (CO2 and CH4), widespread low pH (average 7.7), locally high 222Rn concentrations in the bottom water and a high extent of organic matter maturation in the sediments.

Morphology of pyrite in particulate matter from shallow submarine hydrothermal vents

Abstract Here we present the first study of pyrite textures observed in particulate matter from an active shallow submarine vent site, which is characterized by large spatial and temporal variations in the physico-chemical conditions caused by a combination of geochemical processes and microbial activity. Morphologic characterization of pyrite crystals from suspended particulate matter in the discharged fluid of a shallow submarine vent system near Punta Mita, Nayarit, Mexico, showed diverse crystal morphologies and aggregates; well-defined framboids are only observed within the deposits of finelayered calcite (calcareous tuff) formed around the vents, whereas particulate matter contains diverse pyrite crystal forms as globular, cubic, octahedral, and pyritohedral but no framboids. Available sulfur isotopic data indicate that pyrite is formed as a consequence of microbial sulfate reduction in a hydrothermal reducing environment. The results of our study in a natural system provide evidence of the effect of variations in key parameters, such as redox state, on the pyrite morphologies and framboid size distribution, and support the conclusions of numerous studies that have attempted to explain pyrite crystallization processes that generate different morphologies as a result of large variations in the physico-chemical conditions.

Geological, hydrogeological, and geothermal factors associated to the origin of arsenic, fluoride, and groundwater temperature in a volcanic environment El Bajío Guanajuatense, Mexico

Arsenic and fluoride groundwater concentrations over national standards for drinking water were measured in the regional aquifer of Juventino Rosas, Guanajuato State, Central Mexico. Also anomalous temperature occurs in groundwater wells of the area. Concentrations of total dissolved solids, silica, and chloride are too low to indicate a geothermal heat source. Additionally, isotopic evidence indicates that groundwater from the studied wells is subject to an evaporation process affected by the humid weather of the zone. The chemical characteristics of the water indicate a deep circulation warm water system in normal geothermal gradient. The warm waters of Juventino Rosas are mainly of three types: Water type I: (Na–HCO3), represented by the highest temperature wells and presence of fluoride and arsenic; water type II (Na–Ca–HCO3) that represents a mixing process between water types I and III. In this group, the sample 13JR contained high concentration of F−; water type III (Ca–HCO3), represented only by one sample (Cen 2) located over the outcrop of shales, limestones, and metamorphic rocks. This sample contains the highest concentrations of sulfate, manganese, and iron. All the geological and geochemical evidences indicate that rhyolite units are the most probable source of As and F−. The area corresponds to a low-temperature and low-enthalpy system and not to a well defined geothermal system.

The origin of groundwater arsenic and fluorine in a volcanic sedimentary basin in central Mexico: a hydrochemistry hypothesis

