The first discovery of abnormal (Y+REE)-enriched zircons in rocks of the Baltic shield

Abstract

The content of rare elements in zircon may vary significantly, although it typically does not exceed cer tain threshold values, which have been ascertained empirically. The so called hydrothermal/metasomatic zircons are notable for the highest content of these ele ments, which are almost always present as isomorphic impurities [1]. While zircons enriched in Hf, U, and Th are common [2, etc.], considerable deviations in the Y and REE contents have been less frequent. The Y content typically lies within the range from 10 to 5000 ppm; the total REE content varies from 100 to 2500 ppm [3]. The maximum REE content (98 200 ppm) was registered in zircons from metasomatites of the unique Oklo uranium deposit (Gabon, Equatorial Africa) [4]. For the Baltic shield, an anomalously high content of REE was ascertained in zircons from metabasites of the Kontokki dike complex in the Kos tomuksha structure (29 800 ppm [5]) and rocks of the Panozero sanukitoid complex in Central Karelia (55 300 ppm in lamproites [6] and 68 800 ppm in monzogabbro [7]). REEs are concentrated in local domains and zones of variation of zircon grains, which were of contrasting dark color in the BSE images. A high Y content (up to 5 wt % of Y 2 O 3 ) was revealed in metamict zircons from the metasediments of the Dal radian complex in Scotland, which associate with xenotime and are formed upon intensive low temper ature fluid processing of rocks [8].The anomality of the composition of the zircons considered in this article was revealed by performing U-Pb dating (SHRIMP II, Center of Isotopic Research, All Russia Research Institute of Geology), which was accompanied by studying the distribution of rare and rare earth elements (Cameca IMS 4f, Yaroslavl' Branch of the Physical Technical Institute) according to the procedure described in [9]. After sev eral months the zircon content in the same domains was measured again on an ion microprobe; the results were in a close agreement, and no analytical errors were ascertained. The features of the structure and composition of zircons with respect to the major ele ments were studied on a JEOL JSM 6510LA scanning electron microscope with a JED 2200 energy disper sive spectrometer (Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences); the analysis results were standardized to 100%. The presence of inclusions of foreign mineral phases was verified on an electron microscope upon the maxi mum possible magnification by composition contrast (BSE regime). The correctness of determination of the microelemental composition on an ion microprobe (with a locality of 15-20 µm) was regulated by analyz ing on an electron microprobe (which was less sensi tive, but was characterized by a locality of 1-2 µm only). No foreign mineral phases were revealed in zir cons. When plotting the spectra of REE distribution, the compositions of minerals were normalized to the composition of C1 chondrite [10].Zircons of anomalous composition were found in a thin (up to 20 cm) pegmatite vein (sample 101) cross ing the e...

New data on zircon geochemistry and age (U-Pb, SHRIMP II) of the Yastrebetskoe Zr-REE-Y deposit, Ukrainian shield

“…3b, 3c). Similarly high REE and Y content (up to 96800 and 84800 ppm, respectively) as in the rims of samples 16 and 20 (Table 1) were previously reported only in zircon from a Svecofennian aged zone of intense fluid rework ing at the Fennoscandian shield (Skublov et al, 2011). The Y content in the anomalous zircon from the Yas trebetskoe deposit is more than two times higher than the total REE content, which is very atypical even of metasomatic zircons, where Y content is no more than 10000 ppm.…”

Distribution of Trace Elements Controlled by Sector and Growth Zonings in Zircon from Feldspathic Pegmatites (Ilmen Mountains, the Southern Urals)

“…As total REE concentration increases to 1000-2000 ppm, P concentration of 100-200 ppm remains unchanged. Such a deviation has been revealed earlier for high yttrium zircon which contains large quantities of incompatible elements, including water [59]. In this case, the scheme of isomorphism H + + (REE + Y) 3+ = Zr 4+ is at work [27].…”