Los Humeros Research Articles

Determination of the time-dependent moment tensor using time reverse imaging

We have developed an approach to determine the time-dependent moment tensor and the origin time in addition to commonly derived locations of seismic events using time reverse imaging (TRI). It is crucial to locate and characterize the occurring microseismicity without making a priori assumptions about the sources to fully understand the subsurface processes inducing seismicity. Low signal-to-noise ratios (S/Ns) often force standard methods to make assumptions about sources or only characterize selected larger magnitude events. In TRI, microearthquakes are located by back propagating the full-recorded time-reversed wavefield through a velocity model until it ideally convergences on the source location. Therefore, it is less affected by low S/Ns and potentially locates and characterizes most of the events. After distinguishing artificial convergence locations from source locations, the quality of the source location and the moment tensors are derived by recording the stress at the determined source locations during the back propagation of the time-reversed wavefield. A robust workflow is derived using synthetic test cases in a realistic scenario with velocity models that only approximate the true velocity model and/or noisy displacement traces. The influence of a rudimentary velocity model on the source-location accuracy and characterization is significant. Our workflow handles these less-than-optimal station distributions and velocity models. Finally, the derived workflow is successfully applied to field data recorded at the geothermal field of Los Humeros, Mexico. Although only a 1D velocity model is currently available, source locations and (time-dependent) moment tensors could be determined for selected events.

Analysis of shear wave splitting parameters in los Humeros geothermal field, Puebla, Mexico

A study of anisotropic characteristics based on shear wave splitting (SWS) parameters (fast shear wave polarization and delay time) was carried out on 121 microearthquakes recorded by seismic arrays installed in Los Humeros Geothermal Field, Puebla, Mexico from 2011 to 2016. This analysis determined how both parameters seem to have been influenced by the geothermal field operation and by the spatial variations of the seismic sources with respect to the seismic array. There is a clear evidence of anisotropy since most of the SWS results show a main NW-SE polarization direction, which is not consistent with the maximum horizontal stress orientation in the region. The fast polarizations remained mostly constant for the analyzed period, keeping a preferential direction of NW-SE at the central-southern section of the study region, which in some cases follows the orientation of some proximal local faults. In the northern region, the fast polarizations appear uniform without following the orientations of the local faults. This preferential direction may be due to the shallow structures are being affected by the tectonism inhered from the basement, controlled in part by the Mexican fold and thrust belt whose main orientation is NW-SE and, to a lesser extent, by the pre-existing caldera ring faults.Observations of mean delay times mostly range between 0.025 s and 0.07 s for all seismic stations analyzed, with some individual values reaching up to 0.15 s, but with a total average of 0.041 s for the period analyzed. The maximum anisotropy percentage found was 13.54%, and the variations allowed us to characterize the geothermal field in two main areas with high fracturing. Results show Los Humeros, Antigua, Los Conejos and Las Viboras faults, and other minor faults around them as the main conduits of permeable sources. However, the inferred Antigua and Malpaís faults may also be considered important areas.Delay times δt and their respective error bars, showed an apparent decrease from 2014 to 2016, which may indicate a possible inverse correlation with an increase in fluid injection. However, the lack of data during 2013 and 2014 makes it difficult to robustly prove this hypothesis due to the change in station configuration.

Comment on “Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico)” by Urbani et al. (2020)

Abstract. A multiple shallow-seated magmatic intrusion model has been proposed by Urbani et al. (2020) for the resurgence of the Los Potreros caldera floor, in the Los Humeros volcanic complex (LHVC). This model predicts (1) the occurrence of localized bulges in the otherwise undeformed caldera floor, and (2) that the faults corresponding to different bulges exhibit different spatial and temporal evolution. Published data and a morphological analysis show that these two conditions are not met at Los Potreros caldera. A geothermal well (H4), located at the youngest supposed bulge (Loma Blanca) for which Urbani et al. (2020) calculated an intrusion depth (425±170 m), does not show any thermal and lithological evidence of such a shallow-seated cryptodome. Finally, published stratigraphic data and radiometric dating disprove the proposed common genesis of Holocene resurgence faulting and viscous lavas extruded in the centre of the caldera. Even if recent shallow intrusions do exist in the area, published data indicate that the pressurization of the LHVC magmatic–hydrothermal system driving resurgence faulting occurs at greater depth. Thus, we suggest that the model and calculation proposed by Urbani et al. (2020) are unlikely to have any relevance to the location, age and emplacement depth of magma intrusions driving resurgence at the Los Potreros caldera.

