Chemical evolution and origin of the Luumäki gem beryl pegmatite: Constraints from mineral trace element chemistry and fractionation modeling

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The 1928 Ma old Luumäki gem beryl pegmatite is hosted by rapakivi granites of the Wiborg batholith in southeast Finland. The moderately evolved niobium-yttrium-fluorine (NYF) pegmatite system belongs to the topaz-beryl type of the rare-element pegmatite class. It has a simple major mineral assemblage of K-feldspar + plagioclase + quartz + biotite ± fluorite throughout the main pegmatite zones (border, wall, intermediate, and core zone). It consists of at least two chemically related bodies (Luumäki N and Luumäki S), of which only Luumäki N contains gem beryl (heliodor) bearing miarolitic pockets. We characterize the geology, mineral assemblages, and the major and trace element chemistry of K-feldspar, plagioclase, biotite and quartz from the pegmatite. The mineral chemistry data show a progressive enrichment of Rb, Cs and Tl in K-feldspars, and depletion in Sr and Ba. The K-feldspar from the beryl-bearing pockets records the highest enrichment in incompatible elements, distinct from the data trend shown by K-feldspar from the main pegmatite zones. The REE data for plagioclase show a decrease of the positive Eu-anomaly and then a change to negative Eu anomaly in the more evolved inner zones. This demonstrates an increase of the oxidation state of the pegmatite melt over time, consistent with the abundance of hematite in late-stage mineral assemblages including those of the miarolitic pockets. Fractional crystallization modeling is able to replicate the progressive enrichment of incompatible elements in K-feldspar,
and to predict degrees of crystallization, which are in good agreement with volume estimates for the different pegmatite zones. The modeling results demonstrate that formation of the zoned pegmatite up to the quartz core can be well explained by an igneous crystallization process, leading up to considerable enrichment in incompatible elements. The melt reached saturation with an aqueous hydrothermal fluid only after more than 90 % of the pegmatite melt had already crystallized. The separation of the oxidizing aqueous fluid was critical for the formation of the gem beryl pockets. The mineral trace element data of the Luumäki pegmatite and the host rapakivi granite, in conjunction with geochronological data, demonstrate that the pegmatite melt was derived from the residual melt of the rapakivi granites.
Originalspråkengelska
TidskriftLithos
Volym274-275
Sidor (från-till)147-168
Antal sidor22
ISSN0024-4937
DOI
StatusPublicerad - mar 2017
MoE-publikationstypA1 Tidskriftsartikel-refererad

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@article{1e67068e579247f98f1680a8c4f360a8,
title = "Chemical evolution and origin of the Luum{\"a}ki gem beryl pegmatite: Constraints from mineral trace element chemistry and fractionation modeling",
abstract = "The 1928 Ma old Luum{\"a}ki gem beryl pegmatite is hosted by rapakivi granites of the Wiborg batholith in southeast Finland. The moderately evolved niobium-yttrium-fluorine (NYF) pegmatite system belongs to the topaz-beryl type of the rare-element pegmatite class. It has a simple major mineral assemblage of K-feldspar + plagioclase + quartz + biotite ± fluorite throughout the main pegmatite zones (border, wall, intermediate, and core zone). It consists of at least two chemically related bodies (Luum{\"a}ki N and Luum{\"a}ki S), of which only Luum{\"a}ki N contains gem beryl (heliodor) bearing miarolitic pockets. We characterize the geology, mineral assemblages, and the major and trace element chemistry of K-feldspar, plagioclase, biotite and quartz from the pegmatite. The mineral chemistry data show a progressive enrichment of Rb, Cs and Tl in K-feldspars, and depletion in Sr and Ba. The K-feldspar from the beryl-bearing pockets records the highest enrichment in incompatible elements, distinct from the data trend shown by K-feldspar from the main pegmatite zones. The REE data for plagioclase show a decrease of the positive Eu-anomaly and then a change to negative Eu anomaly in the more evolved inner zones. This demonstrates an increase of the oxidation state of the pegmatite melt over time, consistent with the abundance of hematite in late-stage mineral assemblages including those of the miarolitic pockets. Fractional crystallization modeling is able to replicate the progressive enrichment of incompatible elements in K-feldspar,and to predict degrees of crystallization, which are in good agreement with volume estimates for the different pegmatite zones. The modeling results demonstrate that formation of the zoned pegmatite up to the quartz core can be well explained by an igneous crystallization process, leading up to considerable enrichment in incompatible elements. The melt reached saturation with an aqueous hydrothermal fluid only after more than 90 {\%} of the pegmatite melt had already crystallized. The separation of the oxidizing aqueous fluid was critical for the formation of the gem beryl pockets. The mineral trace element data of the Luum{\"a}ki pegmatite and the host rapakivi granite, in conjunction with geochronological data, demonstrate that the pegmatite melt was derived from the residual melt of the rapakivi granites.",
keywords = "1171 Geosciences",
author = "Michallik, {Radoslaw M.} and Thomas Wagner and Tobias Fusswinkel and Heinonen, {Jussi S.} and Pasi Heikkil{\"a}",
year = "2017",
month = "3",
doi = "10.1016/j.lithos.2017.01.001",
language = "English",
volume = "274-275",
pages = "147--168",
journal = "Lithos",
issn = "0024-4937",
publisher = "Elsevier Scientific Publ. Co",

}

TY - JOUR

T1 - Chemical evolution and origin of the Luumäki gem beryl pegmatite

T2 - Constraints from mineral trace element chemistry and fractionation modeling

AU - Michallik, Radoslaw M.

