Compositional trends in Ba-, Ti-, and Cl-rich micas from metasomatized mantle rocks of the Gföhl Unit, Bohemian Massif, Austria

Abstract

Ba-, Ti-, and Cl-rich micas associated with other Ba-and/or Cl-rich minerals in the rock matrix or in garnet and clinopyroxene hosted multiphase solid inclusions (MSI) are observed in mantle-derived garnet pyroxenites. The micas show extremely high variability in chemical composition ranging between Ba-rich phlogopite, chloroferrokinoshitalite, and oxykinoshitalite. Elemental covariation trends in mineral chemical data reveal the principal substitution mechanisms responsible for the observed chemical variability. The substitution Ba 2+ Al 3+ ↔K 1+ Si 4+ associated with either OH 1-↔Cl 1or Ti 4+ 2O 2-↔Mg 2+ 2OH 1 links phlogopite to chloroferrokinoshitalite and oxykinoshitalite, respectively, whereas the substitution2Cl 1links chloroferrokinoshitalite to oxykinoshitalite. The preferred incorporation of Cl in Fe-rich mica and of Ti+O in Mg-rich mica indicates that XFe (Fe tot /Fe tot +Mg) exerts an important control on mica composition. The positive correlation of XFe with Cl led to the formation of possibly the most Clrich mica so far described classified as chloroferrokinoshitalite(XFe 0.88 , Ba 0.95 K 0.03 Fe 2.68 Mg 0.37 Al 1.91 Si 2.01 Cl 1.98 ) with 10.98 wt% Cl. Substantial substitution of OHby Cland This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

Halogen-bearing metasomatizing melt preserved in high-pressure (HP) eclogites of Pfaffenberg, Bohemian Massif

“…The crystallized MI in PF have a clear granitic composition, with a constant mineral assemblage comprising feldspars, quartz and micas, and lack any evidence of system reopening, e.g., decrepitation cracks. Polycrystalline inclusions (also called multiphase solid inclusions or MSI) have already been reported in HP/UHP rock types and are interpreted as containing metasomatizing agents: garnet pyroxenites (Malaspina et al, 2006(Malaspina et al, , 2009Zelinkova et al, 2023), orogenic garnet peridotites (Malaspina et al, 2009;Čopjaková and Kotková, 2018;Naemura et al, 2018), spinel-bearing harzburgite xenoliths (Schiano et al, 1995), eclogites (Faryad et al, 2013;Gao et al, 2012Gao et al, , 2013Chen et al, 2014;Liu et al, 2018) and paragneisses (Stepanov et al, 2016). However, MSI are generally rather different in phase assemblage with respect to the MI in Pfaffenberg.…”

“…However, MSI are generally rather different in phase assemblage with respect to the MI in Pfaffenberg. For example, some MSI contain mostly hydrous phases (e.g., Malaspina et al, 2006Malaspina et al, , 2009 or mainly phyllosilicates, phosphate and carbonates (Naemura et al, 2018;Zelinkova et al, 2023), and others are completely devoid of OH-bearing phases (e.g., Gao et al, 2012Gao et al, , 2013Gao et al, , 2014Chen et al, 2014;Liu et al, 2018) or contain glass, sulfides and mafic minerals (Schiano et al, 1995). Moreover, the MSI reported in the literature often show decrepitation features, such as irregular shape, protrusion textures, radial cracks surrounding the inclusions and healed cracks in the host mineral (e.g., Chen et al, 2014;Gao et al, 2014;Liu et al, 2018;Brown, 2023), which suggests that they are not fully preserved due to material loss (or gain) via system reopening after entrapment.…”