The origin of the negative niobium tantalum anomaly in subduction zone magmas

In order to determine the distribution of niobium and tantalum between clinopyroxenes and aqueous fluids, we measured in two separate sets of experiments niobium and tantalum solubility in clinopyroxenes and in aqueous fluids, respectively. The solubility of niobium in clinopyroxenes was experimentally investigated in the system CaMgSi2O6–NaAlSi2O6–Nb2O5–H2O at 1.5 GPa and 700–1100 °C using piston-cylinder experiments. In the presence of excess Nb2O5, CaNb2O6 coexists with a clinopyroxene. The solubility of niobium in the pyroxene increases drastically with the aluminum content. While the solubility of niobium is in the order of 100–300 ppm by weight for Al-poor clinopyroxenes, it reaches 4 wt.% for clinopyroxenes containing 10 wt.% Al2O3. Microprobe analyses suggest that Nb is incorporated in the clinopyroxene as NaNbAl2O6 component. The solubility of CaNb2O6 in aqueous fluid was determined by the direct visual observation of the dissolution of CaNb2O6 crystals in an aqueous fluid using an externally-heated diamond anvil cell. At 1.5 to 1.7 GPa and 800–1000 °C, an aqueous fluid saturated with diopside dissolves only 20–100 ppm by weight of CaNb2O6, i.e. niobium solubility in the fluid is orders of magnitude below the solubility in aluminous clinopyroxenes. Experiments on the solubility of CaTa2O6 in clinopyroxenes and in aqueous fluid suggest that Ta behaves generally similar to Nb, but with the notable exception that the solubility of CaTa2O6 in aqueous fluids and aluminous pyroxenes is approximately a factor of five lower than the solubility of the corresponding niobium compound. Our results imply that (1) the fluid/clinopyroxene partition coefficient for Nb and Ta is between 0.1 and 0.001 for aqueous fluids containing little dissolved silicates and for clinopyroxenes of a composition realistic for the subducted slab or the subarc mantle. Only for very Al-poor clinopyroxenes may the partition coefficient approach unity. (2) The depletion of Nb and Ta in subduction zone magmas and fluids is related to the intrinsically low solubility of Nb and Ta in water-rich and solute-poor fluids in the shallow parts of a subduction zone. (3) The presence of rutile in the subducted slab is not a necessary requirement for the development of the negative Nb and Ta anomaly. (4) Fluid transport of niobium and tantalum will affect the Nb/Ta ratio in the subarc mantle only under exceptional circumstances. (5) Previous reports of high fluid/clinopyroxene partition coefficients for Nb and Ta are probably related to the Al-free or Al-poor composition of the systems studied.