H AND CU ISOTOPIC VARIABILITY AND ALTERATION IN TURQUOISE

Turquoise (Cu(Al,Fe³⁺)₆(PO₄)₄(OH)₈·4H₂O) is a semi-precious gemstone common in the American Southwest, which has been extensively mined. Hydrogen and Cu isotope ratios in turquoise have the potential to provide insight on the conditions of formation. We have developed a non-destructive, rapid and precise method for measuring H and Cu isotopes in turquoise by secondary ion mass spectrometry (SIMS). Instrumental mass fractionation (IMF) in turquoise was correlated with H and Fe contents for D/H measurements and Fe content for ⁶⁵Cu/⁶³Cu measurements. Using our method, δD and δ⁶⁵Cu can be measured by SIMS with precision that rivals conventional methods ± 5‰ and ± 0.4‰, respectively. Hydrogen and Cu isotopic ratios were measured in ten turquoise samples from various deposits throughout the American Southwest. Samples showed δD values from -120‰ to -54‰ and δ⁶⁵Cu values from -0.1‰ to 13.8‰. To interpret the wide range in H and Cu isotopic composition, the chemistry and mineralogy of turquoise was investigated using Raman, FTIR and XPS. No relation between the Cu isotopic values and the turquoise chemical composition or mineralogy (e.g., the Fe content) was observed. The wide range of positive δ⁶⁵Cu values suggests turquoise formed from supergene enrichment processes, after leaching Cu from copper-rich porphyritic host rocks by meteoric water. The effects of turquoise alteration on the H and Cu isotopic compositions were also investigated. The H isotopic compositions were affected by up to 100‰ whereas the copper isotopic compositions became 2‰ lighter. Therefore, altered samples should be avoided when characterizing turquoise gemstones provenance using H and Cu isotopes analysis.