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 , , 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 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 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 e foreground. ), both making up part s in the near foreground. ) View to north from near 600 ft elevation. Mt Finch (1567 ft) is a small composite volcano with fumoralic activity , 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 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 i (B) View to south from isthmus between Metrogoon East and West Lakes, which are at Mt Finch. Northern wall of the caldera, and unnamed peak (2322') in the background about 1 km from caldera wall. Breached scoria cone (Nick's Cone) in th 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 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 39 Peninsula (Miller, verbal comm. 7/26/96), making it as regional comparison with other were an important tephrochronologic horizon there provided by T. Miller, M. Mangan, T. Neal, S. Dreher (Carson, 1998). and J. Faust Larsen. The Fisher tephra in the Cold Bay area (110± Phenocrysts of plagioclase, clino- and orthopyrox- 120 km east of Fisher Caldera) is a distinctive ene, olivine and magnetite were hand-picked from sequence of 4 ash/lapilli layers with a total thickness lava, pumice and scoria clasts, welded tuff and ash. of 10±50 cm thick (Fig. 3a), within a thicker package The use of laser ¯uorination (e.g. Valley et al., of up to 10 distinct ash/lapilli layers, with some inter- 1995) allowed us to obtain precise (^0.1½, 1 std. vening soils and sands. The complete assemblage dev.) oxygen isotope analyses of only 1±2 mg of comprises the upper member of the Cold Bay Forma- material (Table 2). In most cases, larger plagioclase tion (Funk, 1973). phenocrysts were analyzed individually. The pumic- A distinctive feature of the Fisher tephra in the Cold eous nature of most tephra samples in general Bay area is a 5±15 cm thick band of black basic scoria makes it impractical to study the whole-rock d 18O overlying a 2±10 cm thick tan dacitic layer, with compositions because vesiculated glass easily mingled compositions at the interface. These two hydrates and/or exchanges oxygen, thus altering layers are immediately underlain by 5±10 cm of its d 18O in surface environments. Instead, we very ®ne, yellow-olive brown dacitic ash, sometimes analyzed d 18O in coexisting phenocrysts. Small strati®ed. The whole assemblage is covered by a 10± values of D18O (Plag 2 Px), typically 1±1.5½, 20 cm heterogeneous mixture of pumice, scoria and indicate the absence of secondary alteration of lithic material. In some locations, a layer may be miss- phenocrysts and for equilibration at magmatic ing. In the Cold Bay area, the tephra is medium-to temperatures (e.g. Chiba et al., 1989). ®ne-grained lapilli, and well-sorted. The same Major element whole rock and/or glass chemical sequence, thicker and with pumice clasts to 6 cm compositions for the samples studied in this paper diameter, is present 12 km east of Fisher, near are given in Table 1.Whole-rock major and trace Shishaldin volcano (Figs. 3b,c). This sequence of element analyses were performed by XRF in the silicic pumice overlain by basic scoriaceous tephra Department of Geosciences at Franklin and Marshall was interpreted as representing the inverted stratigra- College and in the Department of Mineral Sciences of phy of a zoned magma chamber, tapped by a powerful the Smithsonian Institution. Mineral and glass caldera-forming eruption (Miller and Smith, 1977). analyses were made by WDS on the University of Clasts at the pumice±scoria interface have two Wisconsin Cameca SX-51 electron microprobe. quenched glasses, evidence of contemporaneous Analytical conditions were 15 kV and 6 nA, with ma®c and silicic liquids. glass and minerals used as standards. For glass analyses a defocused beam was used.

3. Sample collection, preparation and analytical technique 4. Petrography

Samples of Fisher tephra and lavas were collected Dacite from the Fisher tephra that we examined from Unimak Island during several ®eld seasons: in contains a variable proportion of phenocrysts (from 1984±85 on the northwest ¯ank of Shishaldin and ,3%± , 10%) consisting of tabular plagioclase from adjacent lowlands northeast of Fisher Caldera (0.5±1 mm, An55±35), elongated and slightly (Fournelle, 1988), and in 1989 from within Fisher normally-zoned clinopyroxene (Wo40±38En37±36Fs24± Caldera (Fournelle, 1990). Samples of Fisher tephra 25) and more dominant orthopyroxene (Wo4.1±3.4En51± from the Cold Bay region on the Alaska Peninsula 47Fs36±48), rare Fe-rich olivine (Fo33), magnetite and (Fig. 1) were collected during the 1995±96±97 ®eld minor ilmenite. Quartz is absent. Zircon is also absent, seasons, and studied by Tina Dochat (Dochat, 1997) as was demonstrated by HF dissolution of individual and Eric Carson (Carson, 1998). Several important pumice clasts. samples for wider Fisher tephra distribution as well Basaltic andesite from the Fisher tephra contains 40 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53

Table 1 Major element chemical analyses of pyroclastics and lavas from Fisher Caldera and other volcanoes considered in this paper.

p N Sample Type SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2OK2O

Pre-9100 BP pre-caldera lavas 1 FC-43 WR 48.92 1.07 19.00 9.52 0.16 5.13 12.08 2.59 0.39 9100 BP Fisher eruption 2 SH-6 WR 67.89 0.65 15.29 4.80 0.19 0.89 2.57 5.53 2.28 3 SH-141 WR 64.14 0.72 15.97 4.87 0.22 0.99 2.73 5.54 2.17 GL 69.26 0.59 15.08 4.46 0.21 0.63 2.52 4.54 2.44 4 SH-117 WR 65.75 0.99 14.86 4.71 0.20 0.69 2.42 5.23 2.33 GL 68.12 0.62 14.68 4.08 0.23 0.61 2.38 4.66 2.47 4 SH-118 WR 53.71 1.45 15.91 10.65 0.22 3.40 7.62 3.98 1.06 GL 56.02 1.59 16.01 9.01 0.19 3.46 8.22 3.76 1.18 4 SH-119b WR 58.49 1.18 15.72 8.32 0.22 2.51 5.74 4.45 1.52 GL 60.82 1.05 15.32 8.26 0.24 1.93 5.17 4.23 1.72 4 SH-119w GL 70.58 0.60 15.31 4.26 0.19 0.57 2.30 3.25 2.36 5 FC-61 WR 63.27 0.90 15.43 5.68 0.20 1.06 3.11 5.21 2.02 GL 69.16 0.80 15.72 4.99 0.23 0.79 2.71 3.06 2.18 6 86Amm187 GL 69.83 0.48 15.19 4.00 0.12 0.53 2.28 2.23 2.57 7 96JF-16B WR 60.34 0.71 16.28 5.23 0.24 0.97 2.33 4.58 1.98 GL 69.05 0.57 15.18 4.26 0.17 0.71 2.43 4.92 2.33 7 96JF-16C WR 49.75 1.60 16.64 11.90 0.21 3.61 7.77 2.93 0.77 GL 53.36 1.64 15.89 12.00 0.23 3.80 8.47 2.50 1.05 8 86Amm162B GL 69.28 0.58 15.51 4.40 0.19 0.57 2.32 3.89 2.45 Post-9100 intracaldera lavas 9 FC-7 GL 63.29 1.63 14.28 7.05 0.32 1.60 3.88 5.68 1.84 10 FC-54 WR 52.18 1.21 16.61 8.99 0.19 4.58 8.74 3.48 0.96 11 FC-3 WR 53.27 1.25 16.59 9.06 0.19 3.87 8.46 3.53 1.06 12 FC-30 WR 51.89 1.23 16.99 9.48 0.17 4.30 8.89 3.38 0.93 13 FC-57 WR 48.20 0.70 13.49 8.76 0.15 10.77 11.71 1.48 0.59 Cold bay tephra horizons 14 96Amm3 GL 78.14 0.13 13.35 1.69 0.13 0.14 1.32 2.34 1.47

