Evidence for Zn isotopic fractionation at Merapi volcano

Fumarolic gases, rocks and condensate samples from Merapi volcano (Indonesia) have been analyzed by multiple collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) for their Zn isotopic compositions. The variation observed, expressed as δ66Zn (δ66Zn = [(66Zn/64Zn)sample/(66Zn/64Zn)JMC 3-049L − 1] 103), ranges from + 0.05‰ to + 1.68‰. The overall reproducibility was found to be better than 0.10‰ (2SD). The δ66Zn of andesitic rock samples are homogeneous with a mean value of + 0.24‰, which is comparable to previously reported basalt data. δ66Zn in the Woro fumerolic field are ranging from + 0.05‰ to + 0.85‰ and + 1.48‰ to + 1.68‰ in fumarolic gases and condensates respectively. Upon cooling from 590 to 297 °C, gaseous Zn concentrations decrease and become increasingly enriched in the lighter Zn isotopes, while the condensates become increasingly enriched in the heavier isotopes. Gas and condensate isotopic compositions can be explained with a Rayleigh condensation model with temperature dependent isotope fractionation factor, α(solid/vapor). Over the 574 °C to 297 °C range of observations, 1000 ln(αsolid/vapour) = C1 + C2/T + C3/T2 (with C1 = 0, C2 =− 0.88 × 103, C3 = 1.00 × 106 and T in degrees Kelvin), which corresponds to α(solid/vapor) of 1.00033 to 1.00153 respectively at the indicated temperatures.