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Formation of Hybrid Arc Andesites Beneath Thick Continental Crust

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Formation of Hybrid Arc Andesites Beneath Thick Continental Crust Earth and Planetary Science Letters 303 (2011) 337–347 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Formation of hybrid arc andesites beneath thick continental crust Susanne M. Straub a,b,⁎, Arturo Gomez-Tuena c, Finlay M. Stuart d, Georg F. Zellmer b, Ramon Espinasa-Perena e, Yue Cai a,f, Yoshiyuki Iizuka b a Lamont Doherty Earth Observatory at the Columbia University, 61 Route 9W, Palisades NY 10964, USA b Institute of Earth Sciences, Academia Sinica, 128 Academia Road, Sec. 2, Nankang, Taipei 11529, Taiwan, ROC c Centro de Geociencias, Universidad Nacional Autónoma de México, Querétaro 76230, Mexico d Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, East Kilbride G75 0QF, UK e Instituto de Geofisica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Mexico, D.F. 04510, Mexico f Department of Earth and Environmental Sciences, Columbia University, 61 Route 9W, Palisades NY 10964, USA article info abstract Article history: Andesite magmatism at convergent margins is essential for the differentiation of silicate Earth, but no Received 7 November 2010 consensus exists as to andesite petrogenesis. Models proposing origin of primary andesite melts from mantle Received in revised form 12 January 2011 and/or slab materials remain in deadlock with the seemingly irrefutable petrographic and chemical evidence Accepted 13 January 2011 for andesite formation through mixing of basaltic mantle melts with silicic components from the overlying crust. Here we use 3He/4He ratios of high-Ni olivines to demonstrate the mantle origin of basaltic to andesitic Editor: R.W. Carlson arc magmas in the central Mexican Volcanic Belt (MVB) that is constructed on ~50 km thick continental crust. N Keywords: We propose that the central MVB arc magmas are hybrids of high-Mg# 70 basaltic and dacitic initial mantle helium isotopes melts which were produced by melting of a peridotite subarc mantle interspersed with silica-deficient and high-Ni olivine silica-excess pyroxenite veins. These veins formed by infiltration of reactive silicic components from the andesite formation subducting slab. Partial melts from pyroxenites, and minor component melts from peridotite, mix in variable Mexican Volcanic Belt proportions to produce high-Mg# basaltic, andesitic and dacitic magmas. Moderate fractional crystallization and recharge melt mixing in the overlying crust produces then the lower-Mg# magmas erupted. Our model accounts for the contrast between the arc-typical SiO2 variability at a given Mg# and the strong correlation between major element oxides SiO2, MgO and FeO which is not reproduced by mantle–crust mixing models. Our data further indicate that viscous high-silica mantle magmas may preferentially be emplaced as intrusive silicic plutonic rocks in the crust rather than erupt. Ultimately, our results imply a stronger turnover of slab and mantle materials in subduction zones with a negligible, or lesser dilution, by materials from the overlying crust. © 2011 Elsevier B.V. All rights reserved. 1. Introduction assimiliation of up half of the mass of the erupted melt (e.g. Eichelberger, 1978; Leeman, 1983; Hildreth and Moorbath, 1988; Andesite magmas at convergent margins are enriched in silica Plank and Langmuir, 1988; Streck et al., 2007; Tatsumi et al., 2008; compared to magmas erupting at mid-ocean ridges and intra-plate Reubi and Blundy, 2009). Other models propose primary andesite volcanoes. Determining the cause(s) of silica enrichment is funda- formation beneath the Moho, which may occur by various mecha- mental for models of continental crust formation, arc growth rates nisms such as hydrous melting of peridotite (Hirose, 1997; Moore and and across-arc mass balances (Plank and Langmuir, 1993; Rudnick, Carmichael, 1998; Blatter and Carmichael, 1998b; Carmichael, 2002), 1995; White et al., 2006). Andesite petrogenesis, however, has long slab melting (Defant and Drummond, 1990) or hybridization of slab been controversial, with no consensus even whether andesitic and mantle materials by melt rock-reaction processes (Kay, 1978; magmas form in the subarc mantle or in the overlying crust (Rudnick, Yogodzinksi et al., 1994; Kelemen, 1995; Yogodzinski et al., 1995; 1995; Rudnick and Gao, 2002). Many models assume a basaltic flux Rapp et al., 1999; Kelemen et al., 2003, 2004; Gomez-Tuena et al., from mantle to arc crust, with silicic magmas evolving subsequently in 2007). These two approaches differ substantially with respect to the the upper plate crust through fractional crystallization and crustal turnover of slab and mantle materials in subduction zones, the rate of crustal growth, and the overall connectivity between arc magmatism and the other geochemical cycles of Earth. ⁎ Corresponding author at: Lamont Doherty Earth Observatory at the Columbia A recent study suggests