AU - Wagner, Thomas

AU - Fusswinkel, Tobias

AU - Heinonen, Jussi S.

AU - Heikkilä, Pasi

PY - 2017/3

Y1 - 2017/3

N2 - The 1928 Ma old Luumäki gem beryl pegmatite is hosted by rapakivi granites of the Wiborg batholith in southeast Finland. The moderately evolved niobium-yttrium-fluorine (NYF) pegmatite system belongs to the topaz-beryl type of the rare-element pegmatite class. It has a simple major mineral assemblage of K-feldspar + plagioclase + quartz + biotite ± fluorite throughout the main pegmatite zones (border, wall, intermediate, and core zone). It consists of at least two chemically related bodies (Luumäki N and Luumäki S), of which only Luumäki N contains gem beryl (heliodor) bearing miarolitic pockets. We characterize the geology, mineral assemblages, and the major and trace element chemistry of K-feldspar, plagioclase, biotite and quartz from the pegmatite. The mineral chemistry data show a progressive enrichment of Rb, Cs and Tl in K-feldspars, and depletion in Sr and Ba. The K-feldspar from the beryl-bearing pockets records the highest enrichment in incompatible elements, distinct from the data trend shown by K-feldspar from the main pegmatite zones. The REE data for plagioclase show a decrease of the positive Eu-anomaly and then a change to negative Eu anomaly in the more evolved inner zones. This demonstrates an increase of the oxidation state of the pegmatite melt over time, consistent with the abundance of hematite in late-stage mineral assemblages including those of the miarolitic pockets. Fractional crystallization modeling is able to replicate the progressive enrichment of incompatible elements in K-feldspar,and to predict degrees of crystallization, which are in good agreement with volume estimates for the different pegmatite zones. The modeling results demonstrate that formation of the zoned pegmatite up to the quartz core can be well explained by an igneous crystallization process, leading up to considerable enrichment in incompatible elements. The melt reached saturation with an aqueous hydrothermal fluid only after more than 90 % of the pegmatite melt had already crystallized. The separation of the oxidizing aqueous fluid was critical for the formation of the gem beryl pockets. The mineral trace element data of the Luumäki pegmatite and the host rapakivi granite, in conjunction with geochronological data, demonstrate that the pegmatite melt was derived from the residual melt of the rapakivi granites.

AB - The 1928 Ma old Luumäki gem beryl pegmatite is hosted by rapakivi granites of the Wiborg batholith in southeast Finland. The moderately evolved niobium-yttrium-fluorine (NYF) pegmatite system belongs to the topaz-beryl type of the rare-element pegmatite class. It has a simple major mineral assemblage of K-feldspar + plagioclase + quartz + biotite ± fluorite throughout the main pegmatite zones (border, wall, intermediate, and core zone). It consists of at least two chemically related bodies (Luumäki N and Luumäki S), of which only Luumäki N contains gem beryl (heliodor) bearing miarolitic pockets. We characterize the geology, mineral assemblages, and the major and trace element chemistry of K-feldspar, plagioclase, biotite and quartz from the pegmatite. The mineral chemistry data show a progressive enrichment of Rb, Cs and Tl in K-feldspars, and depletion in Sr and Ba. The K-feldspar from the beryl-bearing pockets records the highest enrichment in incompatible elements, distinct from the data trend shown by K-feldspar from the main pegmatite zones. The REE data for plagioclase show a decrease of the positive Eu-anomaly and then a change to negative Eu anomaly in the more evolved inner zones. This demonstrates an increase of the oxidation state of the pegmatite melt over time, consistent with the abundance of hematite in late-stage mineral assemblages including those of the miarolitic pockets. Fractional crystallization modeling is able to replicate the progressive enrichment of incompatible elements in K-feldspar,and to predict degrees of crystallization, which are in good agreement with volume estimates for the different pegmatite zones. The modeling results demonstrate that formation of the zoned pegmatite up to the quartz core can be well explained by an igneous crystallization process, leading up to considerable enrichment in incompatible elements. The melt reached saturation with an aqueous hydrothermal fluid only after more than 90 % of the pegmatite melt had already crystallized. The separation of the oxidizing aqueous fluid was critical for the formation of the gem beryl pockets. The mineral trace element data of the Luumäki pegmatite and the host rapakivi granite, in conjunction with geochronological data, demonstrate that the pegmatite melt was derived from the residual melt of the rapakivi granites.

KW - 1171 Geosciences

U2 - 10.1016/j.lithos.2017.01.001

DO - 10.1016/j.lithos.2017.01.001

M3 - Article

VL - 274-275

SP - 147

EP - 168

JO - Lithos

JF - Lithos

SN - 0024-4937

ER -