Metagenomic Approach to Bacterial Diversity and Lipolytic Enzymes’ Genes from a Steam Soil of Los Humeros Geothermal Field (Puebla, México)

Los Humeros volcanic complex, located at the eastern end of the Trans Mexican Volcanic Belt, possess superficial thermal manifestations (fumaroles, heat steam-soils and alteration zones), with temperatures between 50 °C and 90 °C. This is the first work to explore the bacterial diversity of a steam soil of Los Humeros geothermal field using a metagenomic 16S RNA gene analysis, and the presence of lipolytic enzymes’ genes after an enrichment culture by a whole-genome shotgun approach. High diversity indexes were found in soil samples, with Firmicutes, Chloroflexi, Proteobacteria, Actinobacteria and Acidobacteria as the most prevalent phyla. Alicyclobacillus was the most prevalent genus at 15 cm and Anoxybacillus at 50 cm depth; anaerobic and obligate thermophilic and hyperthermophilic members were found at 50 cm depth. Firmicutes was favored by the oil-rich medium, and the dominant genera were Geobacillus and Caldibacillus. 165 genes related to lipolytic enzymes were found; carboxilesterases and phospholipases C were the most abundant, and 43% were assigned to uncharacterized lipolytic enzymes. The diversity as well as the high percentage of non-characterized lipolytic enzymes found after soil enrichment in an oil-rich medium suggest the presence of biotechnologically promising organisms and enzymes that deserve to be investigated.

The role of microporosity on the permeability of volcanic-hosted geothermal reservoirs: A case study from Los Humeros, Mexico

Permeability is a key parameter in the analysis of geothermal systems, usually controlled by secondary structures such as fractures, faults or stratigraphic contacts. Contrarily, primary porosity is considered to have a negligible effect on the permeability of those systems, as it is commonly observed that there is low connectivity between macropores (conventionally defined as larger than 1μm). However, we observe that at the micrometric scale this scenario is different, and connectivity is clearly identified. In this study, an updated innovative petrophysical method is developed to calculate microporosity with high precision and simulate permeability on andesite lava flow core-samples from a geothermal production well in Los Humeros Geothermal Field (LHGF), Mexico. The method is based on high-resolution X-ray microCT and numerical post-processing algorithms, producing porosity-permeability estimates, which are in good agreement with other independent petrophysical methods such as Helium porosimetry-permeameter. Our results indicate that macropores account for an average of 3.7 ± 0.9 % of total rock volume fraction, whereas microporosity is generally higher, with a volume fraction in the order of 6.2 ± 0.6 %. Furthermore, when removing microporosity from the pore network, the permeability will account for an average of 3.50 × 10−17 m2, which is only the 17.92 % of the total average permeability for all samples. On the other hand, when microporosity is added, there is an average increment in the permeability of 82.08 %. This means that more than 80 % of the permeability of the samples is due to microporosity, and that microporous space indeed plays a major role in the permeability of these rocks. These results reveal that microporosity plays an important role on the permeability of rocks from volcanic-hosted LHGF. This approach has not been previously fully considered, and in this study, we show that a detailed microporosity analysis is essential for further estimations of permeability of volcanic-hosted geothermal systems.

Local Earthquake Tomography at Los Humeros Geothermal Field (Mexico)

AbstractA passive seismic experiment using 25 broadband and 20 short‐period stations was conducted between September 2017 and September 2018 at Los Humeros geothermal field, an important natural laboratory for superhot geothermal systems in Mexico. From the recorded local seismicity, we derive a minimum 1‐D velocity model and obtain 3‐D Vp and Vp/Vs structures of Los Humeros. We improved the classical local earthquake tomography by using a postprocessing statistical approach. Several inversions were computed and averaged to reduce artifacts introduced by the model parametrization and to increase the resolution of the investigated region. Finally, the resulting Vp and Vp/Vs structures and associated seismicity were integrated with newly acquired geophysical and petrophysical data for comprehensive interpretation. The recorded seismicity is mainly grouped in three clusters, two of which seem directly related to exploitation activities. By combining new laboratory measurements and existing well data with our Vp model, we estimate possible geological unit boundaries. One large intrusion‐like body in the Vp model, together with neighboring high Vp/Vs anomalies, hints at a region of active resurgence or uplift due to the intrusion of new magma at the northern portion of the geothermal field. We interpret high Vp/Vs features as fluid bearing regions potentially favorable for further geothermal exploitation. Deep reaching permeable faults cutting the reservoir unit could explain fluid flow from a deeper local heat source in the area.

