Ignacio S. Torres-Alvarado

SINCLAS: standard igneous norm and volcanic rock classification system

SINCLAS is an efficient CIPW norm and volcanic rock classification system written in MS-VisualBasic®. It reads input data from STATISTICA® or MS-Excel® files and returns the results to them for further use. As an important improvement, the system takes into account minor chemical constituents in the norm computation. Before computing the CIPW norms and final rock classification, this program handles the Fe2O3/FeO ratio according to three different options and recalculates the actual analyses to an anhydrous (volatile-free) 100 % adjusted basis. Based on mass balance principles and variable atomic weights, SINCLAS achieves accurate and highly consistent CIPW norm results, generally better than 0.002%m/m. Furthermore, the program uses the latest IUGS recommendations for classification of volcanic rocks based on the total alkalis versus silica (TAS) diagram. Taking advantage of the MS-Windows® operating system, SINCLAS is simple to run; it generates a file with adjusted SiO2 and Na2O + K2O values for a TAS diagram in any graphics package, and reports the results in ready-to-print tables in MS-Word® format.

Beyond Subduction and Plumes: A Unified Tectonic-Petrogenetic Model for the Mexican Volcanic Belt

The Mexican Volcanic Belt (MVB) is a major linear belt of Miocene to present-day volcanism in southern Mexico. Its origin has been controversial, although the majority opinion views it as a volcanic arc related to the subduction of the Cocos plate under the North American plate. Both calc-alkaline and alkaline volcanism characterize the belt; the latter has been previously cited as indicative of the role of a mantle plume. Here we present objections to these explanations, and conclude on the basis of geological, geochemical, and geophysical data that the MVB is unrelated to subduction or to a mantle plume, and is instead a rift-like structure experiencing active extension. Calc-alkaline or alkaline geochemistry of magmas is not useful for inferring tectonic setting, but reflects source parameters and petrogenetic processes. For the MVB, calc-alkaline geochemistry suggests crustal contamination, and the OIB-like geochemistry suggests an enriched mantle source. Our proposal of a heterogeneous mantle beneath the MVB comprising “normal” mantle and metasomatic, enriched veins, can explain the close association in space and time of calc-alkaline and alkaline volcanism throughout the belt.

The origin of reservoir fluids in the geothermal field of Los Azufres, Mexico — isotopical and hydrological indications

Abstract The calculation of hydrological balance resulted in a potential, average annual infiltration rate of 446±206 mm/m 2 for the Los Azufres geothermal area, which corresponds to a total of 82×10 6 m 3 per a. Due to the highly fractured and faulted structure of the volcanic formations, a considerable potential for the infiltration of recent meteoric water into deeper sections of the volcanic formations can be assumed. Isotopic data indicate the minor importance of recent meteoric water for the recharge of the geothermal reservoir. Very negative δ 13 C values can be explained by the input of organic C from the surface, but the lack of 14 C in the deep fluids reflects a pre-historic age for the infiltration event of fossil meteoric water. The dilution of the meteoric water by 14 C-free CO 2 gas from a shallow magma chamber complicates the exact age determination of the infiltration event, which probably occurred during the Late Pleistocene or Early Holocene glacial period. Strong water–rock interaction processes, such as sericitization/chloritization, caused the primary brine composition to be camouflaged. A preliminary hydrological model of the reservoir can be postulated as follows: the fossil hydrodynamic system was characterized by the infiltration of meteoric water and mixing with andesitic and/or magmatic water. Strong water–rock interaction processes in the main part of the production zone prove the existence of former active fluid circulation systems. Due to changes in pressure and temperature, the rising fluids get separated into liquid and vapour phases at a depth of 1500 m. After cooling, the main portions of both phases remain within the convective reservoir cycle. Isotope analyses of hot spring waters indicate the direct communication of the reservoir with the surface at some local outcrops. A recent reactivation of the hydrodynamic system is caused by the geothermal production, as indicated by the detection of lateral communication between some production and reinjection wells.

Chemical Equilibrium in Hydrothermal Systems: The Case of Los Azufres Geothermal Field, Mexico

This work presents a review of the geological, hydrogeochemical, and mineralogical information related to the Los Azufres geothermal field, along with new isotopic data from hydrothermal calcite and quartz. The equilibrium state between present-day geothermal fluids and hydrothermal mineral phases in this field was examined using mineralogical, thermochemical, as well as isotopic data. Hydrothermal minerals in Los Azufres are typical for neutral to alkaline, high-temperature thermal fluids, forming an incipient to complete propylitic alteration of the primary rocks. The comparison of activity diagrams of common hydrothermal phases to mineralogical observations at Los Azufres shows that the reservoir rocks tend to approach a state of chemical equilibrium with hydrothermal fluids percolating through them. This tendency to equilibrium has been corroborated by analysis of the oxygen isotopic composition of hydrothermal calcite, whose σ18O values are in equilibrium with present geothermal or meteoric waters. Although 18O/16O ratios from some hydrothermal quartz samples show equilibrium with present-day fluids, others indicate the presence of fluids with higher 18O/16O ratios than modern geothermal water. The oxygen isotopic composition of hydrothermal quartz can thus be used as an indicator of isotopic compositions of fluids at the time of quartz precipitation.

