HFSE RESIDENCE AND NB/TA RATIOS IN METASOMATISED, RUTILE-BEARING MANTLE PERIDOTITES
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dc.contributor.author | Kalfoun F. | |
dc.contributor.author | Ionov D. | |
dc.contributor.author | Merlet C. | |
dc.date.accessioned | 2021-09-28T02:28:14Z | |
dc.date.available | 2021-09-28T02:28:14Z | |
dc.date.issued | 2002 | |
dc.identifier | https://elibrary.ru/item.asp?id=14043943 | |
dc.identifier.citation | Earth and Planetary Science Letters, 2002, 199, 1-2, 49-65 | |
dc.identifier.issn | 0012-821X | |
dc.identifier.uri | https://repository.geologyscience.ru/handle/123456789/30612 | |
dc.description.abstract | 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. | |
dc.title | HFSE RESIDENCE AND NB/TA RATIOS IN METASOMATISED, RUTILE-BEARING MANTLE PERIDOTITES | |
dc.type | Статья |
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