Computer Science
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002e%26psl.199...49k&link_type=abstract
Earth and Planetary Science Letters, Volume 199, Issue 1-2, p. 49-65.
Computer Science
17
Scientific paper
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.
Ionov Dmitri
Kalfoun F.
Merlet Claude
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