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Distinguishing Lower and Upper Crustal Processes in Magmas Erupted

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Distinguishing Lower and Upper Crustal Processes in Magmas Erupted Contrib Mineral Petrol (2013) 166:563–585 DOI 10.1007/s00410-013-0891-4 ORIGINAL PAPER Distinguishing lower and upper crustal processes in magmas erupted during the buildup to the 7.7 ka climactic eruption of Mount Mazama, Crater Lake, Oregon, using 238U–230Th disequilibria Meagan E. Ankney • Clark M. Johnson • Charles R. Bacon • Brian L. Beard • Brian R. Jicha Received: 1 November 2012 / Accepted: 24 May 2013 / Published online: 5 July 2013 Ó Springer-Verlag Berlin Heidelberg (outside the USA) 2013 Abstract Uranium-series isotope ratios determined for through dehydration melting of amphibolite that was ini- 35 volcanic rocks and 4 glass separates erupted from *36 tially garnet free. Dehydration melting in the lower crust to 4.8 ka at Mt. Mazama, Crater Lake, Oregon, identify offers a solution to the ‘‘hot-slab paradox’’ of the Cascades, both 230Th-excess and 238U-excess components. U–Th where low volatile contents are predicted due to high slab isotope compositions cover a wide range, exceeding those temperatures, yet higher water contents than expected have previously measured for the Cascade arc. Age-corrected been documented in erupted lavas. The 238U-excess (230Th/232Th) and (238U/232Th) activity ratios range from observed at Mt. Mazama is rare in Cascade lavas, but 1.113 to 1.464 and from 0.878 to 1.572 (44.4 % 230Th- occurs in more than half of the samples analyzed in this excess to 8.8 % 238U-excess), respectively. The most dis- study. Traditionally, 238U-excess in arc magmas is inter- tinctive aspect of the data set is the contrast in U–Th iso- preted to reflect slab fluid fluxing. Indeed, 238U-excess in tope ratios between low and high Sr (LSr, HSr) arcs is common and likely masks 230Th-excess resulting components that have been previously identified in prod- from lower crustal interaction. Isotopic and trace element ucts of the 7.7 ka caldera-forming climactic eruption and data, however, suggest a relatively minor role for slab fluid preclimactic rhyodacite lavas. The LSr component exclu- fluxing in the Cascades. We propose that 238U-excess sively contains 238U-excess, but the HSr component, as reflects melting and assimilation of young, hydrothermally well as more primitive lavas, are marked by 230Th-excess. altered upper crust. The processes related to generating 230Th-excesses such as those recorded at Mt. Mazama are 238U-excess are likely important features at Mt. Mazama commonly observed in the Cascades. Melting models that accompanied development of a large-scale silicic suggest that high 230Th-excesses observed in the more magma chamber that led to the caldera-forming eruption. primitive lavas evolved through mixing of a mantle melt with a partial melt of a mafic lower crustal composition Keywords Mount Mazama Á Crater Lake Á Cascade arc Á that contained garnet in the residuum that was produced Uranium-series isotopes Á Crustal interaction Communicated by M.W. Schmid. Introduction Electronic supplementary material The online version of this article (doi:10.1007/s00410-013-0891-4) contains supplementary The Cascade arc is commonly characterized as a ‘‘hot arc’’ material, which is available to authorized users. based on its slow convergence rates and the youthfulness of the subducting Juan de Fuca and Gorda plates (Wilson M. E. Ankney (&) Á C. M. Johnson Á B. L. Beard Á B. R. Jicha Department of Geoscience, University of Wisconsin-Madison, 1988; Blackwell et al. 1990; Harry and Green 1999; Van 1215 West Dayton Street, Madison, WI 53706, USA Keken et al. 2002; Hildreth 2007). This is consistent with e-mail: [email protected] high melting temperatures that have been estimated for primitive lavas (e.g., Conrey et al. 1997; Elkins-Tanton C. R. Bacon Volcano Science Center, U.S. Geological Survey, et al. 2001; Leeman et al. 2005), which in turn suggests 345 Middlefield Rd, Menlo Park, CA 94025, USA extensive melting and assimilation of significant amounts 123 564 Contrib Mineral Petrol (2013) 166:563–585 of lower crust. The high proportion of mafic lavas in the Cascade arc implies elevated magma fluxes from the 710 mantle, which may be aided by the extensional nature of much of the arc (Hughes and Taylor 1986; Priest 1990; 610 Sherrod and Smith 1990). High magma production rates for the Cascade arc have been inferred based on an estimate of 510 *2,300 volcanoes, indicating a relatively large number of 410 volcanic centers per kilometer of arc length (Hildreth 2007), although other studies contend that the Cascade arc Ba/Th 310 is a relatively low productivity arc based on global surveys (Hughes and Mahood 2011). 