Subduction zones heterogeneities

The mafic magmas from Emici and Roccamonfina volcanoes display variable abundances and ratios of incompatible elements as well as very distinct isotopic signatures. These were probably generated in a vertically zoned heterogeneous mantle source which was modified by two temporally and compositionally distinct metasomatic events. These episodes of mantle metasomatism may be related to subduction of the Adriatic plate beneath central Italy, followed by the arrival of new subduction-related material from the Ionian plate. 

Multiple location methods when used properly will ordinarily produce precise relative earthquake locations. However, these locations will almost always have an imresolved bias, depending on the scale of lateral heterogeneity in the underlying Earth model. Lateral heterogeneity, especially in subduction zones, will also set limitations on the maximum inter-event distance when using multiple-event location methods. If the inter-event distances are too large, then travel-time predictions from events to stations will not be precisely correlated, potentially degrading the precision of relative locations. 

Since hthium and boron isotope fractionations mainly occur during low temperature processes, Li and B isotopes may provide a robust tracer of surface material that is recycled to the mantle (Elhott et al. 2004). Heterogeneous distribution of subducted oceanic and continental crust in the mantle will thus result in variations in Li and B isotope ratios. Furthermore, dehydration processes active in subducdon zones appear to be of crucial importance in the control of Li and B isotope composition of different parts of the mantle. For the upper mantle as a whole Jeffcoate et al. (2007) gave an estimated 8 Li-value of 3.5%o. 

Figure 9 Examples of models proposed for the chemical structure of the terrestrial mantle, (a) Whole mantle convection with depletion of the entire mantle. Some subducted slabs pass through the transition zone to the coremantle boundary. Plumes arise from both the core-mantle boundary and the transition zone. This model is not in agreement with isotopic and chemical mass balances, (b) Two-layer mantle convection, with the depleted mantle above the 660 km transition zone and the lower mantle retaining a primitive composition, (c) Blob model mantle where regions of more primitive mantle are preserved within a variously depleted and enriched lower mantle, (d) Chemically layered mantle with lower third above the core comprising a heterogeneous mixture of enriched (mafic slabs) and more primitive mantle components, and the upper two-thirds of the mantle is depleted in incompatible elements (see text) (after Albarede and van der Hilst, 1999).

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