Project HOTSPOT: Scientific Drilling of the Snake River Plain
John Shervais Utah State University
Project HOTSPOT: Scientific Drilling of the Snake River Plain held its inaugural workshop in Twin Falls, Idaho, on May 18-21, 2006 (Shervais et al 2006a). This inter-disciplinary workshop explored major science issues and logistics central to a comprehensive, intermediate-depth drilling program along the hotspot track. This was followed by two special sessions at Fall 2006 AGU dedicated to SRP drilling, and meetings at GSA Denver 2007 and Fall AGU in 2007-2008.
Figure 1 – HOTSPOT Workshop excursion in southern Idaho, showing massive basalt flows with pillows at base.
The central question addressed by the workshop was: how do mantle hotspots interact with continental lithosphere, and how does this interaction affect the geochemical evolution of mantle-derived magmas and the continental lithosphere? Our hypothesis is that continental mantle lithosphere is constructed in part from the base up by the underplating of mantle plumes that are compositionally and isotopically distinct from pre-Phanerozoic cratonic lithosphere. Plumes modify the impacted lithosphere in two ways: by thermally and mechanically eroding pre-existing cratonic mantle lithosphere, and by underplating plume-source mantle that has been depleted in fusible components by decompression melting to form flood basalts or plume track basalts. The addition of new material to the crust in the form of mafic magma represents a significant contribution to crustal growth, and densifies the crust in two ways: by adding mafic material to the lower and middle crust as frozen melts or cumulates, and by transferring fusible components from the lower crust to the upper crust as rhyolite lavas and ignimbrites, leaving a mafic restite behind. We further hypothesize that the structure, composition, age and thickness of continental lithosphere influence the chemical and isotopic evolution of plume-derived magmas, and localizes where they erupt on the surface.
We know from studies of surface basalts and existing core that these differences reflect in part variations in lithospheric age, composition, and thickness, magma fractionation and recharge in crustal storage systems, and assimilation of older crust, as well as input from the deep-seated mantle plume and adjacent asthenosphere. Concrete scientific questions to be addressed within this context include:
How do the variations in magma chemistry, isotopic composition, and age of eruption constrain the mantle dynamics of hotspot-continental lithosphere interaction?
What do variations in magma chemistry and isotopic composition tell us about processes in the crust and mantle? To what extent is magma chemistry controlled by melting, fractionation, or assimilation of crustal components, and where do these processes occur?
Is the source region predominately lithosphere, asthenosphere, or plume? What are the proportions of each? Are there changes in the magma source/proportions at any one location along the plain through time relative to the position of the hotspot?
How does a heterogeneous lithosphere affect plume-derived mafic magma? Effect of crust-lithosphere age, structure, composition, and thickness on basalt and rhyolite chemistry, from variations in lava chemistry along the plume track.
What is the time-integrated flux of magma in the Snake River-Yellowstone volcanic system? Is it consistent with models of plume-derived volcanism, or is this flux more consistent with other, non-plume models of formation?
Can we establish geochemical and isotopic links between the “plume head” volcanic province (Columbia River Basalts), and the “plume tail” province (Snake River Plain) in the western SRP?
Rhyolites of the SRP are distinct from normal calc-alkaline rhyolites associated with island arc systems: they were very hot (850º-1000ºC) dry melts with low viscosity and anhydrous mineral assemblages. They produced very large volume (>200 km3) low aspect ratio lavas, vast (≈1000 km3) well-sorted, intensely welded ignimbrites and lava-like ignimbrites, and regionally widespread ashfall layers with little pumice. They are the youngest and best-preserved example of this type of volcanism, but the SRP eruptive centers are concealed beneath basalt. They have geochemical affinities to A-type/P-type granites and are common in other plume-related silicic provinces throughout the world. Major issues include:
Origin of the SRP rhyolites: crustal melting or fractional crystallization of mantle-derived basalt?
What are the volumes of the rhyolitic eruptions? What is the eruptive mass flux, and how does this vary with time, as the hot spot tracks across changing lithosphere? Related to this, how much plume-derived mafic magma is required to produce the rhyolites, and what does this tell us about total magma flux in the Snake River-Yellowstone plume system?
Do the rhyolites associated with the older western province differ from those of central and eastern SRP? Does the plume-crust interaction vary across a heterogeneous cratonic margin?
The formation of A-type granitic melts as dry melts of continental crust requires an external heat source capable of transferring immense amounts of heat to the crust – sufficient to form large volumes of high silica rhyolite with magmatic temperatures of 850-1000ºC. Determining the heat budget associated with these melts will be critical to our understanding of plume-continent interaction. In addition, the large volumes of rhyolite preserve a record of magma chamber processes that cannot be seen in surface exposures, but which are critical to understanding the origin and nature of these unique magmas. Proximal rhyolites will also provide a more complete record than distal rhyolites exposed outside the plain.
