Noble gas and boron isotopic signatures of the Bacon‐Manito geothermal fluid, Philippines

Noble gas isotopic composition and abundances were determined on dry gas sampled in geothermal wells from the Bacon-Manito (BGPF) geothermal field in the Philippines. The most significant findings come from the 3He/4He ratio; a mantle-He source is evidenced by ratio close to 7 Ra. Peripheral fluid from the west and south of the geothermal system is relatively enriched in 4He (R/Ra slightly > 2), most probably sourced from U and Th decay in old igneous or crustal rocks. The two end-members mix, producing the range of R/Ra ratios observed in the other wells included in this study. Preliminary data on the δ11B signature of the Bacon Manito fluid separated from vapour range from 7‰ to 9‰. These values suggest that the local magmatic rocks could represent the main boron source, in agreement with the boron isotopic signature of Pacific arc lavas.