Troilite nano-inclusions in apatite : Implications for melt immiscibility from a lamprophyric magma
Martínez, Marina (Universitat Autònoma de Barcelona)
Corbella i Cordomí, Mercè (Universitat Autònoma de Barcelona)

Fecha:	2025
Resumen:	Calcium phosphate apatite is a volatile-rich mineral ubiquitous in terrestrial and planetary igneous 12 rocks and can form by a variety of processes. However, the utilization of this powerful mineral for 13 unraveling complex magmatic systems is challenged due to its crystallographic complexity. 14 Recent work has demonstrated that examination by transmission electron microscopy (TEM) can 15 reveal nanostructures that are of paramount importance for a correct interpretation of its chemistry. 16 Here, we have examined different apatite grains from two thin sections of Les Guilleries 17 lamprophyre dykes (NE Spain), representing the last pulses of deep magma ascension at the end 18 of the Variscan orogeny. We have identified two different generations of F-rich apatites (magmatic 19 and hydrothermal), which have been examined by SEM, EPMA, TEM, and Raman spectroscopy. 20 Primary, magmatic apatites are close to rounded and even-sized, whereas secondary, hydrothermal 21 apatites are highly acicular, cut most of the mineral phases, and contain slightly higher amounts of 22 Cl and detectable rare-earth elements compared to magmatic apatites. Transmission electron 23 microscopy work shows that magmatic apatites contain abundant nano-inclusions (~5-60 nm in 24 size) in their cores, consisting of euhedral, negative crystals of troilite, an amorphous phase, and/or 25 a possible gas phase. We argue that fluctuations of temperature during ascent of the lamprophyric 26 magma triggered S saturation and subsequent unmixing of a Fe-S melt from the silicate magma. 27 The immiscible Fe-S melt was trapped in negative crystals of rapidly growing apatite grains. 28 During ascent of the magma, an additional fluctuation of temperature slowed its growth rate and 29 prevented the generation of new, negative crystals, thus the rims of these apatite grains grew free 30 of inclusions. Upon cooling, troilite crystallized at a temperature of ~950 ºC and relatively high 31 oxygen and sulfur fugacities (log fS2 ≈ -7; log fO2< -8. 5). The crystallizing troilite was likely 32 followed by exsolution of a Si,Cl-bearing aqueous fluid and a gas phase. Finally, some Cl (~0. 43 33 wt%) and Si (~2. 87 wt%) from the aqueous fluid phase within the inclusions migrated outwards 34 and interacted with the host apatite at very localized scales (a few nanometers). This study suggests 35 that the unmixing of Fe-S melts during calc-alkaline magmatism may be more common than 36 previously recognized, which has important implications for potentially concentrate economically 37 valuable elements that may lead to the formation of ore deposits.
Ayudas:	Agencia Estatal de Investigación PID2019-109018RB-I00
Nota:	Altres ajuts: This work was supported by the Margarita Salas postdoctoral grant by the Spanish Minister
Derechos:	Aquest document està subjecte a una llicència d'ús Creative Commons. Es permet la reproducció total o parcial, la distribució, i la comunicació pública de l'obra, sempre que no sigui amb finalitats comercials, i sempre que es reconegui l'autoria de l'obra original. No es permet la creació d'obres derivades.
Lengua:	Anglès
Documento:	Article ; recerca ; Versió acceptada per publicar
Materia:	Apatite ; Troilite ; Melt immiscibility ; Melt inclusions ; Calc-alkaline magmatism
Publicado en:	Lithos, Vol. 494-495 (February 2025) , art. 107896, ISSN 1872-6143

DOI: 10.1016/j.lithos.2024.107896

Disponible a partir de: 2027-02-28 Postprint

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