HR: 14:55h
AN: V33C-06    [Abstracts]
TI: Origin of the negative niobium tantalum anomaly in subduction zone magmas
AU: Baier, J
EM: johannes.baier@uni-tuebingen.de
AF: Institut für Geowissenschaften, Wilhelmstrasse 56, Tübingen, 72074 Germany
AU: * Audetat, A
EM: andreas.audetat@uni-bayreuth.de
AF: Institut für Geowissenschaften, Wilhelmstrasse 56, Tübingen, 72074 Germany
AU: * Audetat, A
EM: andreas.audetat@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, 95440 Germany
AU: Keppler, H
EM: hans.keppler@uni-bayreuth.de
AF: Institut für Geowissenschaften, Wilhelmstrasse 56, Tübingen, 72074 Germany
AU: Keppler, H
EM: hans.keppler@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, 95440 Germany
AB: The partitioning behavior of niobium between clinopyroxenes and aqueous fluids was determined experimentally using an indirect approach in which the solubility of Nb in clinopyroxenes and aqueous fluids was measured in two separate runs that both were saturated in CaNb₂O₆. The solubility of Nb in clinopyroxenes was investigated in the system CaMgSi₂O₆ - NaAlSi₂O₆ - Nb₂O₅ - H₂O at 1.5 GPa and 900 - 1100 °C. At these conditions, clinopyroxene coexists with hydrous melt and CaNb₂O₆. The solubility of niobium in the pyroxene increases drastically with its aluminum content. While the solubility of niobium is in the order of 100 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 direct visual observation of dissolving CaNb₂O₆ crystals in an externally heated hydrothermal diamond anvil cell, using the technique described in [1]. 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 lower than that in aluminous clinopyroxenes. Reconnaissance experiments on the solubility of CaTa₂O₆ in clinopyroxenes and in aqueous fluid suggest that Ta behaves generally similar to Nb, with the notable exception that the solubility of CaTa₂O₆ in aqueous fluid is even lower than that of CaNb₂O₆. Our results imply that fluid/clinopyroxene partition coefficients of Nb and Ta are << 1 for pyroxenes of compositions that are typical for the subducted slab or the subarc mantle. Only for very Al-poor clinopyroxenes, the partition coefficient may approach unity. The relative depletion of Nb and Ta in subduction zone magmas is therefore primarily a result of the intrinsically low solubility of Nb and Ta in the fluid and does not require the presence of rutile in the subducted slab. Even if rutile plays a role in controlling the Nb and Ta content of fluids emanating from the slab, a suitable mechanism is required to maintain this geochemical signature until the fluid components reach the site of magma production. Previous studies have underestimated the role of clinopyroxenes in sequestering Nb and Ta because they were conducted in Al-poor systems. [1] Audetat, A. and Keppler, H., 2005, Earth Planet. Sci. Lett. 232, 393-402.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1038 Mantle processes (3621)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005