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The knowledge of the eruptive history of volcanic centers allows for improving the evaluation of the related risks and hazards in populated areas, but substantially depends on the ability of dating the lava flows. However, traditional methods such as U-Th/He, 40 Ar- 39 Ar, 40 K- 40 Ar and radiocarbon dating are not always suitable. Therefore, an alternative approach based on the combination of in situ -produced 36 Cl based cosmic ray exposure dating, lichenometry and dendrochronology was tested on two lava flows (called Lava flow “A” and “B”) from Pico de Orizaba (Mexico), previously attributed to 16th and 17th century eruptions, respectively. The presented results show that both lava flows are significantly older than their assumed 16th and 17th century ages. Regarding lava flow “A”, the measured in situ -produced 36 Cl concentrations lead to a mean age of 3.03 ± 0.70 ka, while dendrochronology and lichenometry yield minimum ages of 834 and 1130 years, respectively. Regarding lava flow “B”, the measured in situ -produced 36 Cl concentrations lead to a mean age of 1.45 ± 0.35 ka, whereas lichenometry gives a minimum age of ∼1000 years. Overall, this demonstrates that the combination of in situ -produced 36 Cl based cosmic ray exposure dating, lichenometry and dendrochronology has a considerable potential for dating purposes on young deposits and landforms, in particular on lava flows at high elevation sites. [ABSTRACT FROM AUTHOR]

We report on the status and capabilities of the Lamont–Doherty Earth Observatory in situ 14C extraction laboratory. In late 2006 we began, in collaboration with the AMS group at the University of Arizona, construction of a new laboratory to extract in situ cosmogenic 14C from terrestrial silicates. Long-term measurements of the process blank over the last two years give an arithmetic mean and standard deviation of 125 ± 43 × 103 atoms 14C (n = 9) and show significant improvement in the number of atoms, as well as stability compared to initial measurements of the process blank. We report long-term measurements of the intercomparison material CRONUS-A, which has been developed as part of the CRONUS-Earth effort to characterize inter- and intra-laboratory variability. We interpret the standard deviation (5%) of six replicate measurements of CRONUS-A as the reproducibility of in situ 14C extractions in our laboratory. [ABSTRACT FROM AUTHOR]

Despite their significance for estimating hazards and forecasting future activity, dating young volcanic deposits and landforms (<50,000 yrs old) remains a challenge due to the limitations inherent to the different isotopic chronometers used. The Trans-Mexican Volcanic Belt is one of the most active and populated continental arcs worldwide, yet its temporal pattern of activity is poorly constrained. Such deficiency is particularly problematic for the Sierra Chichinautzin Volcanic Field (SCVF) that is located at the doorstep of Mexico City and Cuernavaca and is hence a major source of risk for these cities. Existing ages for this area derive mostly from either radiocarbon on charcoal, which is rare and may be contaminated, or 40Ar/39Ar on rock matrix, which is poorly precise for this time period and rock type. Here, we focus on the Pelado monogenetic volcano, which is located in the central part of the SCVF and erupted both explosively and effusively, producing a large lava shield and a widespread tephra blanket. This unique eruptive event was previously dated at ∼12 calibrated (cal) kyrs BP, using radiocarbon dating on charcoal from deposits related to the eruption. To test alternative dating approaches and confirm the age of this significant eruption, we applied two less conventional techniques, radiocarbon dating of bulk paleosol samples collected below the complete tephra sequence at nine sites around the shield, and in-situ 36Cl exposure dating of two samples of an aphyric lava from the base of the shield. Radiocarbon paleosol ages span a continuous time interval from 13.2 to 20.2 cal kyrs BP (2σ), except for one anomalously young sample. This wide age spread, along with the low organic contents of the paleosols, may be due to erosive conditions, related to the sloping topography of the sampling sites and the cool and relatively dry climate of the Younger Dryas (11.7–12.9 ka), during which the Pelado eruption probably occurred. The two 36Cl-dated lava samples have consistent ages at 1σ analytical errors of 15.5 ± 1.4 ka and 13.2 ± 1.2 ka, respectively, yielding an average age of 14.3 ± 1.6 ka for this lava flow. The high full uncertainty in 36Cl ages (24%) is due to high rock Cl content. We conclude that paleosol radiocarbon dating is useful if numerous samples are analyzed and climatic and relief conditions at the time of the eruption and at the sites of tephra deposition are considered. The 36Cl dating technique is an alternative method to date volcanic eruptions, as it gave consistent results, but in the specific case of Pelado volcano, the high Cl content in the analyzed rocks increases the age uncertainties. [Display omitted] • Catalogue of recent eruptions in Trans-Mexican Volcanic Belt is incomplete. • 14C on paleosol and 36Cl dating are used on a previously dated monogenetic volcano. • New ages are coherent with previous 14C charcoal age. • Spread in paleosol ages likely due to erosive conditions at time of eruption. • 36Cl ages are statistically consistent but poorly precise, due to high Cl in rock. [ABSTRACT FROM AUTHOR]

