LI SUPERSATURATION IN PEGMATITE-FORMING MELTS: THE MOBLAN PEGMATITE

The Moblan spodumene-pegmatite in Quebec is dominated by simple aplite and albite-rich wall zones that enclose a central quartz-spodumene-rich core. The core contains approximately 35% modal spodumene, which corresponds to a concentration of approximately 13000 ppm Li in the rock. This extreme Li enrichment coupled with the relatively simple mineralogy appears inconsistent with equilibrium crystallization of a granitic melt.

New experimental results reveal evidence for lithium supersaturation in the pegmatite-forming melts before the crystallization of Li-aluminosilicates. Dissolution experiments at 500 MPa and 550-750 °C demonstrate that the solubility of lithium in a granitic melt at spodumene (LiAlSi₂O₆) and petalite (LiAlSi₄O₁₀) saturation ranges between 500 and 3000 ppm, with the lower values corresponding to lower temperatures. However, crystallization experiments demonstrate that nucleation of Li-aluminosilicates was inhibited for at least 100 h in a granitic melt with ~12000 ppm Li at 600 °C. Crystallization of a Li-aluminosilicate mineral in experimentally produced pegmatites was delayed until the melt reached 500 °C and yielded a concentration of ~6000 ppm Li in the coexisting melt.

Comparison of the experimental results with the spodumene-rich Moblan pegmatite is consistent with extreme Li enrichment of the pegmatite-forming melt prior to emplacement and leads to a plausible mechanism for the genesis of such simply zoned Li-rich pegmatites: a highly evolved and mobile Li-supersaturated melt is generated through disequilibrium fractional crystallization; this melt subsequently escapes through faults and fissures and eventually crystallizes in narrow dikes, where Li-aluminosilicates are developed following the formation of narrow bands with quartz-feldspar intergrowths and albite-rich wall zones.