Magmatism of the Snake River Plain

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Magmatism of the Snake River Plain Yellowstone
region Implications for continental lithosphere
evolution above a mantle plume

• Bill Leeman
• now at
• National Science Foundation

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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.

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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

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Architecture of the lithosphere - N. Rocky Mtns.
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Isotopes signify distinct mantle sources across
prominent tectonic boundaries
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Pb-Pb systematics imply Archean age for SRP
basalt sources with increasingly radiogenic Pb to
the west
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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.
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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
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OF
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.
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(0-15 Ma)
Archean xenoliths lt 0.5115
From Leeman, Oldow, and Hart (1992) and
unpublished data
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?Volume ca. 10000 km3
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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 Henrys Fork
Huckleberry Ridge Tuff 2.1 2450 15500 85 x 50 Big Bend Ridge, etc. (segments)
Total duration gt2.1 Ma Total AFT eruptive
volume gt 3700 km3 (Total volume of rhyolitic
magma is considerably greater)
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How much basalt are we talking about?

• 1. Yellowstone analog - rhyolites produced by
  crustal melting due to intrusion of basalts
  assuming IE 2 (this could be gt10), 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).

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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
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• 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, 2.20.8 cm/yr).
• 3. Ongoing BR 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?

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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).
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What is the source of Y-SRP basalts?

• Upwelling plume material
• a. If t gt 100 km, a plume is unlikely to melt
  unless Tp gt1500C
• 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 (--gt
  lamproite melts?)
• c. Larger melts of mafic/pyroxenitic veins
  (--gt 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

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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.
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Schematic lithospheric structure, NW USA
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Decompression melting scenario
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Yellowstonevelocity profilesSchutt
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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)

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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

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