Experimental metasomatic alteration of titanite in a series of metamorphic fluids at 700 °C and 200 MPa

• Published in: Mineralogy and petrology 2024-09, Vol.118 (3), p.321-344
• Main Authors: Gmochowska, Wiktoria, Harlov, Daniel, Słaby, Ewa, Jokubauskas, Petras, Sláma, Jiří, Łącki, Marcin
• Format: Article
• Language: English
• Subjects: Calcite; Calcium chloride; Calcium hydroxide; Crystals; Dissolution; Dissolving; Earth and Environmental Science; Earth Sciences; Experiments; Fluids; Garnet; Geochemistry; Hydrothermal activity; Hydrothermal solutions; Inorganic Chemistry; Leaching; Mineralogy; Niobium; Perovskites; Potassium chloride; Rare earth elements; Recrystallization; Silica; Slaked lime; Sodium chloride; Sodium hydroxide; Tantalum; Thorium; Titanite; Trace elements; Wollastonite
• ISSN: 0930-0708; 1438-1168
• DOI: 10.1007/s00710-024-00862-5

Seven experiments exploring the reaction of titanite with various hydrothermal solutions have been carried out at 700 °C and 200 MPa for a run duration of 16 days. In experiments involving fluids consisting of NaCl+H 2 O, KCl+H 2 O, CaCl 2 +H 2 O, 2M NaOH, or 2M KOH, no reaction of the titanite with the fluid was observed other than a slight dissolution of the titanite. Experiments involving NaF+H 2 O and Ca(OH) 2 +H 2 O resulted in visible alteration of the titanite in texture and composition, coupled with the formation of perovskite. In the NaF+H 2 O experiment, perovskite, enriched with rare earth elements (REE), formed as euhedral to subhedral crystals on the surface of the recrystallized titanite. In the Ca(OH) 2 +H 2 O experiment perovskite took in minor amounts of REE, and formed as a reaction rim partially replacing the titanite via a coupled dissolution-reprecipitation reaction. Wollastonite, along with minor calcite, and grossular garnet, formed as an outer rim on the perovskite. In the NaF+H 2 O experiment major and trace elements were leached from the titanite, whereas in the Ca(OH) 2 +H 2 O experiment no leaching of major or trace elements was observed. Nb/Ta, Th/U, and Y/Ho were investigated as potential indicators of hydrothermal processes. While the Nb/Ta ratio was altered in the experimentally metasomatised titanite, the degree of alteration was the same for both fluids. In contrast, only small changes in the Th/U and Y/Ho ratios between the altered and original titanite were seen for either experiment. The formation of perovskite at the expense of titanite in NaF+H 2 O or Ca(OH) 2 +H 2 O fluids demonstrates how titanite reacts with these fluids in simple, low silica activity systems under mid to upper crustal P-T conditions.