that ‘high-Ni’ olivines, that have been by University, 61 Route 9W, Palisades NY 10964, USA. Tel.: +1 845 365 8464; fax: +1 845 365 8155. now reported from several arcs, e.g. Mexico, Cascades, Setouchi, E-mail address: [email protected] (S.M. Straub). Kamchatka and the Aleutians (e.g., GeoROC, 2009) may provide 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.01.013 338 S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347 important insights into the origin of arc andesites (Straub et al., high-Ni olivines may also crystallize in magmas produced through 2008b). High-Ni olivines are characterized by Ni contents of ~2200– mixing of basaltic mantle melts with larger portions of silicic crustal 5400 ppm at low olivine forsterite contents of Fo80 to Fo89 [where melts. In order to test conclusively for the mantle origin of the olivine- Fo=molar ratio of Mg/(Mg+Fe2+)]. In Fo–Ni space, they plot well bearing central MVB magmas, we analyzed He isotope ratios of high- above the field of olivines from mid-ocean-ridge basalts (MORB) Ni olivines. Here, we present the results, together with a first (Sobolev et al., 2005). Experiment and theory rule out that such high- systematic study of olivine zoning patterns. Ni olivines crystallize from partial melts of peridotite mantle where olivine is residual during melting and buffers melt SiO2 (~49–50 wt. 2. Geological setting and samples %), MgO (~10–15 wt.%), Mg# (~71–73) [where Mg#=molar ratio of Mg/(Mg+Fe2+) in bulk rock] and Ni of equilibrium melts (numbers Quaternary volcanism of the central MVB is related to subduction exemplify moderate extents of melting b10–15% of fertile mantle) of the Cocos plate at the Middle American Trench (Fig. 1). Despite the (Langmuir et al., 1992; Sobolev et al., 2005; Straub et al., 2008b). The ca. 50 km thick Paleozoic to Precambrian continental arc crustal melt Ni is limited, because the amount of Ni in mantle olivines is basement (Perez-Campos et al., 2008), basaltic and basaltic andesite limited to ~2000–3000 ppm (Blatter and Carmichael, 1998a; GeoROC, magmas are common next to the ubiquitous andesitic and dacitic 2009). The experimentally well-constrained mineral/melt partition series along the broad arc front (Siebe et al., 2004a; Schaaf et al., 2005; coefficient KdNi of 10–11 (Hart and Davis, 1978; Beattie et al., 1991; Gómez-Tuena et al., 2007). Many basaltic and andesitic magmas of the Wang and Gaetani, 2008) then predicts that melt Ni will not exceed composite volcano Popocatepetl and the adjacent monogenetic Sierra 200–300 ppm in a melt of MgO with 10–12 wt.% (Langmuir et al., Chichinautzin Volcanic Field (Fig. 1) contain olivines as single or 1992; Straub et al., 2008b). Consequently, the maximum Ni content of principal phenocrysts. We selected 25 olivine-rich volcanic rocks with early crystallizing magmatic olivine of ≥Fo89.5, controlled by the same SiO2 =50.0–61.2 wt.%, MgO=3.2–9.7 wt.% and Mg#=50–72 from KdNi, cannot exceed 2000–3000 ppm Ni either. As olivine crystalliza- monogenetic volcanoes Guespalapa (7 samples), Chichinautzin (5), tion rapidly depletes melt Ni, any derivative melt (=melt derived by Texcal Flow (4), Suchiooc (4), Cuatepel (2), and one sample each from fractional crystallization) can only crystallize low-Fo olivines with less Tuxtepec, Yecahuazac and Popocateptl volcanoes (Table 1). The Ni (Straub et al., 2008b). sample set includes high-Nb arc basalts and basaltic andesites (18– High-Ni olivines observed in intraplate basalts (Sobolev et al., 16 ppm Nb, Nb/La~0.7–1.2) next to the volumetrically dominant low- 2005) have been suggested to indicate the presence of secondary Nb (4–14 ppm Nb; Nb/Lab0.1–0.8) calc-alkaline basalts to andesites veins of olivine-free ‘reaction pyroxenites’ in a peridotite mantle with the typical high ratios of fluid mobile large-ion lithophile source. Such veins may form following the infiltration of the silicic element to high-field strengths elements (Table 1). We report bulk melts from subducted eclogite into the surrounding peridotite mantle rock analyses for selected major and trace elements, measured by DCP that would locally transform mantle olivines into reaction pyroxenes and ICP-MS methods, respectively, major element oxides and Ni at sub-solidus conditions (Sobolev et al., 2005). Because reaction abundances in olivines measured by electron microprobe, 3He/4He pyroxenes inherit ca. 80–86% of the Ni content of the original olivine isotope analyses on olivine separates as well as Nd isotopic (as being diluted by the addition of silica, Sobolev et al., 2005), but compositions of bulk rock measured by thermal ionization mass have a three times lower KdNi than olivine (Beattie et al., 1991), their spectrometry. Analytical details are given in the Appendix. The new partial melts are Ni-rich and precipitate high-Ni olivines at shallow data are presented in Tables 1 and 2, and Appendix Table 3. pressures, give or take additional dilution by partial melts from peridotite (Sobolev et al., 2005, 2007).
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