Tracking the Ti^(4+)substitution in phlogopite by spectroscopic imaging:A tool for unravelling the growth of micas at HP-HT conditions  

作　　者：G.Della Ventura N.El Moutaouakkil B.Boukili S.Bernardini A.Sodo L.Pronti M.Cestelli-Guidi F.Holtz F.Lucci 

机构地区：[1]Dipartimento di Scienze,UniversitàRoma Tre,Roma,Italy [2]INFN-LNF,via Fermi 2,00040 Frascati,Roma,Italy [3]INGV,Via di Vigna Murata 605,00143 Roma,Italy [4]Laboratory of Géosciences,Water and Environment.Department of Earth Sciences.Faculty of Sciences,University Mohammed V,Rabat,Morocco [5]Leibnitz Universität Hannover,Institute of Mineralogy,Callinstr 3,30167 Hannover,Germany [6]Dipartimento di Scienze della Terra e Geoambientali,Universitàdi Bari“Aldo Moro”,Bari,Italy

出　　处：《Geoscience Frontiers》2024年第3期1-13,共13页地学前缘（英文版）

基　　金：the grant to Department of Science,Roma Tre University(MIUR-Italy Dipartimenti di Eccellenza,ARTICOLO 1,COMMI 314-337 LEGGE 232/2016)。

摘　　要：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 TiO_(2)≈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℃at P≈0.73±0.13 GPa)for cores whereas lower values(T≈790±54℃at P≈0.29±0.13 GPa)are obtained for the outer rims.Comparable temperatures(T≈1075±54℃)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

关 键 词：Ti-phlogopite Jersey minette(UK) Ti-substitution FTIR imaging Raman imaging Thermobarometric modelling 

分 类 号：P534.44[天文地球—古生物学与地层学] P588.3[天文地球—地质学]