Elsevier

Chemical Geology

Volume 218, Issues 3–4, 25 May 2005, Pages 339-359
Chemical Geology

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

https://doi.org/10.1016/j.chemgeo.2005.01.014Get rights and content

Abstract

Synthesis experiments were conducted on a natural basalt (with 2 or 5 wt.% H2O 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 TiO2 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 H2O content and bulk composition (especially TiO2 and K2O). Its stability increases with increasing pressure and decreasing temperature. H2O helps produce a more mafic melt and so results in dissolution of rutile and shrinkage of the PT field of rutile crystallization.

The rutile/melt partitioning results confirm previous observations [Green and Pearson, 1987, Jenner et al., 1993, Foley et al., 2000, Schmidt et al., 2004], including that rutile is a dominant carrier for Nb and Ta, and that rutile favours Ta over Nb with DNb always lower than DTa for each rutile/melt pair. In addition our experiments demonstrate that both DNb and DTa decrease with increasing H2O 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.

Introduction

Tonalites, trondhjemites, and granodiorites (TTG), arguably the analogue of modern adakites (Martin, 1999), are widespread in the Archean continental crust and are considered by many authors to be products of partial melting of subducted oceanic crust (e.g., Martin, 1986, Drummond and Defant, 1990, Defant and Drummond, 1990, Foley et al., 2002, Barth et al., 2002, Rapp et al., 2003), although Smithies (2000) thought them to be derived from thickened, mafic lower continental crust. These oceanic crust or mafic continental crust-derived melts exhibit enrichment in incompatible elements, strong heavy REE depletion, and negative Nb–Ta and Ti anomalies (Fig. 1). Most previous studies (e.g., Martin, 1986, Martin, 1999, Drummond et al., 1996, Zhang et al., 2001, Rapp et al., 2003, Wang et al., 2003) emphasized the heavy REE depletion characteristics and thus concluded that garnet is a necessary residual phase during the generation of TTG magmas. Previous experiments have also shown that TTG liquids in equilibrium with garnet were produced at pressures of 1 GPa and above (e.g., Sen and Dunn, 1994, Wolf and Wyllie, 1994, Rapp and Watson, 1995, Winther, 1996). Thus it was usually accepted that the depth of melting of basaltic composition would be 33 km and more (e.g., Rapp and Watson, 1995, Xu et al., 2002, Xiong et al., 2001, Xiong et al., 2003), based on the minimum pressure for garnet stability field.

Rutile is a common minor phase in high-grade metamorphic rocks, especially in eclogites (Zack et al., 2002). It has attracted considerable attention as a likely controller of Nb and Ta budgets and Nb/Ta fractionation in subduction zone processes (e.g., Green, 1995, Stalder et al., 1998, Rudnick et al., 2000, Foley et al., 2000, Klemme et al., 2002). The negative Nb–Ta and Ti anomalies are characteristic features in TTG and always appear coupled with heavy REE depletion. Thus rutile stability during the partial melting of subducted oceanic crust and rutile/TTG melt trace element partitioning behavior are of key importance for understanding the genesis of TTG magmas.

Experimental solubility measurements of rutile (Green and Pearson, 1986, Ryerson and Watson, 1987) showed that rutile saturation in mafic–felsic melts at a given pressure mainly depends on temperature and melt composition (SiO2 content). Recent experiments on a dry, Fe-free synthetic basalt (Klemme et al., 2002) demonstrated that rutile stability is a function of both protolith TiO2 content and temperature (or degree of partial melting), and is also influenced by the Ti content of coexisting minerals. The PT stability filed of rutile in the partial melting field of basaltic composition and the effects of pressure and H2O are still unclear. Other experimental studies (Green and Pearson, 1987, Jenner et al., 1993, Foley et al., 2000, Schmidt et al., 2004) documented rutile/melt trace element partitioning, but did not specifically investigate rutile stability, nor Nb and Ta paired rutile/melt partitioning over a range of conditions. Thus, the existing studies are not sufficient to demonstrate rutile stability and the role of rutile in Nb/Ta fractionation during partial melting of subducted oceanic crust or mafic lower continental crust.

