Fernando Velasco-Tapia

Evaluation of the odd-even effect in limits of detection for electron microprobe analysis of natural minerals.

Limit of detection (LOD), being a fundamental quality parameter for analytical techniques, has been recently investigated and a systematic behavior has been observed for most odd-even element pairs for many techniques. However, to the best of our knowledge very few LOD data are available in published literature for electron microprobe analysis; these consist of three papers, two being on rare-earth elements and the third covering a large number of elements of atomic number between 21 and 92. These data confirm the systematic behavior of LODs for many odd-even pairs. To initiate to full this gap, we determined LODs for several major rock-forming chemical elements from Na to Fe with atomic numbers between 11 and 26, during the microprobe analysis of common minerals (olivine, plagioclase, pyroxene, amphibole, quartz, and opaques) in volcanic rocks. The odd-even effect of nuclear stability seems to be present in LOD data for most odd-even pairs investigated. Nevertheless, the experimental strategy concerning the reference materials, calibration procedure, and blank measurements, should be substantially modified to better evaluate the systematic behavior of LOD values in microprobe analysis.

Maturing Arc Signatures Monitored by Trace Element and Hf Isotope Systematics in the Early Cretaceous Zacatecas Volcanic Field, Mexico

Mesozoic growth of continental crust along the southwestern margin of North America and its southern extension in Mexico has been partly explained by the accretion of terranes. These terranes have been considered to be fragments of exotic, intraoceanic island arcs that approached mainland Mexico after the Early Cretaceous. Trace elements and Lu-Hf isotopic systematics for primitive arc successions of the Zacatecas Volcanic Field indicate a close relationship with parts of the northern Guerrero superterrane. Major and trace element systematics of lava flows and dioritic rocks from laccoliths suggest a cogenetic origin of the Zacatecas Formation and Las Pilas Complex rocks, here combined in the Zacatecas Group. This group represents a single arc succession that evolves from a primitive to mature arc. Initial 176Hf/177Hf (age corrected to 130 Ma) ranges from 0.28296 to 0.28307, corresponding to εHf(t) = +9.3 to +13.4, indicating a source related to a depleted mantle wedge with a superimposed subducted sediment contribution. Based on combined field and geochemical evidence, we propose an arc model and suggest a spatial extension of paleoarc spreading north–south from Baja California beyond the present-day Trans-Mexican Volcanic Belt in the Early Cretaceous.

Mantle Xenoliths and Their Host Magmas in the Eastern Alkaline Province, Northeast Mexico

Alkaline magmas along the periphery of Sierra de San Carlos-Cruillas and Sierra de Tamaulipas (Eastern Alkaline Province, northeast Mexico) contain ultramafic xenoliths. Most of the host rocks are mafic (e.g., basalts, basanites, trachybasalts, phonotephrites), and have geochemical characteristics of nearly primary magmas generated in subcontinental zones (SiO2 = 42.6-48.4%, MgO = 6.3-11.2%, Mg# = 59.2-69.7). MORB-normalized patterns are similar to those displayed by extensionrelated mafic magmas. This hypothesis is supported by an enrichment in light REE ([La/Yb]N = 10.8-27.1; chondrite-normalized ratios) and the behavior of relatively immobile trace elements. Using a partial melting model for REE, the source for the mafic magmas apparently was in the garnet P-T stability field. On the other hand, peralkaline phonolites and tephriphonolites (SiO2 = 52.9-54.1%, MgO = 1.5-1.8%, Mg# = 33.1-39.8) also contain mantle xenoliths. These evolved magmas show MORB-normalized multi-element diagrams characterized by enrichment in highly incompatible elements (e.g., Rb, Sr, Ba) and positive anomalies for HFSE. Ultramafic xenoliths are predominantly protogranular spinel-lherzolites as well as harzburgites and rare dunites, websterites, and wherlites. A few samples display transitional and porphyroclastic textures, indicating that the xenoliths were carried to the surface from stable mantle zones with little or no deformation. The mineralogy (olivine + orthopyroxene + clinopyroxene ± spinel) of xenoliths contained in mafic magmas is typical of unaltered mantle nodules. However, small garnet crystals exhibiting disequilibrium textures occur in one sample. The core and rim compositions in olivine range from Fo90 to Fo94, whereas orthopyroxenes are characterized by En88-93 and clinopyroxenes by En44-51Fs3-10Wo43-50. Chromiferous spinels have Mg/(Mg + Fe+2) = 0.76-0.83 and Cr/(Cr + Al) = 0.10-0.25. In comparison to the nodules contained in mafic magmas, mantle xenoliths included in phonolitic liquids show some differences: (1) smaller size (diameter < 1 cm); (2) clinopyroxene is less common (< 5% volume) and spinel is absent; and (3) partial alteration to micaceous minerals. Equilibrium temperatures for mantle xenoliths contained in mafic magmas range from 850 to 1170°C, as calculated applying different geothermometers, whereas mantle nodules sampled by peralkaline liquids show only lower equilibrium temperatures (<900°C). A first approximation to equilibrium pressure, based on mineralogical constrains, indicates values from 10 to 25 kbar. Summing up, we consider that the mafic magmas were generated within the spinel-garnet domain of the lithospheric mantle, having little or no interaction with their wall rocks. The ultramafic xenoliths were probably sampled during magma ascent above its source. In contrast, the geochemistry of phonolitic rocks and their altered mantle xenoliths reveal that such magmas cannot be explained by direct mantle melting. The petrogenesis of these peralkaline magmas can be described by a two-step model: (1) partial melting of metasomatized mantle which produced an alkaline magma enriched in LILE and HFSE; and (2) subsequent fractional crystallization of this magma at upper mantle pressures, producing phonolites. While ascending, the peralkaline magmas sampled altered nodules in a shallow level of the subcontinental mantle. The rise and eruption of the mantle-bearing mafic and evolved magmas were facilitated by the post-Laramide extensional regime established during Tertiary time in northeast Mexico.

