Gabbros upper mantle

The basalt - gabbro (the amphibole - in the case of water presence) transition to eclogite happens in upper mantle at the continental condition. 

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. 

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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