5 th Swiss Geoscience Meeting, Geneva 2007
Annen Catherine*, Leeman William P.**, Dufek Josef***
*Section des Sciences de la Terre, Rue des Maraîchers 13, CH-1205 Genève
(catherine.annen@terre.unige.ch)
** National Science Foundation, Earth Sciences Division, Wilson Blvd. 4201, VA
22230, USA
***Department, of Earth and Planetary Science, University of California, Berkeley, CA
94720, USA
In the central Snake River Plain (Idaho, USA), at least 7,000 km 3 and most probably more than 10,000 km 3 of dry, high temperature rhyolites were erupted over a few million years (Bonnichsen et al. 2007). Isotopic signatures, especially a strong depletion in

18 O, indicate a crustal origin for these rhyolites (Boroughs et al. 2005). To produce such high volumes of rhyolitic melts, a cylinder of the crust 50 km in radius and more than 3.6 km in thickness must be heated over
900°C, requiring the massive input of basalt into the crust.
According to heat transfer calculations, the intrusion of 12 to 20 km of basalts at depths greater than 10 km is needed to provide enough heat to melt the crust over a thickness of several kilometers (Fig. 1). Also, the shallower the depth of basalt emplacement, the more basalt is needed to melt the crust. The geochemistry of the Central Snake River Plain rhyolites indicates that the melts become less differentiated with time (Bonnichsen et al. 2007). This trend can be explained by an increase in the melting degree of the source or by the depletion of the protolith that becomes progressively more refractory. In both cases, the temperatures in the source must increase with time, which is the case if the basaltic body is incrementally emplaced and grows with time. Heat transfer models indicate emplacement rates for the basalt in the range of 3-5 mm/yr
(Fig. 1).
The basalt that is used to melt the crust also generates silicic melts by differentiation. Unless the melting degree of the crustal protolith is more than
40%, the volumes of melt generated by differentiation of the basalt will likely exceed the volume of melt generated by partial melting of the crust. However, geochemical data indicate that the contribution of crust is dominant. This apparent contradiction suggests that the two types of melt do not extensively mingle and that the crustal melt is preferentially extracted and erupted.

5 th Swiss Geoscience Meeting, Geneva 2007
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Thickness of basalt emplaced (km)
4 8 12 16 20 24
4
Depth of basalt top
10 km
15 km
20 km
25 km
30 km
3
2
1
0
1 2 3 4 5 6
Basalt emplacement rate (mm/yr)
Figure 1. The thickness of upper crust that is heated above 900°C for different basalt emplacement rates and corresponding total basalt thicknesses and for different depths of basalt emplacement. The basalt is emplaced over 4 Myrs.
REFERENCES
Boroughs, S., Wolff, J., Bonnichsen, B., Godchaux, M. and Larson, P. 2005.
Large-volume, low-delta O-18 rhyolites of the central Snake River Plain, Idaho,
USA. Geology, 33, 821-824.
Bonnichsen, B., Leeman, W.P., Honjo, N., McIntosh, W.C. and Godchaux, M.M.
2007. Miocene silicic volcanism in southwestern Idaho:geochronology, geochemistry, and evolution of the central Snake River Plain. Bulletin of
Volcanology: DOI 10.1007/s00445-007-0141-6.