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) corresponding 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 (. 1 5.5½). 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
Low-d 18O magmas contain oxygen that was
derived from surface waters. Although low-d 18O
magmas are globally rare, they are abundant in
Iceland (Muehlenbachs et al., 1974; Condomines et
al., 1983) and Yellowstone (e.g. Hildreth et al., 1984;
Bindeman and Valley, 2000). There have been limited
oxygen isotope studies of Late Quaternary volcanic
* Corresponding author. Tel.: 11-608-262-7118; fax: 11-608262-0693.
E-mail address: [email protected] (I.N. Bindeman).
products in other subarctic areas such as the Aleutian
and Kamchatka arcs, that might be loci for low-d 18O
magmas. The generation of low-d 18O magmas will be
enhanced in areas of glaciation where large reservoirs
of low-d 18O water are available. The extent of the last
glaciation on Alaska and the Aleutian Islands is given
in Hamilton (1994) and Mann and Peteet (1994).
During the last glacial maximum (,24,000±
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
glaciers could have survived longer. Glaciers could
have contributed extremely light meteoric waters
0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0377-027 3(01)00219-0
36
I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53
(, 2 25½) to precaldera hydrothermal systems,
promoting wide-spread 18O depletion of country
rocks around the magma chamber. For comparison,
the d 18O of Pleistocene ice in Camp Century and
other ice cores in Greenland (Dansgaard et al.,
1993) is 220½ lower than d 18O of 9100 yr BP ice.
In addition, older fossil hydrothermal systems with
d 18O-depletion down to 25½ are described in the
island of Unalaska (e.g. Per®t and Lawrence, 1979).
During a search for low-d 18O magmas, we
analyzed products of major explosive volcanic eruptions in the eastern Aleutians and southwestern Alaskan Peninsula, which are preserved regionally and
serve as important tephrochronological and tephrostratigraphic markers. We discovered that tephra and
pyroclastic ¯ows produced during the 9100 yr BP
Fisher Caldera formation represent a low-d 18O
magma not previously reported in the Aleutians.
The oxygen isotope composition of associated phenocrysts can be used as a tephrostratigraphic tool for
correlation of distant and sometimes ambiguous ash
layers. The distinct Fisher tephra horizon is useful for
providing ages of sea level stand near Cold Bay
11,000±13,000 yr BP (Jordan and Maschner, 2000)
and may ultimately be helpful for dating and correlating archaeological sites, relevant to human migration
through the lower Alaska Peninsula and the Aleutians.
2. Fisher Caldera and Fisher pyroclastic deposits
Fisher Caldera on Unimak Island (Fig. 1) is the largest
late Quaternary caldera (18 £ 11 km) of the Aleutians
(Fournelle et al., 1994; Miller and Richter, 1994). Funk
(1973) described distinctive tephra near Cold Bay and
suggested the source was one or more volcanic vents on
Unimak about 10,000 yr ago. Miller (pers. commun.,
7/26/96) and Miller and Smith (1977, 1987) studied
proximal and distal deposits (including determining
14
C ages of associated organic material) and concluded
that an eruption of Fisher at ca. 9100 yr BP had produced
ash ¯ow tuffs on Unimak as well as a distinctive regional
air fall strata. For clarity, we will refer to the eruptive
products from the caldera-forming event at ca 9100 yr
BP as the `Fisher tephra'. Further tephrochronologic
work on the lower Alaska Peninsula has yielded more
corroborating data for this tephra, including additional
14
C dates (Dochat, 1997; Carson, 1998).
Fournelle (1990) conducted a reconnaissance study
of a portion of the caldera in 1989. Fisher Caldera
(Fig. 2) was probably preceded by a series of stratocones (R.L. Smith, pers. commun., 5/10/85; Stelling
and Gardner, 2000), such as Eickelberg Peak
(composed of olivine-bearing basalts and basaltic
andesites), which is now truncated by the north
caldera wall. Pleistocene or older rocks are exposed
north of Fisher Caldera in the Tugamak Range (Fig. 1)
and on the Whaleback, and may represent fragments
of an older volcano, or pre-existing caldera.
Postcaldera structures include smaller intracaldera
stratocones (Mt Finch), maars (Pyro Hill), and a
breached scoria cone (Nick's Cone), possibly related
to a ®ssure eruption of Mt. Finch. The cones mostly
consist of basalt and basaltic andesite lavas, and pyroclastic surge deposits are preserved on the ¯anks of
Mt. Finch. Maar deposits occasionally contain highMg cumulate cobbles (11 wt% MgO, 12 wt% CaO).
Fumarolic activity is present inside of the caldera.
Miller and Richter (1994) estimated the edi®ce of
Fisher volcano, before caldera collapse, as more than
300 km 3. Our estimate of the area of the caldera is
110±115 km 2. Using the Smith (1979) correlation
between the caldera area and eruptive volume, and
assuming an eruptive draw-down of 500±1000 m,
the erupted volume of magma could have been 55±
115 km 3. Only a small amount of the Fisher pyroclastic ¯ow and ash has been accounted for Ð mainly due
to lack of mapping Ð but much of it could be covered
by younger eruptive products of Shishaldin and Westdahl/Pogromni volcanoes, or reside under the sea.
The ignimbrite out¯ow sheets are found in all directions around Fisher Caldera and are exposed along the
northern and southern shores of Unimak (Miller and
Smith, 1977, their Fig. 2). Presumably they extend
beneath present sea level, for ignimbrite ¯ows can
penetrate and weld under water (Fisher and Schminke,
1984). The pyroclastic ¯ows that resulted in Fisher
out¯ow sheets demonstrated great mobility,
surmounting the .300 m high Tugamak Range
(Miller and Smith, 1977). We suggest that alpine
glaciers between Fisher and Tugamak might have
lessened the relief of the Tugamak mountains, and
also would help to explain the patchwork pattern of
ignimbrite mapped by Miller and Smith (1977). Fisher
tephra was mostly deposited to the east (current winds
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 geomorphological 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. Breached scoria cone (Nick's Cone) in the foreground.
I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53
Peninsula (Miller, verbal comm. 7/26/96), making it
an important tephrochronologic horizon there
(Carson, 1998).
