HR: 1340h
AN: V13A-1451    [Abstracts]
TI: The Influence of Subducted Sediment Composition on the Volcanic gas Output at Arcs: Comparing the Luzon and Bicol Arcs, the Philippines
AU: * Hilton, D R
EM: drhilton@ucsd.edu
AF: Fluids and Volatiles Lab., Scripps Inst. Oceanography, La Jolla, CA 92093 United States
AU: Fischer, T P
EM: fischer@unm.edu
AF: Dept. Earth and Planet. Sci., University of New Mexico, Albuquerque, NM 87131 United States
AU: Stimac, J A
EM: jstimac@unocal.com
AF: Philippine Geothermal Inc., 12th Floor Citibank Tower, Makati, 8741 Philippines
AU: De Leeuw, D
EM: gdeleeuw@ucsd.edu
AF: Fluids and Volatiles Lab., Scripps Inst. Oceanography, La Jolla, CA 92093 United States
AU: Villasenor, L B
EM: larrybv@unocal.com
AF: Philippine Geothermal Inc., 12th Floor Citibank Tower, Makati, 8741 Philippines
AU: Abrigo, F V
EM: fe.abrigo@unocal.com
AF: Philippine Geothermal Inc., 12th Floor Citibank Tower, Makati, 8741 Philippines
AB: 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 CO$_2$/S ~ 2-37; S/HCl ~ 8-18; N$_2$/He <1,000; CO$_2$/N$_2$ ~ 70-1400 and CH$_4$/He ~67-195. These results are consistent with a varying hydrothermal influence superimposed on arc-type magmatic gas. $^3$He/$^4$He 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 CO$_2$/$^3$He (8 -10 x 10$^9$) whereas Tiwi (n=4) displays much greater variability (11-42 x 10$^9$). In addition, $\delta$$^1$$^3$C values of Bulalo span the range typical of arc-derived carbon (-2.7 to -4.1$\permil$) whereas $\delta$$^1$$^3$C at Tiwi is significantly higher at -0.6 to -0.2$\permil$. Interestingly, Pinatubo gases show CO$_2$/$^3$He values closer to Tiwi (40 x 10$^9$) but $\delta$$^1$$^3$C values closer to Bulalo (-4.8$\permil$). Apportioning CO$_2$ 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, CO$_2$ 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.
DE: 8424 Hydrothermal systems (8135)
DE: 9320 Asia
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting