Fayalite-sekaninaite paralava from the Ravat coal fire ( central Tajikistan)

Abstract

We studied ferrous paralava, a high-temperature rock, produced by complete fusion of the sedimentary protolith in the Ravat natural coal fire which has been on for over two thousand years. The paralava was sampled from the Fan-Yagnob coal deposit at the Kukhi-Malik site in the vicinity of former Ravat Village in central Tajikistan. This rock contains fayalite, sekaninaite, hercynite, Ti-magnetite, tridymite, and siliceous glass. Low-Ca pyroxene (clinoferrosilite), globules of sulfides (mainly pyrrhotite) and Fe-Ti oxides, secondary greenalite (after fayalite) and hematite are minor. Paralava includes xenoliths of partially molten clinkers (up to 20 vol.%) composed of mullite, cordierite, tridymite, and relict detrital quartz. We found relatively high Fe contents (100⋅Fe/(Fe+Mg) > 60) in mafic minerals, high K 2O enrichment (up to 1.4 wt.%) in sekaninaite, and an unusually low CaO content (0.5 wt.%) in the rock. The Ravat paralava appears to be derived from a mixture of pelitic rocks (50–70%) and iron-rich rocks (30–50%), but without participation of calcareous material, which explains the low CaO and the absence of plagioclase and Ca-bearing pyroxene. The primary melt was as hot as >1210 °C, and the coal-fired fayalite-sekaninaite paralava crystallized at 1200–1100 °C, at a relatively low oxygen fugacity (near the QFM buffer), outside the zone of active aeration. Large-scale crystallization of ferrospinels and fayalite led to increasing Al 2O 3 and SiO 2 in the melt whence sekaninaite and tridymite crystallized as later phases. The residual melt progressively acquired a more silicic-aluminous composition, rich in K 2O, CaO, and P 2O 5, and became quenched to glass at >1080–1090 °C, when temperature dropped abruptly, possibly, by roof collapse or opening of large cracks, as it usually happens in underground coal fires.