A pilot rock magnetic and ore microscopy study of xenolith-bearing young basaltic rocks from the Camargo cinder cone field, Chihuahua, Northern Mexico

Initial results of a rock magnetic, paleomagnetic, microscopic study of the Camargo volcanic field from Chihuahua (northern Mexico) are reported. The field is formed by more than 300 cinder cones and extends over an area of 200,000 km2 Mantle and lower crust xenoliths in the alkaline basaltic rocks of La Olivina, in the northern sector of the field, suggest rapid magma ascent rates. This study concentrates on the rock magnetic properties of the cinder cone field. Remanences are carried in most cases by Ti-poor titanomagnetites and magnetites as a result of the oxy-exsolution of original titanomagnetite during initial flow cooling. For a few units, remanence seems to be carried by (titano)hematites, as shown by unblocking temperature spectra and secondary titano(maghemite). Judging from the ratios of hysteresis parameters, all samples fall in the pseudo-single domain state region. Most units (13) show reverse polarity magnetizations, and only 2 show normal polarity. Radiometric dating gives the field’s age range as between 2 and 1.7 Ma, which, together with the magnetostratigraphic results, suggests a relatively short emplacement time for the volcanic field. The mean preliminary paleomagnetic direction obtained for the Camargo volcanic field is I ¼ 30.68, D ¼ 341.38, k ¼ 19, and a95 ¼ 9.88, which deviates counterclockwise from the expected direction estimated from the North American apparent polar wander path. In turn, this suggests a vertical axis tectonic counterclockwise rotation of approximately 208 relative to stable North America

A Tertiary volcanic rocks from the Patagonian plateau, Argentina

1 p.International audienceThe Patagonian plateau basalts are a sequence of flat-lying Late Cretaceous through Tertiary basaltic and andesitic rocks exposed east of the Andes in dissected mesetas of Chilean and Argentine Patagonia. The plateau basalts cover a total area of 120,000 km2 between 40°S and 52°S. These igneous rocks rest on generally flatlying older Mesozoic silicic volcanic or sedimentary rocks. Of major importance to the paleomagnetic investigation is the lack of significant tectonic disturbance of this portion of Patagonia from Late Cretaceous to present. 12 sites (107 cores) have been sampled in the Tertiary (Paleocene to Miocene) volcanic rocks between 43°S and 46°S in latitude and between 68°W and 70°W in longitude. Rocks are usually very fresh but, as they outcrop generally on top of the mesas, few hundred meters above the Plateau, remagnetization by lightning strikes is a major problem of the paleomagnetic analysis. Paleomagnetic, rock magnetic and paleointensity results will be presented

Reassessment of paleointensity estimated of a single lava flow from Xitle volcano, Mexico, by means of multispecimen domain-state corrected

International audienceDetermining paleomagnetic field intensity (paleointensity: PI) for lavas with high reliability and low measurement uncertainty is still difficult to achieve. Some of the factors on which the PI used methods depends are the non-ideal physical and magnetic characteristics of lava sample, grain size, cooling rate effect and thermal stability. Xitle volcano (SW Mexico City) is a good example to illustrate and discuss this problem because several previous PI studies that were carried out on its evolved flow units, have commonly resulted in different mean values with large dispersions. Indeed, 211 published PI data obtained by use of Thellier and microwave experiments gave a mean of 64.1 μT with a standard deviation of 11.0 μT. After a careful evaluation, we found that only 134 of these data can be considered reliable, as they meet a set of selection criteria designed in this study. These evaluated data gave an average mean of 62.0 ± 9.3 μT. In order to strengthen the PI estimates of Xitle, we conducted a multispecimen domain-state corrected (MSP-DSC) method along one vertical (ca. 4.5 m) and three horizontal (ca. 1.25 m, each) profiles. Top horizontal and vertical profiles have fulfilled a stringent criteria set while central and lower profiles exceeded the alteration check criteria limit and thus are considered unreliable. Accordingly, Xitle PI mean derived from MSP-DSC experiment is calculated at 60.5 ± 4 μT, thus in a good agreement with the mean value estimated from previous filtered data. The result and success rate obtained in this study may be ascribed to cooling rate variations commonly found at the lava profile, and indicate that MSP-DSC outcome is governed by the magnetic properties such as the domain-size behavior and the thermal stability of the magnetic carriers present in the treated specimens, as in the conventional Thellier & microwave-style experiments. From these two averages, a combined mean and standard deviation of 61.9 ± 9 μT is calculated, which technically is considered the most probable intensity estimate at the Xitle eruption time, ca 370 AD

Is the cardioprotector effect of estrogens useful in the menopause? [¿Es útil el efecto cardioprotector de los estrógenos en la menopausia?]

