Testing Models for Basaltic Volcanism: Implications for Yucca Mountain, Nevada Eugene Smith, UNLV Clinton Conrad, University of Hawaii Terry Plank, Lamont Doherty Earth Observatory Ashley 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 Km Traditional model Deep Melting model Shallow melting=very little additional activity and lower probability of disruption Deep melting=higher potential of additional activity And 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 Na 2 O. Na 2 O is a function of the degree of melting. Na 2 O behaves as an incompatible element which is 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 Mg 2 SiO 4 (ol) + SiO 2 (melt) = Mg 2 Si 2 O 6 (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.

Olivine-Liquid Geothermometer Sugawara (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 Fo90 High MgO/FeO Melting During ascent-30-40% of olivine removed Remaining olivine becomes Enriched 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 to 1165 º C-- dry 1025 to 1035 º C wt. % water In agreement with 1005 ± 20 º C determined by Nicholis and Rutherford (2004).

Geothermometer Step-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 Temperature Lathrop Wells Basalt Melting temperature º C dry 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 Fo90 High MgO/FeO Melting 1025 to 1035  C Eruption  C melting 1350  C 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-west Mantle 3 cm/yr-east So 5 cm/yr shear at Base of the lithosphere

Lid-driven cavity flow A=W c /H c, T=H asth /(H asth +H c )

200 degrees hotter- 10 to 100 times less viscous 1 cm/yr

Asthenosphere Lithospheric Mantle Crust Next Area of Hot Mantle 2 cm/yr 3 cm/yr ~ 1 cm/yr upwelling A tape recording of mantle Thermal Anomalies? 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 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?

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.