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Fisher Caldera, Unimak Island, Aleutians Journal of Volcanology and Geothermal Research 111 (2001) 35±53 www.elsevier.com/locate/jvolgeores Low-d 18O tephra from a compositionally zoned magma body: Fisher Caldera, Unimak Island, Aleutians Ilya N. Bindeman*, John H. Fournelle, John W. Valley Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, WI 53706, USA Received 13 August 2000; revised 21 January 2001; accepted 20 February 2001 Abstract We present the results of an oxygen isotope study of phenocrysts in pumice clasts and ash layers produced by the 9100 yr BP composite dacite-basaltic andesite climactic eruption that formed Fisher Caldera in the eastern Aleutians. Products of the eruption represent a low-d 18O magma with d 18O plagioclase (14.79 ^ 0.24½) and clinopyroxene (3.81 ^ 0.23½) correspond- ing to equilibrium at magmatic temperatures. Dacitic and overlying basaltic±andesitic tephra of the climactic eruption, subsequent intracaldera basaltic to andesitic lavas, and a cumulate inclusion, are similarly low in d 18O. Other analyzed lavas and pyroclastics of Unimak island and the lower Alaska peninsula, as well as precaldera Fisher basalt, have normal d 18O magmatic values (.15.5½). We propose a model in which prior to 9100 yr BP, normal mantle-derived basaltic magma coalesced in a large shallow precaldera magma chamber during Late Wisconsin glaciation. Lowering of magmatic d 18O resulted then from long-term assimilation of ,5±10% of syn-glacial hydrothermally-altered country rocks. Differentiation of basaltic magma was concurrent with this assimilation and produced low-d 18O Fisher dacites, cumulates, and post-caldera crystal-richer lavas. We propose the use of d 18O values of phenocrysts (especially alteration-resistant pyroxene) in tephra as a tool for tephrochronological and tephrostratigraphic correlation. Distinctly low-d 18O values are useful in identi®cation of the Fisher ash in the eastern Aleutians and in the lower Alaska Peninsula. q 2001 Elsevier Science B.V. All rights reserved. Keywords: oxygen isotopes; assimilation; tephrochronology; tephra; volcanic ash; Fisher Caldera; glaciation; Aleutians; Alaska Peninsula; Unimak Island; Tugamak Range; Cold Bay 1. Introduction products in other subarctic areas such as the Aleutian and Kamchatka arcs, that might be loci for low-d 18O Low-d 18O magmas contain oxygen that was magmas. The generation of low-d 18O magmas will be derived from surface waters. Although low-d 18O enhanced in areas of glaciation where large reservoirs magmas are globally rare, they are abundant in of low-d 18O water are available. The extent of the last Iceland (Muehlenbachs et al., 1974; Condomines et glaciation on Alaska and the Aleutian Islands is given al., 1983) and Yellowstone (e.g. Hildreth et al., 1984; in Hamilton (1994) and Mann and Peteet (1994). Bindeman and Valley, 2000). There have been limited During the last glacial maximum (,24,000± oxygen isotope studies of Late Quaternary volcanic 12,000 yr BP), a 300±500 m ice cap covered the lower Alaska Peninsula and the eastern Aleutians, including Unimak Island, and signi®cant alpine * Corresponding author. Tel.: 11-608-262-7118; fax: 11-608- 262-0693. glaciers could have survived longer. Glaciers could E-mail address: [email protected] (I.N. Bindeman). have contributed extremely light meteoric waters 0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0377-0273(01)00219-0 36 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 (,225½) to precaldera hydrothermal systems, Fournelle (1990) conducted a reconnaissance study promoting wide-spread 18O depletion of country of a portion of the caldera in 1989. Fisher Caldera rocks around the magma chamber. For comparison, (Fig. 2) was probably preceded by a series of strato- the d 18O of Pleistocene ice in Camp Century and cones (R.L. Smith, pers. commun., 5/10/85; Stelling other ice cores in Greenland (Dansgaard et al., and Gardner, 2000), such as Eickelberg Peak 1993) is 220½ lower than d 18O of 9100 yr BP ice. (composed of olivine-bearing basalts and basaltic In addition, older fossil hydrothermal systems with andesites), which is now truncated by the north d 18O-depletion down to 25½ are described in the caldera wall. Pleistocene or older rocks are exposed island of Unalaska (e.g. Per®t and Lawrence, 1979). north of Fisher Caldera in the Tugamak Range (Fig. 1) During a search for low-d 18O magmas, we and on the Whaleback, and may represent fragments analyzed products of major explosive volcanic erup- of an older volcano, or pre-existing caldera. tions in the eastern Aleutians and southwestern Alas- Postcaldera structures include smaller intracaldera kan Peninsula, which are preserved regionally and stratocones (Mt Finch), maars (Pyro Hill), and a serve as important tephrochronological and tephro- breached scoria