Testing Models for Basaltic Volcanism: Implications for Yucca Mountain, Nevada

1. 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

2. Acknowledgements • Nuclear Waste Division of Clark County, Nevada • Nevada Agency for Nuclear Projects

5. Crater Flat-Lunar Crater Volcanic Field Death Valley From Smith et al. (2002) and Smith and Keenan (2005)

6. 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.

7. Models Crust • Deep vs. shallow melting. 30 Km Traditional model LM Shallow melting=very little additional activity and lower probability of disruption 60-100 Km Deep Melting model LC Deep melting=higher potential of additional activity And higher probability of repository disruption Asthenosphere

8. 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

9. Fe-Na Geobarometer Pf- final depth of melting determined by Na2O. Na2O is a function of the degree of melting. Na2O behaves as an incompatible element which is diluted by further increments of melting Po-initial depth of melting determined by FeO

10. 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)

11. 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).

12. 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.

14. 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.

15. Model Eruption, olivine less abundant but has higher FeO/MgO, Fo79 During ascent-30-40% of olivine removed Remaining olivine becomes Enriched in FeO Crust 40 Km ~ 1 MPa Lithospheric Mantle 70 Km ~ 2 MPa Asthenospheric Mantle Melting Peridotite, Olivine Fo90 High MgO/FeO

16. 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).

17. 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

18. 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.

19. Model 1025 to 1035 C Eruption Crust 40 Km ~ 1 MPa 850 C Lithospheric Mantle 1100 C 70 Km ~ 2 MPa 1350C Asthenospheric Mantle Melting Peridotite, Olivine Fo90 High MgO/FeO 1330-1340 C melting TOO HOT to be lithospheric mantle

20. Summary Geobarometers indicate deep melting in the asthenosphere. Geothermometer indicates melting of hot asthenospheric mantle. Next-Mantle flow patterns and the control of volcanism

21. 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

22. NA plate 2 cm/yr-west Mantle 3 cm/yr-east So 5 cm/yr shear at Base of the lithosphere Clint Conrad, Johns Hopkins University

23. Lid-driven cavity flow A=Wc/Hc, T=Hasth/(Hasth+Hc)

24. 1 cm/yr 200 degrees hotter- 10 to 100 times less viscous

26. 2 cm/yr Crust Lithospheric Mantle 3 cm/yr 100-200 km ~ 1 cm/yr upwelling Asthenosphere Next Area of Hot Mantle A tape recording of mantle Thermal Anomalies?

27. 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 

28. 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?

29. Number of Dated Volcanic Events vs. Age 0.5 m.y. bins How can episodic pattern be explained?

33. 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.

34. 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.