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Fluid inclusions in Be–Ta–Li–Sn-bearing pegmatites have been reported in the literature as hypersaline aqueous brines or low-salinity aqueous or aqueous–carbonic fluids. The evolution paths of volatile fluids in equilibrium with the different crystallizing phases of rare-element pegmatites do not seem to be unique. The aim of this work is to contribute to this discussion, on the basis of fluid inclusion information from the Borborema Pegmatite Province (BPP) in Northeast Brazil. Some well-known heterogeneous Be–Ta–(Li–Sn) mineralized pegmatites of this province were systematically sampled along cross sections with the purpose of studying the fluid inclusions in the different pegmatite zones (I, muscovite-rich contact zone; II, homogeneous or graphic pegmatite; III, blocky feldspar zone; and IV, quartz core). The observed fluid inclusions (mainly in quartz) were almost always secondary or pseudosecondary, grouped along healed fractures with random orientation. Even some trails paralleling quartz grain limits were not undoubtedly primary. Almost all these fluid inclusions were filled with low-salinity aqueous or aqueous–carbonic fluids. Primary inclusions were recognized only in some well-formed crystals from druses with erratic distribution in the pegmatites. Microthermometric data from these samples confirm that the various crystallizing phases of the pegmatites were in equilibrium with low-salinity aqueous or aqueous–carbonic fluids (with insignificant N 2 or CH 4 contents) with frequent accidentally trapped solids. The various types of primary inclusions found in these crystals compare well with the types of secondary inclusions of the samples from the systematic cross sections. The relative age of the crystals hosting primary fluid inclusions, the relative age of primary inclusion types in successive crystal-growth zones, the relative age of secondary fluid inclusion trails and the corresponding microthermometric data allowed to establish the most probable evolution of the fluid characteristics during pegmatite crystallization. An early low-salinity aqueous–carbonic fluid with CO 2 contents of 20 mol% was present during the crystallization of early minerals, such as garnet and tourmaline of the border zone and graphic quartz, beryl, manganotantalite, and blue tourmaline of zones II and III. The end of zone III crystallization occurred in the presence of a low-salinity, aqueous fluid with very low CO 2 contents, as indicated by inclusions in the smoky core of quartz crystals at the limit between zones III and IV. Later aqueous fluids barren in CO 2 and with salinities increasing from 5% to 20% coexisted with the formation of the pegmatite nucleus and replacement bodies, as indicated by inclusions in the border of these crystals and in euclase. The estimated trapping conditions of 3.5 kbar and 580°C for the earliest inclusion types agree with the regional metamorphic conditions and with the known liquidus conditions of rare-element-bearing pegmatite crystallization. The trapping conditions of inclusions in euclase and coexisting quartz crystal rims in the pegmatitic core, at 3.5 kbar and 400°C are slightly below the conditions admitted for the solidus of rare-element pegmatites and indicate an isobaric cooling during pegmatite crystallization. |
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