Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

An objective procedure involving fourteen statistical tests (a total of thirty-four variants) for detection and rejection of outliers in a univariate sample was applied to five geochemical Reference Material (RM) databases of intrusive rocks from the Geological Survey of Japan (GSJ). All available concentration data for JG1, JG1a, and JG3 granodiorites, JG2 granite, and JGb1 gabbro were first grouped in eight general analytical methods. These groups were tested for systematic differences using analysis of variance (ANOVA). After this process, the databases were evaluated by means of statistical tests for detection of possible outliers at a strict confidence level of 99%, minimizing the danger of rejecting a valid observation as an outlier. New concentration mean values and other statistical parameters were computed from final normal distributions. Although 23% of mean concentration values were practically identical to those reported in earlier literature, 67% differ by about 1% to 20%. The present statistical scheme provides a better perspective to evaluate existing geochemical databases than probably erroneous two-standard deviation method involving prior subjective judgments used by the U.S. and Japanese scientists. The present mean values with generally lower %RSD may contribute to a better estimation of precision, accuracy, and sensitivity of routine analysis for intrusive rock samples.

Colima volcano is the most active in Mexico. including a significant eruption in 1991. Geochemical monitoring of the volcano started in January  1996 with: 1) airborne measurements of the S02 flux by COSPEC; 2) collection of fumarolic gases from the summit crater every three months, and 3) collection of water samples from three springs on thc S-SW slopes of the volcano every two months. Chemical compositions of volcanic gases did not show systematic trends. except for the isotopic composition of the volcanic vapor. Starting from the su mmer of 1997, the volcanic water became gradually enriched in deuteriurn, which indicates a contribution from deep, less degassed magma. Three other possible precursor signa is were identified for fhe 1998-99 eruption: 1) a two to three-fold "peak" in the boron concentration approximately 3 months before the eruption,  2) a step-wise increase in HC03 and the calculated partial pressure of dissolved C02 , approximately l.5 years before the eruption , 3) until September 1998, the S02 flux from the volcano was at a low level of  less than 100 t/d, often below the detection  limit of  the instrument. A sharp incrcase in the S02 flux was recorded a month before the eruption started, from 400±50 t/d up to 1,600 t/d two days before the new lava, emerged on 20 November, 1998. The S02 flux was in a good correlation with  seismicity.  

Ezequiel Ordóñez is still one of the most important Mexican geologists because of the number and the quality of his scientific publications. The first one appears in 1889, the last one of about 112, in 1947. In more than thirty of his works, he puts a special significance to landforms, most of the time as support to petrology, volcanoes, mineral and petroleum deposits and other subjects. The geo- morphic concepts are evident while he was publishing about small areas and provinces or the whole country, which makes of Ezequiel Ordóñez a Geomorphology pioneer in Mexico. It is interesting that not having been exactly a geomorphologist, he never used the name of that discipline or wrote about the contemporary theories mentioning their authors, the way he described the landforms and inter- preted them in his work, makes of Ezequiel Ordóñez a true connoisseur of the geomorphic method, which he probably reached through intuition and possibly by the influence of the German geologist Emil Böse, a former pupil of one of the most advanced schools of landforms studies in the early 1900s.  

