Cluster analysis using portable X ray fluorescence (pXRF) data: a fast and powerful method for regional correlation of ash fall deposits.

SOLA ALFONSO; BAEZ, WALTER ARIEL; EMILCE BUSTOS; HERNANDEZ R; SAMPIETRO VATTUONE, M.M; PEÑA MONNÉ, J.L; BECCHIO RAÚL

Puerto varas

Congreso; Cities on Volcanoes 9; 2016

IAVCEI

Ash fall deposits, produced by explosive volcanic eruptions are distributed over broad geographic regions,deposited within hours, days or weeks and are distinctive within the sedimentary records. Therefore, ash falldeposits are ideal time planes and valued as important temporal tool in stratigraphic correlation. Since glass andphenocryst phases generated by each volcanic eruption have distinguishable chemical fingerprint, individual ashfall deposits can in many cases be identified unambiguously based on their geochemistry and can be correlatedregionally. Radiometric dating, biostratigraphy and high-precision microprobe analysis on glass and phenocrystsare among the most widely used and effective correlations techniques. However, the main drawback of thesemethods is that they are high-priced, time-consuming and out of reach for field geologists and field mapping.We present a fast, low cost and accessible methodology for ash fall deposit correlations. In the NW Argentina,widespread ash fall deposits occur interbedded with Quaternary sequences e.g. [1,2]. Chemical fingerprintsobtained from pXRF in some of these ash deposits and further statistical treatment provided excellentgeochemical correlations. Trace element ratios measured in the samples (Sr/Rb; K/Sr) indicated values that arewithin the range of Central Andes rhyolites. Three different (rhyolitic) groups resulted from multivariatehierarchical clustering. In order to validate the methodology, we used samples of three ash levels from Tafívalley (Tucuman province) which have good stratigraphic and temporal constraint [3]. Later we applied thesame approach to correlate several samples of Quaternary ash fall deposits from different localities in the NWArgentina (e.g. Tonco valley, Calchaquí valley, Lerma valley). Finally, we compared the geochemical groupsobtained from the statistical analysis with volcanic centres in the Southern Puna that have both well-knownQuaternary activity and rhyolitic composition [3,4,5].