Noble gas evidence for the presence of recycled material in magma sources of the Shatsky Rise

The Shatsky Rise is an oceanic plateau consisting of three main massifs that were constructed in the Pacific Ocean by intense volcanism during the Late Jurassic to Early Cretaceous. In order to explore the sources of this oceanic plateau, we present noble gas compositions from fresh quenched glasses cored by ocean drilling at Integrated Ocean Drilling Program Site U1347 on the Tamu Massif and Site U1350 on the Ori Massif. The studied glasses are normal-type basalts, the most abundant of four types of basalts defined by trace element compositions. Possible disturbances of noble gas compositions by posteruption radiogenic ingrowth in aged glasses are assessed by extraction of gases from glass vesicles by stepwise crushing. The (super 3) He/ (super 4) He ratios in glasses from Site U1347 are lower than atmospheric (super 3) He/ (super 4) He, presumably owing to magma degassing coupled with radiogenic ingrowth of (super 4) He. In contrast, glasses from Site U1350 exhibit a limited range of (super 3) He/ (super 4) He (5.5-5.9 Ra). Uniform (super 3) He/ (super 4) He cannot be achieved if gases in glass vesicles have been affected by secondary contamination or posteruption radiogenic ingrowth. Therefore, the uniform (super 3) He/ (super 4) He in the normal-type basalts from Site U1350 is ascribed to their source characteristics. Relatively low (super 3) He/ (super 4) He among oceanic basalts suggests the involvement of recycled slab material in the source of the normal-type basalts. However, the depleted radiogenic isotope signatures are inconsistent with recycled slab being a distinct melting component. Instead, we propose that the normal-type basalts of the Shatsky Rise were sourced from a domain where subducted fertile material is dispersed in the mantle.