Geol 08 v 36n1 plume USA.pdf

Yellowstone plume–continental lithosphere interaction beneath

the Snake River Plain

Barry B. Hanan Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA

John W. Shervais Department of Geology, Utah State University, Logan, Utah 84322-4505, USA

Scott K. Vetter Department of Geology, Centenary College, Shreveport, Louisiana 71134, USA

ABSTRACT

The Snake River Plain represents 17 m.y. of volcanic activity that took place as the North

American continent migrated over a relatively ﬁ xed magma source, or hotspot. The identi-

ﬁ cation of a clear seismic image of a plume beneath Yellowstone is compelling evidence that

the Miocene to recent volcanism associated with the Columbia Plateau, Oregon High Lava

Plains, Snake River Plain, Northern Nevada Rift and Yellowstone Plateau represents a single

magmatic system related to a mantle plume. A remaining enigma is, why do radiogenic isotope

signatures from basalts erupted over the Mesozoic–Paleozoic accreted terrains suggest a plume

source while basalts erupted across the Proterozoic–Archean craton margin indicate an ancient

subcontinental mantle lithosphere source? We show that ancient cratonic lithosphere like that

of the Wyoming province superimposes its inherent isotopic composition on sublithospheric

plume and/or asthenospheric melts. The results show that Yellowstone plume could have a

radiogenic isotope composition similar to the mantle source of the early Columbia River Basalt

Group and that the plume source composition has persisted to the present day.

Yellowstone system were emplaced through

cratonic lithosphere of Idaho, Montana, and

Wyoming (Leeman et al., 1985; Wooden

and Mueller, 1988). Maﬁ c igneous rocks of

the Beartooth Mountains of Montana and

Wyoming suggest a lithosphere stabilization

age of ca. 2.8 Ga (Wooden and Mueller, 1988;

Mueller and Frost, 2006); farther west stabili-

zation ages are Late Archean to Paleoprotero-

zoic (Foster et al. 2006). Deep crustal xenoliths

conﬁ rm that ancient basement extends beneath

the Snake River Plain and shows a pattern of

decreasing age (ca. 3.2–2.5 Ga) from Archean

in the east to Proterozoic in the west (Leeman

et al., 1985; Wolf et al., 2005).

Basalts from deep drill cores in the eastern

Snake River Plain show that fractionation and

magma recharge took place in sill-like layered

intrusions at mid-crustal depths (Shervais et al.,

2006). The limited range of major and trace ele-

ment composition, mantle δ

Keywords: Yellowstone, Snake River Plain, Pb isotopes, Sr isotopes, Nd isotopes, basalt, mantle

plume.

INTRODUCTION

The Snake River Plain of southern Idaho is the

archetype of a continental hotspot track, formed

by the trace of the Yellowstone plume as it propa-

gated northeastward over the past 17 m.y. (Fig. 1).

Volcanic products erupted within the Snake River

Plain comprise rhyolite ignimbrites overlain by

a thin basalt veneer erupted from small shield

volcanoes and cinder cones. The hotspot track

is underlain at depth by a 10-km-thick maﬁ c sill

complex that contains much of the basaltic melt

introduced into the continental crust, and now

forms a layered maﬁ c intrusion in the middle crust

(Shervais et al., 2006).

The origin of the Snake River–Yellowstone

hotspot is controversial. Major element, trace

element, and He isotope systematics of the

basaltic rocks are consistent with a deep, sub-

lithospheric mantle source, similar to the source

of ocean island basalts (e.g., Craig et al., 1978;

Vetter and Shervais, 1992; Hughes et al., 2002;

Graham et al., 2006). In contrast, the radiogenic

Pb isotopes in these basalts are indistinguishable

from melts derived from the ancient litho-

sphere that underlies the plume track, while Sr

and Nd isotope ratios are intermediate between

depleted mantle and continental crust or litho-

spheric mantle values (Church, 1985; Leeman

et al., 1985; Hughes et al., 2002). This conun-

drum has been a major problem for all plume-

oriented models presented in the past.