The Fisher tephra in the Cold Bay area (110±
120 km east of Fisher Caldera) is a distinctive
sequence of 4 ash/lapilli layers with a total thickness
of 10±50 cm thick (Fig. 3a), within a thicker package
of up to 10 distinct ash/lapilli layers, with some intervening soils and sands. The complete assemblage
comprises the upper member of the Cold Bay Formation (Funk, 1973).
A distinctive feature of the Fisher tephra in the Cold
Bay area is a 5±15 cm thick band of black basic scoria
overlying a 2±10 cm thick tan dacitic layer, with
mingled compositions at the interface. These two
layers are immediately underlain by 5±10 cm of
very ®ne, yellow-olive brown dacitic ash, sometimes
strati®ed. The whole assemblage is covered by a 10±
20 cm heterogeneous mixture of pumice, scoria and
lithic material. In some locations, a layer may be missing. In the Cold Bay area, the tephra is medium-to
®ne-grained lapilli, and well-sorted. The same
sequence, thicker and with pumice clasts to 6 cm
diameter, is present 12 km east of Fisher, near
Shishaldin volcano (Figs. 3b,c). This sequence of
silicic pumice overlain by basic scoriaceous tephra
was interpreted as representing the inverted stratigraphy of a zoned magma chamber, tapped by a powerful
caldera-forming eruption (Miller and Smith, 1977).
Clasts at the pumice±scoria interface have two
quenched glasses, evidence of contemporaneous
ma®c and silicic liquids.
39
as regional comparison with other calderas were
provided by T. Miller, M. Mangan, T. Neal, S. Dreher
and J. Faust Larsen.
Phenocrysts of plagioclase, clino- and orthopyroxene, olivine and magnetite were hand-picked from
lava, pumice and scoria clasts, welded tuff and ash.
The use of laser ¯uorination (e.g. Valley et al.,
1995) allowed us to obtain precise (^0.1½, 1 std.
dev.) oxygen isotope analyses of only 1±2 mg of
material (Table 2). In most cases, larger plagioclase
phenocrysts were analyzed individually. The pumiceous nature of most tephra samples in general
makes it impractical to study the whole-rock d 18O
compositions because vesiculated glass easily
hydrates and/or exchanges oxygen, thus altering
its d 18O in surface environments. Instead, we
analyzed d 18O in coexisting phenocrysts. Small
values of D 18O (Plag 2 Px), typically 1±1.5½,
indicate the absence of secondary alteration of
phenocrysts and for equilibration at magmatic
temperatures (e.g. Chiba et al., 1989).
Major element whole rock and/or glass chemical
compositions for the samples studied in this paper
are given in Table 1.Whole-rock major and trace
element analyses were performed by XRF in the
Department of Geosciences at Franklin and Marshall
College and in the Department of Mineral Sciences of
the Smithsonian Institution. Mineral and glass
analyses were made by WDS on the University of
Wisconsin Cameca SX-51 electron microprobe.
Analytical conditions were 15 kV and 6 nA, with
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
from Unimak Island during several ®eld seasons: in
1984±85 on the northwest ¯ank of Shishaldin and
from adjacent lowlands northeast of Fisher Caldera
(Fournelle, 1988), and in 1989 from within Fisher
Caldera (Fournelle, 1990). Samples of Fisher tephra
from the Cold Bay region on the Alaska Peninsula
(Fig. 1) were collected during the 1995±96±97 ®eld
seasons, and studied by Tina Dochat (Dochat, 1997)
and Eric Carson (Carson, 1998). Several important
samples for wider Fisher tephra distribution as well
Dacite from the Fisher tephra that we examined
contains a variable proportion of phenocrysts (from
,3%± , 10%) consisting of tabular plagioclase
(0.5±1 mm, An55±35), elongated and slightly
normally-zoned clinopyroxene (Wo40±38En37±36Fs24±
25) and more dominant orthopyroxene (Wo4.1±3.4En51±
47Fs36±48), rare Fe-rich olivine (Fo33), magnetite and
minor ilmenite. Quartz is absent. Zircon is also absent,
as was demonstrated by HF dissolution of individual
pumice clasts.
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.
SiO2
TiO2
Al2O3
FeO p
48.92
1.07
19.00
9.52
0.16
5.13
WR
WR
GL
WR
GL
WR
GL
WR
GL
GL
WR
GL
GL
WR
GL
WR
GL
GL