Sr, Nd and Pb isotopic and geochemical constraints for the origin of magmas in Popocatépetl volcano (central Mexico) and their relationship with the adjacent volcanic fields

We present new geochemical data (major- and trace-elements, as well as Sr and Nd isotopic compositions) of volcanic rocks erupted from Popocatépetl volcano during the volcanic event from December 2000 to January 2001. These data along with an exhaustive compilation of geochemical and Sr, Nd, and Pb isotope data reported for Popocatépetl rocks and nearby volcanic areas are used to examine the origin and geochemical evolution of the magmas in the central Mexican volcanic belt (CMVB). During this period of volcanic eruptions Popocatépetl produced ash columns as high as 7 km. Pyroclastic flows and lahars were observed after the completion of the activity. Samples of banded pumice and a bomb fragment transported by the lahar were chemically analysed for this work. Rocks show an andesitic composition with 58.5–61.7 wt.% SiO2 and 5.9–4.0 wt.% MgO. Contents of large ion lithophile elements (LILE), rare-earth elements (REE) and Zr are nearly constant through the compositional range. No significant Eu anomaly is present, but the samples show Nb-anomaly relative to LILE and high-field strength elements (HFSE). Nd- and Sr-isotopic compositions of these samples range from 143Nd/144Nd = 0.51291 to 0.51287 and 87Sr/86Sr = 0.70399 to 0.70422. Comparison of Popocatépetl products with volcanic rocks from the nearby areas shows that the magmas in CMVB were generated in a heterogeneously veined-mantle source enriched in LILE, HFSE, and REE. Additional crustal assimilation as well as fractional crystallization could account for the great chemical variability of rocks in the CMVB. Statistical comparison of the geochemical compositions of the volcanic products ejected from 1994 to 2000 to those ejected during the 2001 event shows that most geochemical parameters (major- and trace-elements, normative minerals, Sr and Nd isotopic composition, as well some elemental ratios) present no statistically significant differences. Statistically significant differences in the mean only were computed for the major-elements SiO2, FeO, MgO, CaO, and K2O, as well as for the rare-earth elements Nd, Sm, Eu, Gd, Dy, Ho, Tm, and Yb.

X-Ray Diffraction Analysis of Hydrothermal Minerals from the Los Azufres Geothermal System, Mexico

Los Azufres is an active geothermal field located in the middle of the Mexican Volcanic Belt (MVB), a vast Miocene-Recent, E-W-oriented volcanic province spanning central Mexico. Three wells—Az-5, Az-28, and Az-31—from this geothermal field were selected to investigate the distribution, alteration sequence, and thermal stability of hydrothermal minerals. Maximum in situ measured temperatures are 280°C at 1493 m, 265°C at 1700 m, and 288°C at 1300 m depths in Az-5, Az-28, and Az-31, respectively. The host rocks in these wells are dominantly andesite followed by dacite, rhyolite, and basalt. Rock cuttings from different depths were analyzed for clay and non-clay minerals by X-ray Diffraction (XRD) methods. Hydrothermal quartz, calcite, and pyrite, as well as other alteration mineral phases (e.g., chabazite and chlorite) that are difficult to identify by traditional petrography were identified and their abundances semi-quantitatively estimated by XRD. We show that these mineral data present a better perception of distribution trends of hydrothermal minerals in geothermal wells than the qualitative mineral identifications generally used for this purpose. Homogenization temperatures measured in fluid inclusions of hydrothermal minerals, in situ measured temperatures in the wells, and K+/H+ vs. Mg2+/(H+)2 activity diagrams for the chemical characteristics of the present geothermal fluids were used to define the thermal regime and the resultant stability conditions of the clay minerals. Smectite, illite, and chlorite are present in the <2 mu;m size fraction. Gradual variations in relative abundances of clay minerals range from smectite dominant at shallow well depths to a combination of smectite, illite, and chlorite at intermediate depths, and to illite and chlorite in the deepest levels. Excellent crystallinity and lack of mixed-layered clay minerals support a model involving a discontinuous change from smectite to chlorite and/or illite, rather than that involving continuous mixed-layering of smectite-illite and/or smectite-chlorite. Mineralogical and fluid inclusion data suggest that mineral distribution trends documented for the Los Azufres geothermal system reflect the prevailing thermal regime. The mineral parageneses of the Los Azufres geothermal field are broadly comparable with those reported in other geothermal systems of the world.

