Troilite nano-inclusions in apatite: Implications for melt immiscibility from a lamprophyric magma

Abstract

Calcium phosphate apatite is a volatile-rich mineral ubiquitous in terrestrial and planetary igneous rocks and can form by a variety of processes. However, the utilization of this powerful mineral for unraveling complex magmatic systems is challenged due to its crystallographic complexity. Recent work has demonstrated that examination by transmission electron microscopy (TEM) can reveal nanostructures that are of paramount importance for a correct interpretation of its chemistry. Here, we have examined different apatite grains from two thin sections of Les Guilleries lamprophyre dykes (NE Spain), representing the last pulses of deep magma ascension at the end of the Variscan orogeny. We have identified two different generations of F-rich apatites (magmatic and hydrothermal), which have been examined by SEM, EPMA, TEM, and Raman spectroscopy. Primary, magmatic apatites are close to rounded and even-sized, whereas secondary, hydrothermal apatites are highly acicular, cut most of the mineral phases, and contain slightly higher amounts of Cl and detectable rare-earth elements compared to magmatic apatites. Transmission electron microscopy work shows that magmatic apatites contain abundant nano-inclusions (∼5–60 nm in size) in their cores, consisting of euhedral, negative crystals of troilite, an amorphous phase, and/or a possible gas phase. We argue that fluctuations of temperature during ascent of the lamprophyric magma triggered S saturation and subsequent unmixing of a FeS melt from the silicate magma. The immiscible FeS melt was trapped in negative crystals of rapidly growing apatite grains. During ascent of the magma, an additional fluctuation of temperature slowed its growth rate and prevented the generation of new, negative crystals, thus the rims of these apatite grains grew free of inclusions. Upon cooling, troilite crystallized at a temperature of ∼950 °C and relatively high oxygen and sulfur fugacities (log fS₂ ≈ −7; log fO₂ < −8.5). The crystallizing troilite was likely followed by exsolution of a Si,Cl-bearing aqueous fluid and a gas phase. Finally, some Cl (∼0.43 wt%) and Si (∼2.87 wt%) from the aqueous fluid phase within the inclusions migrated outwards and interacted with the host apatite at very localized scales (a few nanometers). This study suggests that the unmixing of FeS melts during calc-alkaline magmatism may be more common than previously recognized, which has important implications for potentially concentrate economically valuable elements that may lead to the formation of ore deposits.