67.89
64.14
69.26
65.75
68.12
53.71
56.02
58.49
60.82
70.58
63.27
69.16
69.83
60.34
69.05
49.75
53.36
69.28
0.65
0.72
0.59
0.99
0.62
1.45
1.59
1.18
1.05
0.60
0.90
0.80
0.48
0.71
0.57
1.60
1.64
0.58
15.29
15.97
15.08
14.86
14.68
15.91
16.01
15.72
15.32
15.31
15.43
15.72
15.19
16.28
15.18
16.64
15.89
15.51
4.80
4.87
4.46
4.71
4.08
10.65
9.01
8.32
8.26
4.26
5.68
4.99
4.00
5.23
4.26
11.90
12.00
4.40
0.19
0.22
0.21
0.20
0.23
0.22
0.19
0.22
0.24
0.19
0.20
0.23
0.12
0.24
0.17
0.21
0.23
0.19
GL
WR
WR
WR
WR
63.29
52.18
53.27
51.89
48.20
1.63
1.21
1.25
1.23
0.70
14.28
16.61
16.59
16.99
13.49
7.05
8.99
9.06
9.48
8.76
Cold bay tephra horizons
14
96Amm3
GL
78.14
0.13
13.35
15
16
96AP-19
96JF-9a
GL
GL
64.07
76.14
1.23
0.12
17
96TDS-15A
GL
69.06
18
96JF-8H
GL
GL
N
Sample
Type
Pre-9100 BP pre-caldera lavas
1
FC-43
WR
9100 BP Fisher eruption
2
SH-6
3
SH-141
4
SH-117
4
SH-118
4
SH-119b
4
5
SH-119w
FC-61
6
7
86Amm187
96JF-16B
7
96JF-16C
8
Post-9100
9
10
11
12
13
86Amm162B
intracaldera lavas
FC-7
FC-54
FC-3
FC-30
FC-57
Other volcanoes
19
99S9M1
19
99S9M3
20
99S1M1
21
97AC14
21
97AC19
22
SH-61
23
SH-15
24
SH-5
25
SH-134
26
NW95-1
27
SH-1d
28
SS-2
28
SB8740
29
JLOK42b
29
JLOK42c
29
OA-1
GL
WR
WR
WR
WR
WR
WR
WR
WR
WR
WR
WR
WR
MnO
MgO
CaO
Na2O
K2O
12.08
2.59
0.39
0.89
0.99
0.63
0.69
0.61
3.40
3.46
2.51
1.93
0.57
1.06
0.79
0.53
0.97
0.71
3.61
3.80
0.57
2.57
2.73
2.52
2.42
2.38
7.62
8.22
5.74
5.17
2.30
3.11
2.71
2.28
2.33
2.43
7.77
8.47
2.32
5.53
5.54
4.54
5.23
4.66
3.98
3.76
4.45
4.23
3.25
5.21
3.06
2.23
4.58
4.92
2.93
2.50
3.89
2.28
2.17
2.44
2.33
2.47
1.06
1.18
1.52
1.72
2.36
2.02
2.18
2.57
1.98
2.33
0.77
1.05
2.45
0.32
0.19
0.19
0.17
0.15
1.60
4.58
3.87
4.30
10.77
3.88
8.74
8.46
8.89
11.71
5.68
3.48
3.53
3.38
1.48
1.84
0.96
1.06
0.93
0.59
1.69
0.13
0.14
1.32
2.34
1.47
15.66
13.57
6.11
1.69
0.20
0.19
1.47
0.15
3.87
1.40
4.07
4.60
2.06
1.52
0.57
15.10
4.06
0.16
0.62
2.30
4.68
2.37
63.53
0.92
15.54
6.42
0.17
1.44
3.86
4.40
1.90
76.03
ND
76.39
68.48
67.21
66.93
48.70
49.83
59.84
54.70
46.65
71.0
69.84
68.5
56.0
49.0
0.29
ND
0.27
0.73
0.85
0.57
1.18
1.67
1.07
1.75
1.38
ND
0.62
ND
ND
ND
13.43
ND
13.68
14.87
15.37
15.63
15.26
20.29
16.74
15.89
21.45
ND
14.86
ND
ND
ND
1.77
ND
2.02
3.70
4.07
4.29
9.72
9.70
7.06
10.56
9.43
ND
3.21
ND
ND
ND
0.05
ND
0.07
0.16
0.17
0.11
0.16
0.18
0.18
0.23
0.14
ND
0.11
ND
ND
ND
0.18
ND
0.20
0.74
1.01
0.73
8.53
3.62
1.93
3.30
3.69
ND
0.84
ND
ND
ND
0.95
ND
1.22
2.30
2.87
3.10
10.95
10.49
5.30
7.41
12.51
ND
2.65
ND
ND
ND
2.74
ND
2.85
5.20
5.24
4.91
2.61
3.20
4.51
4.33
2.52
ND
5.33
ND
ND
ND
4.01
ND
3.58
2.98
2.73
2.26
0.71
0.56
1.63
1.21
0.93
ND
2.42
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
Fisher
Caldera, km
Ref
1
0.13
0.81
100.01
basaltic ¯ow
Eickelberg Peak, caldera wall
0
4
2
3
0.35
NA
NA
3.51
NA
1.32
NA
0.72
NA
NA
2.36
NA
NA
7.51
NA
4.32
NA
NA
100.69
97.63
99.71
100.68
97.83
100.58
99.43
100.07
98.73
99.48
100.13
99.82
97.23
100.40
99.81
100.69
98.94
99.38
glassy clast in welded tuff
5±10 cm pumice clasts
NE of Fisher Caldera
NW slope of Shishaldin, Unimak I
NE of Fisher Caldera
NE of Fisher Caldera
12
13
12
NE of Fisher Caldera
12
5±10 cm andesitic scoria clasts
NE of Fisher Caldera
12
white stringers in SH-119b
dacitic ash
NE of Fisher Caldera
on surface, inside Fisher Caldera
12
0
layer of lapilli (2±3 mm)
dacitic ash
Cape Lapin, NNE of Fisher Caldera
Cold Bay, ENE of Fisher Caldera
23
116
basic ash
Cold Bay, ENE of Fisher Caldera
116
8
0.17
0.19
NA
0.16
NA
0.27
NA
0.28
NA
0.08
0.26
0.17
NA
0.23
0.14
0.32
0.31
0.19
®ne dacitic ash
Ukolnoi Island
170
1
1
4
1
4
1
4
4
4
4
4
4
4
3
4
3
4
4
9
10
11
12
13
NA
0.20
0.21
0.21
0.12
NA
2.81
1.03
1.09
4.22
99.21
100.28
98.89
98.94
100.57
andesitic pumice bed
young basaltic scoria cone
basaltic lava
basaltic lava
cumulate xenolith
Fisher Caldera
Fisher Caldera
Mt Finch, bottom
Mt Finch, top
Pyro Hill
0
0
0
0
0
4
4
4
4
4
14
0.00
NA
98.72
3±8 mm lapilli
125
3
15
16
0.42
0.00
NA
NA
99.22
99.42
3±8 mm lapilli
3±8 mm lapilli
84
116
3
3
17
0.18
NA
99.17
3±8 mm lapilli
116
3
18
0.23
NA
98.46
3±8 mm lapilli
Source volcano tephra code
Round top vol. (found at Mt.
