Chemical evolution and origin of the Luumäki gem-beryl pegmatite (SE Finland)

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http://urn.fi/URN:ISBN:978-951-51-6585-5
Title: Chemical evolution and origin of the Luumäki gem-beryl pegmatite (SE Finland)
Author: Michallik, Radoslaw Markus
Contributor: University of Helsinki, Faculty of Science, Department of Geosciences and Geology
Doctoral Programme in Geosciences
Publisher: Helsingin yliopisto
Date: 2021-05-31
Belongs to series: Department of Geosciences and Geography A - URN:ISSN:1798-7911
URI: http://urn.fi/URN:ISBN:978-951-51-6585-5
http://hdl.handle.net/10138/329582
Thesis level: Doctoral dissertation (article-based)
Abstract: The Luumäki gem-beryl bearing pegmatite belongs to the miarolitic pegmatites, a comparatively rare and little studied pegmatite class. However, it is the miarolitic pegmatites that give raise to the scientific hypothesis about the involvement of aqueous fluids during pegmatite formation. The controversy regarding the involvement of a separate aqueous fluid was renewed in view of recent interpretations of giant crystal growth and graphic granite textures, which are both quite unique for pegmatites. And recent understanding favors a model where pegmatites are being formed by rapid cooling and crystallization from a volatile-rich melt, instead of very slow cooling of an undercooled, water-saturated granite melt. Some argue that H2O is essential for reducing the viscosity of granite melts and the element transport to form giant crystals in pegmatites, based on fluid inclusion studies and the water-saturated melt model proposed by Richard Jahns and Wayne Burnham. Recently this model has been redefined by studies on melt and fluid inclusions suggesting a melt-melt immiscibility with subsequent aqueous fluid exsolution, as suggested by Rainer Thomas. Others reason that refining of a melt, similar to the metallurgical process of zone refining, is key for pegmatite consolidation, and a separate H2O phase plays a minor role if any at all. The proposed model is the constitutional zone refining (CZR) by David London. My study supports the view of a separate aqueous phase in the late stage of pegmatite crystallization and demonstrates that in the case of the Luumäki pegmatite, an aqueous phase separated at the onset of pocket formation at about 380 °C and 1.2 kbar. Furthermore, it is shown that a pegmatite melt is very heterogeneous and within the same pegmatite body one miarolitic pocket can clearly show evidence for a hydrothermally dominated system, while another represents a magmatic dominated process of formation without clear evidence of the involvement of an aqueous fluid. Also, pegmatite bodies within the same geological framework can differ in their appearance, showing that within a small area of just a few hundred of meters one pegmatite body can show extensive hydrothermal activity whereas the other lacks any evidence of an aqueous fluid separation. The research of pegmatites and their fluid inclusions is a somewhat complicated endeavor, similar to solving a puzzle, in which most pieces appear very similar to one another, and many more studies need to be conducted to solve such a puzzle. Powerful analytical methods such as in-situ fluid inclusion analysis by means of LA-ICP-MS is a step in the right direction and will hopefully encourage many more to engage in this tedious, yet gratifying search for the bigger picture by looking at the small individual pieces in greater detail as ever before.The Luumäki gem-beryl bearing pegmatite belongs to the miarolitic pegmatites, a comparatively rare and little studied pegmatite class. However, it is the miarolitic pegmatites that give raise to the scientific hypothesis about the involvement of aqueous fluids during pegmatite formation. The controversy regarding the involvement of a separate aqueous fluid was renewed in view of recent interpretations of giant crystal growth and graphic granite textures, which are both quite unique for pegmatites. And recent understanding favors a model where pegmatites are being formed by rapid cooling and crystallization from a volatile-rich melt, instead of very slow cooling of an undercooled, water-saturated granite melt. Some argue that H2O is essential for reducing the viscosity of granite melts and the element transport to form giant crystals in pegmatites, based on fluid inclusion studies and the water-saturated melt model proposed by Richard Jahns and Wayne Burnham. Recently this model has been redefined by studies on melt and fluid inclusions suggesting a melt-melt immiscibility with subsequent aqueous fluid exsolution, as suggested by Rainer Thomas. Others reason that refining of a melt, similar to the metallurgical process of zone refining, is key for pegmatite consolidation, and a separate H2O phase plays a minor role if any at all. The proposed model is the constitutional zone refining (CZR) by David London. My study supports the view of a separate aqueous phase in the late stage of pegmatite crystallization and demonstrates that in the case of the Luumäki pegmatite, an aqueous phase separated at the onset of pocket formation at about 380 °C and 1.2 kbar. Furthermore, it is shown that a pegmatite melt is very heterogeneous and within the same pegmatite body one miarolitic pocket can clearly show evidence for a hydrothermally dominated system, while another represents a magmatic dominated process of formation without clear evidence of the involvement of an aqueous fluid. Also, pegmatite bodies within the same geological framework can differ in their appearance, showing that within a small area of just a few hundred of meters one pegmatite body can show extensive hydrothermal activity whereas the other lacks any evidence of an aqueous fluid separation. The research of pegmatites and their fluid inclusions is a somewhat complicated endeavor, similar to solving a puzzle, in which most pieces appear very similar to one another, and many more studies need to be conducted to solve such a puzzle. Powerful analytical methods such as in-situ fluid inclusion analysis by means of LA-ICP-MS is a step in the right direction and will hopefully encourage many more to engage in this tedious, yet gratifying search for the bigger picture by looking at the small individual pieces in greater detail as ever before.
Subject: geochemistry
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