Geochemical Study of Fluids Across the Philippine Archipelago and Their Link to the Underlying Mantle

The Philippine Archipelago resulted from a series of subduction events, the most recent being still active, and arc-continent collision, forming today a series of active volcanic fields and ophiolites. As such the Philippine Mobile Belt constitutes a unique place where to study the spatial distribution and origin of mantle degassing fluxes during plate convergence.

We present the geochemical composition of 12 gas samples collected from various locations across the Philippine archipelago. These include samples from Luzon Island-specifically five wells from the MakBan volcanic geothermal field- three bubbling springs in the Laguna region (Silva, Hernandez, and Silva 2 springs), and a bubbling seep near a river at Poon Bato in Zambales. Additional samples were collected from three bubbling springs on Palawan Island (Bato Bato, Sta. Lourdes, and Sta. Lucia hot springs).

Preliminary results of major compound geochemistry reveal two main fluid families. Samples from the geothermal area are dominated by isotopically heavy CO₂ (-4 to -2‰ VPDB), with concentrations reaching 97%. These samples also contain H₂S (up to 2%) and H₂ (up to 0.6%) with δD_H₂ values ranging from -490 to -440‰ VSMOW. Their composition, coupled with measured ³He/⁴He ratios of up to 6.8 R/Ra, indicates a mantle origin influenced by volatiles from the subduction zone. In contrast, samples from the Laguna springs are characterized by high N₂ concentrations (up to 78%) and CO₂ (up to 15%) with δ¹³C_CO₂ values around -10‰ VPDB, suggesting significant interaction with shallow aquifers that may contain dissolved air (e.g., ASW-like N₂).

On Palawan Island, the gas compositions vary: CO₂ (60%) and N₂ (29%) dominate at Sta. Lourdes spring, while N₂ and CH₄ dominate at Bato Bato (N₂ = 68%; CH₄ = 20%) and Sta. Lucia springs (N₂ = 85%; CH₄ = 5%). The Poon Bato seep in Zambales consists mainly of N₂ (75%), H₂ (8%), and CH₄ (4%). The emissions from Bato Bato, Sta. Lucia, and Poon Bato are associated with ultramafic rocks of ophiolite complexes. Factors such as elevated pH values at Bato Bato and Sta. Lucia (pH = 9.5–10), high H₂ concentrations with δD_H₂ = -724‰ at Poon Bato, and the presence of potentially inorganic CH₄ (δ¹³C_CH₄ = -40 to -20‰ VPDB) suggest that serpentinization is a likely origin for these fluids.

Further interpretation of gas origins will benefit from ongoing analyses of noble gas data, which are currently in progress.