15 96AP-19 GL 64.07 1.23 15.66 6.11 0.20 1.47 3.87 4.07 2.06 16 96JF-9a GL 76.14 0.12 13.57 1.69 0.19 0.15 1.40 4.60 1.52

17 96TDS-15A GL 69.06 0.57 15.10 4.06 0.16 0.62 2.30 4.68 2.37

18 96JF-8H GL 63.53 0.92 15.54 6.42 0.17 1.44 3.86 4.40 1.90 Other volcanoes 19 99S9M1 GL 76.03 0.29 13.43 1.77 0.05 0.18 0.95 2.74 4.01 19 99S9M3 ND ND ND ND ND ND ND ND ND 20 99S1M1 GL 76.39 0.27 13.68 2.02 0.07 0.20 1.22 2.85 3.58 21 97AC14 WR 68.48 0.73 14.87 3.70 0.16 0.74 2.30 5.20 2.98 21 97AC19 WR 67.21 0.85 15.37 4.07 0.17 1.01 2.87 5.24 2.73 22 SH-61 WR 66.93 0.57 15.63 4.29 0.11 0.73 3.10 4.91 2.26 23 SH-15 WR 48.70 1.18 15.26 9.72 0.16 8.53 10.95 2.61 0.71 24 SH-5 WR 49.83 1.67 20.29 9.70 0.18 3.62 10.49 3.20 0.56 25 SH-134 59.84 1.07 16.74 7.06 0.18 1.93 5.30 4.51 1.63 26 NW95-1 WR 54.70 1.75 15.89 10.56 0.23 3.30 7.41 4.33 1.21 27 SH-1d WR 46.65 1.38 21.45 9.43 0.14 3.69 12.51 2.52 0.93 28 SS-2 WR 71.0 ND ND ND ND ND ND ND ND 28 SB8740 WR 69.84 0.62 14.86 3.21 0.11 0.84 2.65 5.33 2.42 29 JLOK42b WR 68.5 ND ND ND ND ND ND ND ND 29 JLOK42c WR 56.0 ND ND ND ND ND ND ND ND 29 OA-1 WR 49.0 ND ND ND ND ND ND ND ND Notes: glass analysis performed at Department of Geology and Geophysics, UW-Madison. Distance from Fisher Caldera in kilometers. Abbreviations: WR ˆ whole rock; GL ˆ glass, NA±not analyzed; ND ˆ no data available. References: 1 ˆ Fournelle, 1988. 2 ˆ Fournelle et al., 1994. 3 ˆ Carson, 1998. 4 ˆ Fournelle, unpublished data. 5 ˆ S. Dreher, written communication, 2000. 6-Tina Neal 1995, written communication I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 41

N P2O5 LOl SUM Sample description Locality (see Fig. 1) Distance from Ref Fisher Caldera, km

1 0.13 0.81 100.01 basaltic ¯ow Eickelberg Peak, caldera wall 0 4

2 0.17 0.35 100.69 glassy clast in welded tuff NE of Fisher Caldera 12 1 3 0.19 NA 97.63 5±10 cm pumice clasts NW slope of Shishaldin, Unimak I13 1 NA NA 99.71 NE of Fisher Caldera 4 4 0.16 3.51 100.68 5±10 cm dacitic clasts NE of Fisher Caldera 12 1 NA NA 97.83 4 4 0.27 1.32 100.58 NE of Fisher Caldera 12 1 NA NA 99.43 4 4 0.28 0.72 100.07 5±10 cm andesitic scoria clasts NE of Fisher Caldera 12 4 NA NA 98.73 4 4 0.08 NA 99.48 white stringers in SH-119b NE of Fisher Caldera 12 4 5 0.26 2.36 100.13 dacitic ash on surface, inside Fisher Caldera 0 4 0.17 NA 99.82 4 6 NA NA 97.23 layer of lapilli (2±3 mm) Cape Lapin, NNE of Fisher Caldera 23 4 7 0.23 7.51 100.40 dacitic ash Cold Bay, ENE of Fisher Caldera 116 3 0.14 NA 99.81 4 7 0.32 4.32 100.69 basic ash Cold Bay, ENE of Fisher Caldera 116 3 0.31 NA 98.94 4 8 0.19 NA 99.38 ®ne dacitic ash Ukolnoi Island 170 4

9 NA NA 99.21 andesitic pumice bed Fisher Caldera 0 4 10 0.20 2.81 100.28 young basaltic scoria cone Fisher Caldera 0 4 11 0.21 1.03 98.89 basaltic lava Mt Finch, bottom 0 4 12 0.21 1.09 98.94 basaltic lava Mt Finch, top 0 4 13 0.12 4.22 100.57 cumulate xenolith Pyro Hill 0 4 Source volcano tephra code 14 0.00 NA 98.72 3±8 mm lapilli Round top vol. (found at Mt. 125 3 Dutton) 15 0.42 NA 99.22 3±8 mm lapilli tephra layer ªAº, .9100 BP 84 3 16 0.00 NA 99.42 3±8 mm lapilli tephra layer ªBº, .9100 BP, round 116 3 top 17 0.18 NA 99.17 3±8 mm lapilli tephra layer ªCº, ,9100 BP, 116 3 Fisher? 18 0.23 NA 98.46 3±8 mm lapilli tephra layer ªDº, ,9100 BP 116 3