We present here new Pb, Sr, and Nd iso-

topic analyses for basalts from the Snake River

Plain, including the Idaho National Laboratory

in the eastern plain, and the Bruneau-Jarbidge

eruptive center and the Glenns Ferry area, both

in the central plain. Our results show that these

isotope compositions are compatible with a

deep mantle plume source, which interacted

with lithosphere that varies in age, composi-

tion, and thickness from west to east, and are

compatible with a mantle plume origin for the

Snake River Plain basalts.

18 O signatures, and

lack of any correlation between 87 Sr/ 86 Sr (or δ

18 O,

206 Pb/ 204 Pb, and 208 Pb/ 204 Pb) and major and trace

element, and rare earth element abundances in

the basalts indicate minimal crustal interaction

(Menzies et al., 1983; Leeman, 1982; Carlson,

1984; Hart, 1985; Church, 1985; Shervais et al.,

2006). Nash et al. (2006) showed that the early

rhyolites represent mixtures of crustal melts

with evolved plume basalts, but later basalts

passed through the previously depleted crust

with little or no interaction.

BACKGROUND GEOLOGY

The Snake River Plain volcanic province is

part of a regional array of volcanic activity that

includes coeval magmatism in the Columbia,

Oregon, and Owyhee plateaus, and the North-

ern Nevada Rift (Fig. 1). The interrelationships

of the provinces in time, space, and geology

are consistent with a single magmatic sys-

tem resulting from the interaction of a mantle

plume with the continental lithosphere of North

America (Camp and Ross, 2004). Recent geo-

physical investigations have imaged a 100-km-

diameter thermal anomaly in the upper mantle

below Yellowstone that plunges 65° NW and

extends to a depth of ∼500 km, near the top of

the mantle transition zone (Yuan and Dueker,

2005; Waite et al., 2006).

Volcanic activity in the Snake River–

Yellowstone system began with eruption of

the main phase of the Columbia River Basalt

Group ca. 16.5–15 Ma (Camp and Ross, 2004)

through Paleozoic and Mesozoic lithosphere

accreted to the Precambrian continental mar-

gin of North America. Volcanism shifted to the

east, across the cratonic margin into the Snake

River Plain, ca. 15 Ma and advanced with time

to its current position at Yellowstone (Camp

and Ross, 2004). Basalts of the Snake River–

METHODS

The analytical procedures and methods for

preparing the basalts for isotope analysis, the

mass spectrometer analytical methods, and

mixing model parameters are given in the GSA

Data Repository. 1

RESULTS

The Pb, Sr, and Nd isotope ratio data for the

analyzed basalts deﬁ ne quasi-linear arrays in

multi-isotope plots (Fig. 2). The Pb-Pb arrays

have a continental-like signature with more

radiogenic 207 Pb/ 204 Pb, 208 Pb/ 204 Pb, and 87 Sr/ 86 Sr

ratios, and lower 143 Nd/ 144 Nd relative to Paciﬁ c

1 GSA Data Repository item 2008014, Part A:

Data Tables, Methods, and Sample Locations; and

Part B: Model Data Tables and Description, is avail-

able online at www.geosociety.org/pubs/ft2008.htm,

or on request from editing@geosociety.org or Docu-

ments Secretary, GSA, P.O. Box 9140, Boulder, CO

80301, USA.

Geology , January 2008; v. 36; no. 1; p. 51–54; doi: 10.1130/G23935A.1; 3 ﬁ gures; Data Repository item 2008014.

GEOLOGY, January 2008

ID MT

mid-ocean ridge basalts (MORB) derived

from depleted asthenosphere. Except for two

samples, central plain basalts have more radio-

genic Pb isotope ratios than eastern plain lavas

and plot in a separate ﬁ eld. The central and east-

ern basalt Pb-Sr-Nd isotope signatures are simi-

lar to the Saddle Mountains basalts (Columbia

River Basalt Group), but more radiogenic than

lavas from Yellowstone (Doe et al., 1982). The

Yellowstone basalts (Doe et al., 1982) have

continental-like isotope signatures that plot along

an extension of the central and eastern arrays.

