Magmatism of the
Snake River Plain – Yellowstone region:
Implications for continental lithosphere
evolution above a mantle plume
Bill Leeman
now 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 largescale 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
forfor
mid-Miocene
magmatic
flareup
Setting
mid-Miocene
magmatic
flareup
W
I
S
Z
On-craton
Off-craton
realistic
artifact
Isotopes signify distinct mantle sources across
prominent tectonic boundaries
Craton
edge
N Lat. (°)
44.0
<0.704
<0.705
<0.7055
<0.706
<0.707
<0.708
N. Lat. (°)
< 0.706
43.0
< 0.706
off-craton
42.0
> 0.706
on-craton
OR
NV
ID
UT
Sr isotopic compositions of Cenozoic basalts
41.0
120.0
118.0
116.0
W. Long. (°)
E-W Crossection
114.0
112.0
Diamonds < 0.706
Circles ≥ 0.706
Map view
Pb-Pb systematics imply Archean age for SRP basalt sources
with increasingly radiogenic Pb to the west
207Pb/204Pb
15.8
15.7
SRP-YNP basalts
Isochron age = ca. 2.5 Ga
15.6
WSRP
15.5
CSRP
ESRP
15.4
y = 0.1597x + 12.733
YNP
15.3
R2 = 0.9682
15.2
16.0
16.5
17.0
17.5
18.0
206Pb/204Pb
18.5
19.0
Rhyolites
15.9
207Pb/204Pb
15.8
15.7
WSRP
y = 0.181x + 12.348
r2 = 0.95
ca. 2.67 Ga
ESRP
Yellowstone
off-craton
15.6
15.5
SRP
YNP
EOR
15.4
15.3
16.0
17.0
18.0
19.0
20.0
206Pb/204Pb
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 Xenoliths
Vervoort, Wolf & Leeman (unpub.)
Proterozoic sediments
Archean Crust
2.9-3.2 Ga
SRP
2.6 2.8-3.2
0.52
SM
0.48
2600
2500
2400
2300
2200
0.40
2100
206
Pb/238U
0.44
These data enlarge the known extent of the
LeemanArchean
et al.,Wyoming
1985 province
0.36
0.32
Intercepts at
27 ± 100 & 2582.4 ± 8.7 [±11] Ma
MSWD = 0.28
0.28
6.5
7.5
8.5
9.5
207
10.5
235
Pb/ U
11.5
12.5
Post-mid Miocene magmatic progressions
CRB flood lavas
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.
Space-time distribution of
Yellowstone hotspot track silicic
volcanic rocks
2
Yellow boxes = anomalies
(Perkins & Nash, 2002)
MREC
Problems in estimating volcanic
propagation rates
EMBH Tiv
3
• Locations of vents/sources
WSRP
• Correlations of distal units to
source
• Causes for silicic magmatism
• Tectonic displacement (extension)
Extension?
1
Migration of SRP
magmatism
(Armstrong, Leeman &
Malde, 1975)
* *
Main trend
* Anomalous
Basalt
In detail, not a simple age progression!
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
CPT avgs
0
Yellowstone
2
JM
W.-Central SRP
4
Tmr
MREC
6
Age
(m.y.)
OF
YP
Tmr/yt
8
Tyd
Tmc
10
BJ/TF/MBH
OF
12
McD
Tephras
14
BJ Rhy
Juniper Mtn.
McDermitt
16
0
1
2
3
FeO*
4
5
6
RTF
MBH
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.
0.5130
SRP-OR Rhy
0.5125
MREC Rhy
W of
craton
(0-15 Ma)
SRP basalts
YP
143Nd/144Nd
IB
0.5120
MREC
AVT
(ESRP)
Archean xenoliths < 0.5115
0.5115
20
15
10
5
Age
From Leeman, Oldow, and Hart (1992) and unpublished data
0
Ignimbrite
Flare Up
500
Eruption Rate
(km3/Ma)
400
300
200
100
0
Caldera-Forming Stage
100
Rifting Stage
∑Volume = ca. 10000 km3
80
60
Cumulative Volume
(as percent of total)
40
20
0
12
9
Age (M.y.)
6
Comparison of the three ash-flow tuffs of the
Yellowstone Group and resulting calderas
Ash-flow
Tuff
Age
(Ma)
Volume
(km3)
Area
(km2)
Dimensions
(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?
Time (Ma)
0
5
10
15
20
0
Upper crust
Thickness (km)
10
total crust
upper crust
Orig Moho
‘C’
rhyolite
intrusions
basaltic
20
shallow
Lower crust
‘M’
30
Vol. new crust
40
50
New ‘Moho’
basalt
YP
Lithospheric mantle
WCSRP
(Distance ->)
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 intraplated 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 presentday WSRP crust has a lower crustal average P-wave velocity (~6.7 km/sec).
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, BNb, 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.
Rb-depletion in SRP source coupled with elevated 87Sr/86Sr
implies old source (consistent with Pb-Pb model age ca. 2.5 Ga)
1000
Rb-depleted sources
FC
100
OIB avgs.
Th/PM
SRP
10
melting
1
.1
.1
PM
N-MORB avgs.
N-MORB
source
E-MORB
source
1
(Rb/Hf)/PM
MORB avgs
OIB avgs
SRP
SKIP-A
SKIP-B
SKIP-C
SKIP-D
10
SRP basalts and OIB are identical for K-Zr systematics
1000000
K
100000
10000
SROT
SKIP-A
SKIP-B
SKIP-C
SKIP-D
YP Rhyolites
Loihi
Koolau
Rhyolites
UC
Crust
OIB
Lamproites
MORB
LC
Lamproites
FC
Intraplate basalts
1000
50
K/Zr = 20
100
10
100
1000
Zr
10000
1000
super-enriched SCML
melts
100
Rock/PM
SROT
10
HAOTs
1
.1
Relatively flat HREE profiles in SRP basalts
suggest shallow (ca. <70 km) spinellherzolite sources lacking garnet
Rb Th Ba K Nb Ta La Ce Sr P Nd Zr Sm Hf Ti Y
6YC-142
L74-26
N-MORB
Minette
Kimberlite
70-15B
15
Helium Isotope Summary
He isotope data for SRP
basalts (olivines) show
greater 3He enrichment
than in MORB, and overlapping ranges for many
inferred hot spot suites.
Arcs
MORB
Continental basalts
YNP springs
SRP basalts (Reid)
SRP basalts (C&L)
Imnaha basalt
Siletzia basalts
Kerguelen (xenoliths)
Loihi
Hawaii
Iceland
0
0
10
20
3He/4He (R/Ra)
30
Schematic lithospheric structure, NW USA
Mantle melting considerations
0
T (°C)
100
200
ca. 1400°C
adiabat
1600
Thick lithosphere
retards melting of
upwelling mantle;
1200
Melting requires either
higher Tp or
lithospheric thinning
1000
lithosphere lid
0
Z (km)
3
P (GPa)
6
Decompression melting scenario
Yellowstone
velocity
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).