1998 Eruption at Volc�n Cerro Azul, Gal�pagos Islands: I. Syn-Eruptive Petrogenesis

Abstract

The 1998 eruption of Volcµn Cerro Azul in the Galµpagos Islands produced two intra-caldera vents and a flank vent that erupted more than 1.010 8 m 3 of lava. Lava compositions changed notably during the 5-week eruption, and contemporaneous eruptions in the caldera and on the flank produced different compositions. Lavas erupted from the flank vent range from 6.3 to 14.1% MgO, nearly the entire range of MgO contents previously reported from the volcano. On-site monitoring of eruptive activity is linked with petrogenetic processes such that geochemical variations are evaluated in a temporal context. Lavas from the 1998 eruption record two petrogenetic stages characterized by progressively more mafic lavas as the eruption proceeded. Crystal compositions, whole rock major and trace element compositions, and isotope ratios indicate that early lavas are the product of mixing between 1998 magma and remnant magma of the 1979 eruption. Intra-caldera lavas and later lavas have no 1979 signature, but were produced by the 1998 magma incorporating olivine and clinopyroxene xenocrysts. Thus, early magma petrogenesis is characterized by mixing with the 1979 magma, followed by the magma progressively entraining wehrlite cumulate mush.

Lithological structure of the Galápagos Plume

“…Pb isotope ratios of the four isotopic domains in Figure 1. Data from: Allan and Simkin [2000], Blichert-Toft and , Cullen and Mcbirney [1987], Cullen et al [1989], Geist et al [2002], , Handley et al [2011], Harpp and White [2001], Kurz and Geist [1999], Naumann et al [2002], Pik et al [1999], Reynolds and Geist [1995], Saal et al [2007], Standish et al [1998], Teasdale et al [2005], Thompson et al, 2004, Vicenzi et al [1990, White and Hofmann [1978], White [1979], and White et al [1993].…”

“…Figure 9. Plot of mean Sr, Nd, and Pb isotopic ratios and inter-quartile range for each volcano covered in this study against the isotopic range of the Gal apagos Archipelago [Allan and Simkin, 2000;Blichert-Toft and White, 2001;Cullen and Mcbirney, 1987;Cullen et al, 1989;Geist et al, 2002Geist et al, , 2005Handley et al, 2011;Harpp and White, 2001;Kurz and Geist, 1999;Naumann et al, 2002;Reynolds and Geist, 1995;Saal et al, 2007;Standish et al, 1998;Teasdale et al, 2005;Thompson et al, 2004;White and Hofmann, 1978;White, 1979;White et al, 1993]. [36] Relationships between He isotope ratios and source lithology may be obscured by many variables, such as the contamination by depleted upper mantle thought to pervade the sources of the Gal apagos plume [Harpp and White, 2001], and the possibility that helium might diffuse between lithologies [Hart et al, 2008]. Basalts from Midocean ridges are degassed twice, first upon eruption [Craig et al, 1975] and then again during subduction [Moreira and Kurz, 2001].…”

Imaging rapidly deforming ocean island volcanoes in the western Galápagos archipelago, Ecuador

“…The magmas that erupt are strongly evolved tholeiitic basalts, which have undergone substantial fractionation of wehrlite in the middle and deep crust and further fractionation involving plagioclase in the shallow magmatic system [ Reynolds and Geist , ; Geist et al ., ]. These interpretations, along with detailed observations at other Galápagos Volcanoes [ Geist et al ., , , ; Naumann et al ., ; Teasdale et al ., ], have resulted in a petrologic model for the magmatic plumbing systems of the western Galápagos shields. The top of the magma system is a mostly liquid sill at ~2 km, based on models of geodetic data [ Yun et al ., ].…”

The 2005 eruption of Sierra Negra volcano, Galápagos, Ecuador

“…observed and monitored by volcanologists equipped with instrumental and petrological tools (e.g., Allan and Simkin 2000;Teasdale et al 2005). Sierra Negra volcano began its first eruption in 26 years on 22 October 2005.…”