cone (Nick's Cone), possibly related stratigraphic markers. We discovered that tephra and to a ®ssure eruption of Mt. Finch. The cones mostly pyroclastic ¯ows produced during the 9100 yr BP consist of basalt and basaltic andesite lavas, and pyro- Fisher Caldera formation represent a low-d 18O clastic surge deposits are preserved on the ¯anks of magma not previously reported in the Aleutians. Mt. Finch. Maar deposits occasionally contain high- The oxygen isotope composition of associated pheno- Mg cumulate cobbles (11 wt% MgO, 12 wt% CaO). crysts can be used as a tephrostratigraphic tool for Fumarolic activity is present inside of the caldera. correlation of distant and sometimes ambiguous ash Miller and Richter (1994) estimated the edi®ce of layers. The distinct Fisher tephra horizon is useful for Fisher volcano, before caldera collapse, as more than providing ages of sea level stand near Cold Bay 300 km3. Our estimate of the area of the caldera is 11,000±13,000 yr BP (Jordan and Maschner, 2000) 110±115 km2. Using the Smith (1979) correlation and may ultimately be helpful for dating and correlat- between the caldera area and eruptive volume, and ing archaeological sites, relevant to human migration assuming an eruptive draw-down of 500±1000 m, through the lower Alaska Peninsula and the Aleutians. the erupted volume of magma could have been 55± 115 km3. Only a small amount of the Fisher pyroclas- tic ¯ow and ash has been accounted for Ð mainly due 2. Fisher Caldera and Fisher pyroclastic deposits to lack of mapping Ð but much of it could be covered by younger eruptive products of Shishaldin and West- Fisher Caldera on Unimak Island (Fig. 1) is the largest dahl/Pogromni volcanoes, or reside under the sea. late Quaternary caldera (18 £ 11 km) of the Aleutians The ignimbrite out¯ow sheets are found in all direc- (Fournelle et al., 1994; Miller and Richter, 1994). Funk tions around Fisher Caldera and are exposed along the (1973) described distinctive tephra near Cold Bay and northern and southern shores of Unimak (Miller and suggested the source was one or more volcanic vents on Smith, 1977, their Fig. 2). Presumably they extend Unimak about 10,000 yr ago. Miller (pers. commun., beneath present sea level, for ignimbrite ¯ows can 7/26/96) and Miller and Smith (1977, 1987) studied penetrate and weld under water (Fisher and Schminke, proximal and distal deposits (including determining 1984). The pyroclastic ¯ows that resulted in Fisher 14C ages of associated organic material) and concluded out¯ow sheets demonstrated great mobility, that an eruption of Fisher at ca. 9100 yr BP had produced surmounting the .300 m high Tugamak Range ash ¯ow tuffs on Unimak as well as a distinctive regional (Miller and Smith, 1977). We suggest that alpine air fall strata. For clarity, we will refer to the eruptive glaciers between Fisher and Tugamak might have products from the caldera-forming event at ca 9100 yr lessened the relief of the Tugamak mountains, and BP as the `Fisher tephra'. Further tephrochronologic also would help to explain the patchwork pattern of work on the lower Alaska Peninsula has yielded more ignimbrite mapped by Miller and Smith (1977). Fisher corroborating data for this tephra, including additional tephra was mostly deposited to the east (current winds 14C dates (Dochat, 1997; Carson, 1998). are mostly westerly), i.e. on the neighboring Alaska I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 37 Fig. 1. Unimak Island and the western Alaska Peninsula (A), Fisher Caldera (B, C) and the position of analyzed samples. Notice geomor- phological features on synthetic-aperture radar image indicative of pumice deposition around Fisher Caldera. Mt. Finch is a composite volcano and Pyro Hill is a maar; Nick's cone is a young, monogenetic, breached scoria cone; Eickelberg Peak is one of the pre-caldera composite volcanoes cut by the caldera wall. Numbers correspond to sample location in Table 1. 38 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 Fig. 2. Views within Fisher Caldera, Summer 1989. (A) View toward west (from right to left) Eickelberg Peak (3590 ft) and adjacent unnamed peak (2958 ft), both making up part of the caldera's northern wall. To their left, Pogromni volcano (6568 ft), Faris Peak (5426 ft) and Westdahl Peak (5118 ft). The lower slope of Mt Finch is in the near foreground. (B) View to south from isthmus between Metrogoon East and West Lakes, which are at ,600 ft elevation. Mt Finch (1567 ft) is a small composite volcano with fumoralic activity on its west ¯anks. (C) View to east from near the base of Eickelberg Peak: a maar (Pyro Hill) is in the foreground. Shishaldin smokes in the background. (D) View to north from near Mt Finch. Northern wall of the caldera, and unnamed peak (2322') in the background about 1 km from caldera wall.
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