Apatite Partitioning in Intermediate Melts

Apatite is an important accessory phase in igneous and metamorphic rocks and may act as an important reservoir for rare earth elements, phosphorus, and other elements in the lower crust. This study presents new trace element partition coefficients between apatite and intermediate melts at 1 GPa. Our results show that the rare earth elements, Y, and Sr partition strongly into the crystal structure of apatite (ap/meltD values 12.61, 1.32, 18.49 for Y, Sr and Gd, respectively), while large ion lithophile elements and high field strength elements are strongly incompatible (ap/meltD values between 0.03 and 0.48). These results have important implications for geochemical models which rely on the interpretation of rare earth element ratios in melts, such as the amphibole sponge model, as the high partition coefficients between apatite and melts can complicate the signature often attributed to amphibole fractionation. The fractionation of small (<5% modally) amounts of apatite significantly alter the Dy/Yb and Dy/Dy* trajectory of a differentiating melt, with fractionation of 10% apatite causing depletion of Dy/Yb in a melt after 80% crystallization.磷灰石（apatite）是火成岩与变质岩中重要的副矿物相，同时可作为下地壳内稀土元素、磷及其他元素的重要储库。本研究测定了1吉帕斯卡（GPa）条件下，磷灰石与中性熔体间的全新微量元素分配系数。研究结果显示，稀土元素、钇（Y）与锶（Sr）可强烈富集进入磷灰石的晶体结构（钇、锶与钆的磷灰石-熔体分配系数（ap/meltD）分别为12.61、1.32与18.49），而大离子亲石元素与高场强元素则表现出强烈的不相容性（ap/meltD值介于0.03至0.48之间）。上述结果对依赖熔体稀土元素比值开展解释的地球化学模型具有重要启示意义，例如角闪石海绵模型：由于磷灰石与熔体间极高的分配系数，常会使得原本归因于角闪石分馏的地球化学特征变得更为复杂。模态占比低于5%的少量磷灰石分馏作用，可显著改变分异熔体的Dy/Yb与Dy/Dy*演化轨迹；当磷灰石分馏比例达10%时，经历80%结晶作用后的熔体将出现Dy/Yb比值的亏损。