Geology of Las Minas: an example of an exhumed geothermal system (Eastern Trans-Mexican Volcanic Belt)

ABSTRACT The Las Minas area corresponds to an exhumed geothermal system considered a proxy for the deep part of the nearby Los Humeros active geothermal system. The stratigraphic succession is made up of: Palaeozoic-Miocene granitoids, a thick Jurassic- Cretaceous carbonate succession, Neogene lava flows and volcano-sedimentary deposits. Linked to a Miocene magma intrusion, marble and skarn rock-volumes developed by contact metamorphism and geothermal fluid flow. Faults are arranged in SW- and NNW-striking systems. These controlled the morphological evolution and favored Neogene-Quaternary dyke emplacement. Faulting gave rise to a tectonic depression where lacustrine sediments and pyroclastics deposited. Skarn rocks are mainly located at fault intersections and along pre-existing discontinuities, suggesting the role of bedding and/or fractures in channeling deep fluids. Results give inputs for exploration at depth of Los Humeros geothermal system.

The Miocene Tatatila–Las Minas IOCG skarn deposits (Veracruz) as a result of adakitic magmatism in the Trans-Mexican Volcanic Belt

The Cu- and Au-rich Tatatila–Las Minas IOCG skarn deposits in Veracruz (central-east Mexico) are circumscribed to the earliest stages of the Trans-Mexican Volcanic Belt (TMVB) and stand for a metallogenic province directly linked to its tectonomagmatic dynamics. This is the first well-documented case for such metallogenic province. These deposits were formed as skarns between rocks of the Mesozoic carbonate series and Miocene intermediate to acid hypabyssal rocks. New U-Pb zircon and 40Ar/39Ar ages provide evidence for four epochs of magmatic activity in the area: (1) early Permian (Artinskian), in association with the Paleozoic basement, (2) late Oligocene to early Miocene suite of pre-TMVB intrusive rocks, (3) middle to late Miocene suite of early TMVB-related intrusive rocks, and (4) Pliocene intrusive and extrusive rocks of the TMVB, possibly associated with the Los Humeros post-caldera stage. The obtained ages range between 24.60 ± 1.10 and 19.04 ± 0.69 Ma for stage 2, and between 16.34 ± 0.20 and 13.92 ± 0.22 Ma for stage 3. Stage 2 corresponds to a magmatic stage unheard of in the area, until this study. Only stage 3 rocks are associated with the IOCG skarn mineralization, with retrograde stages dated at 12.44 ± 0.09 (chromian muscovite, phyllic association) and 12.18 ± 0.21 Ma (zircon, potassic association). Therefore, the ages of stage-3 intrusive rocks are interpreted to date the formation of the prograde skarn associations (mostly ~15.4 to <14 Ma). The petrogenetic affinity of stage-2 and stage-3 rocks is about the same—the main difference has to do with higher Y and Yb contents in stage-3 rocks (although no affinity with within-plate granites was found), which is suggestive of an interaction of their parental magmas with alkaline magmas that most likely belong to the conterminous and contemporaneous Eastern Mexico Alkaline Province. Petrological indicators (elemental and isotopic) in Cenozoic rocks consistently point to intermediate to acid, metaluminous, I- and S-type rocks that were emplaced in a subduction-related continental arc, within the medium- to high-potassium calc-alkaline series, with high-silica adakitic signatures due associated to deep-sourced magmas that underwent crustal contamination to some degree. The various possible sources for the magmas with adakitic signature in this context can be narrowed down to two of them that are not mutually exclusive: adakitic derived from subducted slab melting and melting-assimilation-storage-homogenization (MASH)-derived adakites. Both sources are, in principle, capable of generating magmas that would eventually produce magmatic-hydrothermal mineralizing systems with an associated variety of ore deposit types, including IOCG. Also, both possible sources for adakites are compatible with the renewed steepening of the subducted slab after a period of flat subduction, for the earliest stage in the evolution of the TMVB.

