Trace-element partitioning between gregoryite, nyerereite, and natrocarbonatite melt: implications for natrocarbonatite evolution

Trace-element partitioning between gregoryite, nyerereite, and natrocarbonatite melt is primordial for understanding trace-element distribution and fractionation in alkali-rich carbonatites. However, trace-element data are scarce for gregoryite and nyerereite. Here, we provide the first partition coefficients and lattice strain model parameters for trace-element partitioning between these carbonate minerals and natrocarbonatite at Oldoinyo Lengai (Tanzania). Nyerereite and gregoryite phenocrysts crystallize within a shallow magmatic reservoir (< 3 km depth, similar to 600 degrees C), and gregoryite continues to crystallize during magma ascent at lower pressures. At these low-temperature and pressure conditions, trace elements behave incompatibly in both gregoryite and nyerereite. Trace-element partitioning is characterized by a parabolic fit between the partition coefficients and ionic radii that is explained by a lattice strain model in which the site radius (r0) decreases with increasing charge from r01+ = 1.1 angstrom to r04+ = 0.75 angstrom. We observed different partition coefficients in gregoryite (Ggy) and nyerereite (Nye): those in nyerereite are greater than those in gregoryite for REEs (D-Nd(Nye) = 0.58 vs. D-Nd(Ggy) = 0.21; D-Nye (La) = 0.27 vs. D-Ggy (La) = 0.12), Sr (D-Nye (Sr) = 0.92 vs. D-Sr(Ggy) = 0.5), Ba (D-Ba(Nye) = 0.22 vs. D-Ba(Ggy) = 0.1), and Rb ( D-Rb(Nye) = 0.35 vs. D-Ggy (Rb) = 0.26), but lower for HFSEs (e.g., D-Nye (Hf) increase in the melt during differentiation and the crystallization of both gregoryite and nyerereite. Due to their different partition coefficients, we can constrain the shallow crustal crystallization history of natrocarbonatite melts at Oldoinyo Lengai: the crystallization of roughly equal proportions of gregoryite and nyerereite can produce aphyric natrocarbonatite compositions from a typical natrocarbonatite composition. The late-stage crystallization of gregoryite alone during magmatic ascent and eruption can significantly impact the concentrations of key elements, such as increasing LREE contents and LREE/HFSE and LILE/HFSE ratios in the residual melt. Our results also highlight that natrocarbonatite melt crystallization during the 2019 eruption proceeded at temperatures from 600 degrees C to as low as 300 degrees C. = 0.13 vs. DGgy Hf = 0.28; DNye Nb = 0.02 vs. DGgyNb = 0.08). Because all trace elements are incompatible, their concentrations

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Trace-element partitioning between gregoryite, nyerereite, and natrocarbonatite melt is paramount for understanding trace-element distribution and fractionation in alkali-rich carbonatites. This study provides the first set of partition coefficients and lattice strain model parameters for trace-element partitioning between these carbonate minerals and natrocarbonatite at Oldoinyo Lengai (Tanzania). The results show that trace element behavior is predominantly incompatible in gregoryite and nyerereite under low-temperature and pressure conditions. Rating: 7/10