HR: 12:10h
AN: V22A-08    [Abstracts]
TI: Are MORB and OIB produced by a hybrid flux-melting process instead of `pure' pressure-release melting?
AU: * Phipps Morgan, J
EM: jp369@cornell.edu
AF: Cornell University, Dept. of EAS, Snee Hall, Ithaca, NY 14853 United States
AU: Connolly, J A
EM: james.connolly@erdw.ethz.ch
AF: ETH, Earth Sciences Department, Zurich, 8092 Switzerland
AB: Decompression melting at mid-ocean ridges is the simplest type of terrestrial melting. It is now widely accepted their major element chemistry results from the melting of a peridotitic mantle source (cf. books by Ringwood (1975); Yoder (1976)), yet their trace element isotope geochemistry is heavily influenced by mantle components that arise from recycling non-peridotitic sediments and basalts into the mantle at subduction zones (cf. Dicken (1995) `Isotope Geochemistry'). How can isotopically varying recycled components often dominate the isotope composition of a basalt and its melt inclusions yet have such a small effect on its major element chemistry? This was fairly surprising to the pioneer isotopologists in the early 1980s, as was the idea that the relatively refractory residues of peridotite melting could somehow, after being subducted back into the mantle, be refertilized to remelt again as fertile peridotite. A two-part mechanism of subsolidus stretching and mixing in the mantle (Allegre&Turcotte) followed by diffusive source homogenization during partial melting (Hofmann&Hart) was developed to explain this puzzle. However recent isotopic observations on melt inclusions in particular appear to refute the idea that diffusive homogenization can occur over a km or larger length-scale during typical MORB melting. If this hypothesis does not apply during mantle melting, then how can both isotope and major element constraints be satisfied? One potential path is to return to variants of the ideas of the existence of pervasive metasomatic `ichors', magmatic fluids that infused into, stagnated, and metasomatized a peridotite that much later would undergo pressure-release melting. However, pressures within the deep mantle prevent the formation of silicate melts - thus inhibiting such metasomatic activity. Also, the observation that the recycled enriched sediment and ocean crust components lie above the depleted mantle of a subducting slab that they must metasomatize makes it difficult for this scenario to work. Here we would like to explore a completely different mechanism that might be able to generate the observed major element chemistry of MORB and OIB - melt/wallrock interaction during the ascent of deeper melts generated by pressure-release melting of non-peridotitic compositions. In its essence, this mechanism would imply that typical peridotite melting beneath a hotspot or mid-ocean ridge is actually quite similar to the `flux melting' commonly believed to cause subduction zone volcanism, with the difference that the `flux-agent' at mid-ocean ridges is initially a rising silicate melt that was produced by pressure-release melting of a lower-solidus plum instead of initially being a rising hydrous fluid. If the mantle is a plum-pudding of different recycled components, then it is likely that during ascent small amounts of deeper decompression melting of volatile and incompatible element rich components will take place prior to the main phase of peridotite pressure-release melting. These incompatible and volatile elements will act as `fluxes' that tend to stabilize a melt instead of solid phase, inducing flux-melting of surrounding wallrock as they ascend. Here we will discuss a series of computational thermodynamic melting calculations that illustrate this hypothesis. This mode of mantle melting can reconcile major element and trace element isotope constraints. It is also consistent with the dunite banding in peridotites proposed by Kelemen and others to be the byproduct of basaltic melt-wallrock interactions and furthermore provides a nice `unification' of MORB, OIB, and arc melting -- all are (hybrid) flux-melts.
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
DE: 8125 Evolution of the Earth
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting