This paper summarizes analytical data accumulated in the world literature and other materials about the regularities of the REE distribution in minerals contained in ultramafic and mafic rocks as accessory phases. These minerals are tentatively divided into two groups. The first includes garnets, zircons, apatites and perovskites, which can accumulate increased amounts of REE in their structure. The second consists of minerals whose structure can accumulate only limited contents of these trace elements. These are chrome-spinels, ilmenites, and micas. These minerals, in respect of REE geochemistry, are studied to a varying degree because of the different levels of accumulations of these elements, different degrees of occurrence in rocks, tiny sizes of their grains and other reasons. The analytical database formed on their basis includes about 600 original analyses. The overwhelming majority of presently available data on REE geochemistry in accessory minerals from ultramafic and mafic rocks have been published only in the recent 15 years. The studies became possible due to the development and introduction of new highly sensible microprobe analyses allowing detection of REE and many other trace elements in minerals grains directly in thin sections. The greatest numbers of these analyses were performed for garnets and zircons, fewer for apatites, and the fewest for chrome-spinels, ilmenites, micas, and perovskites. In general, the regularities of REE distribution in these minerals from ultramafic and mafic rocks are less studied compared to the rock-forming minerals from ultramafic and mafic rocks. Among the analytical methods, which were used to study the REE composition of accessory minerals, the most efficient was the mass-spectrometry with inductively coupled plasma (ICP-MS).Keywords: REE distribution • ultramafic and mafic rocks • accessory minerals © Versita Sp. z o.o.

We have studied a large (12 × 22 × 30 cm) spinel lherzolite xenolith with undeformed margins in alkali basalt (basanite) from the eroded crater of Late Cenozoic Shavaryn Tsaram-1 volcano in western Mongolia. The xenolith was sampled along its median transversal profile, at every 15–20 mm for bulk chemistry of lherzolite and basalt (ICP-MS) and at 4–10 mm for the chemistry of olivine, orthopyroxene, clinopyroxene, and Cr-spinel minerals, and of material filling cracks (LA ICP-MS). Incompatible elements (especially, LREE) are distributed unevenly over the xenolith, both in lherzolite and in its constituent minerals, as well as in crack-filling material, with abnormal LREE enrichment in some specimens. Judging by the measured trace-element spectra compared with the model patterns, incompatible elements reside in different amounts as interstitial impurity in cracks inside and between mineral grains in lherzolite, also being a substitutional impurity in the lherzolite constituent minerals. Experimental acid leaching of specimens from sites of high crack density showed (La/Yb)n ratios in the crack fill to be much higher than in the basalt host and more so in bulk lherzolite (180 against 33 and 1.5–3.6, respectively). The proportional contents of P and Ca in the leaching solution, especially in that from the xenolith’s center mark the presence of an apatite microphase, which can be a LREE repository. The observed patterns of LREE and other incompatible elements in the xenolith and in the host alkali basalt fit a model implying that mobile elements residing as interstitial impurity came with fluids which were released from rising basaltic magma and percolated into the xenolith along cracks.

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