Tracking the Ti4+ substitution in phlogopite by spectroscopic imaging: A tool for unravelling the growth of micas at HP-HT conditions

• Published in: Di xue qian yuan. Vol. 15; no. 3; pp. 101777 - 13
• Main Authors: Della Ventura, G., El Moutaouakkil, N., Boukili, B., Bernardini, S., Sodo, A., Pronti, L., Cestelli-Guidi, M., Holtz, F., Lucci, F.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.05.2024; INGV,Via di Vigna Murata 605,00143 Roma,Italy%Laboratory of Géosciences,Water and Environment.Department of Earth Sciences.Faculty of Sciences,University Mohammed V,Rabat,Morocco%Dipartimento di Scienze,Università Roma Tre,Roma,Italy%INFN-LNF,via Fermi 2,00040 Frascati,Roma,Italy%Leibnitz Universit?t Hannover,Institute of Mineralogy,Callinstr 3,30167 Hannover,Germany%Dipartimento di Scienze della Terra e Geoambientali,Università di Bari"Aldo Moro",Bari,Italy; Dipartimento di Scienze,Università Roma Tre,Roma,Italy; INFN-LNF,via Fermi 2,00040 Frascati,Roma,Italy
• Subjects: FTIR imaging; Jersey minette (UK); Raman imaging; Thermobarometric modelling; Ti-phlogopite; Ti-substitution; Ti-substitution; FTIR imaging; Jersey minette (UK); Raman imaging; Ti-phlogopite; Thermobarometric modelling; Jersey minette(UK)
• ISSN: 1674-9871
• DOI: 10.1016/j.gsf.2024.101777

[Display omitted] •FTIR and Raman imaging to understand chemical evolution of mica composition.•Evaluating the substitutional mechanism of Ti in the mica structure.•The entry of Ti in the mica structure can be explained via a vacancy mechanism.•The chemical evolution of the black mica is influenced by the evolving chemical system. Phlogopite solid-solutions have a wide pressure–temperature (P-T) stability field and are ubiquitous in a wide variety of geological settings, from deep lithosphere magmatic environments to upper crust metamorphic domains. Phlogopite composition represents therefore a valuable physical–chemical archive and may provide important information regarding its crystallization and the petrogenesis of the host-rock. In this paper we examine the phlogopite phenocrysts from the well-known Fort Regent mica-bearing lamprophyre minette from St. Helier (Island of Jersey, UK). Phlogopite phenocrystals from lamprophyres generally show normal-step and continuous compositional zoning, however those from the Fort Regent minette show a peculiar texture characterized by dark brown high-Ti (average TiO2 ≈ 8.5 wt.%) cores enveloped by euhedral low- to mid-amplitude zonation due to oscillatory contents in Ti, Fe and Mg. Thermo-barometry modelling based on biotite-only composition yields relatively high P-T estimates (T ≈ 970 ± 54 °C at P ≈ 0.73 ± 0.13 GPa) for cores whereas lower values (T ≈ 790 ± 54 °C at P ≈ 0.29 ± 0.13 GPa) are obtained for the outer rims. Comparable temperatures (T ≈ 1075 ± 54 °C) but extremely high and anomalous pressure values (P ≈ 1.82 ± 0.13 GPa) are obtained for the yellowish inner rims. The combination of electron micro probe (EMP) analysis and single-crystal infra-red (FTIR) imaging in the OH-stretching region shows that the exceptional and oscillatory Ti contents are due to the Ti-vacancy substitution, typical of crystallization and growth processes of HP/HT environments. Raman imaging provides additional insight for this process, confirming the dominant dioctahedral nature for the Ti-Fe-rich cores and outer rims. Interpretation of thermobaric estimates obtained from the phlogopite composition-only model, based on the fine-scale compositional evolution, shows that pressure–temperature values from low-Ti high-Mg domains should be carefully evaluated because the substitution mechanisms during the dark mica growth are not univocally related to pressure–temperature variation of the crystallizing environment. Our results demonstrate how a multidisciplinary approach based on the combination of chemical investigations and vibrational spectroscopies could represent a valuable tool to evaluate pressure–temperature estimates from biotite composition-only thermo-barometry models and therefore to correctly unravel HP/HT petrogenetic processes at a very fine scale.