Trace element partitioning among high-temperature pegmatite fluid and wallrock biotite and amphibole, Seridó Pegmatite Province, northeastern Brazil: Implications on thermochronometry of fluid exsolution and advection

Abstract

Pegmatites are recognized for generating wallrock dispersion halos because of fluid exsolution and advection from the crystallizing melt. Nonetheless, the mechanisms governing trace element partitioning between wallrock minerals and migrating fluids remain insufficiently understood. This study reports trace element partition coefficients among biotite, amphibole, and fluid, determined through metric concentration vs. distance profiling at the Areias pegmatite sill wallrock gneiss, Seridó Pegmatite Province, northeastern Brazil. The investigated gneiss maintained its metamorphic mineral assemblage and fabric, with only minor occurrences of local tourmalinization observed. LA-ICP-MS analyses of biotite proximal to the pegmatite reveal enrichment in Li, F, Mg, Cu, Zn, and Ba, and depletion in Ti, V, Cs, and W. In contrast, amphibole exhibits enrichment in Li, F, Mn, Cu, Rb, Ba, and Pb, and depletion in Mg, Ca, Sc, and Ti. Partition coefficient data indicate that biotite preferentially incorporates Cs (214), Rb (67), Ba (27), Li (22), Ta (2.6), Ti (2.1), W (1.9), F (1.7), Ni (1.5), Zn (1.4), CoNb (1.3), Ga (1.2), Cr (1.1), and V (1.0). Conversely, Be-Sr-Th (0.1), Sc (0.2), BPb (0.3), Cu (0.4), U (0.5), and Mn (0.7) exhibit greater affinity for amphibole. Biotite/fluid partition coefficients derived from previously modeled Areias pegmatite fluid compositions are Zn (4.63), Cs (1.23), Rb (0.98), F (0.95), Li (0.47), and Nb (0.19); amphibole/fluid values are Zn (3.28), F (0.55), Nb (0.16), LiRb (0.02), Cs (0.01), and U (0.003). The obtained fluid/melt partition coefficients are Li (19.9), F (7.95), Zn (4.6), Rb (0.97), Cs (0.61), and Nb (0.35). Compared to compiled trace element data from fluid inclusions and geothermal waters, the Areias pegmatite fluid high Rb/Cs and Zn/Nb ratios, along with biotite and amphibole Ti geothermometry temperatures near 673 °C, indicate an unfractionated magmatic origin. Additionally, low Cs/Li and Rb/Li ratios imply substantial fractionation of Li from the melt into the exsolved fluid. Thermochronometric analysis indicates that fluid advection persists for 20 to 100 years, representing a timescale 2 to 4 orders of magnitude longer than thermal models predict for pegmatite crystallization, thereby suggesting that fluid migration commences early and continues beyond pegmatite solidification.