Abstract
Carbonatites host Earth’s main REE deposits, with bastnaesite (LREE)CO 3 F being the main economic REE-bearing mineral. However, bastnaesite mineralisation processes are debated between hydrothermal or magmatic origin. This study aims to assess if bastnaesite can be magmatic, and to characterise the REE behaviour during carbonatite crystallisation. Crystallisation experiments have been performed from 900 to 600 °C at 1 kbar, on a REE-rich calciocarbonatitic composition. REE-bearing calcite is the dominant crystallising mineral, driving the residual melt towards natrocarbonatitic compositions. Both halogens (i.e., Cl and F) and water decrease the temperature of calcite saturation. REE are slightly incompatible with calcite: for all REE, partition coefficients between carbonate melt and calcite are comprised between 1 and 11, and increase with temperature decrease. Britholite (REE, Ca) 5 ((Si,P)O 4 ) 3 (F,OH) crystallises at high temperatures (700–900 °C), while pyrochlore (Ca,Na,REE) 2 Nb 2 O 6 (OH,F) crystallises at low temperatures (600–700 °C), as well as REE-rich apatite (600–650 °C). No bastnaesite is found in crystallisation experiments. We thus performed a bastnaesite saturation experiment at 600 °C. The bastnaesite-saturated melt contains 20 wt% of REE: such magmatic saturation is unlikely to happen in nature. Textural evidences imply a Na, Cl, REE-rich fluid at high temperatures and hydrous conditions. We propose that fluids are the main mineralising agent for bastnaesite at hydrothermal stage (<600 °C).
Highlights
Carbonatites are magmatic rocks with more than 50% of carbonates [Le Maitre, 2002] and constitute the main rare earth element (REE) deposits on Earth [Verplanck et al, 2016]
Major elements have been analysed with a CAMECA SXFive electron probe micro analyzer (EPMA) (ISTO, Orléans), using wavelength dispersive spectrometry (WDS), with acquisition parameters equal to 15 kV and 6 nA
Calcite crystals are round to sub-euhedral (Figure 1A), and more abundant with decreasing temperature
Summary
Carbonatites are magmatic rocks with more than 50% of carbonates [Le Maitre, 2002] and constitute the main rare earth element (REE) deposits on Earth [Verplanck et al, 2016]. One occurrence of active carbonatitic magmatism is known: the Ol Doinyo Lengai volcano, in Tanzania [Keller and Zaitsev, 2012]. This volcano mainly produces highly peralkaline silicate lavas such as phonolites and nephelinites, it is the only one to produce alkaline carbonate lavas called natrocarbonatites [Klaudius and Keller, 2006]. Cooper et al [1975] established a phase diagram in the system Na2CO3–CaCO3, showing the saturation of nyerereite Na2Ca(CO3) to divide the system into two subsystems, an alkali- and a calciumdominated one, with their own eutectics. Jago and Gittins [1991] showed that F drastically decreases the temperature of calcite saturation, potentially unifying the two subsystems by suppressing the calcite–nyerereite eutectic
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