Magmatism of the Snake River Plain – Yellowstone region:

Implications for continental lithosphere evolution above a mantle plume

Bill Leemannow at

National Science Foundation

Theme of this presentation• Earthscope and related geophysical investigations

will provide a snapshot of crust-lithosphere structure

• This will be particularly useful in evaluating near real-time geological processes

• A focus on the active Yellowstone-Snake River Plain magma system would provide an unprecedented opportunity to understand large-scale magmato-tectonic processes and their interactions with and effects on existing lithosphere.

Key topics to be addressed

• Nature of the underlying lithosphere - isotope constraints

• Space-time migration of bimodal volcanism - the ‘hot spot track’

• Volumes, rates, and sources of magmatism - geodynamic implications

• Specific problems and the role of Earthscope

Architecture of the lithosphere - N. Rocky Mtns.Setting for mid-Miocene magmatic flareup

WISZ

Setting for mid-Miocene magmatic flareup

WISZ

artifactrealistic

On-craton

Off-craton

Isotopes signify distinct mantle sources across prominent tectonic boundaries

41.0

42.0

43.0

44.0

N L

at.

(°)

112.0114.0116.0118.0120.0

W. Long. (°)

<0.708<0.707<0.706

<0.7055<0.705<0.704

NV

OR

UT

ID

Sr isotopic compositions of Cenozoic basalts

< 0.706> 0.706

< 0.706

Circles ≥ 0.706Diamonds < 0.706

N. Lat. (°)

Map view

Cratonedge

E-W Crossection

off-cratonon-craton

y = 0.1597x + 12.733

R2 = 0.9682

15.2

15.3

15.4

15.5

15.6

15.7

15.8

16.0 16.5 17.0 17.5 18.0 18.5 19.0

206Pb/204Pb

207P

b/20

4Pb

SRP-YNP basaltsIsochron age = ca. 2.5 Ga

YNP

ESRP

WSRPCSRP

Pb-Pb systematics imply Archean age for SRP basalt sourceswith increasingly radiogenic Pb to the west

15.3

15.4

15.5

15.6

15.7

15.8

15.9

16.0 17.0 18.0 19.0 20.0

206Pb/204Pb

207Pb/204Pb

SRPYNPEOR

YellowstoneESRP

WSRPy = 0.181x + 12.348 r2 = 0.95 ca. 2.67 Ga

NHRL (oceanic array)

Rhyolites

off-craton

Pb in SRP rhyolites becomes progressively more radiogenic to west, and also is consistent with an Archean source; compositions dramatically change near the

inferred craton edge.

Zircon Geochronology of Lower Crustal XenolithsVervoort, Wolf & Leeman (unpub.)

Leeman et al., 1985

Proterozoic sediments

Archean Crust

2.6 2.8-3.2

2.9-3.2 Ga

SRP 2600

2500

2400

2300

2200

2100

0.28

0.32

0.36

0.40

0.44

0.48

0.52

6.5 7.5 8.5 9.5 10.5 11.5 12.5

207Pb/235U

206 Pb/238U

Intercepts at 27 ± 100 & 2582.4 ± 8.7 [±11] Ma

MSWD = 0.28

SM

These data enlarge the known extent of the Archean Wyoming province

Post-mid Miocene magmatic progressions

dashed lines mark isotope discontinuites

Following CRB ‘event’, magmatism expanded NE-ward with time into the SRP with a minor bifurcation into SE Oregon.

Early silicic magmatism requires precursor basaltic intrusions.

CRB flood lavas

Migration of SRPmagmatism

(Armstrong, Leeman &Malde, 1975)

MREC

EMBH Tiv

Apparent propagation rate

Extension?

WSRP

Yellow boxes = anomalies

Problems in estimating volcanic propagation rates

• Locations of vents/sources

• Correlations of distal units to source

• Causes for silicic magmatism

• Tectonic displacement (extension)

Space-time distribution of Yellowstone hotspot track silicic

volcanic rocks(Perkins & Nash, 2002)

**

In detail, not a simple age progression!

Main trendAnomalousBasalt

*

1

2

3

Ignimbrite flare-up between 11.7-10.0 Ma coincided with widespread outbreaks of distinct rhyolites

These occurrences signify that: (1) Large pockets of compositionally diverse silicic magmas existed coevally within wide expanses of the crust, and(2) Mafic magmatism must have been similarly widespread

0

2

4

6

8

10

12

14

16

Age(m.y.)

0 1 2 3 4 5 6

FeO*MBH

RTF

BJ Rhy

Tephras

McD

Tyd

Tmr/yt

YP

JM

OF

CPT avgs

Yellowstone

W.-Central SRP

MREC

McDermitt

Juniper Mtn.

