Curie Temperatures and Emplacement Conditions of Pyroclastic Deposits From Popocatépetl Volcano, Mexico

Most pyroclastic deposits of Popocatépetl volcano were emplaced at high temperatures and have similar mafic to more evolved compositions, suggesting a long-lived, interconnected magma environment. We performed a magnetic and microscopic study on different eruptive sequences <14 ky in age and found that temperature and field dependence of magnetic susceptibility are suited to separate eruption phases. We observed homogeneous titanomagnetite with Curie temperatures (T$_{C}$) of 50–200°C and 200–400°C, together with different amounts of oxy-exsolved titanomagnetite with T$_{C}$ ∼ 570°C. Some block-and-ash flow deposits show remarkably irreversible T$_{C}$ in heating and cooling branches with a positive ΔT$_{C}$ (T$_{C}$ $_{heating}$ –T$_{C}$ $_{cooling}$) of up to 130°C in the center. The central part of this sequence is characterized by decreasing magnetic susceptibility and low field dependence of magnetic susceptibility (<10%), which is atypical for ulvöspinel-rich titanomagnetite. The nonreversibility of heating and cooling runs measured with rates of around 10 K/min is probably related to vacancy-enhanced nanoscale chemical clustering, which seems to occur preferentially during rapid quenching, possibly combined with subtle maghemitization. In contrast, pumice layers have the highest field dependence (∼20%) and contain Ti-rich and intermediate titanomagnetite with T$_{C}$ < 100 and ∼300°C, which are in line with mafic and more evolved magma composition. In intermediate phases, irreversibility of T$_{C}$ is more common but with a relatively low ΔT$_{C}$ of ±20°C. We suggest that magneto-mineralogy in pyroclastic density currents is complex but offers a complementary tool to the paleomagnetic directional analysis for emplacement temperature and contribute information on the volcanic material history and their emplacement conditions

Curie Temperatures and Emplacement Conditions of Pyroclastic Deposits From Popocatépetl Volcano, Mexico

Most pyroclastic deposits of Popocatépetl volcano were emplaced at high temperatures and have similar mafic to more evolved compositions, suggesting a long‐lived, interconnected magma environment. We performed a magnetic and microscopic study on different eruptive sequences <14 ky in age and found that temperature and field dependence of magnetic susceptibility are suited to separate eruption phases. We observed homogeneous titanomagnetite with Curie temperatures (TC) of 50–200°C and 200–400°C, together with different amounts of oxy‐exsolved titanomagnetite with TC ∼ 570°C. Some block‐and‐ash flow deposits show remarkably irreversible TC in heating and cooling branches with a positive ΔTC (TC heating–TC cooling) of up to 130°C in the center. The central part of this sequence is characterized by decreasing magnetic susceptibility and low field dependence of magnetic susceptibility (<10%), which is atypical for ulvöspinel‐rich titanomagnetite. The nonreversibility of heating and cooling runs measured with rates of around 10 K/min is probably related to vacancy‐enhanced nanoscale chemical clustering, which seems to occur preferentially during rapid quenching, possibly combined with subtle maghemitization. In contrast, pumice layers have the highest field dependence (∼20%) and contain Ti‐rich and intermediate titanomagnetite with TC < 100 and ∼300°C, which are in line with mafic and more evolved magma composition. In intermediate phases, irreversibility of TC is more common but with a relatively low ΔTC of ±20°C. We suggest that magneto‐mineralogy in pyroclastic density currents is complex but offers a complementary tool to the paleomagnetic directional analysis for emplacement temperature and contribute information on the volcanic material history and their emplacement conditions.Plain Language Summary: Explosive eruptions of volcanoes are a dangerous threat to human settlements. In this study, we investigated pyroclastic material from the last 14 ky of Popocatépetl volcano using magnetic mineral assemblages, hysteresis properties, and temperature‐ and field‐dependent magnetic susceptibility. The data are suited to separate different eruption phases and provide information on the volcanic material history and emplacement conditions. Magnetic susceptibility analyses are suggested to be a complementary tool to the paleomagnetic directional analysis for the determination of emplacement temperatures.Key Points: Several Curie temperatures were observed in pyroclastic deposits. Temperature and field dependence of magnetic susceptibility are suited to separate eruption phases. Irreversible Curie temperatures in heating and cooling curves observed in block‐and‐ash flows may suggest rapid quenching.DGAPA‐UNAMhttps://doi.org/10.17632/9g2tszftvr.2https://earthref.org/MagIC/19591/206809c5-acfc-41e3-b4cb-411630a7025