Multivariate Analysis, Mass Balance Techniques, and Statistical Tests as Tools in Igneous Petrology: Application to the Sierra de las Cruces Volcanic Range (Mexican Volcanic Belt)

Magmatic processes have usually been identified and evaluated using qualitative or semiquantitative geochemical or isotopic tools based on a restricted number of variables. However, a more complete and quantitative view could be reached applying multivariate analysis, mass balance techniques, and statistical tests. As an example, in this work a statistical and quantitative scheme is applied to analyze the geochemical features for the Sierra de las Cruces (SC) volcanic range (Mexican Volcanic Belt). In this locality, the volcanic activity (3.7 to 0.5 Ma) was dominantly dacitic, but the presence of spheroidal andesitic enclaves and/or diverse disequilibrium features in majority of lavas confirms the operation of magma mixing/mingling. New discriminant-function-based multidimensional diagrams were used to discriminate tectonic setting. Statistical tests of discordancy and significance were applied to evaluate the influence of the subducting Cocos plate, which seems to be rather negligible for the SC magmas in relation to several major and trace elements. A cluster analysis following Wards linkage rule was carried out to classify the SC volcanic rocks geochemical groups. Finally, two mass-balance schemes were applied for the quantitative evaluation of the proportion of the end-member components (dacitic and andesitic magmas) in the comingled lavas (binary mixtures).

Numerical Simulation of a Kinetic Cell for in-situ Combustion. A Parametric Study and History Matching Aspects

Abstract The in-situ combustion method is an enhanced oil recovery technique based on the injection of air in petroleum reservoirs with the aim to burn a portion of hydrocarbons. This reduces the oil viscosity improving substantially the oil mobility. Simultaneously other phenomena take place as: distillation, segregation, oil upgrading, among others. In this work, a mathematical model to simulate oil combustion for kinetic cell experiments is presented. The model includes four-phases, nine components and four chemical reactions: coke formation, heavy oil fraction combustion, light oil fraction combustion and coke combustion. This formulation is commonly used to simulate in-situ combustion projects at combustion tubes- and petroleum reservoir-scales. The mass and energy balances were formulated leading to one set of highly coupled ordinary differential equations, which was numerically solved. The predictive model capabilities were tested by comparison with lab data, and it was found that CO and CO2 productions, oxygen uptake and cell temperature evolution agree well with experimental results. At one preliminary stage, the parameters fitting experimental results were inferred by individual manipulation until the best results were found. These parameters were perturbed in order to identify those parameters dominating the global dynamic of process. We found that energy activations and the mass density of oil components are the dominant parameters. We suggest that history matching processes must be focused over these parameters, and for this end, the implementation of advanced computational routines to solve multivariable inverse problems is recommended. In this work, we developed two automatic history matching techniques: one process based on Newton’s method and the second one based on evolutionary algorithms. The Newton’s method showed problems to find the minimum error, meanwhile the evolutionary algorithm was able to optimize the dominant parameters, but at the expense of slow convergence.