In contrast, Columbia River basalts (excluding

Saddle Mountain) that were emplaced through

the Mesozoic–Paleozoic accreted terranes have

isotope signatures more characteristic of oceanic

mantle sources, overlapping the enriched end of

the Paciﬁ c MORB array. Columbia River, Snake

River Plain, and Yellowstone basalts show an

exponential decrease in 206 Pb/ 204 Pb from west

to east, from oceanic island basalt (OIB) like

values in Oregon and Washington toward values

typical of the lower crust and lithosphere of

the Wyoming Province (Leeman et al., 1985;

Church, 1985; Wooden and Mueller, 1988).

Columbia River

Basalt Provence

Mesozoic Accreted

Terranes

Precambrian

North American

Craton

Steens

Mountain

Steens

INL

Yellow-

stone

Mountain

Heise

6.6 Ma

Picabo

10 Ma

2.1 to 0

G GF

16.6 - 15 Ma

B-J

Twin

Falls

10.5

Bruneau-

Jarbidge

12.7 Ma

Owyhee-

Humbolt

15 Ma

NV UT

Figure 1. Relief map of Paciﬁ c Northwest (after Camp and Ross, 2004). Blue dashed line is

boundary between Mesozoic–Paleozoic accreted terrains and Precambrian North American

craton. Locations are shown for the Idaho National Laboratory (INL) core WO-2, Bruneau-

Jarbidge (B-J), Glenns Ferry lavas (GF), and the Beartooth Mountains (BT). Snake River

Plain and Yellowstone Plateau volcanic centers and approximate ages are shown in orange,

Columbia River Basalt Group is shown in yellow.

DISCUSSION

Although Yellowstone and Snake River Plain

basalts have Pb, Sr, and Nd isotope signatures

similar to maﬁ c volcanic rocks and deep crustal

xenoliths derived from the Wyoming craton, the

high 3 He/ 4 He isotope ratios observed at Yellow-

stone (Craig et al., 1978) provide strong evi-

dence that the hotspot represents the manifes-

tation of a deep mantle plume. This conclusion

is reinforced by the major and trace element

compositions of the lavas, which are nearly

identical to those of OIBs (Vetter and Shervais,

1992; Hughes et al., 2002; Shervais et al., 2006).

Helium isotope data for olivine basalts from the

Snake River Plain show the 3 He anomaly to be

long lived and not restricted to the Yellowstone

caldera (Graham et al., 2006).

Lithosphere beneath the Snake River Plain

and Yellowstone stabilized in the Late Archean

to Paleoproterozoic (Mueller and Frost, 2006).

Compared to other Late Archean rocks, the

Pb and Sr initial ratios are higher, and the Nd

initial ratios are lower, than expected for a

depleted upper mantle source (Wooden and

Mueller, 1988; Menzies et al., 1983). These

isotope data, and mantle xenolith Os, Sr, Nd,

and Pb isotopes (Carlson and Irving, 1994),

suggest that crustal material was mixed into the

lithosphere during Late Archean subduction and

later Proterozoic metasomatic events (Church,

1985; Wooden and Mueller, 1988). However,

this enriched lithosphere is not conducive to

preserving ancient high 3 He/ 4 He. An appropriate

mantle with high time-integrated 3 He/(U + Th)