Abstract: Because the intensity and energy spectrum of the cosmic ray flux are affected by atmospheric depth and geomagnetic-field strength, cosmogenic nuclide production rates increase considerably with altitude and to a lesser degree with latitude. The scaling methods used to account for spatial variability in production rates assume that all cosmogenic nuclides have the same altitude dependence. In this study we evaluate whether the production rates of cosmogenic 36Cl, 3He and 21Ne change differently with altitude, which is plausible due to the different threshold energies of their production reactions. If so, nuclide-specific scaling factors would be required. Concentrations of the three cosmogenic nuclides were determined in mafic phenocrysts over an altitude transect between 1000 and 4300m at Kilimanjaro volcano (3°S). Altitude dependence of relative production rates was assessed in two ways: by determination of concentration ratios and by calculation of apparent exposure age ratios for all nuclide pairs. The latter accounts for characteristics of 36Cl that the stable nuclides 3He and 21Ne do not possess (radioactive decay, high sensitivity to mineral composition and significant contributions from production reactions other than spallation). All ratios overlap within error over the entire transect, and altitudinal variation in relative production rates is not therefore evident. This suggests that nuclide-specific scaling factors are not required for the studied nuclides at this low-latitude location. However, because previous studies have documented anomalous altitude-dependent variations in 3He production at mid-latitude sites, the effect of latitude on cross-calibrations should be further evaluated. We determined cosmogenic 21Ne/3He concentration ratios of 0.1864±0.0085 in pyroxenes and 0.377±0.018 in olivines, agreeing with those reported in previous studies. Despite the absence of independently determined ages for the studied lava surfaces, the consistency in the dataset should enable progress to be made in the determination of the production rates of all three nuclides as soon as the production rate of one of the nuclides has been accurately defined. To our knowledge this is the first time that 36Cl has been measured in pyroxene. The Cl extraction method was validated by measuring 36Cl in co-existing plagioclase phenocrysts in one of the samples. [Copyright &y& Elsevier]

Abstract: In-situ cosmogenic 36Cl production rates from spallation of Ca and K determined in several previously published calibration studies differ by up to 50%. In this study we compare whole rock 36Cl exposure ages with 36Cl exposure ages evaluated in Ca-rich plagioclase in the same 10±3ka lava sample taken from Mt. Etna (Sicily, 38°N). The exposure age of the sample was determined by K–Ar and corroborated by cosmogenic 3He measurements on cogenetic pyroxene phenocrysts. Sequential dissolution experiments showed that high Cl concentrations in plagioclase grains could be reduced from 450ppm to less than 3ppm after 16% dissolution. 36Cl exposure ages calculated from the successive dissolution steps of this leached plagioclase sample are in good agreement with K–Ar and 3He age. Stepwise dissolution of whole rock grains, on the other hand, is not as effective in reducing high Cl concentrations as it is for the plagioclase. 330ppm Cl still remains after 85% dissolution. The 36Cl exposure ages derived are systematically about 30% higher than the ages calculated from the plagioclase. We could exclude contamination by atmospheric 36Cl as an explanation for this overestimate. Magmatic 36Cl was estimated by measuring a totally shielded sample, but was found to account for only an insignificant amount of 36Cl in the case of the 10ka whole rock sample. We suspect that the overestimate of the whole rock exposure age is due to the difficulty in accurately assessing all the factors which control production of 36Cl by low-energy neutron capture on 35Cl, particularly variable water content and variable snow cover. We conclude that some of the published 36Cl spallation production rates might be overestimated due to high Cl concentrations in the calibration samples. The use of rigorously pretreated mineral separates reduces Cl concentrations, allowing better estimates of the spallation production rates. In the Appendix of this paper we document in detail the equations used. These equations are also incorporated into a 36Cl calculation spreadsheet made available in the supplementary data. [Copyright &y& Elsevier]

Usual methods are unfortunately unsuitable to accurately date many Holocene deposits and landforms from active Mexican volcanoes. This is notably the case for the Tacaná Volcanic Complex (TVC), located in the State of Chiapas in southern Mexico, and the San Marcos Department in Guatemala. The complex consists of the Chichuj, Tacaná, and San Antonio volcanoes and the Ardillas dome. Tacaná volcano, the main summit of the TVC, collapsed 15 ± 5 ka ago (40Ar/39Ar), producing the Agua Caliente Debris Avalanche. The result was the formation of a horseshoe-shaped crater 600 m wide open to the northwest. Afterward, several undated effusive and explosive eruptions occurred. All these eruptions are likely Late Pleistocene to Holocene in age because the landforms derived from that activity overlap the horseshoe-shaped crater. To corroborate this hypothesis, we used surface exposure dating with in situ- produced cosmogenic 36Cl combined with lichenometry and dendrochronology in the summit domes, the Ardillas dome, the horseshoe-shaped crater's cliff, and an andesitic lava. The 36Cl exposure ages (9.3 ± 1.9 ka; 8.9 ± 0.9 ka; 8.6 ± 1.7 ka) of the summit domes are statistically indistinguishable within the associated uncertainties and suggest that the domes formed during Early Holocene. Instead, the southern part of the Late Pleistocene collapsed crater yielded significantly younger ages ranging from 5.3 ± 0.6 to 7.0 ± 0.8 ka. These ages are assumed to be minimum and might indicate later rockfall activity and instability, affecting the horseshoe-shaped crater. The 36Cl exposure age of an andesitic lava (0.4 ± 0.1 ka), emplaced to the SW of Tacaná, is in good agreement with lichenometry (>347 yr). These ages could represent a gravitational collapse event associated with phreatic explosions vented close to a scar collapse. The 36Cl exposure ages derived from the Ardillas lava dome (0.3 ± 0.1 ka) do not correspond to its emplacement age. The dome must be older than pyroclastic deposits dated at 760 ± 30 yr BP (14C) that cover its surface. Strikingly, the 36Cl, lichenometrical and dendrochronological data of the "Andesitic" lava and the Ardillas dome are associated with NE-SW landslide scars, explosion craters, phreatic vents, fumarolic activity, and tensional fractures. All these features are perpendicular to the NW-SE minimum stress regime (σ3), affecting the TVC and generating gravitational activity. Nowadays, gravitational collapses represent a real threat to the surrounding populations that can be triggered not only by volcanic activity but also by seismicity and extraordinary rains. [ABSTRACT FROM AUTHOR]