BJ/TF/MBH

Tmr

Tmc

Figure X14. Temporal variation in chemistry of West-Central SRP rhyolites (14-3 Ma). Included are data for Bruneau-Jarbidge (CPT and BJ), Mt. Bennett Hills (MBH), and Rogerson/Twin Falls (RTF) areas (our averages), Owyhee front (OF), Magic Reservoir Center (Tmr/yt, Tyd), and regional ashes (Tephras). Comparative data are shown for the younger Yellowstone (YP) and older Juniper Mtn. (JM) and McDermitt (McD) eruptive centers. Regression lines through data from most eruptive centers have negative slopes consistent with magmas becoming more evolved with time. BJ/RTF/MBH data differ dramatically in showing increasing ‘maficity’ with time.

OF

0.5115

0.5120

0.5125

0.5130

143Nd/144Nd

05101520

Age

MREC Rhy

SRP-OR Rhy

MREC

AVT

YP

W ofcraton

SRP basalts

(ESRP)

IB

From Leeman, Oldow, and Hart (1992) and unpublished data

(0-15 Ma)

Archean xenoliths < 0.5115

Caldera-Forming Stage Rifting Stage

IgnimbriteFlare Up

Eruption Rate (km3/Ma)

Cumulative Volume(as percent of total)

500

100

12 9 6

0

Age (M.y.)

80

60

40

20

0

400

300

200

100

∑Volume = ca. 10000 km3

Comparison of the three ash-flow tuffs of the Yellowstone Group and resulting calderas

Ash-flow

Tuff

Age

(Ma)

Volume

(km3)

Area

(km2)

Dimen-sions (km)

Caldera

name

Lava Creek Tuff

 0.640 1000 7500 85 x 45 Yellowstone

Mesa Falls Tuff

1.3 280 2700 16 x 16 Henry’s Fork

Huckleberry Ridge Tuff

2.1 2450 15500 ~85 x 50 Big Bend Ridge, etc.

(segments)

Total duration: >2.1 Ma Total AFT eruptive volume > 3700 km3

(Total volume of rhyolitic magma is considerably greater)

How much basalt are we talking about?

1. Yellowstone analog - rhyolites produced by crustal melting due to intrusion of basalts; assuming I:E = ~2 (this could be >10), volume production is constrained by thermal balances:rhyolite volume = ~10000 km3 (produced over 2 Ma)partial melt zone = 100000 km3 (for 10% melting) thickness of pmz = ~6-13 km (for radii of 70 to 50 km)

2. Heat budget requires crystallization of ~2g of basalt for each 1g of rhyolite produced, or about 20000 km3 over 2 Ma - a supply rate of ~0.01 km3/yr (~1/10 the rate for Kilauea): equivalent total thickness of basalt intruded = ~1.3-2.5 km (for radii of 70 to 50 km), or about 1 km/Ma

3. For a lithosphere block (width = 100 km, thickness = 100 km)migrating over plume heat source at 2-4 cm/yr (20-40 km/Ma), the required volume of basalt amounts to 5% partial melting of SCLM (assuming greater lithosphere volume or faster migration decreases % pm).

Implications and questions 1. Large volume (~10000 km3/Ma) injection of basalt into crust,

with near constant crustal thickness along the SRP, implies accommodation by lithosphere stretching (parallel to SRP axis):

extension = V/(tL• width) = ~1 km/Ma strain rate for SRP = (1 km/Ma • 15 Ma)/500 km = ~3%

2. The inferred magnitude of extension (~1 cm/yr) is similar to the difference between plate velocity estimated from time-distance relations for silicic eruptive centers (~3.5-4 cm/yr) vs. estimates based on other methods (e.g., NUVEL-1 model, 2.2±0.8 cm/yr).

3. Ongoing B&R style extension may account for extended magmatism distal from the plume center.

4. More work is needed to reconcile the inferred basalt production with apparent thermal inertial of either SCLM or a plume deflected by a thick lithosphere. E.g., just how thick is the mechanical boundary layer wherein reside the old isotopic components that contribute to Y-SRP magmatism?

Model for SRP crustal evolution - assuming an averaged crustal extension rate ( ~5%/Ma) and original crustal thickness of 40 km. Original Moho and midcrust (Conrad discontinuity) shallow with time according to lines ‘M’ and ‘C’. To maintain near-constant crustal thickness (based on available seismic refraction data) requires addition of under- or intra-plated basalt over depths equivalent to those between curves ‘M’ and ‘Moho’ (though not restricted to the geometry shown). Final mass distribution is such that ~3/4 of the present-day WSRP crust has a lower crustal average P-wave velocity (~6.7 km/sec).

YP WCSRP (Distance ->)

0

10

20

30

40

50

0 5 10 15 20Time (Ma)

Thi

ckne

ss (k

m)

total crustupper crustOrig Moho

Upper crust

Lower crust‘M’

Lithospheric mantle

Vol. new crust

New ‘Moho’

shallow basaltic

intrusions

‘C’ rhyolite

basalt

What is the source of Y-SRP basalts? • Upwelling plume material

a. If t > ~100 km, a plume is unlikely to melt unless Tp >1500°C

b. Plume could contribute heat to SCLM and volatiles (e.g., He)

c. If melting occurs, expect OIB- or MORB-like magmas

• Lower SCLM (isotopic compositions depend on age of SCLM)

a. If strongly refractory (e.g., residual peridotite), perhaps no melt

b. Low % melts of hydrated lithosphere (--> lamproite melts?)

c. Larger % melts of mafic/pyroxenitic veins (--> basaltic melts?)

