Bulk-rock major and trace element compositions of abyssal peridotites: Implications for mantle melting, melt extraction and post-melting processes beneath ocean ridges

Abstract

This paper presents the first comprehensive major and trace element data for ~ 130 abyssal peridotite samples from the Pacific and Indian ocean ridge-transforms systems. The data reveal important features about the petrogenesis of these rocks, mantle melting and melt extraction processes beneath ocean ridges, and elemental behaviours. While abyssal peridotite are serpentinized and have also experienced seafloor weathering, magmatic signatures remain well preserved in the bulk-rock compositions. The better inverse correlation of MgO with progressively heavier rare earth elements (REEs) reflects varying amounts of melt depletion. This melt depletion may result from recent sub-ridge mantle melting, but could also be inherited from fertile source histories. Light REEs in bulk-rock samples are more enriched, not more depleted, than in the constituent clinopyroxene (cpx) of the same sample suites previously studied. If the cpx light REEs record sub-ridge mantle melting processes, then the bulk-rock light REEs must reflect post-melting refertilization. The significant correlations of light REEs (e.g., La, Ce, Pr, Nd) with immobile high field strength elements (HFSEs, e.g., Nb and Zr) suggest that enrichments of both light REEs and HFSEs resulted from a common magmatic process. The refertilization takes place in the “cold” thermal boundary layer (TBL) beneath ridges where the ascending melts migrate through and interact with the advanced residues. The refertilization apparently did not affect cpx relics analyzed for trace elements. This observation suggests grain-boundary porous melt migration in the TBL. The ascending melts may not be thermally “reactive”, and thus may have only affected cpx rims, which, together with precipitated olivine, entrapped melt, and the rest of the rock, were subsequently serpentinized. The very large variations in bulk-rock Zr/Hf and Nb/Ta ratios are unexpected. The correlation between the two ratios is consistent with the observations in basalts that DZr/DHf <1 and DNb/DTa <1. Given the identical charges (5+ for Nb and Ta; 4+ for Zr and Hf) and essentially the same ionic radii (RNb/RTa = 1.000 and RZr/RHf = 1.006 ~ 1.026), yet a factor of ~ 2 mass differences (MZr/MHf = 0.511 and MNb/MTa = 0.513), it is hypothesized that mass-dependent Ds or diffusion/mass-transfer rates may be important in causing elemental fractionations during porous melt ascent in the TBL. It is also possible that some “exotic” phases with highly fractionated Zr/Hf and Nb/Ta ratios may exist in these rocks, thus having “nugget” effects on the bulk-rock analyses. All these hypotheses need testing by constraining the storage and distributions of all the incompatible trace elements. As serpentine contains up to 13 wt % H2O, and is stable up to 7 GPa before transformed to dense hydrous magnesium silicate phases that are stable at pressures of ~ 5 to 50 GPa, it is possible that the serpentinized peridotites may survive, at least partly, subduction-zone dehydration, and transport large amounts of H2O (also Ba, Rb, Cs, K, U, Sr, Pb etc. with elevated U/Pb ratios) into the deep mantle. The latter may contribute to the HIMU component in the source regions of oceanic basalts.