19 0.01 NA 99.46 ®ne ash Emmons Lake ash, 1,80,000 BP 165 4 19 ND ND ND Ignimbrite above ash Emmons Lake ash, 1,80,000 BP 165 20 0.01 NA 100.30 3 cm rhyolite pumice Emmons Lake ash, 18,000 BP 200 4 21 0.16 NA 99.33 Ignimbrite Aniakchak Caldera 300 5 21 0.22 NA 99.75 Ignimbrite Aniakchak Caldera 300 5 22 0.12 0.31 99.16 welded tuff Round top volcano (False Pass tuff) 50 4 23 0.18 0.95 99.17 basaltic lava Shishaldin volcano 20 2 24 0.27 0.72 100.77 basaltic lava Shishaldin volcano 20 2 25 0.37 1.47 100.32 dense andesite block Shishaldin volcano 20 4 26 0.35 NA 99.73 basaltic andesite lava Westdahl 40 6 27 0.24 0.88 100.00 basaltic lava Whale back 10 1 28 ND ND ND rhyodacite ignimbrite Seguam caldera 300 28 0.13 0.62 98.94 rhyodacite lava Seguam intracaldera dome 300 29 ND ND ND 2049 BP dacite Okmok caldera wall 200 29 ND ND ND 2050 BP andesite Okmok caldera wall 200 29 ND ND ND 1946 basaltic ash Okmok inside of caldera 200 42 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 43 ,5% of the same phenocrysts, but with more calcic respect to the bulk chemical composition, the compo- and complexly zoned plagioclase (An79±40), more Mg- sition of residual liquid, and phenocryst content and rich olivine with normal zoning (Fo76±72). Orthopyr- composition. Whole-rock incompatible trace element oxene (Wo5±3En47±51Fs45±54) and normally zoned concentrations in the dacitic portion of Fisher tephra clinopyroxene (Wo48±38En35±44Fs12±27) have slightly are, on average, twice that of its basic portion (Table more Mg-rich compositions than these phenocrysts 3). Mass balance calculations using the above pheno- in dacites. The glass composition in the basic portion cryst compositions and whole-rock values (Table 1) is typically 56 wt% SiO2 vs. 69 wt% in the dacitic make it possible to obtain SH141 dacite after 53% of portion (see Table 1); both have 3 and 4.9 wt% higher fractional crystallization of basaltic andesite SH118.

SiO2 than their respective whole-rock compositions. These observations suggest that dacites could be Fe±Ti oxide calculated temperature in dacite derived by 45±55% differentiation of basic magma.

(sample SH-141) is 893±8308C and log fO2 is 213 However, complex zoning of many feldspar pheno- to 214.2 bar, plotting along the QFM buffer (Ghiorso crysts suggest that episodes of assimilation and/or and Sack, 1991). The basic portion of Fisher tephra magma mixing occurred as well. (sample SH-118) has, respectively, higher Fe±Ti oxide equilibration temperatures in the range 940 ± 8608C, log fO2 is 212.3 to 214.8 bar, along the same 5. Oxygen isotope results buffer. This observation con®rms that the pre-climac- tic Fisher magma chamber was not only zoned with We analyzed oxygen isotope ratios in 11 samples of respect to compositions of the residual melt and the Fisher tephra from 8 localities which represent phenocrysts, but, expectedly, with respect to tempera- ash-fall, welded ignimbrite, and lapilli-bomb beds ture too. (Table 2). More distant localities (as far as 170 km Post-caldera Fisher basalts and basaltic andesites from the Fisher Caldera) contain only ®ne-grained from the intracaldera Mt Finch are more crystalline ash, but phenocrysts are none-the-less present. We (20±35% phenocrysts), with larger phenocrysts: clin- also studied postcaldera eruptive products within the opyroxene (Wo39±43En43±45Fs13±16), plagioclase (An75± caldera, one sample from the precaldera wall, and 50), and olivine (Fo70±67). Plagioclase has sieved samples from the neighboring strato-volcanoes and morphology and patchy zoning, with intervals of calderas. Four regionally distributed dacitic tephra normal and reverse zoning, suggestive of resorbtion. layers in the neighboring Cold Bay area that occur Cumulate inclusions (i.e. FC-57) contain large (up to above and below the 9100 yr BP Fisher tephra were 14 5 mm) phenocrysts of slightly zoned olivine (Fo89-84) also analyzed (Table 2). Although there are C age and clinopyroxene (Wo46En47Fs7 cores, to constraints on these four layers (Carson, 1998), their Wo41En46Fs13 rims). The groundmass is devitri®ed source volcanoes are not well constrained. Possible and serpentinized. Pre-caldera basalts and basaltic sources of these coarse tephras include the nearby andesites cropping out in the caldera wall at Eickel- dacite-producing volcanoes of Roundtop, Emmons berg Peak contain clinopyroxene (Wo41En46Fs13), Lake and Dutton (Miller et al., 1999). plagoclase (An75-65), and olivine (Fo78-75), and exhibit We ®nd that all analyzed samples from the 9100 yr broad variations (5±25%) in crystal content. BP Fisher tephra are low in their d 18O values. The The compositional diversity of products of 9100 yr d 18O (Plag) and d 18O (Cpx) values in both dacitic BP Fisher eruption, described above, indicates that the and basaltic±andesitic portions of the Fisher tephra pre-climactic Fisher magma chamber was zoned with are nearly identical with the D(Plag 2 Cpx) of ,1½

Fig. 3. Tephra layers from the Fisher climactic eruption. (A) Exposed 4 layers of the Fisher ash from Cold Bay, t115 km from Fisher Caldera. Here, the ®ne tan basal ash (#1) is ,7 cm thick, covered by ,1 cm of tan pumice (#2), then ,2 cm of black tephra (#3), and covered by ,7cm of mixed material (#4). (B) Proximal deposits, 12 km from Fisher Caldera, NW of Shishaldin volcano. Coarse dacite pumices overlain by coarse basaltic tephra. (C) Same location as (B), cleaned up, exposing the multiple layers of surge and airfall deposits below the coarser material. There are approximately 36 cm of the ®ne basal ash (#1), and about 15 cm of the overlying dacitic pumice (#2), overlain by ,18 cm of basaltic tephra (#3), with mingled pumices at the interface of the two. 44 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53

Table 2 Oxygen isotope composition of phenocrysts in Fisher Caldera tephras and lavas, tephras, and other volcanic rocks from the Aleutians and Lower Alaska Peninsula

