A first finding of anomalously Cs-rich aluminosilicate melts in ongonite: Evidence from melt inclusion study

Abstract

The chemical properties of cesium allow accumulation of this element in the late silicic derivatives of igneous 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 concentrates 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 concentrates 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 miarolitic 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 AryBulak 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 distribution in ongonites. The Ary-Bulak Massif is a dome-shaped stock exposed over ~0.8 km 2 among the Devonian volcanosedimentary rocks [5]. The massif is composed largely of porphyritic ongonite. A zone of aphyric rock 50‐100 m wide occurs only near the southwestern contact 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-feldspathic” phases. The “fluoritic” phase is a partly devitrified calcium fluoride melt with the following admixtures (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 calcium 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