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Jicha Et Al 2009 EPSL.Pdf Earth and Planetary Science Letters 277 (2009) 38–49 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Discriminating assimilants and decoupling deep- vs. shallow-level crystal records at Mount Adams using 238U–230Th disequilibria and Os isotopes Brian R. Jicha a,⁎, Clark M. Johnson a, Wes Hildreth b, Brian L. Beard a, Garret L. Hart c, Steven B. Shirey d, Brad S. Singer a a Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison WI 53706, USA b U.S. Geological Survey, 345 Middlefield Road MS910, Menlo Park, CA 94025, USA c School of Earth and Environmental Sciences, Washington State University, 1228 Webster Pullman, WA 99163-2812, USA d Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington DC 20015, USA article info abstract Article history: A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has Received 25 March 2008 been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components Received in revised form 17 September 2008 during magma genesis. All of the lavas have (230Th/238U) N1 and span a large range in (230Th/232Th) ratios, Accepted 26 September 2008 and most basalts have higher (230Th/232Th) ratios than andesites and dacites. Several of the lavas contain Available online 22 November 2008 antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are Editor: C.P. Jaupart indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to Keywords: eruption. One isochron gave a crystallization age that is ~20–25 ka older than its corresponding eruptive age, Mount Adams and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in Cascade arc the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate 230Th excesses – U Th isotopes (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Os isotopes Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas assimilation interacted with young, mafic lower crust, as indicated by 187Os/188Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large 230Th excesses (25–48%) and high 187Os/188Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization. © 2008 Elsevier B.V. All rights reserved. 1. Introduction been attributed to a variety of processes, including modification of the mantle by melts and fluids (Reagan et al.,1994), flux-ingrowth melting Identifying the physical processes and timescales involved in the (Thomas et al., 2002), slab melting (Dossetto et al., 2003; Sigmarsson origin of mafic arc magmas and their subsequent evolution to more et al.,1998), or lower crustal melting of garnet-bearing and garnet-free silicic compositions is critical for understanding eruptive histories of protoliths (Bourdon et al., 2000; Garrison et al., 2006; Jicha et al., 2007). volcanoes located above subduction zones and ultimately the growth The majority of published 230Th excesses in continental arc lavas come of continental crust. U-series nuclides have been used for decades to from the Cascade arc. provide a temporal link between volcanic output and magma The Cascade arc is unusual among convergent margins because of dynamics in the mantle and crust (see Turner et al., 2003 for a review). the warm thermal structure of the subduction zone, the large number In most active oceanic and continental arcs, U-series data have shown of volcanoes (N2000), and compositional diversity of its eruptive that the mantle wedge is metasomatized by U-rich fluids and/or melts products (Leeman et al., 2005; Hildreth, 2007). Numerous geochem- liberated from the downgoing slab and overlying sediments, producing ical and isotopic investigations have been undertaken to identify the lavas that have 238U excesses (e.g., Elliott et al.,1997). Some continental origin of the distinctive types of primitive magmas erupted through- arc lavas, however, often have significant 230Th excesses, which have out the arc (e.g., Leeman et al., 1990; Bacon et al., 1997; Borg et al., 1997). Re–Os isotope studies from the Lassen and Adams regions were aimed at understanding the behavior of Re and Os in the mantle and ⁎ Corresponding author. crust (Borg et al., 2000; Hart et al., 2002, 2003). Hart et al. (2003) E-mail address: [email protected] (B.R. Jicha). interpreted the radiogenic Os isotope compositions of Mount Adams 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.09.035 B.R. Jicha et al. / Earth and Planetary Science Letters 277 (2009) 38–49 39 Fig. 1. Geologic map of the Mount Adams volcanic field showing the map units analyzed in this study. All other Quaternary units are shaded gray. Solid black triangles denote vent locations. Map modified from Hildreth and Fierstein (1995). Map unit designation is that of Hildreth and Fierstein (1995). 