A groundwater sampling campaign was carried out in the summer of 2013 in a low-temperature geothermal system located in Juventino Rosas (JR) municipality, Guanajuato State, Mexico. This groundwater presents high concentrations of As and F− and high Rn counts, mainly in wells with relatively higher temperature. The chemistry of major elements was interpreted with different methods, like Piper and D’Amore diagrams. These diagrams allowed for classification of four groundwater types located in three hydrogeological environments. The aquifers are hosted mainly in alluvial-lacustrine sediments and volcanic rocks in interaction with fault and fracture systems. The subsidence, faults and fractures observed in the study area can act as preferential channels for recharge and also for the transport of deep fluids to the surface, especially in the basin plain. The formation of a piezometric dome and the observed hydrochemical behavior of groundwater suggest a possible origin of the As and F−. Geochemical processes occurring during water–rock interaction are related to high concentrations of As and F−. High temperatures and alteration processes (like rock weathering) induce dissolution of As and F−-bearing minerals, increasing the content of these elements in groundwater.RésuméUne campagne d’échantillonnage des eaux souterraines a été menée durant l’été 2013 dans un système géothermal basse-température situé dans la municipalité de Juventino Rosas (JR), Etat du Guanajuato, Mexique. L’eau souterraine présente de fortes concentrations en As et F− et des teneurs élevées en Rn, surtout dans les puits à température relativement élevée. La chimie des majeurs a été interprétée par différentes méthodes, comme les diagrammes de Piper et D’Amore. Ces diagrammes tiennent compte du classement des 4 types d’eau souterraine, situés dans trois contextes hydrogéologiques. Les aquifères sont contenus principalement dans des sédiments fluvio-lacutres et des roches volcaniques connectées à des systèmes de failles et de fractures. La subsidence, les failles et les fractures observées dans l’aire d’étude peuvent agir comme des chenaux préférentiels pour la recharge ainsi que pour le transfert de fluides profonds vers la surface, particulièrement dans la plaine du bassin. La formation d’un dôme piézométrique et le comportement hyrogéochimique observé de l’eau souterraine suggèrent une origine possible de As et F−. Les processus géochimiques qui se produisent pendant l’interaction eau–roche sont associés aux fortes concentrations en As et F−. Les hautes températures et les processus d’altération (comme l’altération des roches) provoquent une dissolution des minéraux contenant As et F−, augmentant la concentration de ces éléments dans l’eau souterraine.ResumenSe llevó a cabo una campaña de muestreo de aguas subterráneas en el verano de 2013 en un sistema geotérmico de baja temperatura situado en el municipio de Juventino Rosas (JR), Estado de Guanajuato, México. Esta agua subterránea presenta altas concentraciones de As y F− y altos contenidos de Rn, principalmente en pozos con temperatura relativamente alta. Se interpreta la química de los principales elementos con diferentes métodos, como diagramas de Piper y de D’Amore. Estos diagramas permitieron clasificar cuatro tipos de aguas subterráneas ubicadas en tres ambientes hidrogeológicos. Los acuíferos se alojan principalmente en sedimentos aluviales, lacustres y rocas volcánicas en interacción con los sistemas de fallas y fracturas. La subsidencia, fallas y fracturas observadas en el área de estudio pueden actuar como canales preferenciales para la recarga y también para el transporte de fluidos profundos hacia la superficie, especialmente en la llanura de la cuenca. La formación de un domo piezométrico y el comportamiento hidroquímico observado en el agua subterránea sugieren un posible origen del F− y del As. Los procesos geoquímicos que ocurren durante la interacción agua–roca son relacionados con altas concentraciones de As y F−. Las altas temperaturas y los procesos de alteración (como meteorización de las rocas) inducen la disolución los minerales que llevan As y F−, aumentando el contenido de estos elementos en el agua subterránea.摘要2013年夏季在位于墨西哥瓜纳华托州Juventino Rosas市的低温地热系统中进行了地下水采样。地下水中砷、氟和氡的含量很高,主要是在相对较高温度的井中。采用不同的方法如Piper和 D’Amore图标法解译了主要元素的化学成分。这些图表可以对位于三个水文地质环境中的四种地下水类型进行分类。含水层主要赋存于冲积—湖湘沉积物中及具有断层和断裂系统的火山岩中。研究区观测到的沉降、断层和断裂可以充当补给的优先通道,也可以充当深层流体到地表的传输优先通道,特别是在盆地平原更是如此。测压丘的形成及地下水观测到的水文化学特征显示出了砷和氟的可能来源。水岩相互作用期间的地球化学过程与砷和氟的含量高有关。高温度和更替过程(如岩石风化)引起了含砷和含氟矿物的溶解,增加了这些元素在地下水中的含量。ResumoUma campanha de amostragem de águas subterrâneas foi realizada no verão de 2013 em um sistema de baixo gradiente de temperatura geotermal, localizado no município de Juventino Rosas (JR), Estado do Guanajuato, México. As águas do aquífero apresentam alta concentração de As e F− e alta contagem de Rn, principalmente em poços com temperatura relativamente mais elevada. A química dos elementos maiores dissolvidos foi interpretada em diferentes métodos, como diagramas de Piper e D’Amore. Estes diagramas permitiram a classificação de quatro tipos de águas subterrâneas localizadas em três ambientes hidrogeológicos. Os aquíferos estão hospedados principalmente em sedimentos aluvionares-lacustres e rochas vulcânicas de interação com sistemas de falhas e fraturas. A subsidência, falhas e fraturas observadas na área de estudo podem agir como canais preferenciais de recarga e também para o transporte de fluidos profundos para a superfície, especialmente na planície da bacia. A formação de domos piezométricos e o comportamento hidroquímico observado nas águas subterrâneas sugerem uma possível origem de As e F−. Os processos geoquímicos que ocorrem durante a interação água–rocha estão relacionados com a alta concentração de As e F−. Altas temperaturas e processos de alteração (como intemperismo) induzem a dissolução de minerais contendo As e F−, aumentando o teor desses elementos nas águas subterrâneas.

Geochemical processes in an active shallow submarine hydrothermal system, Bahía Concepción, México: mixing or boiling?

Hydrothermal activity at Bahía Concepción, on the western coast of the Gulf of California, is not linked to present volcanic activity. This site is a potential energy source; however, geothermal modelling of the system is needed in order to determine the processes that generated this activity. Two processes might lead to the formation of the secondary fluids that were sampled in this submarine hydrothermal system: the thermal endmember might be undergoing either boiling or mixing with a fluid trapped within the sedimentary strata. However, boiling was ruled out as this process would not produce a highly saline fluid within the temperature range determined for the Bahía Concepción system (∼87°C). We modelled the mixing of a highly saline fluid with a geothermal fluid of meteoric origin. Our results suggest that the composition of the thermal water representing the hot endmember at Bahía Concepción consists of 20–30% highly saline fluid and 70–80% aqueous fluid of meteoric origin. The computed fluid has a chemical composition similar to that determined for the submarine vents. Its derived contents of calcium, chloride, and silica are similar to those we measured for the vent discharge. Our hypothesis of saline water addition is supported by the mineralizing fluid characteristics determined from fluid inclusion microthermometric study of a fossil intertidal system that discharges in the vicinity of the vent areas. This intertidal spring fluid at ∼40°C, according to the geochemical model, would be supersaturated in opaline silica and calcite, consistent with the mineralogy of the stromatolitic sinter encircling the discharges.

Inter-laboratory test for oxygen and hydrogen stable isotope analyses of geothermal fluids: Assessment of reservoir fluid compositions

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.

FIRST STAGE IN THE CONSTRUCTION OF A MAP OF GEOTHERMAL PROVINCES FOR MEXICO

1 1 1 1 1 2 2 Ruth Esther Villanueva-Estrada , Juan Ramón Armenta Ruiz , Roberto Gerardo Rocha-Miller , Augusto Antonio Rodríguez-Díaz , Sandra Beatriz Briceño Prieto , Carlos Pita de la Paz , Luis Alfredo Sánchez Galindo , 3 4, 1 1 María Teresa de Jesús Rodríguez-Salazar , Isabel Pérez Martínez Patricia Jacqueline Ramos Chaparro , Patricia Cruz Martín . 1 2 Recursos Naturales, Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, Ciudad de México, México city, 04510, Mexico, Delegacion Tlalpan, Alheli Mz. 7 3 Lt 13A Col. Ejidos San Pedro Martir, Delegacion Tlalpan, Mexico, 14640, Mexico, Química Analítica, Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán,