The Ti-Substitution in Phlogopite from the Jersey Minette (Channel Islands, UK) as Revealed by FTIR Spectroscopy

Phlogopite solid-solutions have a wide physico-chemical stability field; they are thus ubiquitous in a wide variety of geological environments. Phlogopite may provide important insight into the physico-chemical parameters of its crystalization conditions thus representing a tool for understanding magma storage conditions and petrogenesis. This paper focuses on the study of some intriguing single-crystals occurring within a lamprophyre minette from Fort Regent, St. Helier (Jersey Island, UK). The primary assemblage is represented by phlogopite + clinopyroxene + alkali-feldspar + apatite. Phlogopite occurs as both euhedral to subhedral phenocrysts and as euhedral microcrystals. Phlogopite phenocrystals associated with mica-lamprophyres generally present normal and continuous compositional zonings, however, the crystals from the studied minette are characterized by dark chestnut patchy cores enveloped by euhedral oscillatory rims. Preliminary results indicate (i) cores with a Mg# = Mg/(Mg + Fetot) of ≈ 0.73 and high TiO2 contents of ≈ 10%; (ii) inner rims having Mg# ≈ 0.90 with TiO2 not exceeding 2%; and (iii) external rims with Mg# ≈ 0.67 and TiO2 contents up to 5.5%. Combination of EMP and single-crystal FTIR results in the OH-stretching region shows that, differently to what observed in other occurrences, the substitution mechanism responsible for the exceptional Ti zoning during the biotite crystallization is the Ti-vacancy mechanism that is known to be typical of HP/HT crystallization environments.