Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.v33c..05g&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #V33C-05
Physics
1031 Subduction Zone Processes (3060, 3613, 8170, 8413), 3621 Mantle Processes (1038), 3630 Experimental Mineralogy And Petrology
Scientific paper
A compositional feature that distinguishes subduction-related lavas from oceanic basalts is depletion of the high field strength elements (HFSE), such as Ti, Zr, Nb, and Ta [1,2]. Similar depletions also characterize many continental basalts [3,4], and have been inferred for the bulk continental crust [5,6]. Because the HFSE are compatible in rutile (TiO2), it has been posited that their depletion in island arc basalts (IAB) is due to its presence as a residual phase, either in the subducted oceanic crust or the mantle wedge. Here I present results from new experiments that investigate the influences of pressure, temperature, and composition on the partitioning of Zr4+, Nb5+, Hf4+, and Ta5+ between rutile and silicate melt. These results demonstrate that low-degree partial melting of rutile-bearing subducted oceanic crust would produce significant, identifiable fractionations among the HFSE, providing a test for the nature of the subduction component. Experiments were carried out on 2 SiO2-Al2O3-MgO-CaO-Na2O-K2O base melt compositions (rhyodacite; basalt). Rutile saturation was achieved by adding 10-40 wt% TiO2. Each starting composition was doped with ZrO2, Nb2O5, HfO2, and Ta2O5. Low-pressure experiments were carried out using sealed Pt capsules in a vertical quenching furnace. High pressure experiments were carried out in graphite capsules using a solid-medium piston-cylinder device. The major element composition of glass and rutile, as well as the trace element content of the rutile, were determined by electron microprobe. The trace element content of the glass was determined SIMS. At 1 bar and temperatures of 1250 ° to 1450 °C the concentration of TiO2 at rutile saturation is significantly higher in the basalt (17-38 wt%) than the rhyodacite (5-12 wt%). Rutile-melt partition coefficients for the HFSE are higher for the rhyodacite than for the basalt by a factor of ~2-5. Partition coefficients for Nb5+ and Ta5+ are larger than those for Zr4+ and Hf4+ in all experiments. All partition coefficients decrease strongly with increasing temperature, while those for Nb5+ and Ta5+ have a strong, positive pressure dependence. Therefore, if the subduction component is a small-degree melt of the subducted oceanic crust, IAB will show an increase in the fractionation of Nb5+ and Ta5+ from Zr4+ and Hf4+ with increased depth to the subducted slab. References: [1] Pearce J. A. and Cann J. R. (1973) Earth Planet Sci Lett 19, 290-300. [2] Perfit M. R. et al. (1980) Chem Geol 30, 227-256. [3] Thompson R. N. et al. (1983) In Continental basalts and mantle xenoliths, 158-185. [4] Fitton J. G. (1995) Earth Planet Sci Lett 136, 715-721. [5] Taylor S. R. and McLennan S. M. (1985) The Continental Crust: Its Composition and Evolution. [6] Rudnick R. L. and Fountain D. M. (1995) Rev Geophys 33, 267-309.
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