Técnicas analíticas desarrolladas en Laboratorio de Estudios Isotópicos (LEI) para el reconocimiento de procesos involucrados en la generación y evolución de la corteza terrestre

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Statistics for Fission-Track Thermochronology

This chapter introduces statistical tools to extract geologically meaningful information from fission-track (FT) data using both the external detector and LA-ICP-MS methods. The spontaneous fission of 238U is a Poisson process resulting in large single-grain age uncertainties. To overcome this imprecision, it is nearly always necessary to analyse multiple grains per sample. The degree to which the analytical uncertainties can explain the observed scatter of the single-grain data can be visually assessed on a radial plot and objectively quantified by a chi-square test. For sufficiently low values of the chi-square statistic (or sufficiently high p values), the pooled age of all the grains gives a suitable description of the underlying ‘true’ age population. Samples may fail the chi-square test for several reasons. A first possibility is that the true age population does not consist of a single discrete age component, but is characterised by a continuous range of ages. In this case, a ‘random effects’ model can constrain the true age distribution using two parameters: the ‘central age’ and the ‘(over)dispersion’. A second reason why FT data sets might fail the chi-square test is if they are underlain by multimodal age distributions. Such distributions may consist of discrete age components, continuous age distributions, or a combination of the two. Formalised statistical tests such as chi-square can be useful in preventing overfitting of relatively small data sets. However, they should be used with caution when applied to large data sets (including length measurements) which generate sufficient statistical ‘power’ to reject any simple yet geologically plausible hypothesis

Datación mediante trazas de fisión y análisis multielemental con LA-ICP-MS del fluorapatito de Cerro de Mercado (Durango, México)

Este artículo describe la datación mediante trazas de fisión y el análisis multielemental del fluorapatito de Cerro de Mercado (Durango, México) por medio de ablación láser asociada con un espectrómetro de masas con fuente de plasma de acoplamiento inductivo (LA-ICP-MS). Para este apatito se obtuvo una edad promedio de 31.2 ± 0.2 Ma (1σ) utilizando 104 edades de trazas de fisión determinadas en 12 fragmentos diferentes, lo que concuerda perfectamente con el valor aceptado de 31.4 ± 0.5 Ma (1σ). Se realizó una revisión de la literatura sobre la composición química y edad del apatito de Durango, y los resultados obtenidos a lo largo de este estudio (elementos traza, las Tierras Raras y edades de trazas de fisión) son coincidentes con algunos datos publicados. En este trabajo se presenta también una ecuación nueva para calcular la incertidumbre de las edades de trazas de fisión tomando en cuenta todos los errores analíticos. Los resultados obtenidos validan el procedimiento utilizado y permiten que esta metodología pueda ser usada rutinariamente en otros apatitos

New fission-track results from the northern Chiapas Massif area, SE Mexico: trying to reconstruct its complex thermo-tectonic history

The Chiapas Massif Complex, which represents the crystalline basement of the southern Maya block within the North American plate, records numerous thermo-tectonic and magmatic events that occurred in southern Mexico at least since the late Mesoproterozoic. The present study was performed across the northern Chiapas Massif region to reconstruct its complex thermo-tectonic history from Mesozoic to present times. Basement samples and sandstones of the San Ricardo Formation derived from the Chiapas Massif Complex source area were analyzed by in situ apatite fission-track dating. The new fission-track results obtained in this study, together with previously published data, indicate that the Chiapas Massif Complex, or rather the whole Maya terrane, have experienced a complex long-term geodynamic evolution with at least five post-Permian tectonic and magmatic events: (1) a Late Triassic cooling event, likely related to the initial breakup of Pangea; (2) Early Jurassic volcanism that can be linked to the Nazas volcanic arc; (3) a Middle Jurassic tectonic event that was triggered by continental rifting at the beginning of the opening of the Gulf of Mexico; (4) a Late Cretaceous to Paleocene orogeny that may actually represent the southernmost continuation of the Laramide sensu lato which affected central and northern Mexico; and (5) the middle–late Miocene Chiapanecan event that is tectonically controlled by the interaction of the North American, Caribbean, and Cocos plates. This interpretation could be useful towards a better understanding of the geological history of southern North America. Some recommendations on sampling and analytical strategies are also given for consideration in further thermochronological studies in Chiapas.</jats:p

Technical note: on LA–ICP-MS U–Pb dating of unetched and etched apatites

Abstract. The same unetched and chemically etched apatites from five rock samples were dated with U–Pb using laser ablation inductively coupled plasma mass spectrometry. The objective of this study is to demonstrate whether or not the etching, needed for the apatite fission track analysis, impact on the obtaining of apatite U–Pb ages. The results of this experiment indicate that the etching has no effect on the determination of apatite U–Pb ages by the laser ablation inductively coupled plasma mass spectrometry technique. Thus, laser ablation inductively coupled plasma mass spectrometry may be used safely for simultaneous apatite fission track in-situ and U–Pb double dating. </jats:p

Technical note: LA–ICP-MS U–Pb dating of unetched and etched apatites

Abstract. The same unetched and chemically etched apatite crystals from five rock samples were dated by the U–Pb method via laser ablation inductively coupled plasma mass spectrometry (LA–ICP-MS). The objective of this study is to test whether chemical etching required for apatite fission track analysis impacts the precision and accuracy of apatite U–Pb geochronology. The results of this experiment suggest that etching has insignificant effects on the accuracy of apatite U–Pb ages obtained by LA–ICP-MS. Therefore, LA–ICP-MS is reliable for U–Pb analysis as part of apatite fission track and U–Pb double dating. </jats:p