Rutile stability and rutile/melt HFSE partitioning during partial melting of hydrous basalt: Implications for TTG genesis

Abstract

Synthesis experiments were conducted on a natural basalt (with 2 or 5 wt.% H 2O added) at 1.0–2.5 GPa and 900–1100 °C to investigate the stability field of rutile and rutile/liquid HFSE partitioning during partial melting of hydrous basalt. The basalt chosen has TiO 2 content close to average N-MORB. 100 ppm of Ta, Nb, Hf, Zr, etc., were added to the starting composition in order to improve analytical precision with the LAM-ICP-MS and the electron microprobe. Rutile occurs in the partial melting field of hydrated basalt at pressures higher than approximate 1.5 GPa, depending on H 2O content and bulk composition (especially TiO 2 and K 2O). Its stability increases with increasing pressure and decreasing temperature. H 2O helps produce a more mafic melt and so results in dissolution of rutile and shrinkage of the P– T field of rutile crystallization. The rutile/melt partitioning results confirm previous observations [ Green, T.H., Pearson, N.J., 1986. Ti-rich accessory phase saturation in hydrous mafic–felsic compositions at high P, T. Chem. Geol. 54, 185–201; Jenner, G.A., Foley, S.F., Jackson, S.E., Green, T.H., Fryer, B.J., Longerich, H.P., 1993. Determination of partition coefficients for trace elements in high pressure–temperature experimental run products by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM–ICP–MS). Geochim. Cosmochim. Acta 57, 5099–5103; Foley, S.F., Barth, M.G., and Jenner, G.A., 2000. Rutile/melt partition coefficients for trace elements and assessment of the influence of rutile on the trace element characteristics of subduction zone magmas. Geochim. Cosmochim. Acta 64, 933–938; Schmidt, M.W., Dardon, A., Chazot, G., and Vannucci, R., 2004. The dependence of Nb and Ta rutile–melt partitioning on melt composition and Nb/Ta fractionation during subduction processes. Earth Planet. Sci. Lett. 226, 415–432 ], including that rutile is a dominant carrier for Nb and Ta, and that rutile favours Ta over Nb with D Nb always lower than D Ta for each rutile/melt pair. In addition our experiments demonstrate that both D Nb and D Ta decrease with increasing H 2O content but increase with decreasing temperature. Rutile is a necessary residual phase during the generation of Archean tonalite– trondhjemite–granodiorite (TTG) magmas to account for the negative Nb–Ta anomaly of the magmas. The depth for TTG production via melting of subducted oceanic crust must be more than 45–50 km based on the approximate 1.5 GPa minimum pressure for rutile appearance. Rutile fractionates Nb from Ta and will result in slightly higher Nb/Ta in coexisting liquids. Archean TTG magmas with subchondritic Nb/Ta must, therefore, have been derived from low Nb/Ta source regions [cf. Rapp, R.P., Shimizu, N., Norman, M.D., 2003. Growth of early continental crust by partial melting of eclogite. Nature 425, 605–609] unless alternative magmatic processes have lowered the Nb/Ta ratio. Also rutile-bearing residues should display lower Nb/Ta after TTG liquids are extracted. Hence, the present data do not support the view that subducted rutile-bearing eclogitic oceanic crust is a superchondritic Nb/Ta reservoir on Earth.