40 B.R. Jicha et al. / Earth and Planetary Science Letters 277 (2009) 38–49 lavas to reflect assimilation of young, mafic lower crust that is not Most of the more than 120 exposed and inferred vents of the Mount detectable using Sr, Nd, or, Pb isotopes. Adams volcanic field (Hildreth and Lanphere, 1994; Hildreth and In this contribution, we integrate the large geologic, chemical, Fierstein, 1995, 1997) lie within a 6 km wide belt of mafic magmatism isotopic, and geochronologic database of the Mount Adams volcanic that extends north–south through the volcanic field for over 50 km, field with new U–Th isotope measurements of lavas and their consti- and this zone may reflect a region where mantle-derived basaltic tuent minerals, as well as new 40Ar/39Ar ages, to explore the timescales magmas are focused through the crust (Hildreth and Fierstein, 1995). of magmatic processes in the mantle wedge and the overlying litho- sphere over the last 350 kyr. Previous U–Th isotope studies of Cascade 3. Petrology of sample suite arc lavas primarily focused on constraining the timescales of crustal processes (e.g., fractional crystallization, mixing, wallrock assimilation) Twenty three samples from the Mount Adams volcanic field were (Bennett et al., 1982; Trimble et al., 1984; Newman et al., 1986; Volpe, analyzed, all of which erupted b342 ka, and include eight basalts, four 1992; Volpe and Hammond, 1991; Reagan et al., 2003) or re-examining basaltic–andesites, eight andesites, and three dacites (Fig. 2) that were 226Ra–230Th crystallization ages (Cooper and Reid, 2003). Our focus at also analyzed by Hart et al. (2003) and Jicha et al. (2009). All lavas have Mount Adams is understanding the ascent history of the magmas. been previously analyzed for major- and trace-element contents by These results are compared to other long-lived arc volcanoes for XRF and INAA methods, as described by Bacon and Druitt (1988) and which 40Ar/39Ar and U–Th mineral isochrons are available, such as those Baedecker (1987). Mount Adams basalts are olivine phyric, and most in the Aleutians (Jicha et al., 2005) and Southern Volcanic Zone of Chile contain plagioclase and clinopyroxene (Hildreth and Fierstein, 1995). (Jicha et al., 2007). Quartz, sanidine, and biotite are absent, and amphibole is present in only two lavas, both of which predate the inception of the 2. Mount Adams stratovolcano (Hildreth and Fierstein, 1995). Andesites and dacites have varying percentages of olivine, clinopyroxene, orthopyroxene, The Mount Adams volcanic field is located at 46.2° N in southern plagioclase, and Fe–Ti oxides. Modal percentages of phenocrysts can Washington, ~50 km east of Mount St. Helens and ~75 km SSE of be found in Hildreth and Fierstein (1995). Porphyritic pyroxene an- Mount Rainier. It has produced the largest volume of Quaternary desite (56–62 wt.% SiO2; Hildreth and Fierstein, 1995) is the dominant eruptive material among Cascade arc stratovolcanoes in Washington lithology at Mount Adams, and samples of this lava type were taken and Oregon (315±84 km3)(Hildreth and Fierstein, 1997), and is only from geographically dispersed locations. surpassed in volume by Mount Shasta in northern California (Sherrod and Smith, 1990). Of the total eruptive volume, basalt constitutes 4. Analytical methods 9–15%, and is primarily restricted to peripheral vents, basaltic andesite and andesite account for 84–89%, and dacite only 1–2% (Hildreth and 4.1. 40Ar/39Ar geochronology Lanphere, 1994). Mount Adams basalts have remarkable composi- tional diversity, including coeval alkalic, low-potassium tholeiitic, 40Ar/39Ar incremental heating experiments were undertaken on and calc-alkaline basalts. Their geographic distribution shows no eleven Mount Adams lavas to confirm the K–Ar ages of Hildreth and relation between composition and distance from the trench. The Lanphere (1994) and improve age uncertainties of selected samples compositional diversity of Mount Adams lavas has been interpreted to for which U–Th mineral isochrons were obtained, which is important reflect variable contributions from MORB- and intraplate-like mantle for discriminating between eruptive and crystallization ages. Holo- sources, modified slightly by a subducted sediment component crystalline groundmass separates were prepared from lava samples (Jicha et al., 2009).
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