National Museum of Nature and Science, Japan

Shatsky Rise east of Japan is a unique oceanic plateau with characteristics of both "plume head" and non-plume origin [1-3] which was cored during IODP Expedition 324 in 2009. Igneous rocks sampled at four sites are variably altered pillows and massive lava flows of olivine phyric and plagioclase-clinopyroxene microphyric basalts. On-board geochemical data [4] show that the basalts are broadly MORB-like, similar to those from the Ontong Java Plateau (OJP) and likely originating from a slightly enriched MORB-like source. We report here the first results of geochemical study of olivine-hosted melt inclusions from moderately altered olivine-phyric basalts cored at the Site U1349 (Ori massif, Sample 324U1349-12R4/37-39). Melt inclusions and olivines were analyzed for major elements by electron microprobe and for trace elements by LA-ICPMS. Olivine phenocrysts range in composition from Fo81 to Fo87 (NiO up to 0.4 wt%) and contain small glassy (<20 mu m in size) and relatively large partly crystallized (30-60 mu m) inclusions. Before analyses, partly crystallized inclusions were homogenized in internally heated pressure vessel at 200 MPa and 1210 degrees C and 1280 degrees C and subsequently quenched after 5 and 20 minutes, respectively. Melt inclusions obtained after both heating treatments and corrected for post-entrapment re-equilibration with host olivine have primitive, broadly MORB-like compositions and highly depleted trace element patterns compared to NMORB and the Kroenke-type basalts (OJP) with respect to Ba, Th, Nb, Ta, LREE, Sr, Zr and Hf (e.g., La (sub N) /Yb (sub N) = 0.2-0.4 in inclusions vs. approximately 0.7 in NMORB). The presence of highly depleted melts trapped in olivine suggests strong geochemical heterogeneity of the Shatsky Rise mantle sources ranged from less to significantly more depleted than average DMM. [1] Duncan & Richards (1991) Rev. Geophys 29, 31-50. [2] Sager et al. (1999) J. Geoph. Res. 104 (B4), 7557-7576. [3] Fougler (2007) Spec. Pap. Geol. Soc. Am., 430, 1-28. [4] Sager et al. (2010) Proc. IODP, 324: Tokyo.