Dutton)
tephra layer ªAº, .9100 BP
tephra layer ªBº, .9100 BP, round
top
tephra layer ªCº, ,9100 BP,
Fisher?
tephra layer ªDº, ,9100 BP
116
3
19
19
20
21
21
22
23
24
25
26
27
28
28
29
29
29
0.01
ND
0.01
0.16
0.22
0.12
0.18
0.27
0.37
0.35
0.24
ND
0.13
ND
ND
ND
NA
ND
NA
NA
NA
0.31
0.95
0.72
1.47
NA
0.88
ND
0.62
ND
ND
ND
99.46
ND
100.30
99.33
99.75
99.16
99.17
100.77
100.32
99.73
100.00
ND
98.94
ND
ND
ND
®ne ash
Ignimbrite above ash
3 cm rhyolite pumice
Ignimbrite
Ignimbrite
welded tuff
basaltic lava
basaltic lava
dense andesite block
basaltic andesite lava
basaltic lava
rhyodacite ignimbrite
rhyodacite lava
2049 BP dacite
2050 BP andesite
1946 basaltic ash
Emmons Lake ash, 1,80,000 BP
Emmons Lake ash, 1,80,000 BP
Emmons Lake ash, 18,000 BP
Aniakchak Caldera
Aniakchak Caldera
Round top volcano (False Pass tuff)
Shishaldin volcano
Shishaldin volcano
Shishaldin volcano
Westdahl
Whale back
Seguam caldera
Seguam intracaldera dome
Okmok caldera wall
Okmok caldera wall
Okmok inside of caldera
165
165
200
300
300
50
20
20
20
40
10
300
300
200
200
200
4
4
4
4
4
5
6
7
7
5±10 cm dacitic clasts
4
5
5
4
2
2
4
6
1
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
,5% of the same phenocrysts, but with more calcic
and complexly zoned plagioclase (An79±40), more Mgrich olivine with normal zoning (Fo76±72). Orthopyroxene (Wo5±3En47±51Fs45±54) and normally zoned
clinopyroxene (Wo48±38En35±44Fs12±27) have slightly
more Mg-rich compositions than these phenocrysts
in dacites. The glass composition in the basic portion
is typically 56 wt% SiO2 vs. 69 wt% in the dacitic
portion (see Table 1); both have 3 and 4.9 wt% higher
SiO2 than their respective whole-rock compositions.
Fe±Ti oxide calculated temperature in dacite
(sample SH-141) is 893±8308C and log fO2 is 213
to 214.2 bar, plotting along the QFM buffer (Ghiorso
and Sack, 1991). The basic portion of Fisher tephra
(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
buffer. This observation con®rms that the pre-climactic Fisher magma chamber was not only zoned with
respect to compositions of the residual melt and
phenocrysts, but, expectedly, with respect to temperature too.
Post-caldera Fisher basalts and basaltic andesites
from the intracaldera Mt Finch are more crystalline
(20±35% phenocrysts), with larger phenocrysts: clinopyroxene (Wo39±43En43±45Fs13±16), plagioclase (An75±
50), and olivine (Fo70±67). Plagioclase has sieved
morphology and patchy zoning, with intervals of
normal and reverse zoning, suggestive of resorbtion.
Cumulate inclusions (i.e. FC-57) contain large (up to
5 mm) phenocrysts of slightly zoned olivine (Fo89-84)
and clinopyroxene (Wo46En47Fs7 cores, to
Wo41En46Fs13 rims). The groundmass is devitri®ed
and serpentinized. Pre-caldera basalts and basaltic
andesites cropping out in the caldera wall at Eickelberg Peak contain clinopyroxene (Wo41En46Fs13),
plagoclase (An75-65), and olivine (Fo78-75), and exhibit
broad variations (5±25%) in crystal content.
The compositional diversity of products of 9100 yr
BP Fisher eruption, described above, indicates that the
pre-climactic Fisher magma chamber was zoned with
43
respect to the bulk chemical composition, the composition of residual liquid, and phenocryst content and
composition. Whole-rock incompatible trace element
concentrations in the dacitic portion of Fisher tephra
are, on average, twice that of its basic portion (Table
3). Mass balance calculations using the above phenocryst compositions and whole-rock values (Table 1)
make it possible to obtain SH141 dacite after 53% of
fractional crystallization of basaltic andesite SH118.
These observations suggest that dacites could be
derived by 45±55% differentiation of basic magma.
However, complex zoning of many feldspar phenocrysts suggest that episodes of assimilation and/or
magma mixing occurred as well.
5. Oxygen isotope results
We analyzed oxygen isotope ratios in 11 samples of
the Fisher tephra from 8 localities which represent
ash-fall, welded ignimbrite, and lapilli-bomb beds
(Table 2). More distant localities (as far as 170 km
from the Fisher Caldera) contain only ®ne-grained
ash, but phenocrysts are none-the-less present. We
also studied postcaldera eruptive products within the
caldera, one sample from the precaldera wall, and
samples from the neighboring strato-volcanoes and
calderas. Four regionally distributed dacitic tephra
layers in the neighboring Cold Bay area that occur
above and below the 9100 yr BP Fisher tephra were
also analyzed (Table 2). Although there are 14C age
constraints on these four layers (Carson, 1998), their
source volcanoes are not well constrained. Possible
sources of these coarse tephras include the nearby
dacite-producing volcanoes of Roundtop, Emmons
Lake and Dutton (Miller et al., 1999).
