The evolutional behaviors of rare earth elements the carbonatite system: Evidences from elemental geochemical study of apatites in the Fengzhen carbonatites, Inner Mongolia, China

The behaviors of rare earth elements (REE) in the carbonatite magmatic evolution process is critical for the petrogenesis and mineralization of carbonatite. However, it is very difficult to study the carbonatite magmatic evolution process, as most carbonatites observed currently on the Earth’s surface are either cumulates formed in the carbonatite magmatic evolution process or altered rocks which had undergone intensive post-magmatic hydrothermal alteration. The Fengzhen carbonatites in Inner Mongolia, China, have a series of well developed and preserved carbonatites with the complete magmatic evolutional sequence, in which apatites crystallized continuously from the magmatic to hydrothermal stages, and recorded the evolutional process of compositional variations of melts. This study investigates the mineralogical, major and trace elemental geochemical characteristics of apatites formed at different carbonatite crystallization stages in the Fengzhen carbonatites, in order to trace the REE fractionation and enrichment behaviors. Apatites were primarily formed during the magmatic and hydrothermal stages. Based on the crystallization sequence of magmatic apatites, they can further be identified as four phases including the olivine-stage apatite (Ap-1), pyroxene-stage apatite (Ap-2), feldspar-stage apatite (Ap-3), and quartz-stage apatite (Ap-4). Especially, the Ap-1 apatites have relatively high contents of Na and S but relatively low Si contents comparing to other phases of apatites. Compositional characteristics of the Ap-1 apatites are consistent with those of the alkali-rich and Si-poor initial carbonatitic melt. From the olivine-stage apatites to quartz-stage apatites, their total REE contents (particularly HREE) are increased from 4589×10–6– 6262×10–6 for the Ap-1 ones, 9905×10–6– 10791×10–6 for the Ap-2 ones, 11 854×10–6– 13 975×10–6 for the Ap-3 ones to 19 490×10–6– 25 672×10–6 for the Ap-4 ones), while the degrees of differentiation between light and heavy rare earth elements (La/Yb)N are gradually decreased from 226-284 for the Ap-1 ones, 132-146 for the Ap-2 ones, 111-126 for the Ap-3 ones, to 27-33 for the Ap-4 ones. Hydrothermal-stage apatites (Ap-5) have the lowest REE contents (94.1×10–6– 111×10–6), indicating that they were formed by the precipitation in a residual REE-depleted fluid. These findings show that rare earth elements were gradually enriched in the residual melt with the crystallization differentiation of REE-poor calcite and silicate minerals in the carbonatite magmatic evolution process. Therefore, the intensive crystallization differentiation in the carbonatite magmatic evolution process is probably a key mechanism driving the REE enrichment in the residual carbonatite magma.