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

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 degrees C at P ⠁ 0.73 +/- 0.13 GPa) for cores whereas lower values (T ⠁ 790 +/- 54 degrees C at P ⠁ 0.29 +/- 0.13 GPa) are obtained for the outer rims. Comparable temperatures (T ⠁ 1075 +/- 54 degrees 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 compositiononly 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. (c) 2024 China University of Geosciences (Beijing) and Peking University. Published by Elsevier B.V. on behalf of China University of Geosciences (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).