Magma genesis of Ojin Rise Seamounts, Northeast of Shatsky Rise

Shatsky Rise in the Northwest Pacific is one of the largest oceanic plateaus. It consists of three major edifices (Tamu, Ori, and Shirshov Massifs), a linear volcanic ridge (Papanin Ridge), and a cluster of several dozen seamounts (Ojin Rise Seamounts). Edifice sizes and ages imply a trend of decreasing volume and age from southwest to northeast, and the Papanin Ridge is a potential hotspot track that connects to Hess Rise, another large oceanic plateau (Sager et al., 2016, Earth-Sci Rev; Tejada et al., 2016, Geochim Cosmochim Acta). The Ojin Rise Seamounts are another possible candidate for a hotspot track (Sager et al., 1999, JGR), but most of these seamounts are poorly mapped and sampled. We collected basement rock samples as well as bathymetry, geomagnetic, and gravity data, on seamounts close to the Shirshov Massif during cruise KR14-07 in 2014 to gain a better understanding of the Ojin Rise Seamounts origin and relationship with Shatsky Rise.The bathymetry data show that many of the seamounts are flat-topped guyots, with summit depths of 3000 m. Most guyots are likely capped by shallow-water carbonate platform sediments overlying volcanic substrates. Also, many seamounts are aligned in a northwest-southeast direction that is oriented parallel to "Hawaiian" magnetic lineations. Examinations of the bathymetry and gravity data suggest that crustal thickness beneath the Ojin Rise Seamounts is about 12 km and the seamounts formed on or near the spreading ridge (Shimizu et al., 2016, JpGU meeting). Since the magnetic chrons parallel to the seamounts alignment is M1-M10 (Nakanishi et al., 1999, JGR), their formation ages are estimated to be 125-135 Ma. In contrast to their apparently ridge-controlled formation, the seamounts rocks are trachytic basalts and Nd-Sr-Pb-Hf isotope compositions are similar to enriched compositions of Shatsky Rise (Heydolph et al., 2014, Lithos), suggesting that the origin of the Ojin Rise Seamounts is closely related to Shatsky Rise.