that would allow preservation of ancient 3 He

enrichments cannot exist within or below the con-

tinents, and is unlikely to exist within the upper

40.0

0.5135

G GF

39.5

B-J

Pacific MORB

Saddle Mountains

CRB

Steens

0.5130

39.0

SFV

Steens

CRB

INL

38.5

0.5125

38.0

SFV

Saddle Mountains

37.5

Pacific MORB

0.5120

37.0

36.5

0.5115

16.0

16.5

17.0

17.5

18.0

18.5

19.0

19.5

0.700

0.705

0.710

0.715

206 Pb/ 204 Pb

87 Sr/ 86 Sr

15.8

0.5135

B-J

Pacific MORB

SFV

Saddle Mountains

15.7

CRB

INL

Steens

0.5130

SFV

Steens

15.6

CRB

15.5

0.5125

INL

B-J

G GF

Y YP

15.4

Pacific MORB

0.5120

15.3

Saddle Mountains

15.2

0.5115

16.0

16.5

17.0

17.5

18.0

18.5

19.0

19.5

16.0

16.5

17.0

17.5

18.0

18.5

19.0

19.5

206 Pb/ 204 Pb

Figure 2. Pb, Sr, and Nd isotope data for the central plain Bruneau-Jarbidge (B-J, red dia-

mond), Glenns Ferry (GF, brown), and eastern plain Idaho National Laboratory (INL, blue)

basalts. Also shown for comparison are Paciﬁ c mid-oceanic ridge basalts (MORB; Hanan

and Graham, 1996), Yellowstone basalts (YP, green box; Doe et al., 1982; Hildreth et al., 1991);

Columbia River Basalt group (CRB, black outline; GEOROC database), Saddle Moun-

tains lavas (orange outline; GEOROC database), the Steens basalts (Camp and Hanan,

2007), and for comparison, basalts from the Stonyford Volcanic Complex (SFV, red out-

line; Shervais et al., 2005). Stonyford ﬁ eld represents isotope composition of alkali basalts

from Mesozoic oceanic lithosphere, associated with Coast Range Ophiolite. GEOROC—

http://georoc.mpch-mainz.gwdg.de/georoc/start.asp.

GEOLOGY, January 2008

ID MT

Precambrian

North American

Craton

Mesozoic Accreted

Terranes

Yellow-

stone

INL

Steens

Mountain

2.1 to 0

Heise

6.6 Ma

Picabo

10 Ma

B-J

16.6 - 15 Ma

Twin

Falls

10.5

Bruneau-

Jarbidge

12.7 Ma

Owyhee-

Humbolt

15 Ma

NV UT

37.0

mantle based on the 3 He/ 4 He in MORB

(Day et al., 2005). The high 3 He of the Snake

River–Yellowstone province suggests that

there is a ﬂ ux of deep mantle material across

the 660 km mantle transition zone into the

upper mantle plume imaged at Yellowstone.

We propose that the apparent conﬂ ict

between the isotopic and chemical data can be

resolved by considering mass balance between

the contrasting components. The Pb, Sr, and

Nd concentrations in the plume source are low

compared to subcontinental mantle lithosphere,

and melts derived from this source are expected

to have concentrations of Pb, Sr, and Nd on

the order of 0.3–3, 90–660, and 7–40 ppm,

respectively, based on observed concentra-

tions in OIBs and on trace element melting

models of inferred plume sources (e.g., Sun

and McDonough, 1989; see the Data Reposi-

tory [Part B] for melting models). In contrast,

low percent fractional melts of ancient sub-

continental lithosphere, such as the Leucite

Hills volcanics (Vollmer et al., 1984; Mirnejad

and Bell, 2006), are strongly enriched in Pb

(23–120 ppm), Sr (1652−7233 ppm), and Nd

(97−300 ppm), and provide an estimate for

these concentrations in melts assimilated

by the plume basalts as they rise through the

lithosphere (data references in the Data Reposi-

tory [Part B]). Lithosphere:plume proportions

implied by these concentrations are ~38–400

for Pb, ~11–80 for Sr, and 11–41 for Nd. As

a result, the assimilation of lithospheric melts

into partial melts derived from plume-source

mantle will result in hybrid magmas whose

isotopic compositions are controlled by the iso-

topic composition of the Archean lithosphere.