Fluid geochemistry of the Los Humeros geothermal field (LHGF - Puebla, Mexico): New constraints for the conceptual model

Geothermal power in Mexico is mainly produced in four geothermal fields operated by the Comision Federal de Electricidad (CFE): Cerro Prieto, Los Azufres, Los Humeros, and Las Tres Virgenes. The Los Humeros Geothermal Field (LHGF) is ranked third in terms of generated capacity, and in the last decade its installed capacity has doubled (up to 95.0 MW). Further increases in the geothermal power generation capacity in Mexico are planned, and thus the LHGF warrants further examination. The development and growth phases of any geothermal project must start from an awareness of the conceptual model of the natural system studied. The recharge mechanism, feeding zones, and fluid flow-path must be identified, along with the estimation of the temperature at the productive level and of phase separation (liquid – steam). To accomplish this, detailed fluid geochemical surveys were carried out in June 2017 and March 2018, in which 57 and 87 samples were collected, respectively, from cold and thermal springs, water wells and maar lakes located around and inside the LHGF. Samples from fumaroles inside the producing area were also collected for the first time, together with fluid from re-injection wells. The presence of a meteoric component, which plays an important role at the regional scale, is confirmed by the chemical and isotope data, and its contribution in terms of recharge may be higher than previously assumed. The Sierra Madre Oriental, on the west side of the LHGF, is characterized by widespread outcrops of limestone belonging to the same geological formation as those at the bottom of the LHGF. The isotope composition (δD and δ18O, respectively -77.3‰ and -10.50‰ for the hypothetical Infiltration Water - IW) is similar to that observed in cold springs located in the Sierra Madre Oriental, and from this the evolution of isotopes in the liquid-rock-steam system during water-rock interaction and phase separation processes can be modelled. Thus, the experimental data obtained for natural gas emissions (fumarolic condensates) and for geothermal fluids can be reproduced. These findings suggest that geothermal fluids in the LHGF are likely to be derived from meteoric water infiltrating (IW) the limestone outcrops of the Sierra Madre Oriental. During their flow-path, the infiltrating waters exchange isotopes at a high temperature with the crustal rocks, which have a much higher 18O/16O ratio, resulting in a shift towards higher δ18O (-4.35‰ ± 1) as the water O exchanges with rock O. The vapor phase can be separated from this deep water (DW) and it is discharged from the fumarolic effluents of Loma Blanca. Single Step Vapor Separation (SSVS) and Continuous Steam Separation processes (CSS) were modelled using stable isotopes of water. The results of geochemical modeling agree with available data for geothermal liquids discharged from several geothermal wells, suggesting that steam separation may be interpreted either as SSVS or CSS. Other processes can affect the chemistry and isotope composition of geothermal fluids (e.g. phase segregation, gas exchange, contributions from magmatic-volcanic deep fluids and re-injection fluids). The proposed conceptual model is consistent with both the geochemical data and the geological setting, and provides a useful point of reference for examining the fluid flow-path and geochemical processes active in the LHGF, at least at a general level.An involvement of magmatic-volcanic deep fluids in the feeding mechanism of the geothermal system cannot be excluded at priori, but the regional meteoric end-member is supported by the data and it seems the most important component.

Distribution and origin of organic compounds in the condensates from a Mexican high-temperature geothermal field

We report herein for the first time, the presence and relative composition of medium volatility organic compounds in samples from a super-hot geothermal field in Mexico. We sampled ten water/vapor condensates from active exploitation wells from Los Humeros Geothermal field. We identified 48 compounds, being alkanes, aromatics and S-bearing the predominant homologues, followed by O-bearing. Evidence suggests that the origin of the hydrocarbons is mainly thermogenic derived from hydrothermal alteration of buried organic matter, supported by the presence of aromatics, S-bearing and an alkane CPI below 1. The results of this study increase the scarce knowledge of dissolved organic fraction in hydrothermal systems.

Facies variations and permeability of ignimbrites in active geothermal systems; case study of the Xáltipan ignimbrite at Los Humeros Volcanic Complex