Compilation of radiogenic isotope data in Mexico and their petrogenetic implications

Seven hundred and twenty-five Sr, two hundred and forty-three Nd and one hundred and fifty-one Pb isotopic ratios from seven different Mexican magmatic provinces were compiled in an extensive geochemical database. Data were arranged according to the Mexican geological provinces, indicating for each province total number of analyses, range and mean of values and two times standard deviation (2σ). Data from seven provinces were included in the database: Mexican Volcanic Belt (MVB), Sierra Madre Occidental (SMO), Baja California (BC), Pacific Ocean (PacOc), Altiplano (AP), Sierra Madre del Sur (SMS), and Sierra Madre Oriental (SMOr). Isotopic values from upper mantle and lower crustal xenoliths, basement outcrops and sediments from the Cocos Plate were also compiled. In the MVB the isotopic ratios range as follows:87Sr/86Sr 0.703003-0.70841;143Nd/144Nd 0.512496-0.513098;206Pb/204Pb 18.567-19.580;207Pb/204Pb 15.466-15.647;208Pb/204Pb 38.065-38.632. The SMO shows a large variation in87Sr/86Sr ranging from ∼0.7033 to 0.71387.143Nd/144Nd ratios are relatively less variable with values from 0.51191 to 0.51286. Pb isotope ratios in the SMO are as follows:206Pb/204Pb 18.060-18.860;207Pb/204Pb 15.558-15.636;208Pb/204Pb 37.945-38.625. PacOc rocks show the most depleted Sr and Nd isotopic ratios (0.70232-0.70567 for Sr and 0.512631-0.513261 for Nd). Pb isotopes for PacOc show the following range:206Pb/204Pb 18.049-19.910;207Pb/2047Pb 15.425-15.734;208Pb/204Pb 37.449-39.404. The isotopic ratios of the AP rocks seem to be within the range of those from the PacOc.Most samples with reported Sr and Nd isotopic data are spread within and around the “mantle array”. The SMO seems to have been formed by a mixing process between mantle derived magmas and continental crust. The MVB appears to have a larger mantle component, with AFC as the dominant petrogenetic process for the evolved rocks. There is still a need for Pb isotopic data in all Mexican magmatic provinces and of Nd isotopes in BC, AP, SMS, and SMOr.

Magnetic susceptibility of volcanic rocks in geothermal areas: application potential in geothermal exploration studies for identification of rocks and zones of hydrothermal alteration

Magnetic susceptibility and petrographic studies of drilled rock cuttings from two geothermal wells (Az-26 and Az-49) of the important electricity-generating geothermal system, Los Azufres, Mexico, were carried out to determine the relation between the magnetic susceptibility of rocks, the concentration of magnetic minerals and hydrothermal alteration. For this purpose, low-frequency magnetic susceptibility (χlf) was measured and compared its distribution trends with those of magnetic and Fe–Mg silicate minerals, and with the extent of hydrothermal alteration in rocks of the two geothermal wells. The study indicates a decrease in χlf values with depth in the two geothermal wells corresponding with: (1) an increase in the reservoir temperature and hydrothermal alteration; and (2) a decrease in the concentrations of Fe–Mg silicates and opaque minerals. The data suggest that ferromagnesian minerals and opaque minerals like ilmenite are the main contributors to the χlf of rocks. The decrease in χlf, ilmenite, and Fe–Mg mineral contents with an increase in the hydrothermal alteration degree, pyrite and haematite contents suggests the hydrothermal alteration of ilmenite and Fe–Mg minerals (characteristic of high χlf values) to pyrite, haematite and other opaque minerals (with low χlf values). The interaction of hydrothermal fluids with rocks results in the hydrothermal alteration of primary minerals. In a geothermal area, an anomaly of low magnetic susceptibility values of rocks in a homogenous litho unit characterized by high magnetic susceptibility may suggest hydrothermal alteration. Magnetic susceptibility can be a useful parameter, during the initial stages of geothermal exploration, in identifying hydrothermally altered rocks and zones of hydrothermal alteration both at the surface and from drilled wells in geothermal systems.

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.

Discussion and reply: Neogene volcanism at the front of the central Mexican Volcanic Belt: Basaltic andesites to dacites, with contemporaneous shoshonites and high-TiO2 lava - Discussion

We read with interest the contribution by [Blatter et al. (2001)][1] on magmatism in the central part of the Mexican Volcanic Belt. Coexisting subalkaline and alkaline oceanic-island basalt (OIB)-type magmas in both space and time have been reported all along the belt ([Blatter et al., 2001][1]; [