We ®nd that all analyzed samples from the 9100 yr
BP Fisher tephra are low in their d 18O values. The
d 18O (Plag) and d 18O (Cpx) values in both dacitic
and basaltic±andesitic portions of the Fisher tephra
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 ,7 cm
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
N
Sample
d 18O, ½
SiO2, wt%
WR
PI
Cpx
Pre-9100 BP pre-caldera lavas
1
FC-43
48.9
5.79
4.80
9100 BP
2
3
4
4
4
4
5
6
7
7
8
67.9
64.1
65.8
53.7
58.5
65.7
63.3
64.9
60.3
49.8
64.4
4.61
4.60
4.79, 4.51
4.98, 4.93
4.85
4.79
4.59
4.85
4.83, 4.95
4.71
4.98
4.75 ^ 0.05 (10)
4.87 ^ 0.06 (4)
Post-9100 intracaldera lavas
9
FC-7 a
10
FC-54
11
FC-3
12
FC-30
13
FC-57
58.4
52.2
53.3
51.9
48.2
4.58
5.1, 4.78
4.92
5.19
Tephra horizons
14
96AMm3 a,b
15
96AP-19 a
16
96JF-9a a,b
17
96TDS-15A a
18
96JF-8H a
19
99S9M1 a
20
99S1M1 a
73.2
59.2
71.2
64.2
58.6
71.1
71.5
6.33
5.87
6.22
4.69
6.29
6.61
5.90
Other volcanoes
19
99S9M3 a
21
97AC14
21
97AC19
22
SH-61
23
SH-15
24
SH-5
25
SH-134
26
NW95-1
27
SH-1d
28
SS-2 a
29
JLOK42b a
29
JLOK42c
29
OA-1
71.1
68.5
67.2
66.9
48.7
49.8
59.8
54.7
46.7
71.0
68.5
56.0
49.0
6.39
6.63
6.52
6.30
a
Fisher Eruption
SH-6
SH-141
SH-117
SH-118
SH-119b
SH-119w a
FC-61
86AMm187 a
96JF-16B
96JF-16C
86AMm162B a
average, silicic
average, basic
5.62
6.18
5.81
5.64
6.14
4.99
4.98
3.42
3.88
3.89
3.88
4.04
3.70 ^ 0.11 (5)
4.04
3.94
3.86
Opx
Ol
4.15
4.15
2.16
3.98
3.34
glass
4.80
4.95
3.12
4.59
4.35
4.41
Mt
3.32
5.13
4.82
4.84
5.14
5.07
4.06
3.95
4.06
5.23
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
Cr
(ppm)
Ni
(ppm)
Rb
(ppm)
Pre-9100 BP pre-caldera lavas
1
FC-43
48.92
147
55
27
9100 BP Fisher Eruption
2
SH-6
67.89
3
SH-141
64.14
4
SH-117
65.75
4
SH-118
53.71
4
SH-119b
58.49
5
FC-61
63.27
7
96JF-16B
60.34
7
96JF-16C
49.75
864
878
891
414
5
23
1
21
43
23
6
20
Post-9100 intracaldera lavas
10
FC-54
52.18
11
FC-3
53.27
12
FC-30
51.89
13
FC-57
48.20
351
392
353
206
Other volcanoes
22
SH-61
23
SH-15
24
SH-5
25
SH-134
26
NW95-1
27
SH-1d
28
SB8740
670
227
231
550
420
405
870
N
Sample
SiO2
(wt%)
66.93
48.70
49.83
59.84
54.70
46.65
69.84
Ba
(ppm)
Sr
(ppm)
V
(ppm)
Y
(ppm)
7
419
308
23
2
11
3
6
62
56
59
26
264
255
246
431
407
314
223
371
8
28
7
305
139
27
33
330
74
53
52
431
21
13
16
140
19
21
12
9
499
496
552
564
374
22
5
4
8
56
100
12
7
0
16
56
15
17
42
24
25
116
253
530
629
429
368
748
189
and D(Plag 2 Opx) of ,0.6½ (Table 2, Fig. 4 A and
B), being consistent with equilibration at magmatic
temperatures of .8008C (Fig. 5). Basaltic andesites
have smaller oxygen isotope fractionations between
coexisting minerals than dacites, consistent with equilibration at ,70±3008C higher temperatures. Postcaldera andesitic and basaltic lavas inside of the caldera
(Table 2) have similar low d 18O values for phenocrysts. A cumulate (45% crystalline) olivine±clinopyroxene±phyric xenolith, found in a maar inside of
Fisher Caldera (FC-57) has d 18O nearly identical to
both the Fisher tephra and to postcaldera lavas. In
contrast, phenocrysts in the pre-caldera edi®ce
(exposed lava in the caldera wall) have normal d 18O
values (sample FC-43). Olivine phenocrysts in the
basaltic±andesitic scoria of the 9100 yr BP Fisher
tephra, the cumulate xenolith, and postcaldera lavas
are d 18O ˆ 3.32±3.98½ approximately 1½ lower
compared to olivine phenocrysts in island arcs (Eiler
et al., 2000).
Zr
(ppm)
Zr/R
Zr/Ba
Ba/Rb
67
9.6
0.46
21.0
50
46
44
26
280
263
268
129
201
256
280
107
4.5
4.7
4.5
5.0
0.32
0.30
0.30
0.31
13.9
15.7
15.7
15.9
284
258
239
240
29
36
27
14
110
111
110
56
5.8
5.3
9.2
6.2
0.31
0.28
0.31
0.27
18.5
18.7
29.4
22.9
300
240
64
276
302
48
14
23
40
39
19
32
275
86
76
246
152
81
180
4.9
5.7
4.5
5.9
6.3
3.2
1.6
0.41
0.38
0.33
0.45
0.36
0.20
0.21
12.0
15.1
13.6
13.1
17.5
16.2
7.5
Phenocrysts in lavas of the neighboring Shishaldin
stratovolcano, located 20 km to the east of Fisher
Caldera, and in dacites from Roundtop volcano, as
well as basaltic andesites of Westdahl to the west
(Table 2) have normal d 18O values, typical for island
arc volcanics at a given SiO2 content (e.g. Matsushita,
1979). Rhyodacites of Emmons Lake (ca. 180,000 and
18,000 yr BP) and Aniakchak (ca 3300 yr BP)
caldera-forming eruptions also have normal-d 18O
magmatic values, as do the high-silica rhyolitic to
andesitic 1912 eruptive products of the Valley of
Ten Thousand Smokes (Katmai) (Hildreth, 1987),
Seguam lavas, and three dacitic ash samples which
Carson (1998) de®ned as additional tephrochronological markers exposed in the Cold Bay area. The only
other similarly low-d 18O values are for phenocrysts in
andesites and dacites of a complex 2050 yr BP
caldera-forming eruption at Okmok Caldera.
The d 18O values of individual plagioclase phenocrysts 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 plagioclase show D 18O(Plag 2 melt) < 0.3½ (Eiler et al.,
2000). For granitic/rhyolitic magmas, lower temperatures and more sodium-rich compositions of plagioclase 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 compositions, like those found in the Fisher tephra, the
D 18O(Plag 2 melt) is close to zero.
The average d 18O of individual plagioclase phenocrysts 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
(Sample SH6), which has d 18O ˆ 14.80½ The d 18O
(Plag) of the coexisting Fisher basaltic andesite is
14.87 ^ 0.06 (n ˆ 4) and d 18O(O1) ˆ 3.98½. Assuming D(Plag 2 melt) ˆ 0.3 and D(melt 2 O1) ˆ 0.36½
Fig. 5. d 18O (Plag) vs. d 18O (Cpx, Opx, Ol) diagram indicates that
isotopic fractionations between minerals are consistent with equilibration at magmatic temperatures; ®lled and open symbols denote
phenocrysts in basic and silicic rocks respectively; shown temperatures are based on An40-Cpx fractionation from Chiba et al. (1989).