Ignimbrites’ facies are key for interpreting the dynamics of explosive eruptions. Furthermore, they provide evidence of the physicochemical processes that acted during and after their deposition. Nevertheless, ignimbrites commonly present complex facies associations that require an integrative analytical methodology over a robust stratigraphic framework for their proper characterization. This work presents a facies characterization of the caldera-forming Xáltipan ignimbrite at Los Humeros Volcanic Complex, by integrating a combination of petrophysical and geochemical analyses aimed at its outflows (exposed) and intracaldera (buried) deposits. Based on the petrophysical properties, a three-facies association is identified controlled by welding and secondary mineralization effects: (1) The basal facies, characterized in the outflows by highly welded deposits (ranks V-VI), with low porosity (~4–8%), and practically impermeable conditions (<0.01–0.05 mD). In the intracaldera deposits, this facies shows pervasive geothermal-related propylitic mineralization that can reduce porosity up to ca. 80%. (2) A middle facies, with moderately welded deposits (ranks III-IV) and porous conditions consisting of a combination of intershard, vesicular, and fracture spaces, partially filled by secondary minerals, which reduce the porosity (6–11.5% in the outflows; 16–28% for the intracaldera), resulting in semi-permeable conditions (6.11–46.38 mD). (3) The upper facies are defined by being not to poorly welded deposits (ranks I-II), and characterized by being highly porous (dominated by intershard spaces) and permeable (310–1557 mD), and barely affected by secondary mineralization. The geochemical results (major and trace element) indicate that the Xáltipan ignimbrite shows a vertical compositional zonation, from rhyolitic (70.9–76.2 wt% SiO2) in the lower flow unit to rhyolitic-trachydacitic (67–70.7 wt% SiO2) composition in its upper flow unit. This last unit shows a geochemical affinity with the subsequent pyroclastic deposits. Geochemical analyses of intercalated lavas from well logs also provided evidence of unprecedented breccias formed during the caldera collapse, discarding the existence of previously assumed thick lava flows. Facies correlation, among other well logs, shows that the facies distribution is directly controlled by the ignimbrite thickness. Where the ignimbrite is > 400 m thick, the entire facies association is observed, while when the thickness is < 400 m, the upper and middle facies occurs preferentially. This facies analysis may contribute to the overall model of Los Humeros subsurface stratigraphy, by defining the Xáltipan ignimbrite distribution and its porosity-permeability-based zones, which are of particular interest for geothermal purposes. Finally, this work provides the first example of a caldera-forming ignimbrite that presents analog facies associations between its outflows and intracaldera deposits that may be applied for other ignimbrites worldwide.

Imaging the Deep Structures of Los Humeros Geothermal Field, Mexico, Using Three-Component Seismic Noise Beamforming

AbstractWe present a 1D shear-velocity model for Los Humeros geothermal field (Mexico) obtained from three-component beamforming of ambient seismic noise, imaging for the first time the bottom of the sedimentary basement ∼5 km below the volcanic caldera, as well as the brittle-ductile transition at ∼10 km depth. Rayleigh-wave dispersion curves are extracted from ambient seismic noise measurements and inverted using a Markov chain Monte Carlo scheme. The resulting probability density function provides the shear-velocity distribution down to 15 km depth, hence, much deeper than other techniques applied in the area. In the upper 4 km, our model conforms to a profile from local seismicity analysis and matches geological structure inferred from well logs, which validates the methodology. Complementing information from well logs and outcrops at the near surface, discontinuities in the seismic profile can be linked to geological transitions allowing us to infer structural information of the deeper subsurface. By constraining the extent of rocks with brittle behavior and permeability conditions at greater depths, our results are of paramount importance for the future exploitation of the reservoir and provide a basis for the geological and thermodynamic modeling of active superhot geothermal systems, in general.

Shallow structure of Los Humeros (LH) caldera and geothermal reservoir from magnetotellurics and potential field data

SUMMARYThis study focuses in the analysis of the internal structure of the upper 3 km of Los Humeros (LH) caldera and the relation of electrical and hydrothermal anomalies. For this purpose, we measured, processed and interpreted 78 broad-band magnetotelluric (MT) soundings. We performed a 3-D inversion of the data set (ModEM) using all MT soundings, although only half of the available frequencies per sounding due to limited computed power. We also carried out the 2-D inversions (NLCG) of the invariant determinant along two orthogonal profiles (EW and NS) crossing the caldera structure; their comparison yields similar resistivity and structural models results. The resistivity modelling is complemented with the results of a joint 3-D inversion of an accurate gravity database of 720 stations, and total field aeromagnetic data (SGM) from the caldera crater. The combined results provide novel details about the structure of the shallow geothermal reservoir of the resurgence caldera complex hosting the active hydrothermal system. Density and resistivity models show the existence of a composed crater basin structure separated by an EW high-density structure; the northern basin is associated to the LH crater, whereas the southern basin associates to the emergent Los Potreros (LP) caldera basin. The magnetization model indicates that there is a common source for the magnetic volcanic products observed at the caldera surface, and that the LP fault is the more magnetized fault of the geothermal system. The propylic zoning under the geothermal field, which according to the MT model results has resistivities above ∼100 Ω-m, was extrapolated using this and additional criteria to obtain the distribution of other hypothetical propylitic zones of hydrothermal potential.

