LAUSR

Abstract To better understand the transport of Mo and W in granitic melts and the formation mechanism of porphyry ore deposits, we have investigated the diffusivities of Mo and W in granitic melts with 0.04–5.1 wt% H2O at 1000–1600 °C and 1 GPa using a diffusion couple approach and a Mo saturation approach with Mo sheet serving as the source. The Mo and W diffusivities obtained from diffusion profiles measured by LA-ICP-MS can be described as: DMo,anhy=10−1.47±0.73exp[−(387±25)/RT],DW,anhy=10−1.28±1.05exp[−(396±35)/RT],DMo,2.7wt%H2O=10−5.37±0.52exp[−(211±18)/RT],DMo,5.1wt%H2O=10−6.87±0.69exp[−(133±20)/RT],$\begin{align}& {{D}_{\text{Mo},\text{anhy}}}={{10}^{-1.47\pm 0.73}}\exp \left[ -{\left( 387\pm 25 \right)}/{\text{R}T}\; \right], \\ & {{D}_{\text{W},\text{anhy}}}={{10}^{-1.28\pm 1.05}}\exp \left[ -{\left( 396\pm 35 \right)}/{\text{R}T}\; \right], \\ & {{D}_{\text{Mo},2.7\text{wt }\text{%}{{\text{H}}_{2}}\text{O}}}={{10}^{-5.37\pm 0.52}}\exp \left[ -{\left( 211\pm 18 \right)}/{\text{R}T}\; \right], \\ & {{D}_{\text{Mo},5.1\text{wt }\text{%}{{\text{H}}_{2}}\text{O}}}={{10}^{-6.87\pm 0.69}}\exp \left[ -{\left( 133\pm 20 \right)}/{\text{R}T}\; \right], \end{align}$ where D is diffusivity in m2/s (with the subscripts denoting water contents and “anhy” representing nominally anhydrous melt), R is the gas constant, T is the temperature in K, and the activation energies in the exponential are in kJ/mol. When the influence of H2O is incorporated, Mo diffusivity in granitic melts with <5.1 wt% H2O can be modeled as: logDMo=−(1.94±1.58)−(0.87±0.36)w−[(19341±2784)−(2312±620)w]/T$log {{D}_{\text{Mo}}}=-\left( 1.94\pm 1.58 \right)-\left( 0.87\pm 0.36 \right)w-{\left[ \left( 19341\pm 2784 \right)-\left( 2312\pm 620 \right)w \right]}/{T}\;$ where w is H2O content in the melt in wt%. The diffusion behavior (low diffusivities, high activation energies, and strong H2O effects) of Mo and W indicates that they exist and diffuse in the melt in the form of hexavalent cations. Their low diffusivities imply that the bulk concentrations of Mo and W in exsolved hydrothermal fluid and those in the melt are probably not in equilibrium. However, because of the large fluid-melt partition coefficients of Mo and W, they can still be enriched in the hydrothermal fluid, although to a lesser extent than equilibrium partitioning would allow. Slow Mo and W diffusion can be a significant rate-limiting step for the formation of porphyry Mo/W deposits.