18 N Sample SiO2, wt% d O, ½

WR PICpx Opx Ol Mt glass

Pre-9100 BP pre-caldera lavas 1 FC-43 48.9 5.79 4.80 9100 BP Fisher Eruption 2 SH-6 67.9 4.61 4.80 3 SH-141 64.1 4.60 3.42 4.15 2.16 4 SH-117 65.8 4.79, 4.51 3.88 4 SH-118 53.7 4.98, 4.93 4.15 3.98 3.34 4.95 4 SH-119b 58.5 4.85 4 SH-119wa 65.7 4.79 5 FC-61 63.3 4.59 6 86AMm187a 64.9 4.85 3.89 7 96JF-16B 60.3 4.83, 4.95 3.88 7 96JF-16C 49.8 4.71 4.04 3.12 8 86AMm162Ba 64.4 4.98 4.59 average, silicic 4.75 ^ 0.05 (10) 3.70 ^ 0.11 (5) average, basic 4.87 ^ 0.06 (4) 4.04 Post-9100 intracaldera lavas 9 FC-7a 58.4 4.58 10 FC-54 52.2 5.1, 4.78 11 FC-3 53.3 4.92 3.94 4.35 12 FC-30 51.9 5.19 4.41 13 FC-57 48.2 3.86 3.32 Tephra horizons 14 96AMm3a,b 73.2 6.33 5.13 15 96AP-19a 59.2 5.87 16 96JF-9aa,b 71.2 6.22 17 96TDS-15Aa 64.2 4.69 18 96JF-8Ha 58.6 6.29 19 99S9M1a 71.1 6.61 4.82 20 99S1M1a 71.5 5.90 Other volcanoes 19 99S9M3a 71.1 6.39 21 97AC14 68.5 6.63 21 97AC19 67.2 6.52 22 SH-61 66.9 6.30 23 SH-15 48.7 4.84 24 SH-5 49.8 5.62 25 SH-134 59.8 6.18 26 NW95-1 54.7 5.81 5.14 27 SH-1d 46.7 5.64 28 SS-2a 71.0 6.14 5.07 29 JLOK42ba 68.5 4.99 4.06 29 JLOK42c 56.0 4.98 3.95 29 OA-1 49.0 4.06 5.23

a SiO2 is calculated as glass composition minus 4.9 wt% SiO2 as de®ned by ®ve samples where both whole rock SiO2 and SiO2 in the glass is available (see Table 1). Where marked by a, plagioclase was analyzed as a single phenocryst. N± see Fig. 1. b The normal d 18O (Plag) as well as rhyolitic glass composition of 96Amm3 (sample collected on the ¯ank of Dutton volcano by T. Miller and identi®ed by him as coming from Roundtop volcano) match closely the d 18O of Carson (1998) Carson (1998a) Cold Bay tephra `layer B' (sample 96JF9a). This provides evidence that Carson's (1998) Layer B was produced by an eruption of Round Top volcano on Unimak between 9300 and 10,200 yr BP. I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 45

Table 3 Trace element concentrations of samples considered in this paper

N Sample SiO2 Ba Cr Ni Rb Sr V Y Zr Zr/R Zr/Ba Ba/Rb (wt%) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Pre-9100 BP pre-caldera lavas 1 FC-43 48.92 147 55 27 7 419 308 23 67 9.6 0.46 21.0 9100 BP Fisher Eruption 2 SH-6 67.89 864 5 2 62 264 8 50 280 4.5 0.32 13.9 3 SH-141 64.14 878 23 11 56 255 28 46 263 4.7 0.30 15.7 4 SH-117 65.75 891 1 3 59 246 7 44 268 4.5 0.30 15.7 4 SH-118 53.71 414 21 6 26 431 305 26 129 5.0 0.31 15.9 4 SH-119b 58.49 43 407 139 201 5 FC-61 63.27 23 314 27 256 7 96JF-16B 60.34 6 223 33 280 7 96JF-16C 49.75 20 371 330 107 Post-9100 intracaldera lavas 10 FC-54 52.18 351 74 21 19 499 284 29 110 5.8 0.31 18.5 11 FC-3 53.27 392 53 13 21 496 258 36 111 5.3 0.28 18.7 12 FC-30 51.89 353 52 16 12 552 239 27 110 9.2 0.31 29.4 13 FC-57 48.20 206 431 140 9 564 240 14 56 6.2 0.27 22.9 Other volcanoes 22 SH-61 66.93 670 56 253 48 275 4.9 0.41 12.0 23 SH-15 48.70 227 374 100 15 530 300 14 86 5.7 0.38 15.1 24 SH-5 49.83 231 22 12 17 629 240 23 76 4.5 0.33 13.6 25 SH-134 59.84 550 5 7 42 429 64 40 246 5.9 0.45 13.1 26 NW95-1 54.70 420 4 0 24 368 276 39 152 6.3 0.36 17.5 27 SH-1d 46.65 405 8 16 25 748 302 19 81 3.2 0.20 16.2 28 SB8740 69.84 870 56 116 189 32 180 1.6 0.21 7.5 and D(Plag 2 Opx) of ,0.6½ (Table 2, Fig. 4 A and Phenocrysts in lavas of the neighboring Shishaldin B), being consistent with equilibration at magmatic , located 20 km to the east of Fisher temperatures of .8008C (Fig. 5). Basaltic andesites Caldera, and in dacites from Roundtop volcano, as have smaller oxygen isotope fractionations between well as basaltic andesites of Westdahl to the west coexisting minerals than dacites, consistent with equi- (Table 2) have normal d 18O values, typical for island libration at ,70±3008C higher temperatures. Postcal- arc volcanics at a given SiO2 content (e.g. Matsushita, dera andesitic and basaltic lavas inside of the caldera 1979). Rhyodacites of Emmons Lake (ca. 180,000 and (Table 2) have similar low d 18O values for pheno- 18,000 yr BP) and Aniakchak (ca 3300 yr BP) crysts. A cumulate (45% crystalline) olivine±clino- caldera-forming eruptions also have normal-d 18O pyroxene±phyric xenolith, found in a maar inside of magmatic values, as do the high-silica rhyolitic to Fisher Caldera (FC-57) has d 18O nearly identical to andesitic 1912 eruptive products of the Valley of both the Fisher tephra and to postcaldera lavas. In Ten Thousand Smokes (Katmai) (Hildreth, 1987), contrast, phenocrysts in the pre-caldera edi®ce Seguam lavas, and three dacitic ash samples which (exposed lava in the caldera wall) have normal d 18O Carson (1998) de®ned as additional tephrochronolo- values (sample FC-43). Olivine phenocrysts in the gical markers exposed in the Cold Bay area. The only basaltic±andesitic scoria of the 9100 yr BP Fisher other similarly low-d 18O values are for phenocrysts in tephra, the cumulate xenolith, and postcaldera lavas andesites and dacites of a complex 2050 yr BP are d 18O ˆ 3.32±3.98½ approximately 1½ lower caldera-forming eruption at Okmok Caldera. compared to olivine phenocrysts in island arcs (Eiler The d 18O values of individual plagioclase pheno- et al., 2000). crysts are plotted against whole-rock SiO2 content 46 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53

