Geochemical constraints on source nature and recycled oceanic crust in the mantle of the Celebes Sea

Abstract

Indian-type mantle is extensively distributed in the marginal basins of the western Pacific; however, the origin of this mantle remains unclear. The Celebes Sea is located between the Pacific and Indian oceans, and its igneous crustal rocks provide crucial constraints on if and on how this anomalous mantle developed in the region. However, limited knowledge has been obtained on the origin of mantle source of the Celebes Sea because only normal-type mid-ocean ridge basalts (N-MORBs) at ODP Site U770 in this basin have been examined. In this study, both N-MORBs and enriched (E)-MORBs were identified at Site U770, which were analyzed for whole-rock major and trace elements, Sr–Nd–Pb–Hf isotopes, and in situ olivine geochemistry. Site U770 basalt samples are Indian-type (i.e., plot above the Northern Hemisphere reference line [NHRL] on plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb) MORBs, based on their Sr–Nd–Pb–Hf isotopic compositions. An enriched mantle end-member exists in the MORB-type depleted mantle source, as evidenced by correlations between ɛNd and SmN/YbN, Sr/Sr*, and Eu/Eu*. Recycled igneous oceanic crust that experienced plagioclase fractionation can account for the geochemically enriched end-member in the mantle source, based on the positive correlation of ɛNd vs. Sr/Sr* and Eu/Eu*. The recycled igneous oceanic crust plus minor amounts of pelagic sediments can also explain the origins of Indian-type mantle in the Celebes Sea. The E-MORBs have higher Zn/Mn and Zn/Fe ratios, and similar olivine Ni, Ca, and Mn contents as compared to global MORBs, which is also consistent with a recycled oceanic crust-derived, eclogite-rich, mantle source. This suggests recycled components contribute to the lithological and geochemical heterogeneity in the mantle beneath the Celebes Sea.