Volatile Characteristics of Apatite in Dacite from the Eastern Manus Basin and Their Geological Implications

As one of the youngest back-arc basins, the evolutionary behavior of magmatic volatiles in the Eastern Manus Basin has been poorly studied. Recently, apatite has received widespread attention for its powerful function in recording information on magmatic volatiles. In this paper, by determining the major element compositions and primary volatile abundances (F, Cl, SO3) of apatites in dacite, we analyze the compositions of volatiles before magma eruption in the Eastern Manus Basin, as well as their indications for magmatic oxygen fugacity and petrogenesis, so as to improve the study about the evolution of magmatic volatiles in this region. Experimental data indicate that apatite saturation temperatures range from 935 to 952 degrees C. All the apatites are magmatic apatites with F contents of 0.87-1.39 wt.%, Cl contents of 1.24-1.70 wt.%, and SO3 <= 0.06 wt.%. Analysis reveals that the apatites in this study crystallized from volatile-undersaturated melts, so their chemical compositions can be used as indicators of magmatic compositions. According to calculations, the minimum S concentrations of the host melts range from 2-65 ppm or 8-11 ppm. The crystallization and separation of magnetite caused the reduction state of melts, and the relatively low oxygen fugacity (Delta FMQ = -0.2 +/- 0.9) caused low SO3 contents in apatites. In addition, F and Cl contents of the host melt were calculated to be 185-448 ppm and 1059-1588 ppm, corresponding to the H2O contents of 1.4-2.1% and 1.2-1.5% (error +/- 30-40%), respectively. The high Cl/F ratio and H2O contents of samples indicate the addition of slab-derived fluids in the mantle source region of the Eastern Manus Basin. High F contents of the melts may be influenced by F-rich sediments, as well as the release of F from lawsonite and phengite decomposition. High Cl appears to originate from the dual influence of subduction-released fluids and Cl-rich seawater-derived components. Further, it is estimated that 14-21% of the total Cl concentrations in melts were added directly from subduction-released fluids, or higher.

Automated summary

This study investigates the magmatic volatiles in the Eastern Manus Basin by analyzing apatites in dacite, providing insights into the compositions of pre-eruption volatiles and their implications for magmatic oxygen fugacity and petrogenesis. The experimental data suggest that the apatites crystallized from volatile-undersaturated melts, with indications of the addition of slab-derived fluids and F-rich sediments in the mantle source region of the basin. The study also estimates a significant proportion of Cl concentrations in the melts being added directly from subduction-released fluids.