Revealing the role of crystal chemistry in REE fractionation in skarn garnets: insights from lattice-strain theory

Abstract

Garnet is a prominent mineral in skarn deposits and its rare earth elements (REE) geochemistry is pivotal for understanding skarn mineralization and fluid evolution. In contrast to magmatic and metamorphic garnets, skarn garnets are mainly grossular-andradite in composition. They exhibit variable REE patterns, spanning from notable heavy (H)-REE enrichment to significant light (L)-REE enrichment, accompanied by negative to positive europium (Eu) anomalies. However, the key factors governing REE fractionation in skarn garnets remain uncertain. This study applies the lattice-strain theory (LST) to investigate the influence of crystal chemistry and structure on REE fractionation in garnets from the Lazhushan Fe skarn deposit in eastern China. Our results demonstrate that the garnet-liquid partition coefficient ratios of D_La/D_Yb significantly increase (up to 5–7 orders of magnitude) with rising andradite content in garnet. This variation underscores the pivotal role of garnet structure in controlling LREE/HREE fractionation. The results further show that partition coefficient ratios of D_La/D_Sm are strongly dependent on andradite content in garnets, whereas the D_Gd/D_Yb ratios only show a weak correlation to the garnet composition. This contrast suggests that fractionation of LREE in garnet is more sensitive to variations of andradite content than HREE. Data compilation of major elements and REE for garnet from the Lazhushan Fe skarn deposit and other skarn deposits worldwide shows that the garnet REE patterns vary from positive through concave to negative shapes with the garnet ranging from grossularitic to andraditic compositions. Such variations in garnet REE patterns are consistent with the results of geochemical modeling based on the LST. This study demonstrates that, through LST equations, the shape of fluid REE patterns can be predicted from garnet REE patterns, and vice versa. Furthermore, the Eu anomaly (Eu/Eu*_Grt) in skarn garnet depends mainly on fluid Eu anomaly (Eu/Eu*_fluid) and garnet-fluid partition coefficient ratio of D(Eu²⁺)/D(Eu³⁺) with the latter being influenced by garnet composition. These findings highlight the critical role of crystal chemistry and structure in garnet REE fractionation, enhancing our ability to utilize garnet REE in tracing the origin and evolution of skarn-forming fluids.