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
18
for island arc basalts (Eiler et al., 2000), the d O of the
Fisher basaltic andesite magma is ca. 14.4± 1 4.6½.
This is about 1½ lower than typical MORB or OIB
magmas (Eiler et al., 1996).
Fig. 6 highlights two trends in and around Fisher
Caldera. Normal d 18O magmas from other nearby
volcanoes de®ne a steep positive slope of 0.35½
increase in d 18O/10 wt% SiO2 increase, similar to
that in other island arcs (e.g. Matsushita, 1979; Taylor
and Sheppard, 1986). Fisher Caldera magmas plot 1±
1.5½ to the left of this line, and their genesis requires
either direct origin from low-d 18O mantle-derived
magma, or assimilation of low-d 18O rocks at shallow
levels. Fisher precaldera lavas, and lavas and tephras
from neighboring volcanoes on Unimak and the
nearby lower Alaska Peninsula have normal d 18O
values. This suggests shallow level assimilation of
low-d 18O hydrothermally-altered rocks at Fisher
volcano, a locally focused process, rather than differentiation of a mantle-derived basic magma that has a
low-d 18O character over a larger (i.e. .20 km)
geographical extent.
47
A similar 0.35½ increase per 10 wt% SiO2 is
expected for Fisher magmas if dacites were derived
solely by low-d 18O basic magma fractionation. In this
case, the original basalt should be around 4½, or 0.7±
0.9½ lower than the observed (Fig. 6), thus excluding
simple fractionation. These higher d 18O values in
basic rocks of Fisher tephra and post-caldera basic
rocks suggest that either normal-d 18O basic magma
was periodically admixed to a low-d 18O basic magma
at the bottom of the magma chamber, and not to its
dacitic portion, or it is a result of a speci®c interplay of
assimilation and fractionation.
For our samples, we evaluated major and trace
element ratios, such as Ba/Rb, Zr/Rb, Zr/Ba, which
should be insensitive to crystal fractionation of the
zircon-undersaturated Fisher magmas (Table 3) and
hence might serve as an additional chemical ®ngerprint of the low-d 18O Fisher magmas, or possible
sources of assimilation. Rb and K are mostly concentrated in the glass and are more easily leached by
hydrothermal ¯uids, than the more insoluble Zr.
However, there are no discernible differences in
Fig. 6. d 18O (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%
increments of assimilant added. Numbers on each curve are d 18O 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 25 to 215½ 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
sensitive to its SiO2 content. Line with positive slope shows the fractionation trend for plagioclase in Japanese normal-d 18O arc magmas
(Matsushita, 1979); all plagioclase from normal-d 18O Aleutian volcanoes and calderas plot near this trend. Dashed line is a parallel fractionation 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
these trace element ratios between the low-d 18O
Fisher Caldera and normal-d 18O rocks from the
nearby Shishaldin volcano and other studied Aleutian/Alaska Peninsula volcanoes. Not only are the
trace element ratios in the Fisher low-d 18O magma
similar to that of normal-d 18O magmas of Unimak
(i.e. Shishaldin, Westdahl, and Roundtop volcanoes),
both low and normal-d 18O rock suites form overlapping trends on Harker diagrams (not shown). This
implies that assimilation of chemically similar material occurred and that small water/rock ratios have
modi®ed the d 18O of the assimilant but not the cation
abundances:
6. Genesis of low-d 18O magmas of Fisher caldera
In attempting to understand possible mechanisms
for the genesis of low-d 18O magmas at Fisher, we
emphasize six key observations:
(1) d 18O depletion is observed in all phenocrysts
from basic and silicic portions of the Fisher tephra,
and they preserve equilibrium fractionations consistent with magmatic temperatures.
(2) The normal d 18O values of olivine phenocrysts
in pre-caldera Fisher lavas and in adjacent volcanoes
suggest that the low-d 18O values are not mantlederived, but were acquired in a shallow crustal
magma chamber.
(3) It is permissive to produce 9100 yr BP Fisher
dacite by 45±55% fractional crystallization of coeval
basaltic andesite alone. Thus, AFC of precursor basalt
predated formation of basaltic andesites. Since lowd 18O magmas are geochemically similar to normal
magmas as de®ned by trace element concentrations
and ratios (see Tables 1 and 3), the low-d 18O assimilant was likely represented by geochemically-similar
volcanics.
(4) There is a clear crystal fractionation relationship
between the glass and whole rock compositions of the
Fisher 9100 yr BP tephras: the glass SiO2 content of
the most ma®c tephra 96JF16C matches the whole
rock value of basaltic andesite SH118 (Table 2); the
glass of SH118 is close to the whole rock value of
andesite SH119. Mass balance calculations can also
be used to show that dacite SH141 could be produced
by 45±55% crystal fractionation of basaltic andesite
SH118.
(5) Comparing d 18O with magma SiO2 content for
the Fisher samples yields a nearly vertical trend (Fig.
6) of increasing magma SiO2 at constant (and low)
d 18O. This suggests that an initial basaltic intrusion
might have rapidly assimilated a ®xed amount of lowd 18O country rocks immediately surrounding the
magma chamber as well as stoped material. Reiners
et al. (1995) showed that the lower latent heats of
assimilation following olivine crystallization moderate assimilation in ma®c systems. Rapid initial assimilation by basalt would have exhausted the low-d 18O
component around the magma chamber and the
magma itself became surrounded by a rind of cumulates (e.g. FC-57), protecting it from further assimilation. Collectively, a rapidly decreasing rate of
assimilation relative to internal differentiation led to
nearly a vertical fractionation trend (Fig. 6), creating
the Fisher low-d 18O silicic magma erupted at 9100 yr
BP. An AFC process by normal-d 18O mantle-derived
magma can be modelled to reproduce the observed
Fisher 9100 yr BP mineral oxygen isotopic values
(Fig. 6). It would require only 5±10 wt% of an 18Odepleted component (25 to 215½), to cause the
observed 1.2±1.5½ depletion in d 18O of the magma
with an initial d 18O of 15.8± ˆ 6.3½.
