Cosmogenic 3He and 21Ne surface exposure dating of young basalts from Southern Mendoza, Argentina

Abstract

Southern Mendoza, Argentina, is characterised by abundant Pleistocene to Holocene volcanism associated with back-arc magmatism, influenced by the subducting Nazca plate. Age determinations in this volcanic area have been improved during the last 5 years. However, there are some volcanic features especially in the Payunia Volcanic Field (PVF) which suggest fairly recent eruptions and which have not been chronologically determined. Recent studies on the Llancanelo Volcanic Field (LLVF) and PVF have determined volcanic activity mainly using K-Ar and 40Ar/39Ar as well as cosmogenic 3He. However, K-Ar and 40Ar/39Ar fail to produce reliable ages in Holocene basaltic flows. To better constrain the younger volcanic activity in the LLVF and especially in the PVF, surface exposure dating using cosmogenic 3He and 21Ne was applied to five volcanic features. By applying this method, 3He and 21Ne ages ranging from late Pleistocene to mid Holocene were obtained for basalts from the area of Los Volcanes, from the PVF. The youngest age acquired is significant as it supports previous evidence for mid Holocene volcanic activity in the PVF and constitutes the first noble gas cosmogenic surface exposure age obtained from a basaltic bomb. This paper illustrates the advantages of using two nuclides (3He and 21Ne) for cosmic-ray exposure ages in the study of recent volcanic eruptions. The results in the present study indicate that the PVF was active in the last 5 ka.

Redetermination of the 21Ne relative abundance of the atmosphere, using a high resolution, multi-collector noble gas mass spectrometer (HELIX-MC Plus)

“…The integrated 21 Ne/ 20 Ne ratio measured in the sample was 0.003237 ± 0.000067. The concentration of excess 21 Ne, relative to the atmospheric 21 Ne/ 20 Ne ratio determined by Eberhardt et al [10] of 0.00296, 9.3 × 10 5 atoms/g gives a cosmogenic 21 Ne surface exposure age of 4.0 ± 1.4 ka (based on a cosmogenic 21 Ne production rate of 232 atom/g/a for the site [23]). When the new atmospheric 21 Ne/ 20 Ne value of 0.0029051 is used as a reference value, the concentration of excess 21 Ne increases to 1.1 × 10 6 atoms/g, corresponding to a cosmogenic 21 Ne surface exposure age of 4.8 ± 1.7 ka, which is approximately 20% older than the value determined using the Eberhardt et al value.…”

“…Thus, if the fractions of 20 Ne 1 H relative to 21 Ne in the samples measured by Espanon et al [23] are the same as that incorporated into the atmospheric 21 Ne measurements of Eberhardt et al [10], the contributions of 20 Ne 1 H can be subtracted from the cosmogenic 21 Ne contents of the samples. On this basis, cosmogenic 21 Ne surface exposure ages reported by Espanon et al [23] may be close to the true values. Nonetheless, confirmation of this interpretation is required by analysing Ne isotope compositions from the same samples using the HELIX-MC Plus.…”

“…Espanon et al [23] determined cosmogenic 21 Ne surface exposure ages on young basalts from Southern Mendoza, Argentina. The youngest 21 Ne surface exposure age from this study was obtained from sample PY-4 from the Payunia Volcanic Field.…”

“…However, it must be stressed that the Ne isotope compositions reported by Espanon et al [23] were determined using the VG5400 noble gas mass spectrometer with mass resolution of 600 [24]. With such a low mass resolution, 20 Ne 1 H and 21 Ne were measured as a combined peak at mass 21.…”

“…Instrumental mass discrimination values for the VG5400 were determined relative to the atmospheric Ne isotope compositions of Eberhardt et al [10]. Thus, if the fractions of 20 Ne 1 H relative to 21 Ne in the samples measured by Espanon et al [23] are the same as that incorporated into the atmospheric 21 Ne measurements of Eberhardt et al [10], the contributions of 20 Ne 1 H can be subtracted from the cosmogenic 21 Ne contents of the samples. On this basis, cosmogenic 21 Ne surface exposure ages reported by Espanon et al [23] may be close to the true values.…”