HR: 0800h
AN: U21B-0401    [Abstracts]
TI: Marble Cake Perspectives
AU: * Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, 20 Oxford St., Cambridge, MA 02138,
AB: Since the original suggestions by Hanson (Geol. Soc. London, 1977) and Allegre and Turcotte (Nature, 1986), the concept of a "veined" or "marble cake" mantle has gained wide acceptance as a paradigm for mantle composition and components. The "veined mantle" was conceived thinking of the mantle as an ultramafic migmatite with many types of veins, but emphasized metasomatic components contained in hydrous phases as an explanation for alkali basalts. The "marble cake" mantle emphasized recycled oceanic lithosphere. Both types of veins are inevitable consequences of mantle convection. Oceanic lithosphere is recycled and stretched; low melting components of the mantle are inevitably melted in ascending mantle flow, even beneath thick lithsosphere. Both vein types have been widely invoked to explain incompatible element enriched basalts from the mantle. Most recently, disequilibrium melting of veined mantle sources by various mechanisms have become a popular suggestion to explain diverse aspects of mantle geochemistry (e.g. Sobolev et al., Nature, 2005; Phipps Morgan et al., EPSL, 1999). The physical mechanisms that would allow disequilibrium melting of fine scale veins, however, remain to be demonstrated. Average upper mantle composition is residual to continents and requires removal of low F melts to generate the depleted MORB source, and enrichment by low F melts to create the enriched source. Such a process is also necessary in the Sobolev et al model for Hawaii, which generates the equivalent of a low F melt by two stages of larger degree melting. Enriched sources are not restricted to ocean islands, and the name "OIB source" is a misnomer. Enriched basalts occur on normal ridges, in back-arc basins, behind subduction zones, in continental rifts and in isolated volcanic cones. Most of these are not mantle plumes. Enriched components have been ascribed to recycled ocean lithosphere, but recycled ocean crust is depleted, not enriched. Therefore the isotopic signature of crustal components are needed, but the trace element chemistry requires the influence of low degree (low F) melts. These observations can be reconciled by the recognition that low degree melting of recycled eclogite (as well as hydrous peridotite) is inevitable. One likely location is in subduction zones, particularly in light of the recent evidence for hot mantle wedges. At depth in subduction zones, where rutile is not residual, eclogite would melt and metasomatize overlying peridotite. Various flavors of this material are suitable for enriched sources. This environment also provides a much more physically plausible mechanism for creating large volumes of enriched pyroxenite lithologies and separating them from their eclogite source. If eclogite melts at subduction zones, it is even less suitable as an ultimate source of enriched basalts. Tests of this mechanism of mantle heterogeneity come from the occurrence of enriched basalts in arc back-arc environments. Enriched basalts in back-arcs are very common, more abundant than on normal ocean ridges, and often have an isotopic composition that requires very recent low degree melt addition. The recycled eclogite, depleted by low degree melt, then provides fertile major elements and depleted trace elements that when added to depleted peridotite is suitable for the depleted MORB source.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
SC: Union [U]
MN: 2007 Fall Meeting