Site U1442: Expedition 352

Abstract

The Izu-Bonin-Mariana (IBM) fore arc is believed to have formed during the period of seafloor spreading that accompanied the rapid rollback and sinking of the newly subducting Pacific plate immediately following subduction initiation at 51–52 Ma. The deepest and oldest volcanic rocks appear to be fore-arc basalt (FAB), a distinctive type of volcanic rock akin to mid-ocean-ridge basalt (MORB) that has been recovered by drilling at Sites U1440 and U1441. More strongly subduction-affected magnesian lavas called boninites also erupted in the IBM nascent arc. These lavas are named after the type section on the island of Chichijima in the Bonin Islands and were also drilled at Site U1439. Before drilling started at Site U1442, end-member hypotheses for explaining the nature of the IBM volcanic stratigraphy were (1) that FAB is overlain by the initial products of arc volcanism, namely lavas with compositions that are transitional between FAB and boninite, boninite lavas themselves, and then by more typical island arc lavas, and (2) that this stratigraphy was offset at progressive distances westward from the trench, such that each lava type was underlain solely by its own plutonic equivalents. Hypothesis 1 has the implication that melting sources evolved through time beneath specific forearc locations, whereas Hypothesis 2 implies that the strongly subduction-affected, and younger, boninites erupted to the west of FAB crust, which itself was produced shortly after subduction initiation and nearer to the nascent subduction zone. Site U1442 was chosen for the final drilling location because it was significantly more likely to provide lavas with compositions between those of FAB and boninite than the other remaining approved site. As with Site U1441, Site U1442 aimed to determine whether the compositions of IBM fore-arc lavas are entirely gradational between FAB and boninite or whether a compositional gap exists between these two end-members.