HR: 0800h
AN: V41A-1422    [Abstracts]
TI: Volatiles in Primitive, High-K Basanite Magma From the Colima Graben, Western Trans-Mexican Volcanic Belt
AU: * Vigouroux, N
EM: nvigouro@darkwing.uoregon.edu
AF: Dept. of Geological Sciences, Unviersity of Oregon, Eugene, OR 97403 United States
AU: Wallace, P J
EM: pwallace@uoregon.edu
AF: Dept. of Geological Sciences, Unviersity of Oregon, Eugene, OR 97403 United States
AU: Linares, C
EM: linaresc@tonatiu.igeofcu.unam.mx
AF: Instituto de GeofA-sica, Universidad Nacional AutA3noma de MAcxico, Ciudad Universitaria, D.F 04510 Mexico
AB: Volcan Apaxtepec is the youngest of eleven cinder cones that are associated with the andesitic stratovolcanoes Nevado and Fuego de Colima at the southern end of the Colima graben in west-central Mexico (Luhr and Carmichael, 1981). This region lies approximately above the junction of the subducting Cocos and Rivera Plates and is part of the E-W-trending, subduction related Trans-Mexican Volcanic Belt (TMVB). Nine of these cinder cones erupted K-rich basanites and minettes. We analyzed the volatile contents of olivine-hosted melt inclusions from Volcan Apaxtepec, a basanite cone, to test ideas about the relationship of these high-K magmas to subduction processes beneath western Mexico. We sampled tephra from both the early and late stages of the eruption of Apaxtepec. Olivine phenocrysts are uniformly Mg-rich (Fo89-91) in both samples. Melt inclusion volatile contents are variable and extend to high values (0.42 to 6.3 wt.% H₂O, 501 to 6398 ppm CO₂). There are no systematic differences between early and late erupted material. The high water contents and variable crystallization pressures (700 bars to >5 kbars) inferred from the melt inclusion data suggest that basanites are water-rich, vapor-saturated melts that ascend, degas and crystallize starting at mid to lower crustal depths. Previous petrologic studies on whole-rock samples suggested that the basanites are water-poor equivalents of minettes (perhaps due to loss of water upon ascent) that originally formed by decompression melting in the upper metasomatised mantle (Luhr and Carmichael, 1981; Luhr et al., 1989; Carmichael et al., 1996; Hochstaedter et al., 1996; Luhr, 1997). The higher K₂O/H₂O ratio of the basanites compared to more common, subalkaline basalts from elsewhere in the TMVB is consistent with the presence of phlogopite in the mantle source region for basanitic magmas in the Colima graben. Our results suggest that H₂O contents were sufficient for crystallization of phlogopite at crustal depths during ascent. However, phlogopite is lacking from the basanites although it occurs as phenocrysts in neighboring minette cinder cones. A possible explanation is that the high CO₂ contents in the basanites would have resulted in H₂O activities < 0.5 at mid to upper crustal pressures, which may be below the critical value necessary for phlogopite stability.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8428 Explosive volcanism
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005