We have sampled a sequence of 45 late Miocene consecutive lava flows in the Tepic area (Nayarit State, Mexico). The age of the volcanic units lies between 8 and 9 million years (Ma) according to available radiometric data. All samples were step-wise demagnetized, partly with alternating field (AF), partly thermally with very similar results. Most of the rocks exhibited well-defined one component remanent magnetisation with high unblocking temperatures (mostly above 525 °C) and high median destructive fields (MDF) (40-50 mT). Rock-magnetic experiments combined with microscopy show that, in most cases, the main magnetic mineral is Ti-poor titanomagnetic associated with exsoluted ilmenite. Continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases nearly reversible curves with Curie points close to that of magnetite and pseudo-single-domain characteristics. Characteristic remanent magnetisations (ChRM) isolated after the first steps of demagnetisation are all normal polarity. According to the dispersion of virtual geomagnetic pole (VGP) directions, paleosecular variation was abnormally lower than the one observed in general during Miocene. Considering our paleomagnetic results together with available radiometric data, it seems that the volcanic units have been emplaced during a very short time span of about 0.08 million years. The mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle Miocene. Thirty-one samples from eight individual flows yielded acceptable paleointensity estimates. The site mean paleointensities range from 27.8 0.9 to 42.0 7.9 ?T. The virtual dipole moments (VDM) range from 5.9 to 9.5 1022 Am2. This corresponds to a mean value of 7.6 1.4 1022 Am2, which is higher than the average VDM value for late Miocene. Altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations. These results support the theoretical suggestion about an inverse relationship between secular variation and local field strength as result of electromagnetic coupling between the solid inner core and liquid outer core, with the inner core tending to stabilise core convection, and hence the field, when intensity is high. Some fluctuation of absolute intensity was detected within the same directional group (DG) indicating that the intensity of the geomagnetic field varies faster than its direction. " 2002 Elsevier Science B.V. All rights reserved.",,,,,,"10.1016/S0031-9201(02)00096-1",,,"http://hdl.handle.net/20.500.12104/43473","http://www.scopus.com/inward/record.url?eid=2-s2.0-0037191693&partnerID=40&md5=4e54fabefcf2b5ee4ce3f29d7730ec7c",,,,,,"01-feb",,"Physics of the Earth and Planetary Interiors",,"7

Combined paleomagnetic and petromagnetic study of the Upper Cretaceous volcanic sequence in Western Mexico: Implications for tectonics/magnetostratigraphy of the Jalisco block

Results of detailed paleomagnetic studies of the Upper Cretaceous Autlan volcanic sequence from the Sierra Cacoma area (Jalisco block, western Mexico) are reported. The 67.4 ± 1.2 Ma whole-rock K-Ar date for these lavas and the magnetic polarity stratigraphy indicate that flows were emplaced after the Cretaceous Normal Superchron and just prior to the Cretaceous-Tertiary boundary. Fifteen sites (∼ 150 oriented samples) were collected along two vertical profiles. Rock-magnetic experiments permit identification of magnetic carriers and assessment of the paleomagnetic stability. Continuous susceptibility measurements versus temperature in most cases yield reasonably reversible curves with Curie points close to magnetite. Judging from the ratios of hysteresis parameters, all samples fall in the pseudo-single domain grain-size region, probably indicating a mixture of multi-domain and a significant amount of single-domain grains. Reliable paleomagnetic directions were obtained for 14 sites, corresponding to 2 normal and 12 reverse polarity sites. Lowermost flows in the sequence yield reverse polarity magnetization, and probably formed during Chron 31r of the reference geomagnetic polarity time scale. The remaining flows, including those belonging to radiometrically dated sites, correspond to Chron 30n. These tentative magnetic correlations suggest that the entire volcanic sequence was emplaced during a time span of about 2 Ma. The mean paleodirection obtained from 14 sites is Inc = 44.2°, Dec = 320.6°, k = 45, α 95 = 6.0°. The mean inclination is in reasonably good agreement with the expected value for 65 and 70 Ma, as derived from reference poles for the North American craton. Declination is significantly different from expected (D = 345.8° for 65 Ma and D = 346.2° for 70 Ma), which suggests a counterclockwise tectonic rotation of about 25° for this region within the Jalisco block

An attempt to determine the microwave paleointensity on historic Paricutín volcano lava flows, Central Mexico

We report a preliminary rock-magnetic and microwave paleointensity study of historic lava flows between 1943 and 1948 of Paricutín volcano. These lava flows show well-preserved, well-exposed, fresh and extensive outcrops. Most samples are characterized by simple uni-vectorial plots. The isothermal remanent magnetization curves show saturation at low to moderate fields, suggesting titanomagnetite series. From hysteresis experiments magnetic carriers are likely iron-rich titanomagnetites with single-domain or pseudo-single-domain behavior. Microwave paleointensity technique was applied to three selected samples using Kono and Ueno's (1977) method; i.e., the direction of applied laboratory field was perpendicular to the direction of remanent magnetization. The samples yielded paleointensity values of 11.38, 26.37 and 51.6 micro Tesla, which are significantly different from expected values. The observed scatter is likely to be caused by the small fraction of natural remanent magnetization used for paleointensity determination, or from not using a cooling rate correction for natural samples

An attempt to determine the microwave paleointensity on historic ParicutZapotitlánn volcano lava flows, Central Mexico

We report a preliminary rock-magnetic and microwave paleointensity study of historic lava flows between 1943 and 1948 of ParicutZapotitlánn volcano. These lava flows show well-preserved, well-exposed, fresh and extensive outcrops. Most samples are characterized by simple uni-vectorial plots. The isothermal remanent magnetization curves show saturation at low to moderate fields, suggesting titanomagnetite series. From hysteresis experiments magnetic carriers are likely iron-rich titanomagnetites with single-domain or pseudo-single-domain behavior. Microwave paleointensity technique was applied to three selected samples using Kono and Ueno's (1977) method; i.e., the direction of applied laboratory field was perpendicular to the direction of remanent magnetization. The samples yielded paleointensity values of 11.38, 26.37 and 51.6 micro Tesla, which are significantly different from expected values. The observed scatter is likely to be caused by the small fraction of natural remanent magnetization used for paleointensity determination, or from not using a cooling rate correction for natural samples

Inverse optimal control with speed gradient for a power electric system using a neural reduced model

Ash flowtuffs, or ignimbrites have been recently proposed to be a good material for palaeointensity determination. In this paper, we present a multidisciplinary study, combining geochronology, petrology, rock magnetism and palaeomagnetism, carried out on Oligocene to Early Miocene ignimbrites and related flows from the southern part of the Sierra Madre Occidental. Two new 40Ar/39Ar ages were determined for ignimbrites; 20.4 0.2 Ma (the youngest age obtained so far in this area) and 29.2 0.5 Ma. Density measurements, as a proxy for welding, proved to be extremely useful to estimate the emplacement temperature and the origin of the magnetizations carried by the ignimbrites. After alternating field and thermal demagnetizations, the mean palaeomagnetic pole (Lat = 66.8 ; Long = 180.5 Kappa = 142; A95 = 6.3 ), calculated for the period 28-31 Ma, is in close agreement with our only Miocene determination. Comparison with the North America Synthetic Apparent Polar Path indicates a net counter-clockwise vertical axis rotation of about 10 4 compared to stable North America, which occurred likely during the last extensional episode in the Late Miocene (ca. 12-9 Ma). Palaeointensity estimates, obtained with the Thellier-Coe method, are mainly questionable and should not be used for global interpretation. Therefore, these ignimbrites are not a viable material for reliable palaeointensity determinations. " The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.",,,,,,"10.1093/gji/ggt079",,,"http://hdl.handle.net/20.500.12104/43464","http://www.scopus.com/inward/record.url?eid=2-s2.0-84877932531&partnerID=40&md5=90a683da71065dc127326c40c4887b5a",,,,,,"3",,"Geophysical Journal International",,"125