by John W. Shervais, Barry B. Hanan, Michael John Branney, Dennis J. Geist, Scott S. Hughes, Alexander A. Prokopenko, Francois Holtz, Donald Bruce Dingwell, Jörg Erzinger, Cristina Maria Pinheiro De Campos, Douglas Ray Schmitt, Neil R. Banerjee, Lisa Ann (Morzel) Morgan

Mantle plumes are thought to play a crucial role in the Earth’s thermal and tectonic evolution. They have long been implicated in the rifting and breakup of continents, and plume-derived melts play a significant role in the creation and modification of sub-continental mantle lithosphere. Much of our understanding of mantle plumes comes from plume tracks in oceanic lithosphere, but oceanic lithosphere is recycled back into the mantle by subduction, so if we are to understand plume-related volcanism prior to 200 Ma, we must learn how plume-derived magmas interact with continental lithosphere, and how this interaction effects the chemical and isotopic composition of lavas that erupt on the surface. Hotspot volcanism in oceanic lithosphere has been the subject of intense recent and ongoing studies (HSDP, IODP). These studies will provide base-line information about where mantle plumes originate, how they behave, and the volcanic products of these processes. However, hotspot volcanism within continental lithosphere has not been studied in such detail, and is potentially more complex. Most researchers believe that Yellowstone is the world’s best example of a mantle plume beneath continental crust. The Snake River Plain volcanic province, which represents the track of the Yellowstone plume, consists of basalts that are compositionally similar to ocean island basalts and rhyolites caldera complexes that herald the onset of plume-related volcanism. The Snake River Plain preserves a record of volcanic activity that spans over 12 Ma and is still active today (the last volcanic eruption was circa 2 ka BP). Thus, the Snake River Plain is unique and represents the world-class example of active intra-continental plume volcanism. Further, because it is young and tectonically undisturbed, the complete record of volcanic activity can only be sampled by drilling. This proposal seeks approval to pursue funding for an comprehensive, inter-disciplinary, intermediate-depth drilling scientific program in the Snake River Plain. This project was the subject of a 4-day ICDP-funded workshop in May 2006 that focused on the scientific basis for a formal drilling proposal, site selection and site selection criteria, and the logistics of coordinating this project. We envisage a transect along the axis of the SRP that will target the origin and evolution of plume-related volcanism in both space and time. Our plan is to leverage existing drill core and cuttings minimize costs and maximize scientific return. We propose drilling two new core holes to complete this transect: (1) 2.0-2.5 km hole in the central plain between Twin Falls and the INL site and (2) 1.5-2.0 km hole in the western SRP that penetrates the entire section of Miocene to Pliocene basalt which underlies Pliocene-Pleistocene lake sediments, and recovers rhyolite or basement from underneath. These drill holes will complement geophysical studies of continental dynamics planned by Earthscope, as well as current studies centered on Yellowstone.

• The central question we plan to address is: how do mantle hotspots interact with continental lithosphere, and how does this interaction affect the geochemical evolution of mantle-derived magmas and continental lithosphere? Our hypothesis is that continental lithosphere is constructed in large part from the base up by 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 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 or 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. This results in the well-known “Wilson-cycle” effect whereby continents commonly rift along old suture zones (=former rifted margins.
• To address these fundamental questions, we plan a transect of the continental margin that begins with lavas erupted through Mesozoic-Paleozoic accreted terranes of oceanic provenance that lie west of the craton margin, as defined by the Sr=0.706 line, and continues through progressively thicker and older lithosphere of Proterozoic to Archean age. The rationale is to examine how basalt chemistry varied through time at different locations along this transect in response to changes in the thickness, age, and composition of the underlying mantle lithosphere and the age of the erupted basalt. Our goal is to understand how plumes react to continental lithosphere, how plume derived magmas are affected by continental lithosphere, and how continental lithosphere responds to mantle plumes.

Basalts, Climate Change, Continental Evolution, Global Environment, Hot Spots, HOTSPOT, ICDP-2007/14, Idaho, Mantle Plumes, Snake River, SNAKERIVER, Thermal Regimes, U.s.a., Volcanic Systems, YELLOWSTONE

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