210 Three major types of mantle-derived basaltic magma are 110 recognized in the Cascades: high-alumina olivine tholeiites/ low-K tholeiites (HAOT/LKT), calc-alkaline arc basalts 10 (CAB), and highly enriched intraplate basalts (IPB) (Bacon 0.6 0.8 1.0 1.2 1.4 1.6 1.8 238 230 et al. 1997). These magmas represent a range of melting ( U/ Th)0 conditions in terms of depth and temperature and point to 238 230 heterogeneity in the mantle source regions, as well as pos- Fig. 1 ( U/ Th)0-Ba/Th variations for the Cascades relative to other volcanic arcs. Note the dominance of Th-excess [low sible variation in the extent of interaction with the lower 238 230 ( U/ Th)0] and lower Ba/Th ratios at the Cascades relative to crust. From these magmas, three components have been these other arc settings. Data sources: New Britain (NB, Cunningham proposed to exist in the sub-arc mantle of the Cascades: (1) et al. 2009), Aleutians (Al., Turner et al. 1998; Finney et al. 2008), a depleted source, similar to that which produces MORB, Sunda (Su., Hoogewerff et al. 1997), Lesser Antilles (LA, Turner et al. 1996), Tonga-Kermadec (T-K, Turner et al. 1997), and Central (2) subduction components, including those of modern and America (CA, Tepley et al. 2006; Singer et al. 2011). Data for ancient age, and (3) an enriched source that has OIB or Cascades includes this study and data from Jicha et al. (2009b) and IPB affinities (Morris et al. 1990; Bacon et al. 1997; Borg Newman et al. (1986). Relative to many arcs, the Cascades have et al. 1997, 2002; Conrey et al. 1997; Reiners et al. 2000; strong Th-excess and low subduction fluid components Grove et al. 2002; Leeman et al. 2004, 2005; Smith and Leeman 2005; Hildreth 2007; Schmidt et al. 2008; Rowe 44 km thick (Leaver et al. 1984). Seismic velocity mea- et al. 2009; Ruscitto et al. 2010). Trace element and isotopic surements are interpreted to suggest that the lower crust is compositions of recent primitive Cascade lavas suggest around 15 km thick and likely mafic in composition smaller inputs from subduction fluid components as com- (Leaver et al. 1984; Yang et al. 2008; Gao et al. 2011). The pared to other arcs (Morris et al. 1990; Leeman et al. 2004, lower crust may be equivalent to that which forms the 2005; Smith and Leeman 2005; Ruscitto et al. 2010). The basement to the Klamath Mountains or may reflect un- chemical and isotopic compositions of B, Be, and Li do not derplated Cenozoic basaltic rocks (Bacon 1990). A low provide evidence for extensive slab fluid fluxing (Morris P-wave velocity zone measured in the lower crust beneath et al. 1990; Leeman et al. 2004). Moreover, the lack of Crater Lake by Harris et al. (1991) is supportive of lower U-excess in 238U–230Th isotope compositions of most crustal melting and is interpreted to represent the roots of Cascade lavas (e.g., Newman et al. 1986; Volpe 1992; Jicha the volcano. Uncertainty exists, however, as to the degree et al. 2009b) is consistent with a minor subduction fluid of interaction of Cascade magmas with the lower crust and component, which stands in contrast to arcs that contain the role of the lower crust in magma evolution. 238U-excess, and high abundances of fluid-sensitive ele- Determining the roles of amphibole versus garnet in ments, such as Ba (e.g., Turner et al. 1996, 1997; Hoo- deep crustal evolution provides constraints on the temper- gewerff et al. 1997; Tepley et al. 2006). Compared to arc atures, water contents, and bulk composition of the lower magmas worldwide, the Cascade arc generally defines an crust during melting. Important deep crustal roles for these 238 230 end member for low Ba/Th and ( U/ Th)0 (U–Th minerals may exist even when such minerals are not activity ratio corrected for ingrowth of 230Th since the time common or present as phenocrysts in specific suites of of eruption), the latter indicating Th-excess, which may magmas (e.g., Arculus and Wills 1980; Hildreth and broadly indicate a relatively low subduction fluid compo- Moorbath 1988; Koyaguchi and Kaneko 1999; Petford and nent for the modern Cascades (Fig. 1). Gallagher 2001; Annen and Sparks 2002, 2006; Dufek and Crustal thickness beneath the Cascade arc varies from Bergantz 2005). The process of ‘‘cryptic’’ fractionation of 35 to 45 km (Mooney and Weaver 1989), and crustal amphibole in arc magmas in the lower crust, which can interaction is variable along strike of the arc. Near Crater occur near the liquidus at H2O [ 5 % and T \ 1,100 °C, Lake, the continental crust is estimated to be approximately has been invoked to explain the compositions of many arc 123 Contrib Mineral Petrol (2013) 166:563–585 565 magmas (e.g., Davidson et al. 2007; Hildalgo and Rooney The 238U–230Th isotope system, when combined with 2010; Kratzmann et al. 2010; Larocque and Canil 2010).
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