In this paper, we report experimental results on a natural basalt that possesses TiO2 content close to average MORB. Our experiments, in conjunction with previously published data, are used to constrain the PT stability field of rutile during partial melting of hydrated basalt and to reassess the depth of generation for TTG magmas. We also report HFSE (Nb, Ta, Zr, Hf, and V) partition coefficients (D-values) between rutile and coexisting melts under our experimental conditions to complement existing rutile/melt partitioning data. Our data allow a better understanding of the genesis and formation conditions of TTG magmas. The simultaneously measured D-values for Nb, Ta, Zr, and Hf also allow a better assessment of the role of rutile in fractionation of Nb from Ta and Zr from Hf during partial melting and magmatic differentiation.

Section snippets

Experimental and analytical methods

All the experiments and analyses were conducted in the GEMOC National Key Centre at Macquarie University. We have conducted experiments on a natural basalt at 1.0–2.5 GPa and 900–1100 °C designed to produce minerals and melts representative of the partial melting of basaltic composition under conditions existing in subduction zones and the lower continental crust. The basalt (Table 3), a potassic basalt from Chinese Tianshan, is similar in TiO2, FeO, and MgO contents to average N-MORB (1.62% TiO

Results

Experimental conditions and products are given in Table 1. Phases present in the products include quenched glass, amphibole, garnet and clinopyroxene, and accessory rutile, Ti-magnetite (haemo-ilmenite in one case), apatite, and titanite. Plagioclase, orthopyroxene, and olivine were also observed in the 1.0 GPa runs. Further experimental details and results for trace element partitioning between silicate minerals (garnet and amphibole) and melts from these experiments will be given elsewhere;

Equilibrium and Henry's law considerations

Beard and Lofgren (1991) reversed vapor absent partial melting experiments on amphibolites. Their results demonstrated that isothermal runs of 96 h duration were sufficient to produce a reasonable approach to equilibrium in melts and mineral assemblages at temperatures as low as 900 °C. The duration of our runs used to determine phase relationships ranged from 123 h at 925 °C to 43 h at 1075 °C (Table 1) and all the experiments were of the synthesis type with glass as starting material and 2%

Factors controlling the stability of rutile

During partial melting of basalt, Ti-rich accessory phases such as rutile, sphene (titanite), ilmenite, and Ti-magnetite are possible residual phases, in addition to major minerals. The stability and modal abundance of these accessory phases are controlled by many factors, including bulk composition, competition for TiO2 by major phases, TiO2 solubility in melt, pressure, temperature, and H2O, etc. In general, rutile is present at relatively high pressures, whereas other Ti-rich accessory

Concluding remarks

The stability field of rutile and rutile/TTG melt HFSE D-values were experimentally determined on a lightly doped (Ta, Nb, Hf, Zr, etc.) natural basalt with 5 wt.% or 2 wt.% added H2O at 1.0–2.5 GPa and 900–1100 °C. The results can be applied to melting and fractional crystallization processes at depths corresponding to the upper mantle and lower crust, and are especially relevant to hydrous conditions in subduction zones at relatively low temperatures and pressures (the initial stages of

Acknowledgements

This work has been supported by grants from the Macquarie University and the Chinese Academy of Sciences (KZCX3-SW122, KZCX2-SW117, KZCX2-102, and GIGCX-04-03) and the National Nature Science Foundation of China (40172029-40373035). We thank Dr. N. Pearson and Ms. C. Lawson for help with the electron microprobe, and Ms. S. Elhou for help with the LAM-ICP-MS at Macquarie University. X.L. Xiong wishes to thank the GEMOC National Key Centre for research hospitality. The reviews from Dr. T. Zack

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