Fig. 4. Oxygen isotopic composition of phenocrysts in samples from Fisher Caldera and other Aleutian volcanoes. Data are from Table 2, three analyses of Shishaldin lavas are from Singer et al. (1992). Katmai data are from Hildreth (1987). A-B: Histograms of d 18O (Plag) and d 18O (Cpx) in Fisher tephra and postcaldera lavas de®ne the distinct d 18O signature of Fisher magmas. Also shown in distinct patterns are data from other volcanoes and calderas, plus Cold Bay regional tephras of uncertain sources. The low d 18O (Plag) of one sample suggests a previously unidenti®ed eruption of Fisher.

(Fig. 6). The d 18O value of andesine and labradorite plagioclase phenocrysts can be taken as a proxy for d 18O of their host dacitic to basaltic andesitic magmas. Oxygen isotope plagioclase and quenched glass analyses in basalts with calcium-richer plagio- clase show D18O(Plag 2 melt) < 0.3½ (Eiler et al., 2000). For granitic/rhyolitic magmas, lower tempera- tures and more sodium-rich compositions of plagio- clase lead to a `crossover' where plagioclase becomes a fraction of one per mil lower in d 18O than melt (i.e. 0.1±0.5½: Taylor and Sheppard, 1986). It can be expected that for andesitic to dacitic magma composi- tions, like those found in the Fisher tephra, the D18O(Plag 2 melt) is close to zero. The average d 18O of individual plagioclase pheno- crysts in the Fisher 9100 yr BP dacitic tephra is 14.75 ^ 0.05½ (n ˆ 10) which can be inferred to re¯ect that of the original dacitic magma. This is con®rmed by the d 18O of glass in a sample of black, densely welded obsidian in a pyroclastic ¯ow northeast of Fisher Caldera Fig. 5. d 18O (Plag) vs. d 18O (Cpx, Opx, Ol) diagram indicates that (Sample SH6), which has d 18O ˆ 14.80½ The d 18O isotopic fractionations between minerals are consistent with equili- bration at magmatic temperatures; ®lled and open symbols denote (Plag) of the coexisting Fisher basaltic andesite is phenocrysts in basic and silicic rocks respectively; shown tempera- 14.87 ^ 0.06 (n ˆ 4) and d 18O(O1) ˆ 3.98½. Assum- tures are based on An40-Cpx fractionation from Chiba et al. (1989). ing D(Plag 2 melt) ˆ 0.3 and D(melt 2 O1) ˆ 0.36½ Enclosed area denotes 9100 yr BP Fisher and post-caldera lavas. I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 47

18 for island arc basalts (Eiler et al., 2000), the d Oofthe A similar 0.35½ increase per 10 wt% SiO2 is Fisher basaltic andesite magma is ca. 14.4± 1 4.6½. expected for Fisher magmas if dacites were derived This is about 1½ lower than typical MORB or OIB solely by low-d 18O basic magma fractionation. In this magmas (Eiler et al., 1996). case, the original basalt should be around 4½, or 0.7± Fig. 6 highlights two trends in and around Fisher 0.9½ lower than the observed (Fig. 6), thus excluding Caldera. Normal d 18O magmas from other nearby simple fractionation. These higher d 18O values in volcanoes de®ne a steep positive slope of 0.35½ basic rocks of Fisher tephra and post-caldera basic 18 18 increase in d O/10 wt% SiO2 increase, similar to rocks suggest that either normal-d O basic magma that in other island arcs (e.g. Matsushita, 1979; Taylor was periodically admixed to a low-d 18O basic magma and Sheppard, 1986). Fisher Caldera magmas plot 1± at the bottom of the magma chamber, and not to its 1.5½ to the left of this line, and their genesis requires dacitic portion, or it is a result of a speci®c interplay of either direct origin from low-d 18O mantle-derived assimilation and fractionation. magma, or assimilation of low-d 18O rocks at shallow For our samples, we evaluated major and trace levels. Fisher precaldera lavas, and lavas and tephras element ratios, such as Ba/Rb, Zr/Rb, Zr/Ba, which from neighboring volcanoes on Unimak and the should be insensitive to crystal fractionation of the nearby lower Alaska Peninsula have normal d 18O zircon-undersaturated Fisher magmas (Table 3) and values. This suggests shallow level assimilation of hence might serve as an additional chemical ®nger- low-d 18O hydrothermally-altered rocks at Fisher print of the low-d 18O Fisher magmas, or possible volcano, a locally focused process, rather than differ- sources of assimilation. Rb and K are mostly concen- entiation of a mantle-derived basic magma that has a trated in the glass and are more easily leached by low-d 18O character over a larger (i.e. .20 km) hydrothermal ¯uids, than the more insoluble Zr. geographical extent. However, there are no discernible differences in

18 Fig. 6. d O (Plag) vs. SiO2 plot for rocks from Fisher Caldera and other volcanics from Unimak Island and adjacent Alaskan Peninsula. Data are from Table 2. Thin lines are assimilation±fractional crystallization curves at rate of assimilation ˆ rate of fractionation, tick marks are 5% 18 increments of assimilant added. Numbers on each curve are d O of the assimilant with 55 wt% SiO2. Thick grey line is our ®t to the data points. These results suggest that after assimilating 5±10% of 25to215½ country rocks, assimilation ceased and fractional crystallization continued yielding a nearly vertical trend. The proportion of assimilation is dependent on the initial value of d 18O of the assimilant, and is less 18 sensitive to its SiO2 content. Line with positive slope shows the fractionation trend for plagioclase in Japanese normal-d O arc magmas (Matsushita, 1979); all plagioclase from normal-d 18O Aleutian volcanoes and calderas plot near this trend. Dashed line is a parallel fractiona- tion trend for the hypothetical basic low-d 18O magma (see text). 48 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53

