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 (3)He/(4)He 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 (4)He (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 delta(11)B signature of the Bacon Manito fluid separated from vapour range from 7 parts per thousand to 9 parts per thousand. 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.