Late-stage fluid exsolution and fluid phase separation processes in granitic pegmatites: Insights from fluid inclusion studies of the Luumaki gem beryl pegmatite (SE Finland)

The fluid inclusion record of the miarolitic gem beryl-bearing Luumaki pegmatite (SE Finland) has been studied by petrography, microthermometry and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Four types of fluid inclusion assemblages could be distinguished, which record the nature and timing of fluids exsolved in the pegmatite. Type I and type II fluid inclusions are two-phase (LV) aqueous with intermediate salinities of 8.0 +/- 2.7 and 8.1 +/- 0.9wt% NaCl, respectively. Type I fluids occur at the transition between the quartz core and euhedral smoky quartz crystals formed in miarolitic cavities. Type II inclusions are present throughout smoky quartz and gem beryl crystals in miarolitic cavities. Type III are three-phase (LLV), low-salinity (1.2 +/- 1.1 wt% NaCl), aqueous-carbonic inclusions, whereas type IV are three-phase (LVS) aqueous inclusions with halite daughter crystals and high salinity (39.6 +/- 4.5 wt% NaCl equiv.). Both type III and type IV inclusions occur texturally late in the smoky quartz crystals and are closely related to each other, sometimes coexisting on the same inclusion trail. The chemical data show that type I, type III and type IV inclusions lie on linear trends in many elemental variation diagrams, where type I inclusions plot between the data for type III and type IV inclusions. The saline type IV inclusions are strongly enriched in many elements, such as alkalis, alkaline earths and transition metals, whereas the aqueous-carbonic type III inclusions are depleted in these metals. The chemical data, in conjunction with petrographic features, demonstrate that type III and type IV inclusions were formed by phase separation of the type I intermediate-salinity fluid. The metal and halogen content of all four fluid types suggests that they are of magmatic-hydrothermal origin and supports the model that the Luumaki pegmatite reached fluid saturation at the transition between core quartz crystallization and formation of miarolitic cavities. Representative fluid inclusion isochores calculated for type II fluids preserved in gem beryl, in conjunction with the P-T slope constructed from the water content in beryl, yield a best P-T estimate for pocket formation of 380 +/- 80 degrees C and 1.2 +/- 0.7 kbar. This P-T estimate constrains the range of pressure-depth conditions proposed for the rapakivi granites that host the Luumaki pegmatite, and is consistent with recent estimates suggesting a maximum pressure of emplacement of 1.9 kbar. (C) 2020 Published by Elsevier B.V.

Automated summary

The fluid inclusion record of miarolitic gem beryl-bearing Luumaki pegmatite in SE Finland reveals four types of fluid inclusion assemblages, indicating the nature and timing of exsolved fluids in the pegmatite. These inclusions exhibit specific characteristics and relationships, shedding light on the magmatic-hydrothermal origin of the fluids and the conditions of pocket formation.