18 18 these trace element ratios between the low-d O (5) Comparing d O with magma SiO2 content for Fisher Caldera and normal-d 18O rocks from the the Fisher samples yields a nearly vertical trend (Fig. nearby Shishaldin volcano and other studied Aleu- 6) of increasing magma SiO2 at constant (and low) tian/Alaska Peninsula volcanoes. Not only are the d 18O. This suggests that an initial basaltic intrusion trace element ratios in the Fisher low-d 18O magma might have rapidly assimilated a ®xed amount of low- similar to that of normal-d 18O magmas of Unimak d 18O country rocks immediately surrounding the (i.e. Shishaldin, Westdahl, and Roundtop volcanoes), magma chamber as well as stoped material. Reiners both low and normal-d 18O rock suites form overlap- et al. (1995) showed that the lower latent heats of ping trends on Harker diagrams (not shown). This assimilation following olivine crystallization moder- implies that assimilation of chemically similar mate- ate assimilation in ma®c systems. Rapid initial assim- rial occurred and that small water/rock ratios have ilation by basalt would have exhausted the low-d 18O modi®ed the d 18O of the assimilant but not the cation component around the magma chamber and the abundances: magma itself became surrounded by a rind of cumu- lates (e.g. FC-57), protecting it from further assimila- tion. Collectively, a rapidly decreasing rate of 6. Genesis of low-d 18O magmas of Fisher caldera assimilation relative to internal differentiation led to nearly a vertical fractionation trend (Fig. 6), creating In attempting to understand possible mechanisms the Fisher low-d 18O silicic magma erupted at 9100 yr for the genesis of low-d 18O magmas at Fisher, we BP. An AFC process by normal-d 18O mantle-derived emphasize six key observations: magma can be modelled to reproduce the observed (1) d 18O depletion is observed in all phenocrysts Fisher 9100 yr BP mineral oxygen isotopic values from basic and silicic portions of the Fisher tephra, (Fig. 6). It would require only 5±10 wt% of an 18O- and they preserve equilibrium fractionations consis- depleted component (25to215½), to cause the tent with magmatic temperatures. observed 1.2±1.5½ depletion in d 18O of the magma (2) The normal d 18O values of olivine phenocrysts with an initial d 18Oof15.8± ˆ 6.3½. in pre-caldera Fisher lavas and in adjacent volcanoes (6) AFC could have operated long enough in the suggest that the low-d 18O values are not mantle- pre-9100 yr BP magma chamber to promote differen- derived, but were acquired in a shallow crustal tiation of many tens (if not hundreds) of cubic km of magma chamber. dacite, and homogenization of oxygen isotopes (3) It is permissive to produce 9100 yr BP Fisher throughout the whole magma chamber. In particular, dacite by 45±55% fractional crystallization of coeval the fact that the earlier crystallizing olivine pheno- basaltic andesite alone. Thus, AFC of precursor basalt crysts in the Fisher 9100 yr BP and post-9100 yr BP predated formation of basaltic andesites. Since low- rocks are in equilibrium with plagioclase and pyrox- d 18O magmas are geochemically similar to normal enes suggests that these phenocrysts had enough time magmas as de®ned by trace element concentrations to exchange oxygen by diffusion with a progressively and ratios (see Tables 1 and 3), the low-d 18O assim- 18O-depleting magma. ilant was likely represented by geochemically-similar Therefore, a long-term residence is a preferred volcanics. mechanism for the genesis of low-d 18Omagmas (4) There is a clear crystal fractionation relationship rather than a syn-eruptive incorporation of meteoric between the glass and whole rock compositions of the water into the magma chamber during Fisher

Fisher 9100 yr BP tephras: the glass SiO2 content of Caldera formation. These data also preclude a tradi- the most ma®c tephra 96JF16C matches the whole tional model in which basic magma shortly rock value of basaltic andesite SH118 (Table 2); the preceded or even triggered the volcanic eruption glass of SH118 is close to the whole rock value of and caldera-formation. A model in which silicic andesite SH119. Mass balance calculations can also magma replenished a basic magma chamber (e.g. be used to show that dacite SH141 could be produced Eichelberger et al., 2000) is also unlikely for Fisher, by 45±55% crystal fractionation of basaltic andesite since both basic and silicic portions of Fisher tephra SH118. are similarly low in d 18O, and we have shown that I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 49 the d 18O signature in central Unimak Island is not survey more volcanoes, the analyses reported in this of a deep origin. paper show that other magmas of Unimak and the The above observations lead us to suggest the Alaska Peninsula are normal in d 18O (see Table 2). model of evolution for the magma erupted at Fisher In particular, calderas comparable in size and age to 9100 yr BP, presented on Fig. 7. Before 9100 yr BP, at Fisher, such as Aniakchak and Emmons Lake, the site of today's Fisher Caldera there existed a series produced normal-d 18O magmas, as well as the Valley of stratocones (Stelling and Gardner, 2000) possibly of Ten Thousand Smokes (Hildreth, 1987). Okmok within an older caldera. The topographic evidence Ð (Umnak Island) is the only other multi-caldera the south scarp of the Tugamak Range (Fig. 1) Ð was volcano in the Aleutians, with d 18O values in between observed over two decades ago by J.R. Hein and D.W. those de®ned by normal magmas and the low-d 18O Scholl, who had noted `an apparent circular structure Fisher magmas. Like the Fisher tephra, the 2050 yr BP on MSS band 4 of LANDSAT photography' (quote Okmok ignimbrite comprise of zoned dacite±andesite from Miller and Smith, 1977). We speculate that this tephra set, with similarly low-d 18O values. Post may be the remaining wall of an older and larger 2050 yr BP intracaldera volcanics at Okmok are also (.30±40 km?) caldera, which would encircle the low-d 18O magmas (Table 2). In contrast to the Aleu- 9100 yr BP caldera, and serve as a hydrogeological tians, low-d 18O magmas are much more abundant boundary for water circulation. among the Holocene and Pleistocene caldera-forming The Fisher Caldera eruption of 9100 yr BP eruptions of Kamchatka (Bindeman et al., 2001) and occurred several thousand years following the end in Iceland Muehlenbachs et al., 1974). of the Late Wisconsin glacial period. For two exten- We believe that the ultimate reason for appearance sive periods of the Pleistocene: the last glacial maxi- of most low-d 18O magmas is related to melting or mum (Late Wisconsin) from (,21,000±14,000) yr assimilation of geochemically similar, but hydrother- BP (cf. Jordan and Maschner (2000), and from mally-altered volcanics of the previous volcanic cycle ,190,000±130,000 yr BP (marine oxygen isotope which hosted a hydrothermal system (e.g. Bindeman stage 6, Dansgaard et al., 1993), the Fisher volcanic and Valley, 2000; Bindeman et al., 2001). The occur- center was probably under a glacial ice with extre- rence of low-d 18O magmas is probably related to the mely low-d 18O (ca. 235½) (e.g. Dansgaard et al., combination of at least several necessary factors, 1993). This water would have participated in hydro- which can be classi®ed into magmatic, tectonic, thermal circulation through, and the alteration of, the hydrologic, and climatic. Shallow position of sub-volcanic crust and depletion of 18O in those rocks, magma chambers, extended magmatic and hydrother- including during long time periods when the area's mal pre-history, and high temperatures of the assim- surface was ice-free. Hydrothermally-altered rocks ilating magma favor lowering of d 18O. Among in similar geothermal areas are typically 10±15½ tectonic factors, an earlier caldera-formation, like in higher in d 18O than the local meteoric water (Criss Yellowstone (e.g. Bindeman and Valley, 2000), is the and Taylor, 1986). Hydrothermally-altered volcanic most important process of burying hydrothermally minerals in the Kra¯a, Iceland drillhole (similar in altered low-d 18O rocks deep in the crust. A high preci- age and latitude to Fisher 9100 yr BP rocks) are as pitation rate, and the light d 18O values of high- low as 212½ and these depletions could have latitude, high-altitude, and intracontinental waters resulted from exchange with meteoric water of 222 (e.g. Yurtsever and Gat, 1981), are favorable hydro- to 227½ (Hattori and Muehlenbachs, 1982). logic and climatic factors. The signi®cantly lower d 18O values of local meteoric waters during and following glaciations is a prime example of the 7. Comparison with other Aleutian and Alaska climatic factor. The role of hydrothermal alteration Peninsula volcanic centers of island arc crust by sea water with d 18O of around 0½ is unknown. If groundwaters in smaller islands The low-d 18O magmas of the Fisher Caldera event are mixtures of meteoric and sea waters, the depleting at 9100 yr BP are the ®rst reported in the Aleutian/ effect of meteoric waters on magmas might be Alaska Peninsula volcanic arc. While there is need to subdued. That may explain the greater abundance of 50 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53

