Cumulate granites: A perspective from new apatite MgO partition coefficients

Abstract Apatite is a powerful research tool because it is common in various rocks and incorporates many chemical elements. Understanding how elements partition between apatite and melt and the controls on the partition are critical for applications of apatite. It has been recently proposed that MgO content in magmatic apatite is proportional to that of the melt, highlighting the potential importance of this oxide as a critical parameter. A comprehensive compilation of experimental data in this study shows that the relationship between apatite MgO and melt MgO defines two distinct equilibrium trends, indicating that melt composition or, more specifically, melt polymerization strongly controls the partition of MgO. These findings have great implications for using apatite in the study of magma petrogenesis and sediment provenance. We show that when apatite MgO is plotted against host-rock MgO, volcanic and plutonic systems have different behaviors. The volcanic data plot around the experimental apatite-melt equilibrium trends, while apatites in most plutonic rocks have lower MgO than expected if they were in equilibrium with the host bulk rock. For granites with high apatite saturation temperatures, this indicates that apatites crystallized from low-MgO parent melts but later became part of a cumulate with higher MgO than the parental melt, possibly due to extraction of evolved interstitial melt. The contrast between apatite MgO in volcanic and plutonic rocks therefore can provide a new perspective on the debate about volcanic-plutonic connection.