Monitoring of temporal and spatial variations in fumarole helium and carbon dioxide characteristics at Poas and Turrialba volcanoes, Costa Rica (2001-2009)

We report results of a 9-year monitoring program that took place from 2001 to 2009 of the helium and carbon isotope (He-3/He-4, delta C-13) and relative abundances ratios (CO2/He-3) of fumarole sites at Pods and Turrialba volcanoes, Costa Rica. Over the monitoring period, helium isotopes (He-3/He-4), delta(13)((CO2)) and CO2/He-3 values varied between 6.7-7.6 R-A (where R-A = 1.4 x 10(-6)), -5.5 to -1.3 parts per thousand (vs. PDB) and 8.2-59.5 (x10(9)), respectively, at Pods Volcano. Corresponding values for Turrialba Volcano were 6.9-8.0 R-A, -4.4 to -2.7 parts per thousand and 9.4-19.6 (x10(9)), respectively. Notably, fumarole sites at both volcanoes underwent changes in temperature, intensity and areal extent during the 9-year period, and Pods Volcano experienced hydrophreatic eruptions and structural damage induced by a nearby earthquake. At both volcanoes, there were significant and sympathetic temporal changes involving all three geochemical parameters notably in 2001 and 2006 at Pods and in 2001 and between 2005-2007 at Turrialba. We dismiss increased hydrothermal interaction, magma degassing and calcite precipitation as likely causes of the observed variations. Instead, by ascribing endmember compositions to the three principal contributors to the CO2 inventory mantle wedge as well as limestone and organic (sedimentary) carbon (both slab and crust derived) we show that changes in observed He-CO2 relationships mainly reflect enhanced crustal contributions of CO2 and increased inputs from magma degassing. Such changes in the relative roles of crust and magma as suppliers of CO2 are readily apparent in the He-CO2 temporal record. This work supports calls for long-term geochemical monitoring to be included within hazard assessment and mitigation studies at active volcanoes.