This process is well illustrated quantitatively

by mass balance calculations for the radiogenic

isotopes of Pb assuming a lithosphere:plume

ratio of 100. Figure 3 shows the effects of mix-

ing a plume component similar in Pb isotopes

to the Steens basalts, the earliest Columbia

River eruption, and to Paciﬁ c Mesozoic oceanic

crust (e.g., Shervais et al., 2005) with fractional

melts derived from Archean lithosphere with

Pb isotopic composition represented by the

ca. 2.8 Ga isochron for the Beartooth Mountain

igneous rocks (Wooden and Mueller, 1988).

Heterogeneity in the lithosphere is expected to

be much greater than for the plume source (see

Fig. 2). For ease in modeling and visualization

we have chosen a single point located in the

overlap between the Steens and Stonyford ﬁ elds

to represent the plume isotope composition.

The Beartooth rocks are a good proxy for the

Paleoproterozoic–Archean lithosphere because

they show the whole range of 206 Pb/ 204 Pb iso-

tope compositions observed in the Snake River

Plain basalts and crustal xenoliths, the Saddle

Mountain lavas, and the Yellowstone plateau

basalts. Two effects are observed. First, the

mass fraction of plume component increases

16.0

SRP

Yellowstone

INL WO-2

Bruneau-Jarbidge

15.8

Glenns Ferry

SRP Xenoliths

West

Plume

15.6

15.4

100

[ P b ] Lithos p here = 100 x [Pb ] Plume

15.2

95%

97%

98%

99% Plume Component

SCLM

206 Pb/ 204 Pb

Plume

15.0

14.5

15.5

16.5

17.5

18.5

19.5

206 Pb/ 204 Pb

Figure 3. The 206 Pb/ 204 Pb and 207 Pb/ 204 Pb ratios for the Yellowstone Plateau, Idaho National

Laboratory (INL), Bruneau-Jarbidge, and Glenns Ferry lavas decrease from west to east.

Isochron and data (small open diamonds) for Beartooth Mountains maﬁ c igneous rocks

(Wooden and Mueller, 1988) represent Pb isotope composition of lithosphere underlying

Yellowstone Plateau and Snake River Plain (SRP). Deep crustal xenoliths of Leeman et al.

(1985) plot about the Beartooth 2.8 Ga isochron, and like the Snake River Plain basalts,

the 206 Pb/ 204 Pb ratios of the xenoliths decrease from west to east. Field for the Columbia

River Steens basalts and Stonyford Volcanic Complex (black line ﬁ eld labeled Plume; Camp

and Hanan, 2007; Shervais et al., 2005) represents plume component. Red, brown, blue,

and green lines represent mixing tie lines between average plume and distinct lithospheric

Pb reservoirs along the Beartooth isochron for Snake River Plain and Yellowstone Plateau

basalts. Solid lines labeled 95%–99% Plume component indicate proportion of plume com-

ponent in the basalt mixes where the lines intersect the tie lines. Note that Snake River Plain

data deﬁ ne pseudo-isochrons with slopes lower than the 2.8 Ga Beartooth reference iso-

chron. Inset plot shows mixing curve between enriched lithosphere with 206 Pb/ 204 Pb = 16.87

and 207 Pb/ 204 Pb = 15.44 and plume source with 206 Pb/ 204 Pb = 19.0 and 207 Pb/ 204 Pb = 15.55. Pb

concentration in the lithosphere is 100× that of the plume. Note that with plume proportions

>95% the isotope composition of the mix is dominated by lithosphere Pb signature. SCLM—

subcontinental lithospheric mantle.

from east to west: Yellowstone ~95%–98%,

eastern plain ~97%–98.5%, and central plain

~98%–99% plume component (Fig. 3). Sec-

ond, the Pb isotopic composition of the litho-

spheric component changes from east to west:

Yellowstone 206 Pb/ 204 Pb ∼15.5–17.0, central

basalts 206 Pb/ 204 Pb ∼17.1–17.8, and eastern lavas

206 Pb/ 204 Pb ∼18.0–18.5 (Fig. 3). Modeling Snake

River Plain basalt Sr and Nd isotopes using these

end-member compositions gives results consistent

with plume mass fractions >95%.

