The Influence of Subducted Sediment Composition on the Volcanic gas Output at Arcs: Comparing the Luzon and Bicol Arcs, the Philippines

We asses the influence of subducted sediment composition on the volcanic gas output (chemistry and isotope characteristics) by contrasting two juxtaposed volcanic arcs in the Philippines: the western Luzon Arc sampled at the Bulalo geothermal field (flank of Mt. Makiling) and Mt. Pinatubo, and the eastern Bicol Arc sampled at the Tiwi geothermal field (Mt. Malinao). Gases show no distinction in general chemical characteristics between the two arcs with overlapping values in CO₂/S ~ 2-37; S/HCl ~ 8-18; N₂/He <1,000; CO₂/N₂ ~ 70-1400 and CH₄/He ~67-195. These results are consistent with a varying hydrothermal influence superimposed on arc-type magmatic gas. ³He/⁴He ratios are also similar between the two arcs (Bulalo ~ 6.9 R_A, n=6; Pinatubo ~ 7.1 R_A, n=2; and Tiwi ~ 6.3 R_A, n=3). There is, however, a marked difference between the arcs when comparing carbon characteristics. Geothermal gases from Bulalo (n=4) are tightly constrained in CO₂/³He (8 -10 x 10⁹) whereas Tiwi (n=4) displays much greater variability (11-42 x 10⁹). In addition, δ ¹³C values of Bulalo span the range typical of arc-derived carbon (-2.7 to -4.1‰ ) whereas δ ¹³C at Tiwi is significantly higher at -0.6 to -0.2‰ . Interestingly, Pinatubo gases show CO₂/³He values closer to Tiwi (40 x 10⁹) but δ ¹³C values closer to Bulalo (-4.8‰ ). Apportioning CO₂ between slab-derived limestone (L) and organic sediment (S), and mantle wedge (M) (see Sano and Marty, Chem. Geol., 1995) then the Luzon Arc is similar to other arcs worldwide (L = 70-80%; M = 10-20% and S = 8-12%). This likely reflects heterogeneity in sediment sources derived from Eurasia and subducted via the Manila Trench. In contrast, CO₂ from Tiwi can be described as a binary mixture between L and M (~88:12) with no evidence of organic sediment (S) in its source. Pelagic sediment of the Philippine Plate is the principal carbon-bearing species subducted via the East Luzon Trench and the organic C-content is low. Hence, the Philippines provide a clear example where source composition of the down-going sediment exerts the primary control on the carbon output characteristics: this is not the case for the general gas chemistry. We compare these findings with other arcs worldwide.