A Lower Mantle Origin for the World's Biggest LIP? A High Precision Os Isotope Isochron From Ontong Java Plateau Basalts Drilled on ODP Leg 192

Basalts drilled on ODP Leg 192 from the Earth's largest igneous province, the Ontong Java Plateau (OJP) yield a Re-Os isochron age of 121.5+/-1.7 Ma and an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.1295+/-0.0011. The basalts are from four ODP sites located over a 1000 km apart. The generation of such a high precision isochron using the Re-Os isotope system indicates the source of the OJP is remarkably homogeneous, with an Os isotope composition very similar to that of the primitive mantle. Their high Re contents (1000-1600 ppt) and relatively high Os contents suggest that they are produced by melting beyond sulphide-out in the mantle. Other trace element data support the conclusion that these basalts are derived by high degrees (25-30%) of partial melting of the mantle. Such high degrees of partial melting may assist in homogenising the Os isotopic signature of the source region. The Os isotope data indicate that there is no core signature in the source of the OJP basalts analysed in this study, but that they are more likely derived from entrained lower mantle.