HFSE residence and Nb/Ta ratios in metasomatised, rutile-bearing mantle peridotites

We have constrained the residence of high field strength elements (Nb, Ta, Zr, Hf, Ti) in metasomatised peridotite xenoliths in basalts from SE Siberia using high-precision electron-microprobe analyses of accessory Ti-rich oxides and solution inductively coupled plasma mass spectrometry analyses of whole rocks and clinopyroxene. Highest Nb abundances (0.9–4.5%) were found in rutile, compared with <0.5% in armalcolite and loveringite and <0.1% in ilmenite. Mass-balance calculations indicate that only 1–5% of Nb and Ta in the rocks reside in major minerals and that the rest may be hosted by the Ti-oxides. The Nb/Ta values in the Ti-oxides (±2–5% accuracy at Ta≥1000 ppm) range significantly between individual grains in each sample (e.g. 11–37) but their averages are close to Nb/Ta in the bulk rock. Thus, the whole-rock Nb/Ta can be constrained from analyses of the Nb-rich phases. High ZrO2 (1–7%) was found in loveringite and rutile. However, these minerals alone do not control whole-rock Zr/Hf in the peridotites because, unlike the Nb–Ta pair, much of Zr and Hf also resides in pyroxenes. Loveringite typically has high La and Ce (up to 1.6 wt%) and may be an important light rare earth element host. Overall, the Ti-oxide micro-phases may be essential components in nondescript grain boundary materials that are believed to host much of the highly incompatible elements in some mantle rocks and play a role in the behaviour of those elements during melting and metasomatism. Whole-rock Nb/Ta values in most of the peridotites are higher than the chondritic ratio (17.5). A literature review finds largely chondritic and subchondritic Nb/Ta and Zr/Hf in abyssal and massif peridotites, consistent with an origin as partial melting residues (based on peridotite/melt partition coefficients). By contrast, superchondritic Nb/Ta, as well as high La/Yb, is common in mantle xenoliths, indicating that metasomatism may increase Nb/Ta, together with La/Yb, in the initially depleted peridotites. If the high Nb/Ta predominates in the lithospheric mantle (assuming most of it has been metasomatised), it may provide a reservoir complementary to those of asthenospheric mid-ocean ridge basalt-type mantle and continental crust, which both have subchondritic Nb/Ta. However, the lithospheric mantle is not likely to counterbalance the subchondritic reservoirs in the bulk earth, firstly, because of a much higher mass of the asthenospheric mantle and higher Nb and Ta in the crust, and secondly, because many metasomatised peridotites (including all samples in this study) have subchondritic Nb/La.