Estudio de minerales de fierro y titanio y magnetismo de rocas en el distrito ferrífero El Encino, Jalisco, México

Sc presentan resultados de un estudio mineralógico-paleomagnético de la mayoría de las unidades volcano-sedimentarias y de mineralización de hierro en el Distrito Minero de El Encino, Jalisco. Los principales objetivos del estudio son: (a) investigar las relaciones entre parámetros paleomagnéticos y observaciones mineragráficas (minerales opacos) y (b) analizar sus implicaciones sobre origen, tipo y grado de oxidación, tamaños de granos magnéticos, razón de enfriamiento, metamorfismo y alteración hidrotermal. Los resultados para las fases del sistema Ti02 -FeO-Fc203 analizados por el microscopio mineragráfico, muestran que los óxidos magnéticos son abundantes, de grano grande y estado magnético de dominio múltiple, que han sufrido enfriamiento lento y oxidación deutérica de bajo grado. Estos óxidos incluyen predominantemente titano-magnetitas, con menores cantidades de ilmenita y hematita. La pequeña variación en reflectividad dentro de los granos de titano-magnetita y el color, sugieren la ocurrencia de titano-magnetita pobre en titanio así como un grado de oxidaci6n constante, lo cual es consistente con análisis químicos de óxidos mayores (menores cantidades de Ti02 respecto a FeO y Fe203). La polaridad para la mayoría de las unidades estudiadas es normal, lo que concuerda con la escala de polaridad para el Crétacico, en el Cron Normal Aptiano-Albiano. La dispcrsi6n angular de las direcciones de magnetización para la mineralización de hierro es mayor que la observada en las otras litologías. Esto se debe probablemente a procesos de oxidación, metamorfismo y alteración hidrotermal ocurridos postmincralización. Las propiedades magnéticas de la mineralización esuin principalmente controladas por titano-magnetita de dominio múltiple. La susceptibilidad en los cuerpos intrusivos (sitios 8 y 9) es relativamente baja, en el orden de 1.3 x 10-3 Sl, en el rango reportado para granitos de tipo ilmenita. Esto también puede ser resultado de los procesos mencionados arriba. Los resultados paleomagnéticos, que comprenden espectros de desmagnctizaci6n térmica, diagramas vectoriales y de adquisición de magnctizaci6n remanente isotermal, muestran una correspondencia directa con las observaciones mineragráfica indicando un claro predominio de minerales de la serie de titano-magnetitas, con contenidos variables de magnetita y ulvoespincla. Asimismo, se documentan diversos registros de componentes secundarias de magnetización asociados a procesos entre los que predominan eventos de metamorfismo y alteración hidrotermal, identificados en los estudios microscópicos con luz transmitida y luz ref1ejada. Además existe la posible inf1uencia de magnetizaciones remanentes viscosa e isotermal

First archeointensity determinations on Maya incense burners from Palenque temples, Mexico: New data to constrain the Mesoamerica secular variation curve