Wide-area observations of surface deformation in Mexican urban areas and geothermal fields using ENVISAT InSAR

There is an increasingly growing literature on the use of advanced multi-temporal Interferometric Synthetic Aperture Radar (InSAR) to monitor surface deformation with millimeter precision in urban and rural environments, including applications over wide areas spanning hundreds to thousands of square kilometers. In this work, we survey the eastern sector of the Trans-Mexican Volcanic Belt, extending from the state of Hidalgo, through Tlaxcala, Puebla and up to Veracruz, for a total of 19,000 km2. The surveyed area includes both the geothermal exploration site of the Acoculco caldera complex, and Los Humeros geothermal field, one of the largest geothermal exploitation sites of Mexico. Using the multi-temporal Small BAseline Subset (SBAS) InSAR method, we process the whole ENVISAT Advanced SAR (ASAR) IS2 image archive acquired in 2003-2010. Estimated subsidence rates are as high as 3.1 cm/year within the region, and a number of land deformation hotspots are identified within the urban areas of Tulancingo, Tlaxcala and Apizaco, the Tecamachalco valley, as well as within Los Humeros caldera. The surveying of such a scientifically interesting test region provides a proof of concept on how to investigate natural and anthropogenic processes in urban and rural areas using big SAR datasets and InSAR methods.

Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico)

Abstract. Resurgent calderas are excellent targets for geothermal exploration, as they are associated with the shallow emplacement of magma, resulting in widespread and long-lasting hydrothermal activity. Resurgence is classically attributed to the uplift of a block or dome resulting from the inflation of the collapse-forming magma chamber due to the intrusion of new magma. The Los Humeros volcanic complex (LHVC; Mexico) consists of two nested calderas: the outer and older Los Humeros formed at 164 ka and the inner Los Potreros formed at 69 ka. The latter is resurgent and currently the site of an active and exploited geothermal field (63 MWe installed). Here we aim to better define the characteristics of the resurgence in Los Potreros by integrating fieldwork with analogue models and evaluating the spatio-temporal evolution of the deformation as well as the depth and extent of the intrusions responsible for the resurgence, which may also represent the local heat source(s). Structural field analysis and geological mapping show that the floor of the Los Potreros caldera is characterized by several lava domes and cryptodomes (with normal faulting at the top) that suggest multiple deformation sources localized in narrow areas. Analogue experiments are used to define the possible source of intrusion responsible for the observed surface deformation. We apply a tested relationship between the surface deformation structures and depth of elliptical sources to our experiments with sub-circular sources. We found that this relationship is independent of the source and surface dome eccentricity, and we suggest that the magmatic sources inducing the deformation in Los Potreros are located at very shallow depths (hundreds of metres), which is in agreement with the well data and field observations. We propose that the recent deformation at LHVC is not a classical resurgence associated with the bulk inflation of a deep magma reservoir; rather, it is related to the ascent of multiple magma bodies at shallow crustal conditions (&lt;1 km depth). A similar multiple source model of the subsurface structure has also been proposed for other calderas with an active geothermal system (Usu volcano, Japan), suggesting that the model proposed may have wider applicability.

Sensitivity maps for time-reverse imaging: an accuracy study for the Los Humeros Geothermal Field (Mexico)

SUMMARY The estimation of the source–location accuracy of microseismic events in reservoirs is of significant importance. Time-reverse imaging (TRI) provides a highly accurate localization scheme to locate events by time-reversing the recorded full wavefield and back propagating it through a velocity model. So far, the influence of the station geometry and the velocity model on the source–location accuracy is not well known. Therefore, sensitivity maps are developed using the geothermal site of Los Humeros in Mexico to evaluate the spatial variability of the source–location accuracy. Sensitivity maps are created with an assumed gradient velocity model with a constant vp–vs ratio and with a realistic velocity model for the region of Los Humeros. The positions of 27 stations deployed in Los Humeros from September 2017 to September 2018 are used as surface receivers. An automatic localization scheme is proposed that does not rely on any a priori information about the sources and thus negates any user bias in the source locations. The sensitivity maps are created by simulating numerous uniformly distributed sources simultaneously and locating these sources using TRI. The found source locations are compared to the initial source locations to estimate the achieved accuracy. The resulting sensitivity maps show that the station geometry introduces complex patterns in the spatial variation of accuracy. Furthermore, the influence of the station geometry on the source–location accuracy is larger than the influence of the velocity model. Finally, a microearthquake recorded at the geothermal site of Los Humeros is located to demonstrate the usability of the derived sensitivity maps. This study stresses the importance of optimizing station networks to enhance the accuracy when locating seismic events using TRI.

