Rare earth element mobility in and around carbonatites controlled by sodium, potassium, and silica

Abstract

Carbonatites and associated rocks are the main source of rare earth elements (REEs), metals essential to modern technologies. REE mineralization occurs in hydrothermal assemblages within or near carbonatites, suggesting aqueous transport of REE. We conducted experiments from 1200°C and 1.5 GPa to 200°C and 0.2 GPa using light (La) and heavy (Dy) REE, crystallizing fluorapatite intergrown with calcite through dolomite to ankerite. All experiments contained solutions with anions previously thought to mobilize REE (chloride, fluoride, and carbonate), but REEs were extensively soluble only when alkalis were present. Dysprosium was more soluble than lanthanum when alkali complexed. Addition of silica either traps REE in early crystallizing apatite or negates solubility increases by immobilizing alkalis in silicates. Anionic species such as halogens and carbonates are not sufficient for REE mobility. Additional complexing with alkalis is required for substantial REE transport in and around carbonatites as a precursor for economic grade-mineralization.

Thallium Isotopes Reveal Brine Activity During Carbonatite Magmatism

“…ε 205 Tl and REE systematics are coupled in Khanneshin carbonatites. REE incompatibility in solid phases during igneous differentiation can explain why REEs became concentrated in evolved Khanneshin magmas (e.g., Anenburg et al., 2020). As in carbonatite complexes elsewhere (e.g., Thompson et al., 2002; Woolley, 1982), REE enrichment coincided with Fe and Mg enrichment in residual ankerite‐barite magmas compared to the less evolved sövites.…”

“…Khanneshin carbonatites have alkali element (K and Na) concentrations too low to be in equilibrium with immiscible alkaline silicate magmas (e.g., Hamilton et al., 1979), which is a common geochemical feature of carbonatite complexes worldwide (Ol Doinyo Lengai is a notable exception). Loss of K and Na during metasomatism of wall rock (also referred to as fenitization) causes lower‐than‐expected abundances of these elements in carbonatites (Anenburg et al., 2020; Rubie & Gunter, 1983; Weidendorfer et al., 2016). The copious mica‐rich xenoliths in Khanneshin sövite samples attest to metasomatism (e.g., Giebel et al., 2019) that began prior to eruption.…”

“…For this reason, the fractional crystallization of calcite can explain elevated REE and Tl concentrations in residual magmas and fluids, but not the elevated La/Yb ratios. Instead, the observed increase of La/Yb is probably due to the fact that HREEs are preferentially delivered to the wall rock during metasomatism by alkali-rich brines (Anenburg et al, 2020). Such fluids are commonly associated with carbonatite magmatism, but whether they are end products of fractional crystallization (Anenburg et al, 2020), liquid immiscibility (Newton & Manning, 2002, 2010, or both remains controversial.…”

“…Instead, the observed increase of La/Yb is probably due to the fact that HREEs are preferentially delivered to the wall rock during metasomatism by alkali-rich brines (Anenburg et al, 2020). Such fluids are commonly associated with carbonatite magmatism, but whether they are end products of fractional crystallization (Anenburg et al, 2020), liquid immiscibility (Newton & Manning, 2002, 2010, or both remains controversial. In any case, REEs complex with alkali elements in carbonatite-derived brines until interactions with wall rock in fenite zones consume the alkalis, causing REE precipitation in alkali-free phases (Anenburg et al, 2020).…”

“…Hydrous saline fluids-the probable agents of metasomatism-can form due to immiscibility with carbonatitic magmas (Newton & Manning, 2002, 2010 or as the end products of fractional crystallization (Anenburg et al, 2020). Fluid inclusions in carbonatites are evidence of these fluids in subvolcanic settings (Morogan & Lindblom, 1995;Rankin & Le Bas, 1974;Walter et al, 2020).…”