Abstract:
A total of 16 Ontong Java Plateau (OJP) basalt samples from Ocean Drilling Program Legs 192 and 130 were analysed for major, trace and platinum-group elements (PGEs; Ir, Ru, Rh, Pt and Pd). Major-and trace-element compositions determined by our study confirm Leg 192 shipboard analyses that indicated a new group of more primitive or "Kroenke-type" basalts, with higher MgO, Ni and Cr, and lower incompatible-element, abundances than the more common Kwaimbaita-type basalts. The PGE abundances quantified here extend the range of the continuum of compositions found in previously analysed OJP basalts and are similar to those present in some komatiites. The PGEs, therefore, cannot be used to differentiate definitively between OJP basalts groups. The two samples analysed from Leg 130 (one from Site 803 and one from Site 807) are akin to the Kwaimbaita-type basalts. Low-temperature alteration has not affected Pd abundances in the Leg 192 basalts as it has in the Solomon Island and the Leg 130 samples. Elemental abundances and ratios along with petrography reveal that the OJP basalts have not experienced sulphide saturation. Positive correlations of Ir and Ru with Cr and Ni attest to the lithophile behaviour of the PGEs and lend more credence to studies suggesting compatibility of these elements in oxide and silicate phases, such as Cr-spinel and olivine. Estimates of sulphur abundance in the mantle, degree of partial melting and pressure of melt initiation were used in conjunction with the model of Mavrogenes & O'Neill to calculate a minimum initial excess temperature of +185 to +235 degrees C (1465-1515 degrees C at 3.5-4.0 GPa) above ambient mantle for the OJP source. This is in broad agreement with a fossil geotherm preserved in megacrysts and peridotite xenoliths found in pipe-like intrusives of alnoite that outcrop on the island of Malaita, Solomon Islands. Using the PGEs as a guide, the OJP basalts were modelled using a three-source component melt mix: a 10% garnet peridotite melt of primitive mantle composition, which then passed through the garnet-spinel transition and melted a further 20%, a 30% partial melt of fertile upper mantle and 0-1% of outer core material. The core component was included only in the plume source, and the ratio of plume source to upper mantle source was 19:1. It is evident from this study that the PGE contents of at least some of the OJP basalts are too high to be generated by primitive mantle sources alone. A PGE-enriched component is required and we suggest that this is outer core material. While a sulphide-rich mantle component could also increase the PGE abundances (assuming that the sulphide is exhausted during partial melting), the sulphur-undersaturated nature of these basalts argues against this. Variations in the amount of outer core in the source (from 0 to 1 wt%) and degree of fractional crystallization can account for the entire range in PGE abundances of OJP basalts.