DEEP MELTING OF HOT MANTLE: IMPLICATIONS FOR VOLCANIC HAZARD STUDIES AT THE PROPOSED NUCLEAR WASTE REPOSITORY AT YUCCA MOUNTAIN, NEVADA

Alkali basalt volcanoes near Yucca Mountain (YM) are part of a zone of basaltic volcanism that stretches from Death Valley to the Lunar Crater field (LC). In this zone, volcanism is episodic with three peaks of volcanism since 9.5 Ma. A common driving force for magmatism is suggested by coeval volcanism along the length of the zone. Smith et al. (2002, GSA Today, v. 12, no. 4, p. 4-11) proposed that hot mantle exists beneath the zone and provides the impetus for volcanic activity. Modeling by Wang et al. (2002, JGR, v. 107, 10.1029/2001JB000209) indicates that melting was hot and deep beneath LC and Crater Flat (CF). These results are for dry mantle; if LC-CF lavas could be explained by wet melting, this would change interpretations from a hot spot, to cooler melting of wet mantle. Tectonic history and geochemical data, however, argue for minimal volatiles in the mantle source. Although the subducted slab retreated from beneath the central Great Basin > 20 my ago, volcanism has been active for the past 20 my. It is unlikely that the mantle would remain wet until Quaternary time without replenishment of volatiles. Rare hornblende phenocrysts from CF and amphibole xenocrysts from LC suggest that the mantle contained up to 0.5 wt% water. This modest amount of water, as well as trace element ratios, are similar to other hot spot type alkali basalts, and very unlike arc or lithospheric melts. Finally, melting of hydrous peridotite produces basalt with low FeO and high SiO2, unlike the high FeO (> 11 wt%) and low SiO2 (< 47 wt%) basalts in this region. While the mantle source may contain some volatiles, the major and trace element composition of these lavas requires deep, hot melting during the Quaternary. If CF and LC are linked by a common area of hot mantle then LC recurrence rates (11-15 events/my) may be applicable to the CF area and the probability of magmatic disruption of the repository may be significantly higher than previous estimates. A criticism of this linkage is that eruption rates at LC are the highest in the Basin and Range while rates near YM are among the lowest. Overall, the number of vents and eruption volumes are lower near YM than LC; however, in the past, eruption rates were similar (e.g., between 4.5 and 3.5 Ma, 8 events occurred near YM and 9 at LC). LC recurrence rates provide an upper bound for calculating eruption probabilities for YM.