Gabbros lower mantle

Similar results have been reported by Mattern et al. (2002), using more recent equations of state for lower-mantle minerals and incorporating the solubility of alumina in silicate perovskite. They also used a three-layered slab model (midocean ridge basalt (MORE) over harzburgite over pyrolite), but with a MORE composition (Si/(Mg - - Fe) = 2.29) intermediate between our extreme end-members of the Helffrich et al. (1989) eclogite (1.65) and the Helffrich and Stein (1993) gabbro (2.58). 

The evidence from seismic experiments is fully in agreement with the model outlined above. In the Fra Mauro region of Oceanus Procellarum the crust is about 65 km thick and overlies a solid mantle 126 The crust, in turn, is divided into two parts covered with a thin regolith. For comparison with the returned lunar samples, the upper layer has been inferred to be mare basalt and the lower one anorthositic gabbros. 

Kelemen P. B., Koga K., and Shimizu N. (1997) Geochemistry of gabbro sills in the crust-mantle transition zone of the Oman Ophiolite implications for the origin of the oceanic lower crust. Earth Planet. Sci. Lett. 146, 475-488. 

Figure 16 Schematic illustration of mechanisms for transfer of sediments, volcanics, and/or lower erustal gabbros into the mantle wedge from the subducting plate and the base of arc crust. Dark black line indicates position of the subduction zone below this line, material subducts at the convergence velocity. Above this line, material is carried downward more slowly. Any process leading to slow transport of low-melting point metasediment, metabasalt, or metagabbro into the mantle wedge would lead to partial melting of this material beneath an arc.

In summary, for a variety of reasons, it is unwarranted to use geodynamical models to rule out partial melting of eclogite facies sediment, basalt, or lower crustal gabbro beneath arcs. Meanwhile, we believe that geochemical data on primitive arc lavas are best understood if a partial melt of subducted sediment and/or basalt (and/or gabbro) is the primary medium for transport of in compatible trace elements into the mantle wedge. 

Taylor 1981 Alt et al. 1986 Muehlenbachs 1986 Stakes 1991 Staudigel et al. 1995 see Fig. 7a). It is also worth noting that pore-waters in the oceanic crust are initially low in 5 0 (0 near the sediment-water interface to -3 %o at depths of several hundred meters Perry et al. 1976), although it would be reasonable to assume that these waters are quantitatively expelled and/or isotopically equilibrated with co-existing rocks before the oceanic lithosphere is deeply subducted. Because lower crustal gabbros (and pore fluids) on one hand and upper crustal basalts and sediments on the other deviate in o from typical peridotites (5 0 = 5.5 0.2 detailed below) in opposite directions, the oxygen isotope system has the potential to discriminate between subducted components derived from different parts of the oceanic crust. Of course, it is also possible that a mixture of ocean crust components could balance to have no contrast in with the mantle. However, this case would require fortuitous mixtures of these materials and is not to be generally expected. 

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