Figure 6
Along-arc variation of crustal thicknesses along the Izu-Ogasawara arcs.
Partial melting of mantle wedge can result in the production of mantle diapirs, which ascend into the mantle wedge. Where the crust is thin, these diapirs could rise to shallower depths than where the crust is thick. The pressure where primary magmas separate from mantle peridotite would therefore be low where the crust is thin and higher where the crust is thick. In the former scenario andesite primary magmas are produced through incongruent melting of magnesian pyroxene; in the latter primary basalt magmas are produced through congruent melting of the same starting material. The Early Earth, the IOM arcs during the Oligocene, the western Aleutian, west of Adak, could have been similar to the southern segment of the Izu-Ogasawara arcs.
