Testing Models for Basaltic Volcanism: Implications for Yucca

Mountain, Nevada

Eugene Smith, UNLVClinton Conrad, University of Hawaii

Terry Plank, Lamont Doherty Earth ObservatoryAshley Tibbetts, UNLV

Deborah Keenan, Geoscience Consultants

Acknowledgements

• Nuclear Waste Division of Clark County, Nevada

• Nevada Agency for Nuclear Projects

Crater Flat-Lunar Crater Volcanic Field

From Smith et al. (2002) and Smith and Keenan (2005)

Death Valley

Main Point

• It is important to understand the process of volcanism before calculating the probability of future events.

• Understanding the process is especially important for 1,000,000 year compliance periods.

Models

• Deep vs. shallow melting.

Crust

LM

Asthenosphere

LC

30 Km

60-100 Km

Traditional model

Deep Melting model

Shallow melting=very little additional activity and lower probability of disruption

Deep melting=higher potential of additional activityAnd higher probability of repository disruption

Is melting deep or shallow?What is the temperature of melting?

•Use Geobarometers to estimate depth of magma generation.

•Use Geothermometer to estimate melting temperature

Fe-Na Geobarometer

Po-initial depth of melting determined by FeO

Pf- final depth of melting determined by Na2O.Na2O is a function of the degree of melting. Na2Obehaves as an incompatible element whichis diluted by further increments of melting

High melting temperatures and asthenospheric melting

Blue LM from Jones et al. (1996). Z boundary from Zandt et al. (1995). References in Wang et al. (2002).

From Wang et al. (2002)

Si melt Barometer

• Being developed by Cin-Ty Lee (Rice) and Terry Plank (Lamont-Doherty).

• Technique has not been published but is based on the reaction Mg2SiO4 (ol) + SiO2 (melt) = Mg2Si2O6 (opx).

Si melt Barometer

• The notion of this barometer is not new but experimental data is now coming available to calibrate it.

• Appears to be independent of temperature and composition.

shear velocity anomaly (%)

6 0 -6

100

50

150

Dep

th (

km)

-120 -118 -116

Great Valley

Sierra Nevada

Owens Valley

Big PineW E

new

Wang et al

Cross section of seismic shear wave velocity model at ~ 36.5 N across California from Yang and Forsyth (2006). Big Pine volcanic field located by longitude along profile. Moho is crust-mantle boundary from Fleidner et al (1996). Boxes indicate pressure of melting estimate from Wang et al (2002), based on Fe-Na systematics, and new estimate based on silica barometer. Both estimates overlap with the region of large negative shear velocity anomaly which may reflect the presence of melt in the mantle. The convergence of seismic and chemical observations point to a constrained mantle melting region beneath Big Pine at 50-75 km depth. Longitude

Moho

Olivine-Liquid GeothermometerSugawara (2000)

• Basalt from Lathrop Wells near Yucca Mountain is ideal for calculating temperatures– Limited range of FeO and MgO (<0.5 wt. %)– Few crystals (2 to 4 vol % olivine only) – Olivine core compositions show limited range

(Fo76-79) and reflect equilibrium with the host liquid.

Model

Crust

Lithospheric Mantle

Asthenospheric Mantle

40 Km ~ 1 MPa

70 Km ~ 2 MPa

Peridotite,Olivine Fo90High MgO/FeO

Melting

During ascent-30-40% of olivine removedRemaining olivine becomesEnriched in FeO

Eruption, olivine less abundant but has higher FeO/MgO, Fo79

Geothermometer Step 1-Crystallization (Eruption temperatures)

• Line shows liquids in equilibrium with Fo79 olivine with temperatures calculated by the Sugawara (2000) thermometer.

•1155 to 1165 º C-- dry•1025 to 1035 º C --4.6 wt. % water•In agreement with 1005 ± 20 º C determined by Nicholis and Rutherford (2004).

GeothermometerStep-2 Melting temperature

• Lathrop Wells basalt only contains olivine crystals, so add olivine (changing its composition) until it is in equilibrium with an average mantle olivine of Fo90.

• Requires 38 to 40 % olivine addition and assumes a Fe/Mg exchange coefficient of 0.3

Melting TemperatureLathrop Wells Basalt

• Melting temperature• 1440-1450 º C dry

• 1330-1340 º C wet

• These temperatures are typical of the asthenosphere (1350 ºC and are too high for lithosphere.

Model

Crust

Lithospheric Mantle

Asthenospheric Mantle

40 Km ~ 1 MPa

70 Km ~ 2 MPa

Peridotite,Olivine Fo90High MgO/FeO

Melting

1025 to 1035 CEruption

1330-1340 C melting

1350C

850 C

1100 C

TOO HOT to be lithospheric mantle

Summary

Geobarometers indicate deep melting in the asthenosphere.

Geothermometer indicates melting of hot asthenospheric mantle.

Next-Mantle flow patterns and the control of volcanism

Deep Melting

• Must explain:– Hotter mantle temperatures– Narrow belt of volcanism– Episodic pattern with basaltic volcanism occurring

in same belt for as long as 11 Ma

Clint Conrad, Johns Hopkins University

NA plate 2 cm/yr-westMantle 3 cm/yr-eastSo 5 cm/yr shear atBase of the lithosphere

Lid-driven cavity flow

A=Wc/Hc, T=Hasth/(Hasth+Hc)

200 degrees hotter- 10 to 100 times lessviscous

1 cm/yr

Asthenosphere

Lithospheric Mantle

Crust

Next Area of Hot Mantle

2 cm/yr

3 cm/yr

~ 1 cm/yrupwelling

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4

6

8

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Age

Nu

mb

er o

f E

ven

ts

Lunar Crater-ReveilleCrater Flat

A tape recording of mantleThermal

Anomalies?

100-200 km

Western US relative P-velocity variations

Low velocity zones (red) may be areas of hotter lithosphere or asthenospheric.

Spaced 100 to 200 km apart

From presentation by K. Dueker,University of Wyoming 

Spacing of Thermal Pockets

• Thermal Pockets spaced 100-200 km apart and are 50 to 300 km wide.

• At shear rate of 5 cm year, 2-4 million years between pockets and 1 to 6 million years for pocket to pass a specific point.

• Do we observe these patterns in the geologic record?

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AgeN

um

ber

of

Eve

nts

Lunar Crater-Reveille

Crater Flat

Number of Dated Volcanic Events vs. Age

0.5 m.y. bins

How can episodic pattern be explained?

Summary• Melting is deep and in the asthenosphere. • Location of volcanic field controlled by mantle

processes.• Another peak of activity may occur at Yucca

Mountain. Timing and size of peak depend on the size of the next mantle thermal anomaly.

Summary

• High-quality geophysical data required to test models and predict next eruptive period.

• Probability of disruption of repository may be 1-2 orders of magnitude larger than presently calculated.