by John Shervais, Barry Hanan, Michael Branney, Francois Holtz, Joerg Erzinger, Douglas Schmitt

Hotspot volcanism in oceanic lithosphere has been the subject of intense recent and ongoing studies (HSDP, IODP). These studies 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 rhyolite 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 Addendum to our Full Proposal (submitted January 2008) seeks partial funding support for a comprehensive, inter-disciplinary, intermediate-depth drilling scientific program in the Snake River Plain that will explore the entire history of volcanism associated with this hotspot at locations in the central and western Snake River Plain. 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 (from five sites) to minimize costs and maximize scientific return. We propose drilling three new core holes to complete this transect: (1) 1.5 km hole in the central plain between Twin Falls and the Great Rift (near Kimama, Idaho), (2) a 1.8 km hole that penetrates the rhyolites under the basalt to sample sub-rhyolite basalt (near Kimberly, Idaho), and (3) 0.7 km hole in the western SRP that penetrates the upper section of to Pleistocene basalt and Pliocene-Pleistocene lake sediments (near Mountain Home, Idaho). These drill holes will complement the DOE-funded project "The Snake River Geothermal Drilling Project – Innovative Approaches to Geothermal Exploration," which is being carried out in collaboration with Project Hotspot, and shares the same PI/Project Director. The DOE-funded project has a budget of $2.97M for drilling; we request an additional $1.54M from ICDP to complete drilling and to support wireline logging. The US PI-group have also submitted a $4.1M proposal to NSF to support science investigations of the recovered core, as well as expanded geophysical site surveys. This project complements 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.
• 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, Hot Spots, HOTSPOT, ICDP-2010/00, Idaho, Mantle Plumes, Snake River, Thermal Regimes, U.s.a., Volcanic Systems, YELLOWSTONE

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