Fig. 7. A model of evolution of Fisher Caldera. (a) The shallow level crust around the pre-9100 yr. BP magma chamber was `pre-treated' by earlier hydrothermal circulation (possibly inside of previous caldera) of strongly d 18O depleted syn- or post glacial waters. Mantle-like, normal- d 18O(16½) basaltic magma intruded into this strongly d 18O-depleted crust, stoping some of the low-d 18O crustal rocks as it moved, and produced a large magma chamber. (b) initial assimilation of these rocks around the magma chamber leads to differentiation and creates low- d 18O cumulate layers that shield the magma chamber from further assimilation; AFC causes production of a compositionally-zoned magma chamber. Fractionation of basaltic andesite to dacite is largely a process of internal differentiation without signi®cant assimilation of low-d 18O country rocks. Replenishments of fresh portions of mantle-derived normal-d 18O basic magmas could have admixed with lower and more basic portions of the strati®ed magma chamber, elevating its d 18O by a fraction of one per mil (see Fig. 6), and sustaining a relatively sharp ma®c± silicic discontinuity between dacitic and basic layers. (c) Explosive eruption and caldera formation 9100 yr BP: eruptive draw-down evacuates most dacitic magma and taps the upper portion of underlying basic magma layer. (d) Post-caldera volcanic activity. Crystal-richer basaltic and andesitic magmas erupted inside the caldera after its formation, and, because they are equally 18O-depleted, are likely to represent the remaining cumulate-richer margins of this layer. I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 51 low-d 18O magmas in larger land masses such as ary alteration longer than the major element composi- Kamchatka (Bindeman et al., 2001) and Iceland tion of glass shards, which are traditionally used for (Muehlenbachs et al., 1974), where meteoric waters correlation purposes. Since feldspars may suffer some (d 18O ,2 9½) dominate over sea waters secondary alteration after deposition, we recommend (d 18O ˆ 0½). that several feldspar analyses be made from a sample, Why are low-d 18O magmas are only found at Fisher or that analyses of coexisting pyroxene be made to and Okmok? The current sample set includes six ensure the preservation of magmatic D(Plag 2 Px). calderas (Fisher, Emmons Lake, Aniakchak, Seguam, The low d 18O signature of the Fisher 9100 yr BP Okmok, and VTTS-Katmai), and tephra layers from tephra makes it a distinct regional tephrochronologi- unknown sources near Cold Bay (Table 2). They all cal/tephrostratigraphic marker. Four other dacitic ash presumably experienced similar glaciations and layers de®ned by Carson (1998) as tephrochronologi- would have similar d 18O depleted water. cal markers in the Cold Bay area were analyzed and We speculate that the condition for appearance of three found to contain normal-d 18O phenocrysts. low-d 18O magmas is the existence of an older over- Signi®cantly, one layer (younger than 6070 yr BP lapping or enclosing caldera which hosted a hydro- but older than 3600 yr BP: Dochat, 1997; Carson thermal system prior to the most recent caldera event. 1998) is similar to Fisher dacite low-d 18O. The source One isolated caldera event does not seem to be enough volcano for this ash layer is not established but the low to produce detectable change in magmatic d 18O. At d 18O value leads us to suggest that it could be from an Yellowstone, the younger 0.6 Ma Yellowstone unidenti®ed eruption of Fisher. Caldera is enclosed in an older 2.0-Ma-old Caldera. Deposition of Fisher 9100 yr BP tephra on Unimak Oxygen isotopes in 0.6 Ma caldera-forming tuff are and the Alaska Peninsula, could have been cata- about 1½ depleted in d 18O relative to the 2.0 Ma strophic to humans and animals. Archaeological ash-¯ow tuff, while post 0.6 Ma intracaldera lavas sites as old as ,8500 yr BP are present on Umnak are 5½ depleted relative to 0.6 Ma tuff (Hildreth et Island in the Aleutians (McCartney and Veltre, al., 1984; Bindeman and Valley, 2000). At Okmok 1996) and ,9000 yr BP on the Alaska Peninsula Caldera, the younger 2050 yr BP caldera is enclosed (Ugashik River: Dumond, 1981). At 8500±9000 yr in an older 8050 yr BP caldera (Miller and Smith, BP, Aleut people had the seagoing technology to 1987) and analyzed mineral separates are 1½ travel throughout, and exploit the resources, of all depleted relative to normal magmas (no 8050 yr BP the islands and lower Alaska Peninsula. At the time tephra is available for d 18O measurement). We of the climactic 9100 yr BP eruption, sea level at east- suggest that Tugamak Range south scarp may be the ern Unimak and the lower Alaska Peninsula was 16± remnant of an older Fisher caldera in which the 25 m above present sea level (Jordan and Maschner, 9100 yr BP caldera is enclosed. 2000), and thus any occupation sites would be much higher than the coastal sites traditionally studied by archaeologists. 