HR: 09:15h
AN: V31H-06 INVITED    [Abstracts]
TI: Is new Plinian Eruption Imminent at Colima Volcano in Mexico? Insights From Mineral Chemistry, Melt Inclusion Volatiles and Bulk Rock B-Li-Sr-Nd-Hf-Pb Isotopic Systematics
AU: * Savov, I P
EM: savov@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism 5241 Broad Branch Road, N.W., Washington, DC 20015, United States
AU: Cai, M Y
EM: cai@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Shirey, S B
EM: shirey@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism 5241 Broad Branch Road, N.W., Washington, DC 20015, United States
AU: Ryan, J G
EM: ryan@cas.usf.edu
AF: University of South Florida, Geology Department 4202 E.Folwer Ave, SCA 528, Tampa, FL 33620, United States
AU: Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism 5241 Broad Branch Road, N.W., Washington, DC 20015, United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu>
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Navarro-Ochoa, C
EM: naoc@cgic.ucol.mx
AF: Observatorio Vulcanológico de la Universidad de Colima, Av. 25 de Julio # 965 Colonia Villa San Sebastián, Colima, 28045, Mexico
AB: Volcan Colima is among the largest and most active volcanoes on the planet. The extensive and detailed work of James F. Luhr has made Volcan Colima a "living petrological laboratory" on the compositional evolution of an active volcanic complex through time. The possibility that the current eruptive cycle (1961-2007) will culminate in a major Plinian (VEI 4) explosive event similar to those of 1818 and 1913 is an important focus of ongoing work. Lavas and ash erupted since 1961 show increasing compositional similarity to the mafic 1913 scoria, with trends towards lower SiO₂ (from ~ 61.5 to 59 wt% in whole-rock; 72 to 66 wt% in groundmass), lower Ba (500 to 400 ppm), and higher MgO (2.7 to 4.5 wt% in whole-rock). Phenocryst rims mimic these trends with opx and cpx increasing in Mg# from 71 to 73 and 73 to 76 (respectively) and plagioclase increasing in An content (47 to 54 %). Unlike the 1913 scoria, all recent eruptive products lack the micro-vesiculated groundmass textures (signifying expansion of in-situ magmatic gas) that characterized the 1913 tephra. Samples from 1961-2007 are poor in hornblende, have melt H₂O contents that decline with time (from 4 to 1 wt %.) and lower volatile contents in olivine hosted melt inclusions (H₂O=2.1-2.3 wt%;CO₂=17 to 122 ppm;F=1147 to 1354 ppm;Cl=1657-1771 ppm,S=539-675 ppm) as compared to the 1913 scoria. The current magma batch is relatively poorer in water than its predecessor and it appears that volatile-poor mafic mantle melts are currently reaching the base of the Colima conduit system. Although these melts are similar in major and trace element composition to the 1913 scoria they fortunately enter the upper volcano conduit system in a significantly degassed state. Thus the observed Vulcanian (dome blasting) summit explosions could be phreatomagmatic, rather than a product of magmatic degassing. Fluid and crustal assimilation-sensitive isotopic tracers such as B, Li, Sr, Pb, Nd, and Hf isotopes can be used to examine the source of volatiles and differences between the 1913 scoria and more recent magmatism. All of the Colima samples examined show low ⁸⁷Sr/⁸⁶Sr (<0.704), MORB-like Li isotope ratios,ε_Nd (+3 to +6) and ε_Hf (+7 to +10) i.e. representing mantle melts with little or no sign of crustal contamination. LA-MC-ICP-MS B isotope measurements show that the 1961-2007 Colima andesites possess typical arc-like values (δ¹¹B = + 2 to +3‰) similar to samples from the basaltic cinder cone Jorullo (+ 2.1‰). However, these lavas are much lighter than those of nearby alkaline mantle-derived lavas erupting from older small volume monogenetic cinder cones within the Colima graben (up to +10 ‰). Their heavy B isotope ratios may represent fluid-enriched mantle source. The spikes in volatile- rich mafic melts at Colima seem to occur episodically (every ~ 100 years). These variations in fluid sensitive B isotope ratios and melt volatile content reveal episodicity of slab fluid releases below the Colima graben. Coincidence of mafic magmatism and slab fluid release may be necessary for a new Plinian eruption at Colima.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3617 Alteration and weathering processes (1039)
DE: 3619 Magma genesis and partial melting (1037)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting