HR: 0800h
AN: U21A-0007    [Abstracts]
TI: Formation of the Oceanic Lithosphere from the Upper Asthenosphere
AU: * Presnall, D C
EM: dpresnall@ciw.edu
AF: University of Texas at Dallas, Department of Geosciences P. O. Box 830688, Richardson, TX 75083-0688, United States
AU: * Presnall, D C
EM: dpresnall@ciw.edu
AF: Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, D-95440, Germany
AU: * Presnall, D C
EM: dpresnall@ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Rd., N. W., Washington, D. C., 20015, United States
AU: Gudfinnsson, G H
AF: Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, D-95440, Germany
AB: In a global examination of the chemistry of MORBs, we find that Na8-Fe8-axial depth data do not support large variations in the temperature and pressure of MORB extraction from the mantle. Instead, the complete absence of high-pressure (> ~1.5 GPa) olivine-controlled crystallization of MORBs combined with solidus phase relations in the CaO-MgO-Al₂O₃-SiO₂-Na₂O-FeO system indicate that the inverse and positive Na8-Fe8 variations are produced from a heterogeneous source by melt extraction over a very narrow range of P and T (~1.2-1.5 GPa and 1250-1280°C) at the plagioclase-spinel lherzolite transition. This is inconsistent with the existence of hot mantle plumes (Easter, Galapagos, Iceland, Azores, St. Helena, Tristan, Afar) on or close to ridges. However, it is consistent with the very flat 410 km discontinuity beneath the East Pacific Rise, which does not permit the existence of even a single hot plume (Easter) beneath the ridge (Melbourne and Helmberger, 2002, JGR, 107, doi:10.1029/2001B000332). The global absence of MORBs with a high-pressure major-element signature implies that the isolation of the East Pacific Rise from the deeper mantle applies to all ridges. A new model is developed (Presnall and Gudfinnsson, in press, Origin of the Oceanic Lithosphere, J. Petrol.) that explains the formation of new seismic lithosphere (~70 km thickness) by lateral and upward migration of the slightly melted upper part (~70-140 km depth) of the low-velocity zone toward the ridge. Although decompression melting occurs over a large pressure range, melt extraction is constrained to the very narrow P-T range given above by the maximum T in the mantle at which CO₂ vapor can be extracted. This condition occurs at a pressure just below that of the abrupt 280°C temperature decrease of the carbonated lherzolite solidus at the base of the seismic lithosphere. The constant association of strombolian and effusive eruptions at ridges (Clague, 2007, Geophys. Res. Abstr., 9, EUG, 02096) supports the view that explosive escape of CO₂ from the upper part of the seismic low-velocity zone is the vehicle that facilitates transport of MORB melts to the surface. In this model, a new ridge is born when stresses on the lithosphere cause fracturing that penetrates the entire lithosphere. This allows explosive flashing of carbonate-rich melt to CO₂ vapor, which escapes to the surface. Erupted melts carried with the CO₂ vapor are initially low- volume carbonatitic melts that probably exist on the ocean floor ahead of propagating rift tips. These melts change progressively to alkalic basalt, and finally to MORB tholeiite as the ridge matures. The depleted upper asthenosphere rises and becomes new lower lithosphere that moves away from the ridge axis with the new crust. When changing lithospheric stresses shift fracture formation to a different locality, melts change back through alkalic basalt to carbonatite, venting of CO₂ shuts down, and the ridge dies. In support of this modeling, alkalic lavas are found at the southern tip of the southward-propagating Eastern Volcanic Zone in Iceland. Carbonatitic melts farther south are unknown but would be hidden from view on the ocean floor.
UR: http://petrology.oxfordjournals.org/papbysection.dtl
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
SC: Union [U]
MN: 2007 Fall Meeting