Mantle Wedge Evolution beneath the Northern Mariana Forearc

Subduction of the Pacific Plate beneath the Philippine Sea Plate has created one of the largest intra-oceanic convergent systems on the Earth – the Izu-Bonin-Mariana arc (IBM). To understand the geochemical characteristics of a supra-subduction zone mantle wedge and how it has evolved in response to subduction of the Pacific Plate, forearc peridotites (dunites and harzburgites) were collected by dredging the landward slope of the Northern Mariana trench wall. High concentration of fluid mobile elements (Ba, Th, La, Sr, and U) in the bulk rock indicates interaction of slab-derived fluids with the mantle wedge. Major element analyses of primary minerals reveal a wide range of variations in the Cr# of spinels from 0.28-0.83 indicating 16-23% of melt fraction generation. Though the extremely low TiO2 content (25-485 ppm) of the spinels indicates no melt-rock interaction, the presence of tiny (20-50 m) interstitial clinopyroxene (cpx) along with very high Cr# in the same sample opposes that view. Bulk rock 187Os/188Os ratios range from 0.1133 to 0.1281 making them mostly sub-chondritic. However, compared to abyssal peridotites, some of the samples are more radiogenic. Predominantly, the rocks display PGE patterns reflective of partial melting, where Pt, Pd, and Re go into the produced melt - typical of abyssal peridotites. However, the majority of the dunite and harzburgite show evidence of Re addition to the system without any increase in Pt and Pd. A two component mixing model indicates that the Re addition did not happen solely because of sea-water interaction. Sub-chondritic 187Os/188Os values can be attributed to the samples having evolved from a Re-depleted mantle source. The Re-depletion model ages of 48 Ma to 2 Ga indicate that the early melting events long predate the inception of subduction of the Pacific plate. The late re-fertilization event might be a result of the subduction initiation and subsequent migration of melts in the mantle wedge. Thus, the mantle wedge represents extremely depleted and residual part of the upper mantle, and forearc peridotites can be considered as the furthest end member of the abyssal peridotites.