The origin of nelsonite constrained by melting experiment and melt inclusions in apatite: The Damiao anorthosite complex, North China Craton

Models for the nelsonite formation are currently highly contentious, with liquid immiscibility and fractional crystallization as frequently proposed formation mechanisms. The nelsonites in the Damiao massif anorthosite complex in the North China Craton are revisited here together with experimental evidence for the existence of silica-free CaO-FeO-Fe₂O₃-TiO₂-P₂O₅ immiscible nelsonitic liquids. Our results of differential scanning calorimetry (DSC) and internally heated pressure vessel (IHPV) demonstrate that nelsonite with the composition of one-third apatite and two-thirds Fe-Ti oxides by weight 1) completely melts well above 1450 °C at dry condition, which is in good agreement with numerous experimental studies of the CaO-P₂O₅-Fe_xO system for metallurgical purposes; 2) does not melt at the temperature up to 1200 °C with presence of considerable amount of volatiles, e.g., fluorine and water. Therefore, the composition of the nelsonite cannot be molten at temperatures relevant for crystallization of the Damiao magma. A review of experimental studies of liquid immiscibility and analyses of natural immiscible glasses show that all the liquids on the Fe- and P-rich side of the miscibility gap have at least 20 wt.% of aluminosilicate components. Analyses of apatite-hosted melt inclusions from Damiao nelsonite provide further constraints on the nelsonite formation. The inclusions range from ~ 3 to 200 μm in diameter. They are ubiquitous and meet all the morphological criteria of primary melt inclusions crystallized into assemblages of daughter minerals. Almost all assemblages contain vermiculite and/or chlorite, and some contain biotite, amphibole and Fe-Ti oxide. Melt compositions of individual inclusions has been estimated based on modal proportions and mineral compositions of daughter phases. The inclusions show large variations in SiO₂ (20.8–50.2 wt.%) and FeO_t (13.4–32.8 wt.%). With a few exceptions, the inclusions are very low in CaO (< 1.5 wt.%) and high in Al₂O₃ (10.0–21.2 wt.%). The low total values indicate very hydrated nature of the melt inclusions. The melt inclusions were also re-heated at 1200 °C by IHPV. Glasses from partly to fully homogenized melt inclusions show similar compositional characteristics as the bulk compositions from the crystallized melt inclusions but incorporated excessive melting of the host apatite. It appears that the cumulus apatite crystallized from hydrated late-stage immiscible Fe-rich melt formed by liquid immiscibility occurred in the relatively early stage, i.e. prior to the arrival of aqueous fluid phase. We propose that the inclusions at Damiao record a trend of late-stage melt evolution, which have been through silicate liquid immiscibility but most likely have been strongly affected by separation of a magmatic fluid phase and loss of alkali and Ca silicate components from the melt into the fluid.