These mass-balance models can be under-

stood in terms of two processes. First, the

increase in plume component from east to west

most likely reﬂ ects a progressive decrease in

the thickness of the cratonic lithosphere from

east to west as the craton margin is approached.

This decrease in thickness will result in a

decrease in the volume of lithosphere avail-

able to react with the plume-derived melts, and

hence a decrease in the proportion of litho-

sphere component assimilated. The depth of

plume melting will also decrease, resulting in

a higher degree of melting of the plume. Sec-

ond, the regular increase in 206 Pb/ 204 Pb isotopic

composition of the lithosphere component in

the mix, from east to west, results from the

concomitant decrease in lithosphere age and

from compositional heterogeneity of the litho-

sphere. Spatial compositional differences are

reﬂ ected by the lack of regular covariation

between 208 Pb/ 204 Pb, 87 Sr/ 86 Sr, and 143 Nd/ 144 Nd

ratios along the Snake River Plain (Fig. 2), cor-

responding to heterogeneity in Th/Pb, U/Pb,

Rb/Sr, and Sm/Nd and the time-integrated

effect of radioactive decay in the lithosphere.

The similarity between the radiogenic isotope

signatures of the Snake River Plain and the

Saddle Mountain basalts, and their contrast

to the Yellowstone basalts, indicates that the

lithosphere between the Yellowstone Plateau

and the craton margin is not the same as that

underlying the Archean Wyoming province,

but likely a transitional lithosphere containing

a complex mixture Archean and Proterozoic

components (e.g., Foster et al., 2006).

GEOLOGY, January 2008

East

CONCLUSIONS

The ﬁ nal conclusion that can be drawn from

this mass-balance model of isotope mixing is

that we cannot use the radiogenic (Pb, Sr, Nd)

isotopic composition of basalts erupted through

thick cratonic lithosphere as a reliable indicator

of their provenance. Ancient cratonic litho-

sphere like that of the Wyoming Province will

superimpose its inherent isotopic composition

on sublithospheric plume or asthenospheric

melts, until that ancient lithosphere becomes

sufﬁ ciently thinned by thermal or mechanical

erosion, or depleted in low-temperature melting

components, so that sublithospheric melts may

pass through with little or no pollution. This

is apparently the case beneath the Great Basin

today, where lithospheric thinning has proceeded

to the extent that sublithospheric melts arrive at

the surface with isotopic compositions simi-

lar to their primary source region (e.g., Fitton

et al., 1991). These results suggest that the Pb,

Sr, and Nd isotope signature of the Yellowstone

plume is represented by the mantle source of

the early Columbia River main phase basalts

and has remained constant since ca. 16.4 Ma.

The spatial and temporal variability shown by

the Yellowstone plume, from the Columbia River

to Yellowstone, is due to shallow mantle plume-

lithosphere interaction. The Yellowstone plume,

like the Iceland plume, has an isotope signature

similar to the common component C (Hanan and

Graham, 1996). Similarly, temporal and spatial

variability in Iceland basalts (0–16 Ma) is attrib-

uted to shallow interaction with North Atlantic

upper mantle, which is polluted by continental

material (Hanan et al., 2000).

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ACKNOWLEDGMENTS

This work was supported by collaborative National

Science Foundation grants EAR- 9526723 (Hanan),

EAR-9526594 (Shervais), and EAR-9526722 (Vetter).

Insightful reviews by Dennis Geist, Paul Mueller,

Bruce Nelson, and an anonymous reviewer helped to

clarify our thinking.

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Manuscript received 27 March 2007

Revised manuscript received 4 September 2007

Manuscript accepted 6 September 2007

Printed in USA

GEOLOGY, January 2008

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