• Combination models?

a. Plume melts modified systematically during ascent & storage by SCLM-derived melts

b. Hybrid source consisting of plume mantle & thermally eroded SCLM material

Arguments for a lithospheric mantle source

• Pb isotope array and Archean isochron age• Enriched Sr isotope ratios with low Rb/Sr• All radiogenic isotopes consistent with ingrowth

within an isolated Archean source• Similarities to OIB-MORB wrt K-Zr, Ba-Th, B-

Nb, etc. trace element systematics (precludes crustal contamination)

• HREE profiles are flat, and inconsistent with melting of deep mantle (garnet-bearing)

It appears that if an asthenospheric mantle plume is involved, it cannot contribute significant amounts of melt. However, elevated 3He/4He could signify outgassing of volatiles from a deep mantle domain.

101.1.1

1

10

100

1000

MORB avgsOIB avgsSRP

SKIP-D

SKIP-ASKIP-B

SKIP-C

(Rb/Hf)/PM

Th/PM

PM

E-MORB sourceN-MORB

source

OIB avgs.

N-MORB avgs.

SRP

FC

melting

Rb-depleted sources

Rb-depletion in SRP source coupled with elevated 87Sr/86Srimplies old source (consistent with Pb-Pb model age ca. 2.5 Ga)

10000100010010100

1000

10000

100000

1000000SROT

SKIP-B

SKIP-CSKIP-DLoihi

KoolauRhyolitesCrustOIBLamproitesMORB

SKIP-A

Zr

K

UC

LC

YP Rhyolites

Lamproites

50

K/Zr = 20

Intraplate basalts

FC

SRP basalts and OIB are identical for K-Zr systematics

RbTh Ba K Nb Ta La Ce Sr P Nd ZrSmHf Ti Y.1

1

10

100

1000

6YC-142L74-26N-MORBMinetteKimberlite70-15BR

ock/PM

SROT

HAOTs

super-enriched SCMLmelts

Relatively flat HREE profiles in SRP basalts suggest shallow (ca. <70 km) spinel-lherzolite sources lacking garnet

30201000

15

3He/4He (R/Ra)

Arcs

MORB

Continental basalts

YNP springs

SRP basalts (Reid)

SRP basalts (C&L)

Imnaha basalt

Siletzia basalts

Kerguelen (xenoliths)

Loihi

Hawaii

Iceland

Helium Isotope Summary He isotope data for SRP basalts (olivines) show greater 3He enrichment than in MORB, and over-lapping ranges for many inferred hot spot suites.

Schematic lithospheric structure, NW USA

200

150

100

50

Asthenosphere

WISZ YP CAS

Lithosphere

Crust

Accreted

SCLM

NE SW

NW USA Mantle Structure Plume (?)

TBL

SZ

Subducting

Slab

Precambrian craton Accreted terranes

SRP

magma

transport &

storage

pmz

T (°C)

P (GPa)

ca. 1400°C adiabat

0 3 6

100 2000 Z (km)

1200

1600

1000

Thick lithosphere retards melting of upwelling mantle;

Melting requires either higher Tp or lithospheric thinning

Mantle melting considerations

mantle

solidus

lithosphere lid

Decompression melting scenario

Yellowstonevelocity profiles

Schutt

Controls on eruptions & ‘out of sequence’ events?

1. Oceanic hot spot volcanism displays a simple time-volume relation, SRP volcanism does not. This could be explained by different lithosphere structures.

2. Assuming existence of a sufficient magma supply, and ascent by bouyant forces, to get eruptions through continental crust requires a minimum depth (~50 km) to magma reservoir.

3. Shallower reservoirs (e.g., near Moho) cannot support eruption of basalt through normal continental crust, but can support intrusion at shallower levels (est. intrusion of basalt is equivalent to ~1 km thickness/Ma).

4. Magmatic processes gradually increase crustal density thus increasing likelihood of basalt eruptions from increasingly shallower reservoirs. Petrologic constraints suggest that typical SROTs are fed from mid-crust reservoirs (≤ 25 km)

Suggested research goals• High-resolution reflection/refraction seismology - determine geometry

of intrusive structures, mass distribution within crust• Anisotropy and 3-D structure - constraints on deformation style and

magnitude along and adjacent to SRP track• Nature of inferred lithosphere boundaries - isotope contrasts• Attenuation - melt distributions with depth within the crust• Definition of base of lithosphere as a physical/chemical/thermal entity• Modelling deformation of weakened crust (due to magma injection) -

contributions to regional tectonics• Petrology-geochemistry - understanding processes of continental

evolution• Development and extrapolation of understanding of large igneous

systems

Image of compressional-wave velocity structure at 100 km depth(Dueker et al., 2001).

SRP