The Fluid History of Jadeitites Near the Motagua Fault, Guatemala, as Revealed by Stable Isotope and Elemental Geochemistry

Abstract:

Stable isotopes and elemental geochemistry reveal a complex fluid history for jadeitites exposed both north and south of the Motagua Fault Zone (MFZ) in Guatemala. A companion study (Niespolo et al., 2014) utilized these data to source Mesoamerican jadeitite artifacts, as this region is the only source for such materials. Materials studied include jadeitites that range from 71% to 95% jadeite with minor albite and white mica. Jadeitites north of the MFZ have δ¹⁸O_Jadeite = +9.2±0.6‰ (n=6), δ¹⁸O_Albite = +9.4±0.3 (n=2), δ¹⁸O_WhiteMica = +9.8±0.3 (n=3), and δD_WhiteMica = –52± 4‰ (n=3). Jadeite δ¹⁸O values are 2-5‰ higher than those reported by Johnson and Harlow (1999), suggesting a diverse fluid history north of the MFZ. Water in apparent equilibrium at 400°C with north MFZ jadeitite has δ¹⁸O = +8.5‰ and δD = –25‰. Jadeites from Carrizal Grande south of the MFZ have δ¹⁸O_Jadeite = +6.8±0.6‰ (n=7), δ¹⁸O_WhiteMica = +9.0±1.2 (n=6), and δD_WhiteMica = –42±2‰ (n=5), with δ¹⁸O_H2O = +6.4‰ and δD_H2O = –15‰. In contrast, jadeitites from La Cieba south of the MFZ have δ¹⁸O_Jadeite = +7.5±1.4‰ (n=5), δ¹⁸O_Albite = +9.0, δ¹⁸O_WhiteMica = +9.7±0.1 (n=2), and δD_WhiteMica = –42±0‰ (n=2), with δ¹⁸O_H2O = +7.2‰ and δD_H2O = –14‰. Calculated fluids are consistent with those associated with the alteration of serpentinite in a subduction zone setting (e.g., Harlow and Sorenson, 2005). White mica, albite and jadeite are out of ¹⁸O/¹⁶O equilibrium. This suggests post-jadeite isotopic exchange, possibly during exhumation. North MFZ jadeites are enriched in Rb, Ba, Sc, Pb and Nd, and depleted in Sr, Ti, V, and Cr relative to chondrites. Carrizal Grande jadeites are depleted in Sr, Zr, V, and Cr, and enriched in REE and Rb.