(6) AFC could have operated long enough in the
pre-9100 yr BP magma chamber to promote differentiation of many tens (if not hundreds) of cubic km of
dacite, and homogenization of oxygen isotopes
throughout the whole magma chamber. In particular,
the fact that the earlier crystallizing olivine phenocrysts in the Fisher 9100 yr BP and post-9100 yr BP
rocks are in equilibrium with plagioclase and pyroxenes suggests that these phenocrysts had enough time
to exchange oxygen by diffusion with a progressively
18
O-depleting magma.
Therefore, a long-term residence is a preferred
mechanism for the genesis of low-d 18O magmas
rather than a syn-eruptive incorporation of meteoric
water into the magma chamber during Fisher
Caldera formation. These data also preclude a traditional model in which basic magma shortly
preceded or even triggered the volcanic eruption
and caldera-formation. A model in which silicic
magma replenished a basic magma chamber (e.g.
Eichelberger et al., 2000) is also unlikely for Fisher,
since both basic and silicic portions of Fisher tephra
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
18
the d O signature in central Unimak Island is not
of a deep origin.
The above observations lead us to suggest the
model of evolution for the magma erupted at Fisher
9100 yr BP, presented on Fig. 7. Before 9100 yr BP, at
the site of today's Fisher Caldera there existed a series
of stratocones (Stelling and Gardner, 2000) possibly
within an older caldera. The topographic evidence Ð
the south scarp of the Tugamak Range (Fig. 1) Ð was
observed over two decades ago by J.R. Hein and D.W.
Scholl, who had noted `an apparent circular structure
on MSS band 4 of LANDSAT photography' (quote
from Miller and Smith, 1977). We speculate that this
may be the remaining wall of an older and larger
(.30±40 km?) caldera, which would encircle the
9100 yr BP caldera, and serve as a hydrogeological
boundary for water circulation.
The Fisher Caldera eruption of 9100 yr BP
occurred several thousand years following the end
of the Late Wisconsin glacial period. For two extensive periods of the Pleistocene: the last glacial maximum (Late Wisconsin) from (,21,000±14,000) yr
BP (cf. Jordan and Maschner (2000), and from
,190,000±130,000 yr BP (marine oxygen isotope
stage 6, Dansgaard et al., 1993), the Fisher volcanic
center was probably under a glacial ice with extremely low-d 18O (ca. 235½) (e.g. Dansgaard et al.,
1993). This water would have participated in hydrothermal circulation through, and the alteration of, the
sub-volcanic crust and depletion of 18O in those rocks,
including during long time periods when the area's
surface was ice-free. Hydrothermally-altered rocks
in similar geothermal areas are typically 10±15½
higher in d 18O than the local meteoric water (Criss
and Taylor, 1986). Hydrothermally-altered volcanic
minerals in the Kra¯a, Iceland drillhole (similar in
age and latitude to Fisher 9100 yr BP rocks) are as
low as 212½ and these depletions could have
resulted from exchange with meteoric water of 222
to 227½ (Hattori and Muehlenbachs, 1982).
7. Comparison with other Aleutian and Alaska
Peninsula volcanic centers
The low-d 18O magmas of the Fisher Caldera event
at 9100 yr BP are the ®rst reported in the Aleutian/
Alaska Peninsula volcanic arc. While there is need to
49
survey more volcanoes, the analyses reported in this
paper show that other magmas of Unimak and the
Alaska Peninsula are normal in d 18O (see Table 2).
In particular, calderas comparable in size and age to
Fisher, such as Aniakchak and Emmons Lake,
produced normal-d 18O magmas, as well as the Valley
of Ten Thousand Smokes (Hildreth, 1987). Okmok
(Umnak Island) is the only other multi-caldera
volcano in the Aleutians, with d 18O values in between
those de®ned by normal magmas and the low-d 18O
Fisher magmas. Like the Fisher tephra, the 2050 yr BP
Okmok ignimbrite comprise of zoned dacite±andesite
tephra set, with similarly low-d 18O values. Post
2050 yr BP intracaldera volcanics at Okmok are also
low-d 18O magmas (Table 2). In contrast to the Aleutians, low-d 18O magmas are much more abundant
among the Holocene and Pleistocene caldera-forming
eruptions of Kamchatka (Bindeman et al., 2001) and
in Iceland Muehlenbachs et al., 1974).
We believe that the ultimate reason for appearance
of most low-d 18O magmas is related to melting or
assimilation of geochemically similar, but hydrothermally-altered volcanics of the previous volcanic cycle
which hosted a hydrothermal system (e.g. Bindeman
and Valley, 2000; Bindeman et al., 2001). The occurrence of low-d 18O magmas is probably related to the
combination of at least several necessary factors,
which can be classi®ed into magmatic, tectonic,
hydrologic, and climatic. Shallow position of
magma chambers, extended magmatic and hydrothermal pre-history, and high temperatures of the assimilating magma favor lowering of d 18O. Among
tectonic factors, an earlier caldera-formation, like in
Yellowstone (e.g. Bindeman and Valley, 2000), is the
most important process of burying hydrothermally
altered low-d 18O rocks deep in the crust. A high precipitation rate, and the light d 18O values of highlatitude, high-altitude, and intracontinental waters
(e.g. Yurtsever and Gat, 1981), are favorable hydrologic and climatic factors. The signi®cantly lower
d 18O values of local meteoric waters during and
following glaciations is a prime example of the
climatic factor. The role of hydrothermal alteration
of island arc crust by sea water with d 18O of around
0½ is unknown. If groundwaters in smaller islands
are mixtures of meteoric and sea waters, the depleting
effect of meteoric waters on magmas might be
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, normald 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 lowd 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
18
low-d O magmas in larger land masses such as
Kamchatka (Bindeman et al., 2001) and Iceland
(Muehlenbachs et al., 1974), where meteoric waters
(d 18O , 2 9½) dominate over sea waters
(d 18O ˆ 0½).
Why are low-d 18O magmas are only found at Fisher
and Okmok? The current sample set includes six
calderas (Fisher, Emmons Lake, Aniakchak, Seguam,
Okmok, and VTTS-Katmai), and tephra layers from
unknown sources near Cold Bay (Table 2). They all
presumably experienced similar glaciations and
would have similar d 18O depleted water.
