Geochemistry (TE, REE, Oxygen) of Zircon from Leucogranites of the Belokurikhinsky Massif, Gorny Altai, as Indicator of Formation Conditions

Abstract

The present contribution deals with the isotopic-geochemical study (by Electron Probe Microanalysis (EPMA) and Method of Secondary Ion Mass Spectroscopy (SIMS) of zircons from Belokurikhinsky massif leucogranites, which resulted in the determination of two morphological varieties of zircons (zonal and porous). Zonal zircons exhibit low content of trace elements, a “magmatic” type of REE distribution spectra, and geochemical indices consistent with those of magmatic zircons. Anomalously high REE (up to 38 800 ppm), Y (up to 50 700 ppm) and U (up to 24 700 ppm) contents, as well as elevated Hf, P, F, Cl, and water contents (up to 3.5 wt %) were found in porous zircons. They display gently flattened REE distribution spectra with no Ce-anomaly and the persistence of negative Eu-anomaly, as well as geochemical parameters that differ from typical zircons of magmatic origin. The obtained geochemical characteristics of porous zircons reflect the process of enrichment of the magmatic melt with rare and rare earth elements, and indicate a high fluid saturation of the melt. The δ18О determined value for porous zircons is in the range of 9.22 to 12.54‰, which significantly exceeds a mantle value and seems to be caused by influence of external fluids interacting with sedimentary rocks surrounding leucogranites. The detailed isotopic-geochemical study of zircon samples suggests that their crystallization from leucogranites of a third (final) phase in the evolution of the Belokurikhinsky massif took place from fluid-saturated melt with elevated incompatible element concentrations.

Th-Rich Zircon from a Pegmatite Vein Hosted in the Wiborg Rapakivi Granite Massif

“…Such a correlation is not typical of zircons enriched in trace elements. It has been shown earlier that Ba and Sr values in anomalous zircon from Belokurikha leucogranites are correlated both for unaltered low-Ba-and low-Sr zircons and for zircons enriched in incompatible elements [30].…”

“…We mainly followed the analytical procedure described in [23,24]. Concentrations of trace elements were calculated from the normalized to 30 Si + secondary ion intensities using calibration curves based on a set of reference glasses. NIST-610 reference glass was used as a daily monitor for trace element analyses.…”

“…The positive correlation of Y, REE and P and their negative correlation with ZrO 2 and SiO 2 , confirmed by both analytical methods (EPMA and SIMS), supports a xenotime scheme of coupled substitution (Y,REE) 3+ + P 5+ → Zr 4+ + Si 4+ . On the other hand, highly inconsistent (by about one order of magnitude) Y, REE (total on average, 67,157 ppm) and P (on average, 6673 ppm) values, shown repeatedly for Y-rich zircons [30], suggest a different (not xenotime) scheme of coupled substitution.…”

Tetrad Effect of Rare Earth Element Fractionation in Zircon from the Pegmatite of the Adui Massif, Middle Urals

“…The composition spots of the investigated zircon correspond to significantly higher Sm N /La N and La values. The larger range of Sm N /La N -La ratios is often displayed by zircon affected by fluids or zircon which crystallized a fluid-saturated residual melt (for examples of such zircons, see [56]).…”

“…Geosciences 2024, 14, x FOR PEER REVIEW 14 of 21 zircon which crystallized a fluid-saturated residual melt (for examples of such zircons, see [56]). Thus, the composition spots of zircon from the Adui pegmatite in the La-SmN/LaN diagram lie above the accepted boundaries.…”