Major science issues of the paleo-lake Idaho component of SRP drilling include: (1) testing the hypothesis for the role of moisture transport to North America from the Pacific initiation of Northern Hemisphere glaciation; (2) examining the response of the Great Basin hydrological system to the Pliocene climatic optimum; (3) using the high resolution lacustrine records to infer the chronology of biotic recovery in both terrestrial and aquatic ecosystems in the post-eruption intervals following some of the largest explosive volcanic eruptions known; (4) resolving late Neogene record of biotic and landscape evolution in response to tectonic and magmatic processes related to SRP-Yellowstone hotspot evolution; (5) developing a “master reference section” for regional biostratigraphy and hence for sediments inter-bedded in basalts and rhyolites at other HOTSPOT sites.
Boroughs S, Wolff J, Bonnichsen B, Godchaux M, Larson P, 2005, Large-volume, low-δ18O rhyolites of the central Snake River Plain, Idaho, USA. Geology 33; no. 10; p. 821-824; DOI: 10.1130/G21723.1
Branney, M., Bonnichsen, B., Andrews, G., Ellis, B., Barry, T., and McCurry, M., 2008, ‘Snake River (SR)-type’ volcanism at the Yellowstone hotspot track: distinctive products from unusual, high-temperature silicic super-eruptions: Bulletin of Volcanology, v. 70, no. 3, p. 293-314.
Davis, O.K., Ellis, B., Link, P., Wood, S., and Shervais, J.W. 2006. Neogene Palynology of the Snake River Plain: Climate Change and Volcanic Effects. EOS Trans. AGU, 87(52), Fall Meet. Suppl., Abstr. V43D-08
Glen JMG, Payette S, Bouligand M, Helm-Clark C, Champion D, 2006, Regional geophysical setting of the Yellowstone Hotspot track along the Snake River Plain, Idaho, USA. EOS Trans. AGU, 87(52), V54-1698.
Graham, D W, Reid, M R Jordan, B T Grunder, A L Leeman, W P Lupton, J E, 2006, A Helium Isotope Perspective on Mantle Sources for Basaltic Volcanism in the Northwestern US EOS Trans. AGU, 87(52), Fall Meet. Suppl., Abstr. V43D-02.
Haug, G.H., A. Ganopolski, D. Sigman, A. Rosell-Mele, G.E.A. Swann, R. Tiedemann, S. Jaccard, J. Bollmann, M. Maslin, G. Eglinton, 2006, North Pacific seasonality and the glaciation of North America 2.7 million years ago. EOS Trans. AGU, 87(52), Fall Meet. Suppl., Abstr. V43D-08
Hanan, BB Shervais, JW, and Vetter, SK, 2008, Yellowstone plume-continental lithosphere interaction beneath the Snake River Plain, Geology, v. 36, 51-54. DOI: 10.1130/G23935A.1
McCurry, M, Hayden, K, Morse, L, and Mertzman, S, 2008, Genesis of post-hotspot, A-type rhyolite of the Eastern Snake River Plain volcanic field by extreme fractional crystallization of olivine tholeiite: Bulletin of Volcanology, v. 70, no. 3, p. 361-383.
Shanks, W.C., Morgan, L.A., and Bindeman, I., 2006, Geochemical and oxygen isotope studies of high-silica rhyolitic ignimbrites from the Snake River Plain and Yellowstone: Eos, Transactions, AGU.
Shervais, J.W., Branney, M.J., Geist, D.J., Hanan, B.B., Hughes, S.S., Prokopenko, A.A., Williams, D.F., 2006a, HOTSPOT: The Snake River Scientific Drilling Project – Tracking the Yellowstone Hotspot Through Space and Time. Scientific Drilling, no 3, 56-57. Doi:10.2204/iodp.sd.3.14.2006.
Shervais, J.W., and Hanan, B.B., 2008, Lithospheric topography, tilted plumes, and the track of the Snake River-Yellowstone Hotspot, Tectonics, 27, TC5004, doi:10.1029/2007TC002181.
Shervais, J.W., Vetter, S.K. and Hanan, B.B., 2006, A Layered Mafic Sill Complex beneath the Eastern Snake River Plain: Evidence from Cyclic Geochemical Variations in Basalt, Geology, v. 34, 365-368.
Shervais, JW and Vetter, SK, 2009, High-K Alkali Basalts of the Western Snake River Plain: Abrupt Transition from Tholeiitic to Mildly Alkaline Plume-Derived Basalts, Western Snake River Plain, Idaho, Journal of Volcanology and Geothermal Research, in press.
van Keken, P E EM: Lin, S, 2006, Mantle-lithosphere interaction beneath the Yellowstone-Snake River province, EOS Trans. AGU, 87(52), Fall Meet. Suppl., Abstr. V43D-04