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Composition of barian mica in multiphase solid inclusions from orogenic garnet peridotites as evidence of mantle metasomatism in a subduction zone setting

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Rok publikování 2018
Druh Článek v odborném periodiku
Fakulta / Pracoviště MU

Přírodovědecká fakulta

Klíčová slova Ba-rich phlogopite; Kinoshitalite; Multiphase solid inclusions; Orogenic garnet peridotite; Metasomatism; Bohemian Massif
Popis Multiphase solid inclusions in minerals formed at ultra-high-pressure (UHP) provide evidence for the presence of fluids during deep subduction. This study focuses on barian mica, which is a common phase in multiphase solid inclusions enclosed in garnet from mantle-derived UHP garnet peridotites in the Saxothuringian basement of the northern Bohemian Massif. The documented compositional variability and substitution trends provide constraints on crystallization medium of the barian mica and allow making inferences on its source. Barian mica in the multiphase solid inclusions belongs to trioctahedral micas and represents a solid solution of phlogopite KMg3(Si3Al)O-10(OH)(2), kinoshitalite BaMg3(Al2Si2)O-10(OH)(2) and ferrokinoshitalite BaFe3(Al2Si2)O-10(OH)(2). In addition to Ba (0.24-0.67 apfu), mica is significantly enriched in Mg (X-Mg similar to 0.85 to 0.95), Cr (0.03-0.43 apfu) and Cl (0.04-0.34 apfu). The substitution vector involving Ba in the I-site which describes the observed chemical variability can be expressed as (BaFeAlClK-1Mg-1Si-1)-Al-IV(OH)(-1). A minor amount of Cr and Al-VI enters octahedral sites following a substitution vector (VI)(Cr,Al)(2)(VI)(Mg,Fe)(-3) towards chromphyllite and muscovite. As demonstrated by variable Ba and Cl contents positively correlating with Fe, barian mica composition is partly controlled by its crystal structure. Textural evidence shows that barian mica, together with other minerals in multiphase solid inclusions, crystallized from fluids trapped during garnet growth. The unusual chemical composition of mica reflects the mixing of two distinct sources: (1) an internal source, i.e. the host peridotite and its garnet, providing Mg, Fe, Al, Cr, and (2) an external source, represented by crustal-derived subduction-zone fluids supplying Ba, K and Cl. At UHP-UHT conditions recorded by the associated diamond-bearing metasediments (c. 1100 degrees C and 4.5GPa) located above the second critical point in the pelitic system, the produced subduction-zone fluids transporting the elements into the overlying mantle wedge had a solute-rich composition with properties of a hydrous melt. The occurrence of barian mica with a specific chemistry in barium-poor mantle rocks demonstrates the importance of its thorough chemical characterization.
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