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 3 He/ 4 He ratios in glasses from Site U1347 are lower than atmospheric 3 He/ 4 He, presumably owing to magma degassing coupled with radiogenic ingrowth of 4 He. In contrast, glasses from Site U1350 exhibit a limited range of 3 He/ 4 He (5.5–5.9 Ra). Uniform 3 He/ 4 He cannot be achieved if gases in glass vesicles have been affected by secondary contamination or posteruption radiogenic ingrowth. Therefore, the uniform 3 He/ 4 He in the normal-type basalts from Site U1350 is ascribed to their source characteristics. Relatively low 3 He/ 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.

The few geological and geophysical studies of the Lyra Basin at the western margin of the Ontong Java Plateau (OJP; Pacific Ocean) revealed that it is underlain by thicker than normal oceanic crust. The unusually thick oceanic crust is attributed to the emplacement of massive lava flows from the OJP. Dredging was conducted to sample the inferred OJP crust on the Lyra Basin but instead recovered younger extrusives that may have covered the older plateau lavas in the area. The Lyra Basin extrusives are alkalic basalts with ( 87 Sr/ 86 Sr) t = 0.704513–0.705105, ( 143 Nd/ 144 Nd) t = 0.512709–0.512749, ε Nd (t) = +3.0 to +3.8, ( 206 Pb/ 204 Pb) t = 18.488–18.722, ( 207 Pb/ 204 Pb) t = 15.558–15.577, and ( 208 Pb/ 204 Pb) t = 38.467–38.680 that are distinct from those of the OJP tholeiites. They have age-corrected ( 187 Os/ 188 Os) t = 0.1263–0.1838 that overlap with the range of values determined for the Kroenke-type and Kwaimbaita-type OJP basalts, but their ( 176 Hf/ 177 Hf) t = 0.28295–0.28299 and ε Hf (t) = +7.9 to +9.3 values are lower. These isotopic compositions do not match those of any Polynesian ocean island volcanics. Instead, the Lyra Basin basalts have geochemical affinity and isotopic compositions that overlap with those of some alkalic suite and alnöites in the island of Malaita, Solomon Islands. Although not directly related to the main plateau volcanism at 120 Ma, the geochemical data and modeling suggest that the origin of the Lyra Basin alkalic rocks may be genetically linked to the mantle preserved in the OJP thick lithospheric root, with magmatic contribution from the Rarotongan hotspot.