Olivine-enriched melt inclusions in chromites from low-Ca boninites, Cape Vogel, Papua New Guinea: Evidence for ultramafic primary magma, refractory mantle source and enriched components

The composition of primary magmas and their mantle sources can be successfully inferred from the study of melt inclusions trapped in spinel phenocrysts. This is particularly true in the case of severely altered rocks, in which spinel and spinel-hosted melt inclusions usually retain primary magmatic information. We report the results of the study of melt inclusions in high-Cr (Cr# 90-95), primitive (Mg# 65-78) spinel in the Palaeocene low-Ca boninites from Cape Vogel, Papua New Guinea. Melt inclusions are represented by the aggregate of skeletal olivine crystals in the residual glass. Raster beam electron microprobe analyses of melt inclusions demonstrated that they are broadly similar in composition to primitive orthopyroxene and to the most primitive boninites in this area, having (in wt.%): very high MgO (18-30), SiO₂ (53-61) and very low TiO₂ (0.04-0.19), Al₂O₃ (3-9), CaO (2-4), Na₂O (<0.9), K₂O (0.05-0.15) and CaO/Al₂O₃ (0.4-0.6). H₂O abundances in melt inclusions, analysed by an ion probe, are very high (1-2 wt.%), and they could have been even higher ( ∼ 3.5 wt.%) if the melts lost H₂O before crystallisation. Trace elements in melt inclusions, analysed by laser ablation ICPMS, have exceptional depletion in HREE (<1 PM) and significant enrichment in LREE over HREE (La/Yb 5-12), and Pb (Ce/Pb 2-12) and Zr (Zr_n/Sm_n 2-3.4) over REE. The composition of melt inclusions correlate well with the compositions of host spinel showing the fractionation path of initial ultramafic melt. Cape Vogel primary melts could have originated from melting of extremely refractory hot (> 1500 °C) harzburgitic mantle fluxed by subduction-related, H₂O-bearing enriched components. Trace element composition of these enriched components is estimated from melt inclusion compositions by mass balance calculations.