462 ISSN 1028-334X, Doklady Earth Sciences, 2007, Vol. 413A, No. 3, pp. 462–468. © Pleiades Publishing, Ltd., 2007. Original Russian Text © I.S. Peretyazhko, E.A. Tsareva, V.Ye. Zagorsky, 2007, published in Doklady Akademii Nauk, 2007, Vol. 413, No. 6, pp. 791–797. The chemical properties of cesium allow accumula- tion of this element in the late silicic derivatives of igne- ous complexes, in particular, in rare-metal granites, pegmatites, and related metasomatic rocks. However, the Cs content can be high enough only in pegmatites to form its own mineral (pollucite), which occasionally occurs in considerable amounts. In other rocks, Cs con- centrates largely in micas and feldspars. The possible maximal level of Cs accumulation in the melt remains poorly studied. Important new information has been obtained from the study of volcanic glass and melt inclusions (MI) in minerals. The rhyolitic glass concen- trates 220–870 ppm Cs, occasionally up to 3770 ppm [1]. To date, the highest Cs 2 O content (1.2–5.5 wt %) has been detected in MIs captured by quartz from miar- olitic pegmatite of the Malkhan field of the central Transbaikal region [2, 3] and the southwestern Pamirs (Leskhozovskaya and Vezdarinskaya veins). These inclusions contain products of crystallization of the late pegmatitic melt [3] and the inferred high-temperature meltlike gels [4]. While studying ongonite of the Ary- Bulak Massif, we detected MIs filled with a residual glass that contains up to 17 wt % Cs. This is reliable evidence in favor of the existence of natural melts extremely enriched in Cs. In this communication, we describe these unusual inclusions and discuss Cs distri- bution in ongonites. The Ary-Bulak Massif is a dome-shaped stock exposed over ~0.8 km 2 among the Devonian volca- nosedimentary rocks [5]. The massif is composed largely of porphyritic ongonite. A zone of aphyric rock 50–100 m wide occurs only near the southwestern con- tact zone. Beyond this zone, an ongonite variety with anomalously high contents of CaO (3.3–21.8 wt %) and F (2.7–16 wt %) has been found near the same locality. The high CaO (7.8–18 wt %) and F (7.1–15.5 wt %) contents are inherent to aphyric rocks as well. Prosopite CaAl 2 F 4 (OH) 4 has been identified in this rock for the first time as an abundant mineral (6–26 wt %). It was established that interstices between minerals of the groundmass of the Ca- and F-rich rocks are filled with submicrometric intergrowths of “fluoritic” and “K-feld- spathic” phases. The “fluoritic” phase is a partly devit- rified calcium fluoride melt with the following admix- tures (wt %): O (3–12), Al (0.5–3.3), Si (0.2–1.5), Sr (0.3– 0.5, occasionally up to 1.0–1.3), Na (up to 0.5), and S (up to 0.3). The “K-feldspathic” phase is commonly close in composition to sanidine (rims around tabular albite crystals) but characterized by local enrichment in Ca (1.5–4.0 wt %). We provided evidence for the joint crystallization of immiscible aluminosilicate and cal- cium fluoride melts in the presence of HF-bearing aqueous fluid during formation of Ca- and F-rich rocks [6, 7]. The Cs content in rocks from the Ary-Bylak Massif varies from 48 to 386 ppm. The lowest average Cs con- tent is characteristic of the porphyritic ongonite with the “fluoritic” phase (87 ppm, average of 4 samples). Porphyritic ongonite from the central part of the massif contains 104 ppm Cs (average of 35 samples [8]). The aphyric rocks are distinguished by a high dispersion of Cs contents ranging from 82 to 386 ppm (average 159 ppm, based on 9 samples). The main amount of Cs is concentrated in feldspars (largely, sanidine). Mica of the biotite–zinnwaldite series is the only mineral with high Cs content (0.2–0.5 wt % Cs 2 O, on average; up to 2–3 wt % in the narrow outer rims of some phenoc- rysts). However, the percentage of mica phenocrysts does not exceed fractions of a percent. The elevated Cs content (up to 250–300 ppm) in the groundmass indi- cates that this element was gained in the ongonite melt at the final stage of its crystallization. Quartz phenocrysts in porphyritic ongonite contain numerous MIs. Anomalously high Cs contents were detected in some melt inclusions filled with a residual glass. Such unusual melt inclusions have been found in every fourth thoroughly studied rock samples taken A First Finding of Anomalously Cs-Rich Aluminosilicate Melts in Ongonite: Evidence from Melt Inclusion Study I. S. Peretyazhko, E. A. Tsareva, and V. Ye. Zagorsky Presented by Academician M.I. Kuz’min May 18, 2006 Received May 29, 2006 DOI: 10.1134/S1028334X07030324 Vinogradov Institute of Geochemistry, Siberian Division, Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033 Russia; e-mail: pgmigor@igc.irk.ru GEOCHEMISTRY