8. The use of oxygen isotopes in tephrochronology Identi®cation of Fisher tephra at the base or top of anthropological sites may not only be used for dating We propose that oxygen isotope analysis of pheno- purposes, but also can help to assess the impact of the cryst pairs (e.g. Plag±Px) is a useful tool for tephro- 9100 yr BP Fisher eruption on the early human chronological and tephrostratigraphic correlation, in communities of Southwestern Alaska. cases where a distinctive caldera ®ngerprint exists. Oxygen isotope analyses of phenocrysts by laser ¯uorination is a rapid method requiring only ,1± 9. Conclusions 2 mg of material. Oxygen isotope compositions of unaltered phenocrysts, and fractionation ( ˆ Examination of phenocrysts from pyroclastic temperatures) between them can be used for correla- deposits on Unimak Island and the lower Alaska tion. These parameters are not affected by aeolian Peninsula reveals the existence of a previously differentiation, and in many cases can survive second- unrecognized low d 18O magmas. The average 52 I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53 d 18O values of these Fisher Caldera phenocrysts, as References well as inferred parental magmas, are at least 1½ lower than those of the other Unimak Island volca- Bindeman, I.N., Valley, J.W., 2000. The formation of low-d 18O noes and Alaska Peninsula calderas. Samples from rhyolites after caldera collapse at Yellowstone. Geology 28, Fisher intracaldera vents showed the same pattern. 719±722. 18 Bindeman, I.N., Ponomareva, V.V., Valley, J.W., 2001. Oxygen Development of low-d O magma requires incor- isotope study of products of major eruptions in Kamchatka: poration of oxygen derived from surface waters. abundance of low-d 18O magmas with application to tephrochro- Study of other calderas (e.g. Yellowstone) suggests nology (in preparation). that a single caldera event probably does not suf®- Carson, E.C., 1998. Holocene tephrochronology of the Cold Bay ciently lower the d 18O; rather, the hydrothermal area, Southwest Alaska Peninsula. M.S. Thesis, University of Wisconsin-Madison, 178 pp. circulation following caldera collapse provides Chiba, H., Chacko, T., Clayton, R.N., Goldsmith, J.R., 1989. 18 effective lowering of the d O of the shallow crus- Oxygen isotope fractionations involving diopside, forsterite, tal rocks, which subsequent intrusions then can magnetite, and calcite: application to geothermometry. Geochi- assimilate. Based upon this, as well as topographic mica et Cosmochimica Acta 53, 2985±2995. evidence north of Fisher Caldera, we suggest that Condomines, M., Gronvold, K., Hooker, P.J., Muehlenbachs, K., O'Nions, R.K., Oskarsson, N., Oxburgh, E.R., 1983. Helium, Fisher may have had at least two caldera events. oxygen, strontium and neodymium isotopic relationships in 18 The unique d O ®ngerprint of 9100 yr BP Fisher Icelandic volcanics. Earth and Planetary Science Letters. 66, Caldera tephra show that oxygen isotope analyses of 125±136. phenocrysts is a robust and precise technique, Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahljensen, D., requiring a tiny amount of material which is applic- Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Steffensen, J.P., Sveinbjornsdottir, A.E., Jouzel, J., Bond, G., 1993. able to tephrochronologic or tephrostratigraphic Evidence for general instability of past climate from a 250 kyr studies. Our examination of several unidenti®ed ice-core record. Nature 364, 218±220. Alaska Peninsula ash layers found that one had Dochat, T.M., 1997. Quaternary stratigraphy and geomorphology of the low-d 18O Fisher signature, evidence of a more the Cold Bay region of the Alaska Peninsula: a basis for paleo- recent, hitherto unknown, eruption from a Fisher environmental reconstruction. Ph.D. thesis, University of Wisconsin-Madison, 279 pp. intracaldera vent. Dumond, D.E., 1981. Archaeology on the Alaska Peninsula: The Naknek region, 1960±1975. , University of Oregon Anthropo- logical Paper No. 21. University of Oregon, Eugene. Acknowledgements Eichelberger, J.C., Chertkoff, D.G., Dreher, S.T., Nye, C.J., 2000. Magmas in collision: rethinking chemical zonation in silicic magmas. Geology 28, 603±606. We thank Eric Carson, Dave Mickelson, Tina Eiler, J.M., Farley, K.A., Valley, J.M., Hauri, E., Craig, H., Hart, S., Dochat, Jim Jordan, Margaret Mangan, Tom Miller, Stolper, E.M., 1996. Oxygen isotope variations in ocean island Tina Neal, Scott Dreher, Jessica Faust Larsen, Brad basalt phenocrysts. Geochimica et Cosmochimica Acta 61, Singer, and Fred Anderson for sample donation. Mike 2281±2293. Spicuzza for support during isotope analyses. JF Eiler, J.M., Crawford, A., Elliott, T., Farley, K.A., Valley, J.M., Stolper, E.M., 2000. Oxygen isotope geochemistry of oceanic- gratefully acknowledges National Geographic Society arc lavas. Journal of Petrology 41, 229±256. grant #4075-89 for the 1989 expedition to Fisher and Fisher, R.V., Schminke, H.-U., 1984. Pyroclastic Rocks. Springer, ®eld assistance there by Steve McDuf®e, and 1996 Berlin, p. 472. Cold Bay logistical support from Herb Maschner Fournelle, J.H., 1988. The geology and petrology of Shishaldin and funds from the UW-Geology Albert and Alice Volcano, Unimak Island, , Alaska. PhD thesis, Weeks Fund. JF also thanks Bob Tilling for sharing The Johns Hopkins University, 529 pp. Fournelle, J.H., 1990. Geology and geochemistry of Fisher Caldera, his recollections and slides from his 1979 ®eld trip to Unimak Island, Aleutians; initial results. Transactions, Ameri- Fisher Caldera. We thank DOE (93ER14389) and can Geophysical Union 71, 1698±1699. NSF (EAR99-02973) for funding the oxygen isotope Fournelle, J.H., Marsh, B.D., Myers, J.D., 1994. Age, character, and research. Reviews from Tom Miller and an anon- signi®cance of Aleutian arc volcanism. 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