Geochemical constraints on supercritical fluids in geothermal systems

Supercritical fluids with temperatures of ~400–500 °C have been reported from several active geothermal fields worldwide. Although the utilization of such fluids may multiply power production from new and already exploited geothermal systems, the fluid origin and chemical controls on their composition remain unclear. We performed flow-through high-temperature (400–420 °C) experiments at 34–69 bar to study the chemical and mineralogical changes associated with supercritical fluid formation upon boiling of subcritical geothermal fluids of varying chemical composition. Based on geochemical modeling and laboratory results, we propose that an important mechanism of supercritical fluid formation is conductive heating and boiling of subcritical geothermal groundwater by a magmatic intrusion. Such supercritical fluids will display low concentrations of mineral-forming elements (Si, Na, K, Ca, Mg, Al), with their concentrations being controlled by the solubility of salts, oxides, and aluminum silicates in high-temperature (>400 °C) and low-density (ρ < 0.3 g cm−3) fluids. In contrast, supercritical fluids will show elevated concentrations of volatile elements (C, S, B) of crustal and/or mantle origin with their concentrations often being similar to those of subcritical geothermal fluids. Associated mineral deposition, dominated by quartz, aluminum silicates, and salts, may form in the vicinity of the intrusion. Comparison of the modeling and laboratory results with observed chemical composition of natural supercritical fluid discharges indicates that conductive heating and boiling of subcritical geothermal groundwater may indeed be the formation mechanism of such fluids observed for example at Krafla (Iceland), Menengai (Kenya), Los Humeros (Mexico), and Larderello (Italy) with an addition of volcanic gases in many cases. Metal and salt-rich supercritical fluids, for example, at Kakkonda (Japan), may also exist in geothermal systems. However, such supercritical fluids are considered to have been trapped upon crystallization of the magmatic intrusion.

Magmatic volatiles to assess permeable volcano-tectonic structures in the Los Humeros geothermal field, Mexico

Magmatic volatiles can be considered as the surface fingerprint of active volcanic systems, both during periods of quiescent and eruptive volcanic activity. The spatial variability of gas emissions at Earth's surface is a proxy for structural discontinuities in the subsurface of volcanic systems. We conducted extensive and regular spaced soil gas surveys within the Los Humeros geothermal field to improve the understanding of the structural control on fluid flow. Surveys at different scales were performed with the aim to i) identify areas of increased gas emissions (reservoir scale), ii) their relation to (un)known volcano-tectonic structures (fault scale) favoring fluid flow, and iii) determine the origin of gas emissions. Herein, we show results from a CO2 efflux scouting survey, which was performed across the main geothermal production zone (6 km × 4 km) together with soil temperature measurements. We identified five areas with increased CO2 emissions, where further sampling was performed with denser sampling grids to understand the fault zone architecture and local variations in gas emissions. CO2 efflux values range from below detection limit of the device to 1464 g m−2 d−1 with a total output of 87 t d−1 across an area of 13.7 km2. Furthermore, δ13CCO2 and 3He/4He analyses complemented the dataset in order to assess the origin of soil gases. Carbon isotopic data cover a broad spectrum from biogenous to endogenous sources. Determined 3He/4He ratios indicate a mantle component in the samples of up to 65% being most evident in the northwestern and southwestern part of the study area. We show that a systematic sampling approach on reservoir scale is necessary for the identification and assessment of major permeable fault segments. The combined processing of CO2 efflux and δ13CCO2 facilitated the detection of permeable structural segments with a connection to the deep, high-temperature geothermal reservoir, also in areas with low to intermediate CO2 emissions. The results of this study complement existing geophysical datasets and define further promising areas for future exploration activities in the north- and southwestern sector of the production field.

