HR: 14:15h
AN: V43G-03    [Abstracts]
TI: Hot Hawaii, Cold Ridges, Mantle Heterogeneity, and Plumes
AU: * Presnall, D C
EM: d.presnall@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States
AU: Gudfinnsson, G H
EM: g.gudfinnsson@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States
AB: We use model-system phase relations in the CaO-MgO-Al$_{2}$O$_{3}$-SiO$_{2}$ (CMAS) and CaO-MgO-Al$_{2}$O$_{3}$-SiO$_{2}$-Na$_{2}$O-FeO (CMASNF) systems at 1 atm to 6 GPa to compare melt generation and crystallization processes of tholeiitic basalts at Hawaii and oceanic ridges. At both localities, erupted melt compositions are strongly controlled by low-pressure fractional crystallization of magmas generated at greater depths. Also, the Mg numbers of the most primitive melts from each locality are nearly the same (MORB, 72.1; Hawaii, 72.4 - when Fe$^{2+}$/(Fe$^{2+}$ + Fe$^3+}$) = 0.91). However, in other respects, the compositions of these most primitive basalts are quite different, and the phase relations indicate that in both cases they are only slightly less primitive than their respective parental primary melts. At Hawaii, the phase relations support generation of picritic tholeiitic melts at $\sim$5 GPa and $1565\deg$C (Gudfinnsson and Presnall, 2004), whereas at ridges, the conditions are $\sim$0.9-1.5 GPa and 1260-$1280\deg$C (Presnall {\it et al}., 2002). In Hawaii, the trend of picritic melt compositions indicates olivine-controlled fractionation, not a polybaric melting column like that suggested by Klein and Langmuir (1987) for MORBs. For the MORB modeling of Klein and Langmuir (1967) and Langmuir {\it et al}. (1992), which employs polybaric melting columns extending to 4 GPa, the phase relations show that aggregate melts would be produced that require significant low-pressure olivine-controlled fractionation in order to reach the field of observed MORB glasses. No trace of this fractionation has ever been observed in MORBs, even at Iceland. Furthermore, because the phase relations show that an inverse correlation of Na8 with Fe8 can be produced by melting of a heterogeneous mantle in the 0.9-1.5 GPa pressure interval (Presnall{\it et al}., 2002), this correlation cannot be used as an indicator of widely varying temperature. Mantle heterogeneity produced by recycling of oceanic crust and underlying depleted peridotite back into the source region for ridge volcanism would produce little change in the temperatures required for MORB generation in the plag/sp lherzolite transition. However, strong variations in melt productivity would be expected and the compositional range of basalts erupted would be expanded. No petrological evidence for ascending plumes driven by high temperatures appears to exist anywhere along the oceanic ridge system. However, some volcanic centers ({\it e. g}. Galapagos) may be caused by diapirism of low-density, major-element depleted peridotite recycled into the mantle at subduction zones (Presnall and Helsley, 1982). Low-velocity regions extending to depths $>$200 km beneath Iceland, Afar, and Easter (Ritsema and Allen, 2003) could be caused by carbonate-induced melting at low melt-fractions in an eclogite-enriched source rather than by elevated temperature. If temperatures in the central Pacific are generally high due to lithospheric blanketing, the high temperature indicated at Hawaii may not indicate a plume.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting