2011

Òåçèñû ìåæäóíàðîäíîé êîíôåðåíöèè

Ðóäíûé ïîòåíöèàë ùåëî÷íîãî, êèìáåðëèòîâîãî

 è êàðáîíàòèòîâîãî ìàãìàòèçìà

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Geochemical features and the age of baddeleyite from carbonatites of Proterozoic alkaline-ultrabasic Tiksheozero massif (Northern Karelia)

^*Rodionov N.V., ^**Belyatsky B.V. , ^*Kapitonov I.N., *Antonov A.V., ^***Simakin S.G., ^*Sergeev S.A.

^*VSEGEI, Centre for Isotope Researches, S-Petersburg, Russia; rodionov@vsegei.ru

^**VNIIOkeangeologia, S-Petersburg, Russia

^***YaB PhThIAN, Yaroslavl', Russia

It has been obtained U-Pb isotope age for baddeleyite from carbonatites of Tiksheozero massif 1994.8±9.4 Ma and their geochemical characteristics by mean of local analytical methods.

               Recently, baddeleyite (ZrO2) has acquired wide appliance in U-Pb dating of basic, ultra-basic and alkaline intrusive rocks [Amelin et al., 1999; Rasmussen et al., 2008]. Prospects of this mineral as geochronometer are determined by its origin (mainly magmatic), crystal structure peculiarities (monoclinic, very high crystallization temperature, low mechanical stability) and chemical composition (rather high concentrations of U with complete absence of nonradiogenic Pb and, also, high Hf concentrations which allow to apply Lu-Hf isotope dating) [Heaman & LeCheminant, 1993]. It is accepted that the main advantage of baddeleyite in comparison with zircon is the closeness of its U-Pb isotope system [Heaman, 2009], which gives the opportunity to obtain highly precise concordant ages of crystallization of hosting baddeleyite geological bodies of both Archean-Proterozoic and Phanerozoic ages [Bayanova, 2006]. At the same time, there are still unclear problems concerning the genesis of baddeleyite, for example, the origin of baddeleyite at the eclogite formation [Rubatto & Scambelluri, 2003] or laterites over sedimentary carbonate rocks [Nishio & Minakawa, 2004], in metamorphic reactions at the interaction of zircon with metasomatic carbonate fluid [Purtscheller & Tessadri, 1985], as well as for geochemical characteristic of baddeleyite which reflects the peculiarities of its crystallization and genesis is represented by single total analysis of this mineral.

               In carbonatites of Tiksheozero massif (Northern Karelia) baddeleyite is widely observed as independent mineral phase and as inclusions in zircon [Frants et al., 2001], it demonstrates homogeneous inner structure (fig. 1a-c) and clear zoning in CL images (fig.1d), it is noteworthy that, as a rule, inclusions of other mineral phases are absent. Inclusions of baddeleyite in zircon are found in cracks, different defects and edges of zircon grains. We have studied the composition of trace elements and U-Pb systematics of baddeleyite from two carbonatite samples - drill hole 154, depth 26 m (fig.1d) and 210 m (fig.1c). Local analysis was carried out by ion microprobe with high resolution SHRIMP II and Cameca 4F. Distinguishing geochemical peculiarity of the studied baddeleyite is low Hf concentration – in average 3000-4500 ppm and high Nb – up to 8500 ppm, for example, baddeleyite from Phalaborwa carbonatite massif is characterized by Hf 9000-10000 and Nb – 1100 ppm [Reischmann et al., 1995]. Concentrations of the other microcomponents are comparable with those described elsewhere for baddeleyite [Heaman, 2009; Reischmann et al., 1995]: Li: 0.6–2.7, Ti: 865–1760, Sr: 3.34–5.08, Y: 24.4–35.8, Ba: 3.71–4.85, ∑REE: 34.2–56.2 ppm. At the same time, for the studied baddeleyites there are observed considerable variations in concentrations of such elements as U and Th from grain to grain (min concentration 3.2 and 53.9, max 208.9 and 1365.5 ppm, Th and U, respectively) as well as between the studied samples: sample 210 – Th in average 44.1 ppm, U – 488.4 ppm, sample 26 – Th in average 8.4 ppm, U – 167.0 ppm. But the ratio of these elements is practically always the same: Th/U 0.05–0.07. Distribution of REE (fig.1e) in general is similar to the average baddeleyite composition from Phalaborwa massif and is characterized by moderate enrichment in heavy REE: (Lu/La)_n from 2.7 to 68.4 (average 23.4), well pronounced Ce anomaly: Ce/Ce* from 2.73 to 8.9 (average 5.56) and the absence of significant Eu anomaly: Eu/Eu* from 0.75 to 1.14 (average 0.93). It is necessary to point out to at some extent displayed relationships: Pr_n≥Nd_n è Yb_n≥Lu_n (fig. 1e) which are typical for baddeleyite REE normalized distributions. Noteworthy, REE patterns for three analyzed grains is marked by considerably higher concentrations of light REE (fig.1e): (La/Gd)_n 0.99-1.61, it could be connected with microinclusions of silicate minerals, apatite and calcite which is indirectly proved by accompanied higher concentrations of Si, Ca and P.

   Uranium-lead isotope analysis has been carried out for 29 baddeleyite grains from two samples. Concentrations of uranium, thorium and radiogenic lead are considerably different: baddeleyites from sample 210 are characterized by higher Th content – up to 73 ppm, U – up to 94 ppm, radiogenic Pb – 30 ppm (in average); baddeleyites from sample 26 show Th – 1-6 ppm, U – up to 23 ppm, radiogenic Pb – 7.0 ppm (in average). Nevertheless, both U-Pb and Pb-Pb ages of baddeleleyites from two samples coincide within the error (concordant age for sample 26 is 1995±12 Ma and for sample 210 - 1995±16 Ma). For 40 analyses of baddeleyite from two samples the calculated age (fig. 1f) corresponds to ²⁰⁷Pb/²⁰⁶Pb 1993.9±7.5 Ma, and ²⁰⁶Pb/²³⁸U 1978±33 Ma (corrected by ²⁰⁸Pb content) and 1994.4±8.1 Ma by intersection with concordia.

               To compare the results obtained by different methods of dating we have carried out U-Pb analysis of the same baddeleyite grains (from the same mount) by laser ablation and ICP mass-spectrometer Neptune. By this analysis there has been proved the redundant fractionation of uranium relatively lead at baddeleyite analysis [Sylvester et al., 2007], which causes reverse discordant data in U-Pb space. But at the same time, Pb-Pb isotope ratios are marked by high reproducibility of the results. Age evaluation by mean value of ²⁰⁷Pb/²⁰⁶Pb isotope ratio for 11 measurements corresponds to 1994±18 Ma MSWD=0.8, which coincides well with U-Pb concordant age of baddeleyite 1994.8±9.4 Ma (MSWD=0.001), obtained by SHRIMP II (fig.1f).

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