We speculate that the condition for appearance of
low-d 18O magmas is the existence of an older overlapping or enclosing caldera which hosted a hydrothermal system prior to the most recent caldera event.
One isolated caldera event does not seem to be enough
to produce detectable change in magmatic d 18O. At
Yellowstone, the younger 0.6 Ma Yellowstone
Caldera is enclosed in an older 2.0-Ma-old Caldera.
Oxygen isotopes in 0.6 Ma caldera-forming tuff are
about 1½ depleted in d 18O relative to the 2.0 Ma
ash-¯ow tuff, while post 0.6 Ma intracaldera lavas
are 5½ depleted relative to 0.6 Ma tuff (Hildreth et
al., 1984; Bindeman and Valley, 2000). At Okmok
Caldera, the younger 2050 yr BP caldera is enclosed
in an older 8050 yr BP caldera (Miller and Smith,
1987) and analyzed mineral separates are 1½
depleted relative to normal magmas (no 8050 yr BP
tephra is available for d 18O measurement). We
suggest that Tugamak Range south scarp may be the
remnant of an older Fisher caldera in which the
9100 yr BP caldera is enclosed.
8. The use of oxygen isotopes in tephrochronology
We propose that oxygen isotope analysis of phenocryst pairs (e.g. Plag±Px) is a useful tool for tephrochronological and tephrostratigraphic correlation, in
cases where a distinctive caldera ®ngerprint exists.
Oxygen isotope analyses of phenocrysts by laser
¯uorination is a rapid method requiring only ,1±
2 mg of material. Oxygen isotope compositions of
unaltered phenocrysts, and fractionation ( ˆ
temperatures) between them can be used for correlation. These parameters are not affected by aeolian
differentiation, and in many cases can survive second-
51
ary alteration longer than the major element composition of glass shards, which are traditionally used for
correlation purposes. Since feldspars may suffer some
secondary alteration after deposition, we recommend
that several feldspar analyses be made from a sample,
or that analyses of coexisting pyroxene be made to
ensure the preservation of magmatic D(Plag 2 Px).
The low d 18O signature of the Fisher 9100 yr BP
tephra makes it a distinct regional tephrochronological/tephrostratigraphic marker. Four other dacitic ash
layers de®ned by Carson (1998) as tephrochronological markers in the Cold Bay area were analyzed and
three found to contain normal-d 18O phenocrysts.
Signi®cantly, one layer (younger than 6070 yr BP
but older than 3600 yr BP: Dochat, 1997; Carson
1998) is similar to Fisher dacite low-d 18O. The source
volcano for this ash layer is not established but the low
d 18O value leads us to suggest that it could be from an
unidenti®ed eruption of Fisher.
Deposition of Fisher 9100 yr BP tephra on Unimak
and the Alaska Peninsula, could have been catastrophic to humans and animals. Archaeological
sites as old as ,8500 yr BP are present on Umnak
Island in the Aleutians (McCartney and Veltre,
1996) and ,9000 yr BP on the Alaska Peninsula
(Ugashik River: Dumond, 1981). At 8500±9000 yr
BP, Aleut people had the seagoing technology to
travel throughout, and exploit the resources, of all
the islands and lower Alaska Peninsula. At the time
of the climactic 9100 yr BP eruption, sea level at eastern Unimak and the lower Alaska Peninsula was 16±
25 m above present sea level (Jordan and Maschner,
2000), and thus any occupation sites would be much
higher than the coastal sites traditionally studied by
archaeologists.
Identi®cation of Fisher tephra at the base or top of
anthropological sites may not only be used for dating
purposes, but also can help to assess the impact of the
9100 yr BP Fisher eruption on the early human
communities of Southwestern Alaska.
9. Conclusions
Examination of phenocrysts from pyroclastic
deposits on Unimak Island and the lower Alaska
Peninsula reveals the existence of a previously
unrecognized low d 18O magmas. The average
52
I.N. Bindeman et al. / Journal of Volcanology and Geothermal Research 111 (2001) 35±53
18
d O values of these Fisher Caldera phenocrysts, as
well as inferred parental magmas, are at least 1½
lower than those of the other Unimak Island volcanoes and Alaska Peninsula calderas. Samples from
Fisher intracaldera vents showed the same pattern.
Development of low-d 18O magma requires incorporation of oxygen derived from surface waters.
Study of other calderas (e.g. Yellowstone) suggests
that a single caldera event probably does not suf®ciently lower the d 18O; rather, the hydrothermal
circulation following caldera collapse provides
effective lowering of the d 18O of the shallow crustal rocks, which subsequent intrusions then can
assimilate. Based upon this, as well as topographic
evidence north of Fisher Caldera, we suggest that
Fisher may have had at least two caldera events.
The unique d 18O ®ngerprint of 9100 yr BP Fisher
Caldera tephra show that oxygen isotope analyses of
phenocrysts is a robust and precise technique,
requiring a tiny amount of material which is applicable to tephrochronologic or tephrostratigraphic
studies. Our examination of several unidenti®ed
Alaska Peninsula ash layers found that one had
the low-d 18O Fisher signature, evidence of a more
recent, hitherto unknown, eruption from a Fisher
intracaldera vent.
Acknowledgements
We thank Eric Carson, Dave Mickelson, Tina
Dochat, Jim Jordan, Margaret Mangan, Tom Miller,
Tina Neal, Scott Dreher, Jessica Faust Larsen, Brad
Singer, and Fred Anderson for sample donation. Mike
Spicuzza for support during isotope analyses. JF
gratefully acknowledges National Geographic Society
grant #4075-89 for the 1989 expedition to Fisher and
®eld assistance there by Steve McDuf®e, and 1996
Cold Bay logistical support from Herb Maschner
and funds from the UW-Geology Albert and Alice
Weeks Fund. JF also thanks Bob Tilling for sharing
his recollections and slides from his 1979 ®eld trip to
Fisher Caldera. We thank DOE (93ER14389) and
NSF (EAR99-02973) for funding the oxygen isotope
research. Reviews from Tom Miller and an anonymous reviewer, as well as comments by Jim Gardner,
Pete Stelling, and Mike Kaplan, helped improve this
manuscript.
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