International audienceWe present archeointensity data carried out on pieces of incense burners from the ancient Maya city of Palenque, Chiapas, Mexico, covering much of the Mesoamerican Classic period, from A.D. 400 to A.D. 850. We worked on pieces from 24 incense burners encompassing the five Classic ceramic phases of Palenque: Motiepa (A.D. 400-500), Cascadas (A.D. 500-600), Otulum (A.D. 600-700), Murcielagos (A.D. 700-770), and Balunté (A.D. 770-850). All the samples come from highly elaborate, flanged pedestal of incense burners that are undoubtedly assigned to a ceramic phase by means of their iconographic, morphological and stylistic analyses. Archeointensity measurements were performed with the Thellier-Thellier's method on pre-selected samples by means of their magnetic properties. We obtained archeointensities of very good technical quality from 19 of 24 pieces, allowing the determination of a precise mean value for each ceramic phase, between View the MathML source29.1±0.9μT and View the MathML source32.5±1.2μT. The firing temperatures of ceramics were estimated with Mössbauer spectroscopy between 700 °C and 1000 °C. These values ensure that a full thermo-remanent magnetization was acquired during the original heating. Our results suggest a relative stability of the field intensity during more than 400 years in this area. The abundance of archeological material in Mesoamerica contrasts with the small amount of archeomagnetic data available that are, in addition, of uneven quality. Thus, it is not possible to establish a trend of intensity variations in Mesoamerica, even using the global databases and secular variation predictions from global models. In this context, our high technical quality data represent a strong constraint for the Mesoamerican secular variation curve during the first millennium AD. The corresponding Virtual Axial Dipole Moments (VADM) are substantially smaller than the ones predicted by the last global geomagnetic models CALS3k.4, suggesting the need for additional data to develop a regional model and a reference curve for Mesoamerica

Archaeological calibration of remagnetized volcanic rocks from pottery firing kilns in Cuentepec, Morelos, Mexico.

Investigaciones etnoarqueológicas en Cuentepec incluyen experimentos durante la producción de cerámica, de donde es posible extraer conocimientos sociales a partir de la aplicación de técnicas arqueométricas. En este caso, el experimento trata sobre la confiabilidad de técnicas de fechamiento en arqueología. En Cuentepec, se usan pequeños hornos a cielo abierto para la fabricación de cerámica (comales de barro). Se tomaron muestras de roca volcánica que conformaban los hornos para verificar la confiabilidad de la dirección magnética registrada por las mismas y compararla con datos del Observatorio Geomagnético de Teoloyucan localizado cerca a la ciudad de México. Con el objeto de medir sus propiedades magnéticas se perforaron/obtuvieron en el laboratorio 47 núcleos pertenecientes a ocho muestras de bloque orientadas. Las curvas continuas de susceptibilidad magnética con altas temperaturas resultaron en muchos casos razonablemente reversibles, con puntos de Curie sugiriendo titanomagnetita de rica a pobre en titanio. Los parámetros de histéresis indican que todas las muestras caen en la región de tamaño de grano pseudo-dominio-simple, indicando probablemente una mezcla de granos multidominio más una cantidad significante de granos de dominio simple. Las curvas de adquisición de magnetización remanente isotermal fueron muy similares para casi todas las muestras. La saturación se alcanzó en campos moderados del orden de 100-120 mT, lo cual indica algunas espinelas como portadores de la remanencia. Concluimos que las muestras obtenidas de la parte interna de los bloques que forman los hornos, las más cercanas al fuego, guardan los registros más confiables del campo geomagnético. Esto significa que el calor producido por el fuego probablemente sólo remagnetizó las partes internas de los bloques

Rock magnetism and microscopy of the Jacupiranga alkaline-carbonatitic complex, southern Brazil

International audienceThis study of the Cajati deposit provides evidence that the ore was neither purely hydrothermal, nor volcanic in origin, as previous workers have proposed. The ores were formed from magnetite-rich magmas, hydrothermally altered and intruded at an indicated crustal depth in excess of 500 m. The mineralogical and textural association between magnetite and magnesioferrite in the carbonatite, and between the titanomagnetite and magnesioferrite-Ti mineralization in the pyroxenite of hedenbergite, seems to be analog mineralizations strongly related to the ionic substitution of Fe2+ by Mg. Relatively high Q ratios (>= 5) for Jacupirangite-pyroxenite may indicate a thermo remanent magnetization (TRM) by the ore during post-metamorphic cooling, however it call also be developed from chemical remanent magnetization (CRM). Vector plots for the pyroxenite samples show reasonably linear and stable magnetic components. The intensity decay Curves show that only two components of magnetizations are likely present. Continuous susceptibility measurements with increasing temperature show that the main magnetic phase seems to be magnetite. Maghemite is probably produced during the cooling process. Susceptibility recorded from low temperature (liquid nitrogen (- 196 degrees C)) to room temperature produces typical Curves. indicating Verwey transition of magnetite. Hysteresis parameters point Out that nearly all values fill in a novel region of the Day plot, parallel to but below magnetite SD + MD mixing curves