OXIDATION STATE OF PRIMITIVE MAFIC MELT IN WESTERN PHILIPPINES REFLECTS ARC-SCALE OXIDIZATION OF SUB-ARC MANTLE SOURCE

Subduction-related magmas are commonly oxidized, with a fO2 > NNO, higher than mid-oceanic ridge basalts and rift-related rocks. Studies of large porphyry Cu deposits show that the associated magmas are even more oxidized, >NNO+1, than typical subduction-related magmas. Possible causes for the oxidized state of magmas include individual volcanic processes (i.e., degassing of magma reservoirs), regional crustal processes (such as assimilation), and an oxidized source mantle. To evaluate the cause of this oxidation, we examined the composition of primitive magmas quenched in Holocene pyroclastic rocks in the Philippines. Samples were collected from Corregidor volcano, Mount Arayat, and Mount Natib in the Luzon arc, and Mount Parker (Falen volcano), in the Sulu arc of Mindanao. These stratovolcanoes are associated with the Manila and Sulu trenches, respectively.

Mount Parker samples are olivine+pyroxene basalt (MgO = 4.0-4.3 wt%), that show similar chemical compositions, mineral chemistry and textures to eruption products from Mount Pinatubo on Luzon, with moderately high Sr/Y (up to 42 in mafic samples). Olivine has high Mg (Fo = 85.7-86.0), and spinel cores have moderate Cr # (0.48-0.50). The pairs yield ~NNO+1.5 to NNO+2.2. Mount Arayat samples are olivine+pyroxene basalt with MgO = 6.9-9.2 wt%. Olivine has Fo ~87 and spinel cores have high Cr#, ~0.67. Oxidation state calculated from the most primitive olivine+spinel pairs is ~NNO+1.49. A Mount Natib sample (MgO = 4.6 wt%) contains olivine with varying Mg. High-Mg olivine (Fo = 78-85) and associated spinel (Cr# = 0.75-0.81) yielded NNO+0.7-NNO+0.9. The fO2 values are similar to those (~NNO+1.4) of primitive melt enclosed in the eruption product at Mount Pinatubo (deHoog et al., 2004), and higher than many arc magmas. Combined with the data from Iriya volcano (northern extreme of the Luzon arc) and Pinatubo, the results suggest that the oxidized nature of magmas is regional along the subduction zone and likely reflects an oxidized source mantle. This is consistent with the abundance of porphyry copper deposits in the western Philippines, with an oxidized primitive melt being capable of transferring large quantities of metals and sulphur from the mantle to shallow crust relative to more reduced magmas.

Ref: DeHoog, Hattori & Hoblitt, 2004, Contr. Min. Pet.146: 750-761