El objetivo particular de este trabajo es determinar las características de disperción de Ni y Cr totales en sedimentos y suelos superficiales, derivados de rocas ultramáficas en el sistema hidrológico San Juan de Otates, Estado de Guanajuato así como confirmar si existe dispersión de estos elementos hacia las cuencas vecinas. Generalmente ambos elementos se asocian en la naturaleza a piroxenitas, serpentinitas y basaltos, rocas que se encuentran en la parte alta de la subcuenca. Para su análisis, la zona de estudio fue dividida en cinco subcuencas. Para determinar los niveles de estos elementos, se muestrearon 56 sitios de sedimentos y 46 de suelos a una profundidad 0-20 cm; los sitios de muestreo se escogieron con base en la geología, geoforma, pendiente, drenaje, unidades de suelos, uso del suelo, así como tipo y características de obras de ingeniería civil. Para establecer la asociación mineralógica con los iones metálicos, se utilizó la técnica macroscópica. Las muestras fueron digeridas en un horno de microondas, utilizando agua regia para cada muestra, como extractante. Las concentraciones fueron determinadas por espectrometría de absorción atómica de flama, con límite de detección de 0.004 mg/kg-1 para Ni y 0.002 mg/kg-1 para Cr. Para elaborar el mapa de la distribución de iones metálicos se utilizó el sistema de información geográfica ILWIS. El análisis estadístico no mostró diferencias significativas en los contenidos de Ni y Cr tanto en suelos como en sedimentos. Los mayores niveles de Cr se localizaron en la subcuenca  San Juan de Otates; en tanto que el Ni mostró una mayor concentración en tres subcuencas en relación a las otras dos restantes. A partir de la fuente principal de Ni y Cr, la distribución de estos se torna geográficamente irregular. En El Bajío, el canal Santa Ana del Conde, que une los arroyos San Juan de Otates y El Juache, ha visto incrementar últimamente sus contenidos de los elementos en estudio. Finalmente, se encontró que, a nivel de suelos y sedimentos superficiales,  no existe dispersión  de estos elementos hacia subcuencas vecinas.    

El Picacho is an annular Tertiary magmatic complex with a surface of 9 km2 • It is situated in the Eastern part of the Sierra de Tamaulipas, 70 km Southeast of Ciudad Victoria. The complex ha an elliptic shape and is bordered by Lower Cretaceous limestones of the Tamaulipas Superior Fonnation. El Picacho complex consists mainly of alkaline gabbros surrounded irregularly by granites and nepheline syenites. In its central pmt a semicircular body of nephelinitic rocks (nepheline syenites, ijolites and melteigites, among other varieties)  cuts the gabbroic  rocks, forming a fenitization aureole. The geochemical data indicate a differentiation process in the nephelinic rocks from melteigites and ijolites to nepheline syenites. At least two magmatic pulses compose this unit. The gabbros at the contact and the nepheline syenites from the first pulse were later fenitized by alkaline fluids, coming from the ijolitic magma from the second pulse. The alkaline metasomatism has a sodic character. Cutting the nephelinic rocks in the centre of the complex and the gabbros outside of the central body, two Th-Y rich carbonatite dykes were recognized. The first is a coarse grained soevite and consists of calcite, bands of iron oxides, pyrite, siderite, quartz, chalcedony, barite and bastniisite-(La,Ce). The other dyke is alvikitic and is composed of calcite, ankerite, pyrite, plagioclase, ilmenorutile, siderite, fluorapatite, barite, chalcedony and at least three REE-minerals (Th-bastnasite, Y-Th-britholite and cheralite). C and O isotopic determinations point out to a mantle origin of the carbonatites. However, they are fractionated and reequilibrated with hydrothermal solutions at different degrees. Interstitial calcites from the nepheline syenites have isotopic values similar to those of the mantle and are even more primitive than !hose of the carbonatites of El Picacho. The values of δ3C PDB range from -4.029 to -6.133 ‰, while δ180 SMOW vary from 13.020 to 19.530 ‰. We postulate a process of liquid immiscibility with the separation of a carbonate melt from an initially C02 -rich silicate magma. The soevites and the alvikites originated from the first magma, while the nephelinitic rocks represen! the product from the silicate magma. In the same way, a separation of a phosphorous enriched melt from the carbonatitic magma could be responsi ble for the origin of the apatite rich dykes in the centre of El Picacho.

The area lies between 16º35’-16º45’ N Lat. and 93º00’-93º10’ W Long; and consists of 323.3 km2 of moderately rugged terrain set between 380-1,240 mamsl, formed by these units: Sierra Madre Limestone (Middle Cretaceous, light olive gray, thick bedded biomicrite, 800 m thick occurring in the southwest), Angostura Formation (Late Cretaceous, light gray, medium bedded, sparsely fossiliferous biomicrite to biomicrudite and calcarenite, 300-350 m. thick, cropping out both in the southwest and northeast), Soyaló Formation (Paleocene, gray, shaly, thin bedded, arkosic-phyllarenitic, clayey siltstone, 150 m. thick; it is exposed in the outer margins of the lowlands that surround La Mesa de Copoya, which is the outstanding physiographic feature of the area), El Bosque Formation (Early Eocene, grayish red, medium to thick bedded, phyllarenitic sandstone and breccioid conglomerate, 100 m. thick, occurring out in the southern lowlands.), and San Juan Formation (Middle Eocene, marly -quartz phyllarenitic- biomicrite to biosparrudite with biostromic zones, intercalated with biomicrudite-supported, coarse grained sand-granule-to-gravel, quartz phyllarenitic sandstone to conglomerate; the unit becomes less limy upward (upper third), is 450 – 500 m. thick, makes up both La Mesa and the slope around it, and bears the name-sake selachifauna. Quaternary deposits complete the sequence. The Pre-Quaternary units are broadly folded into a NW-SE trending syncline, disrupted by faults. The area largely records Cretaceous-Paleogene marine sedimentation -punctuated by Early Eocene continental deposition- that ceased sometime during the Tertiary, probably by Late Miocene time, because of a change in the tectonic regime that led to folding, fracturing/faulting and regional uplift. Subsequent erosion nearly removed the Tertiary units, the remainder is preserved in the syncline nucleus.

In this work we describe for the first time the mineralogy and very briefly the possible origin of a banded Fe-Mn deposit associated with a Cretaceous volcanosedimentary sequence of the southern Guerrero terrane, near the sulfide massive volcanogenic deposit of La Minita. The deposit is confined within a felsic tuff unit; about 10 meters thick where sampled for chemical analysis. Using XRF, EDS and XRD techniques, we found besides todorokite, cryptomelane, quartz, romanechite (psilomelane), birnessite, illite-muscovite, cristobalite, chlorite, barite, halloysite, woodruffite, nacrite or kaolinite, and possibly hollandite-ferrian, as well as an amorphous material and two unknown manganese phases. Although the manganese and iron minerals that characterize the La Hueca site were apparently somewhat altered by diagenetic or weathering processes, the combined evidence of finely banded to laminated structure, the abundant presence of Fe, Mn, Ba, Zn, and Si, and a mineralogy of probable primary originm here represented by hematite, jasper and manganese minerals enriched in zinc and barium, strongly support the interpretation that La Hueca is essentially of sedimentary origin, precipitated from hydrothermally metal-enriched solutions. The deposit is here interpreted as a distal exhalite derived from the same hydrothermal solutions that formed the barite-sulfides deposits of La Minita.

The objectives of this study are: (1) to determine the main physical, chemical, petrographic and mineralogical properties of the soils and the indurated layers of the study area; (2) to establish the horizon and profile development; and (3) to state hypotheses about the genesis of  both. The main results are: (a) the three studied profiles show poor evolution, because of climate unencouraging to soil formation and felsic nature of the parent material; (b) the roundness and sphericity of the mineral particles are due to their aerial deposition; (c) the resistance to simple compression demonstrates that the studied indurated layers present specific diagnostic characteristics, that do not correspond to reports of other authors; (d) in profile 2, the total oxides losses and gains are consistent with the climate conditions on the zone; (e) petrographic study shows that indurated layers are tuffs moderately consolidated, with secondary pedogenetic processes. The most important conclusions are: (a) the field morphologic classification indicates a limited evolution of the soils; (b) the white fragipans resistance to simple compression presents low values in two cases and intermediate in the third one; (c) the total oxides analysis, in such layers, identifies them as pyroclastic materials of felsic nature that present a low weathering grade; (d) the microanalysis of cement shows that this material is made up of silica gel that cementing  clays.

Preliminary results of Raman spectrometry applied in the search of high pressure silica polymorphs in the impact breccia of the Chicxulub crater, Mexico, are presented. The study was carried directly on a polished slab. The lines corresponding to coesite (one major near to 520 cm-1 and others of minor intensity, about 119, 178, 272 cm-1) and to stishovite (231, 587, 752 cm-1) were observed in some of the spectra. These results allow us to infer the presence of high pressure SiO2 polymorphs in the Chicxulub impactites. This evidence lends support to the origin by impact of the structure.

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