Anatomy of the magmatic plumbing system of Los Humeros Caldera (Mexico): implications for geothermal systems

Abstract. Understanding the anatomy of magma plumbing systems of active volcanoes is essential not only for unraveling magma dynamics and eruptive behaviors but also to define the geometry, depth, and temperature of the heat sources for geothermal exploration. The Pleistocene–Holocene Los Humeros volcanic complex is part of the eastern Trans-Mexican Volcanic Belt (central Mexico), and it constitutes one of the most important exploited geothermal fields in Mexico with ca. 90 MW of produced electricity. With the aim to decipher the anatomy (geometry and structure) of the magmatic plumbing system feeding the geothermal field at Los Humeros, we carried out a field-based petrological and thermobarometric study of the exposed Holocene lavas. Textural analysis, whole-rock major-element data, and mineral chemistry are integrated with a suite of mineral-liquid thermobarometric models. Our results support a scenario characterized by a heterogeneous multilayered system, comprising a deep (depth of ca. 30 km) basaltic reservoir feeding progressively shallower and smaller discrete magma stagnation layers and batches, up to shallow-crust conditions (depth of ca. 3 km). The evolution of melts in the feeding system is mainly controlled by differentiation processes through fractional crystallization (plagioclase + clinopyroxene + olivine + spinel). We demonstrate the inadequacy of the existing conceptual models, where a single voluminous melt-controlled magma chamber (or “Standard Model”) at shallow depths was proposed for the magmatic plumbing system at Los Humeros. We instead propose a magmatic plumbing system made of multiple, more or less interconnected, magma transport and storage layers within the crust, feeding small (ephemeral) magma chambers at shallow-crustal conditions. This revised scenario provides a new configuration of the heat source feeding the geothermal reservoir at Los Humeros, and it should be taken into account to drive future exploration and exploitation strategies.

Anatomy of the Xáltipan ignimbrite at Los Humeros Volcanic Complex; the largest eruption of the Trans-Mexican Volcanic Belt

The Xáltipan ignimbrite (ca. 290 km3 DRE) represents the major explosive event from Los Humeros Volcanic Complex (LHVC) that can be ranked as the largest caldera-forming eruption of the Trans-Mexican Volcanic Belt (TMVB). This eruption marked the onset of an explosive period of LHVC producing the collapse of the main Los Humeros caldera at 164 ± 4.2 ka. This paper provides a detailed characterization of the Xáltipan ignimbrite anatomy and eruptive chronology that records a multi-phase, single eruption. This eruption resulted in more complex architecture and lithofacies variations of the ignimbrite than previously reported. To support these findings, we present two representative composite Xáltipan stratigraphic columns based on two regional reference sections, together with a welding lithofacies map, digital mapping, energy line projections, geologic sections and high-resolution petrophysical characterization (micro-CT, He-pycnometry). Additionally, we have reevaluated the magnitude (DRE volume), and the total erupted mass of the deposit by taking into account critical primary and post-emplacement processes such as lithic concentration, welding grade, and bulk loss due to erosion. The internal architecture of the Xáltipan ignimbrite is composed of two flow units and two pumice fallout deposits (a basal one and an intercalated layer), which formed as a result of a voluminous and sequential eruption that can be resumed in four eruptive phases. The initial stage of the eruption produced a basal pumice fallout (BPF) (phase 1) derived from a buoyant plume that eventually collapsed, generating pyroclastic density current (PDC) deposits with intra-ignimbrite pumice fallout beds. This event was followed by the climactic, caldera-forming, boiling over-type eruptive phase (phase 2) that generated voluminous PDCs of the lower flow unit (LFU). As the eruption waned, a brief eruptive hiatus occurred, immediately followed by a second explosive event in the western rim of the caldera with a boiling-over type eruptive column depositing a discrete pumice fallout layer (IPF) (phase 3), which later collapsed depositing less voluminous PDC (UFU) in the western flank of the Los Humeros caldera (phase 4). The anatomy of the deposits shows that the Xáltipan ignimbrite was emplaced in different behaviors: radially in the proximal parts and channelized and confined in the distal parts, showing significant welding variations that range from non-to-highly welded conditions (welding ranks I-VI) controlled mainly by the paleotopography. The size calculations indicate that the deposits have an aerial extent of ca. 4600 km2, an ejected bulk volume of ca. 330 km3 with a DRE volume of ca. 290 km3, and an ejected mass of 1.1 × 1012 kg, which points out to a paroxysmal eruption (VEI 6-7).