As part of the effort to establish an archeointensity variation curve for Mesoamerica, 13 archeologically well-identified pottery samples belonging to the Ocozocoautla site (Chiapas) were studied. Analyzed samples consist of ‘ofrenda type’ pottery fragments found in several caves. Three archeological intervals are involved: 450-100 B.C., 200–550 A.D. and 550–900 A.D. The Thellier method in its modified form was applied to small fragments previously embedded in salt pellets. Raw intensity values were further corrected for cooling rate effects. The common time-consuming TRM anisotropy correction protocol was substituted by an alternative approach during the paleointensity experiments. Forty-two specimens, belonging to six samples, yielded high-quality Thellier determinations. The NRM fraction f used for paleointensity determination ranges between 0.42 to 0.99, and the quality factor q (Coe et al., 1978) varies from 4 to 59, being normally greater than 5. These results correspond to data of good quality. The mean archeointensity values per pottery fragments range from 14.6±1.5 to 59.5±13.8 μT, while the corresponding virtual axial dipole moments range from 2.5±0.3 to 10.0±2.4 × 10²² A m². These new data, although not numerous, are of high quality and definitively contribute to the Mesoamerican, still insipient, archeointensity database.

We report the first finding of berthierine and chamosite in Mexico. They occur in the iron-ore deposit of Peña Colorada, Colima. Their genetic characteristics show two different mineralization events associated mainly to the magnetite ore. Berthierine is an Fe-rich and Mg-low 1:1 layer phyllosilicate of hydrothermal sedimentary origin. Its structure is 7 Å,d_hkl[10 0] basal spacing and low degree structural ordering. The phyllosilicate has been identified by a lack of 14 Å basal reflection on X-ray diffraction (XRD) patterns. These data were supported by High Resolution Transmision Electron Microscopy (HRTEM) images that show thick packets of berthierine in well defined parallel plates. From the analysis of Fast Fourier Transform (FFT), we found around [1 0 0] reflections of berhierine 7.12 Å and corresponding angles of hexagonal crystalline structure. Berthierine has a microcrystalline structure, dark green color, and high refraction index (1.64 to 1.65). Birefringence is low, near 0.007 to null and it is associated to nanoparticles (<15 nm) and microparticles of magnetite (<25 μm), fine grain siderite, and organic matter. Its texture is intergranular-interstratified with colloform banding. The chamosite Mg-rich is of hydrothermal epigenetic origin affected by low-degree metamorphism. It is an Fe-rich 2:1 layer silicate, with basal space of 14 Å,d_hkl[0 0 1]. The chamosite occurs as lamellar in sizes ranging from 50 to 150 μm. It has intense green color and refraction index from 1.64 to 1.65. The birefringence is near 0.008, with biaxial (-) orientation and a 2V small. It is associated mainly to sericite, epidote, clay, feldspar, and magnetite. Chamosite is emplaced in open spaces filling and linings. Mössbauer spectra of berthierine and chamosite are similar. They show the typical spectra of paramagnetic substances, with two well defined unfoldings corresponding to the oxidation state of Fe⁺²and Fe⁺³. Chemical composition of both minerals was obtained by an electron probe X-ray micro-analyzer (EPMA). The radio Fe+Mg+Mn vs Si and Al show similar chemical compositions and different XRD patterns in the crystalline structure provoked by the environmental conditions of emplacement. A hydrothermal environment was predominant, occurring before, during, and after the magnetite mineralization. The identification of magnetite nanoparticles supports the hypothesis of a marine environment, specifically exhalative sedimentary (SEDEX) for the berthierine.

Summary. We report the results of a detailed rock-magnetic study of basaltic andesites for one of the best documented cases of the first occurrence of a new vo

Subduction of the Farallon plate beneath North America resulted in formation of the Rivera and Cocos oceanic plates, the extensive magmatic arcs of the Sierra Madre Occidental (SMO), and the Trans-Mexican Volcanic Belt (TMVB). Southern Mexico consists of crustal blocks separated by a regional extensional structural system; the latter, called the Guadalajara triple junction, is defined by the Tepic-Zacoalco (TZR), Colima (CR), and Chapala (CHR) rifts. TZR and CHR separate the SMO from the Jalisco and Michoacan blocks, whereas CR is the boundary between the Jalisco and Michoacan blocks. In this study, we carried out combined radiometric and paleomagnetic analyses in the Michoacan block. Radiometric dates of 31.60 to 8.39 Ma confirm both the southern extension of the Sierra Madre Occidental and the early mafic TMVB succession into the Michoacan block. The Oligocene age agrees well with the radiometric dating reported for the southern SMO and the Tertiary volcanic fields of the Sierra Madre del Sur. Paleomagn...