Baudouin, C.; France, L.
Contributions to Mineralogy and Petrology, 2023, 178, 40
Voir en ligne : https://doi.org/10.1007/s00410-023-02021-z
Abstract :
Trace-element partitioning between gregoryite, nyerereite, and natrocarbonatite melt is primordial for understanding traceelement 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, ~ 600 °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 Å to r04+ = 0.75 Å. We observed different partition coefficients in gregoryite (Ggy) and nyerereite (Nye): those in nyerereite are greater than those in gregoryite for REEs ( DNye Nd = 0.58 vs. DGgy Nd = 0.21; DNye La = 0.27 vs. DGgy La = 0.12), Sr ( DNye Sr = 0.92 vs. DGgy Sr = 0.5), Ba ( DNye Ba = 0.22 vs. DGgy Ba = 0.1), and Rb ( DNye Rb = 0.35 vs. DGgy Rb = 0.26), but lower for HFSEs (e.g., DNye Hf = 0.13 vs. DGgy Hf = 0.28; DNye Nb = 0.02 vs. DGgy Nb = 0.08). Because all trace elements are incompatible, their concentrations 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 °C to as low as 300 °C.