Rutile stability and rutile/melt HFSE partitioning during partial melting of hydrous basalt: Implications for TTG genesis
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 P–T 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 P–T 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
References (73)
- et al.
The effects of pressure and temperature on partitioning of Ti, Sr and REE between amphibole, clinopyroxene and basanitic melts
Chem. Geol.
(1994) - et al.
Proton microprobe determined partitioning of Rb, Sr, Ba, Y, Zr, Nb and Ta between experimentally produced amphiboles and silicate melts with variable F content
Chem. Geol.
(1993) - et al.
Partial melting in Archean subduction zones: constraints from experimentally determined trace element partition coefficients between eclogitic minerals and tonalitic melts under upper mantle conditions
Precambrian Res.
(2002) - et al.
Rutile–aqueous fluid partitioning of Nb, Ta, Hf, Zr, U and Th: implications for high field strength element depletions in island-arc basalts
Earth Planet. Sci. Lett.
(1994) - et al.
Experimental determination of trace-element partitioning between pargasite and a synthetic hydrous andesitic melt
Earth Planet. Sci. Lett.
(1995) - et al.
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
(2000) - et al.
Phase equilibria in subducting basaltic crust: implications for H2O release from the slab
Earth Planet. Sci. Lett.
(2003) Significance of Nb/Ta as an indicator of geochemical processes in the crust–mantle system
Chem. Geol.
(1995)- et al.
Pressure effect on Ti- or P-rich accessory mineral saturation in evolved granitic melts with differing K2O/Na2O ratios
Lithos
(2002) - et al.
Ti-rich accessory phase saturation in hydrous mafic–felsic compositions at high PT
Chem. Geol.
(1986)
An experimental study of Nb and Ta partitioning between Ti-rich minerals and silicate liquids at high pressure and temperature
Geochim. Cosmochim. Acta
Origin of early Cretaceous calc-alkaline lamprophyres from the Sulu orogen in eastern China: implications for enrichment processes beneath continental collisional belt
Lithos
The role of sphene as an accessory phase in the high-pressure partial melting of hydrous mafic compositions
Earth Planet. Sci. Lett.
Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust
Earth Planet. Sci. Lett.
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
Role of ‘hidden’ deeply subducted slabs in mantle depletion
Chem. Geol.
A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation
Precambrian Res.
Partitioning of high-field-strength and rare-earth elements between amphibole and quartz-dioritic to tonalitic melts: an experimental study
Chem. Geol.
Experimental constraints on major and trace element partitioning during partial melting of eclogite
Geochim. Cosmochim. Acta
Stability of hydrous phases in subducting oceanic crust
Earth Planet. Sci. Lett.
Adakitic magmas: modern analogues of Archean granitoids
Lithos
Magnetite-melt HFSE partitioning
Chem. Geol.
Trace element evidence from seamounts for recycled oceanic crust in the Eastern Pacific mantle
Earth Planet. Sci. Lett.
The redox state of subduction zones: insights from arc-peridotites
Chem. Geol.
Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites
Precambrian Res.
Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa
Chem. Geol.
Rutile saturation in magmas: implications for Ti–Nb–Ta depletion in island-arc basalts
Earth Planet. Sci. Lett.
Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation
Earth Planet. Sci. Lett.
The dependence of Nb and Ta rutile–melt partitioning on melt composition and Nb/Ta fractionation during subduction processes
Earth Planet. Sci. Lett.
The Archaean tonalite–trondhjemite–granodiorite (TTG) series is not an analogue of Cenozoic adakite
Earth Planet. Sci. Lett.
Mineral–aqueous fluid partitioning of trace elements at 900–1200 °C and 3.0–5.7 GPa: new experimental data for garnet, clinopyroxene, and rutile, and implications for mantle metasomatism
Geochim. Cosmochim. Acta
Nb/Ta Zr/Hf and REEs in the depleted mantle: implications for the differentiation history of the crust–mantle system
Earth Planet. Sci. Lett.
An experimentally-based model for the origin of tonalitic and trondhjemitic melts
Chem. Geol.
Trace element abundances in rutiles from eclogites and associated garnet mica schists
Chem. Geol.
Dehydration melting and water-saturated melting of basaltic and andesitic greenstones and amphibolites at 1, 3, 6.9 kbar
J. Petrol.
Vanadium partitioning and the oxidation state of Archaean komatiite magmas
Nature
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