ORIGIN OF DEPLETED BASALTS DURING SUBDUCTION INITIATION: EVIDENCE FROM IODP SITE 1438 AND OTHER IBM LOCATIONS (Invited Presentation)

Investigations of the Izu-Bonin- Mariana (IBM) forearc have shown that forearc basalts or “FABs” are first in a sequence of magmas generated during the initiation of subduction (Ishizuka et al., 2011; Reagan et al., 2011). Geochemical characteristics of these basalts include unusually low abundances of non-fluid-mobile incompatible elements such as Nb, La and Th, and Ti/V ratios lower than MORB. Basalts of similar age drilled at recent IODP Site 1438, just west of the Kyushu Palau Ridge, are also exceptionally depleted, as are basalts drilled at ODP Site 1201 and DSDP Site 447 in the West Philippine Basin.

Basalts from IODP Site 1438 show variable enrichment in fluid-mobile incompatible trace elements, such as Cs, Rb, K, Ba, Sr, Pb and U. Such enrichments have also been reported in FABs, and in some cases these have been interpreted as evidence for hydrous subduction fluids added to their mantle sources to cause melting. Detailed examination of the Site U1438 sequence, as well as other locations, suggests that these enrichments in fluid-mobile elements are largely the consequence of post-eruptive alteration by seawater. Basalts at Site U1438 have no apparent trace of hydrous subduction inputs based on their trace element signature.

The mantle sources that melted to form these basalts are inferred to be among the most depleted worldwide (e.g., Yogodzinski et al, this session). Given that feature, it is difficult to explain why melting occurred during subduction initiation. Depleted mantle sources may have undergone unusually rapid uplift and decompression as the result of mantle counterflow to the sinking slab, or due to the upwelling of the nearby Oki-Daito mantle plume. Alternatively, flux melting may have occurred with small amounts of hydrous fluid different in composition from normal subduction inputs. In either case, a conclusion is that exceptional melting conditions accompanied the generation of magma across the incipient IBM subduction zone.