Melting controls on the lutetium–hafnium evolution of Archaean crust
Introduction
The processes of partial melting, melt segregation, and assimilation, impose a major control on the chemical and rheological evolution of the lithosphere (Brown, 2007, Sawyer et al., 2007, Yakymchuk and Brown, 2014). The lutetium–hafnium (Lu–Hf) isotopic record, typically measured in zircon crystals, is sensitive to these processes, and thus has the potential to offer fundamental insights into crustal evolution (Belousova et al., 2010, Hawkesworth and Kemp, 2006, Kemp et al., 2005, Patchett, 1983, Scherer et al., 2007, Vervoort et al., 1996). Lutetium is generally more compatible than Hf during melting of both mantle and crust. Thus melt, as newly-formed crust, will have lower Lu/Hf relative to its source. These Lu/Hf compositions define evolution arrays along which different crustal (and mantle) reservoirs evolve with time through the ingrowth of radiogenic 176Hf arising from the isotopic decay of 176Lu (Fig. 1).
Evolution arrays have been used for interpreting the processes of reworking of a crustal source as recorded through Hf isotope data; they chart the isotopic evolution of melt products during these magmatic events (Kemp et al., 2006, Kirkland et al., 2013). During a crustal reworking event, the addition of new mantle-derived (juvenile) magma, or incorporation of existing (evolved) crust, may be inferred where the measured Hf isotopic composition of a magmatic rock departs from the evolution array. The evolution array may also be projected back to intercept a Hf evolution model for either the depleted mantle (DM), or that of new continental crust, establishing an Hf model age (TDM2) which may, under ideal circumstances, provide an estimate of the time of extraction of that crustal packet from the mantle (Dhuime et al., 2011, Kemp et al., 2006).
Hf isotope studies use estimates of 176Lu/177Hf to define evolution arrays for the continental crust. However, there are likely variable compositions of crustal melts generated within different settings, and the values commonly used (e.g., 0.012–0.015; Chauvel et al., 2014, Goodge and Vervoort, 2006, Griffin et al., 2002, Rudnick and Gao, 2014) may be inappropriate for modelling the evolution of Archaean continental crust. Such ancient crust is typified by low-K tonalite–trondhjemite–granodiorite (TTG) suites mostly formed from the partial melting of garnet-bearing hydrated mafic source rocks (i.e. garnet amphibolites), which are progressively reworked to generate increasingly K-rich granites (TTG + G suites; Champion and Smithies, 2007). The source of TTG + Gs is therefore distinct to that of modern andesitic crust.
Lutetium and hafnium have different compatibilities in different minerals and so, during partial melting, melt Lu/Hf will be mediated by the presence of certain minerals in the residual source (Vervoort and Patchett, 1996). In particular, the mineral garnet, a phase whose stability is pressure-dependent, and which is a proxy for melting depth, strongly controls the partitioning of Lu. Consequently, the trajectories of consecutive evolution arrays based on source Lu/Hf, arising from successive melt events of a package of crust, are influenced by the nature of these successive melting events. Modelling a single evolution array projecting through successive melt events as linear is simplistic.
In summary, interpretation of Hf isotope datasets and calculation of Hf model ages using evolution arrays is potentially compromised through uncertainties arising from the variability in source Lu/Hf composition, especially with respect to formation of felsic continental crust in the early Earth, and changes in melt Lu/Hf due to different degrees of partial melting mediated by the presence of key minerals in the residue. It is important to quantify these effects, and to establish the role of melting in the development of Hf isotope evolution arrays.
In this contribution, we demonstrate an approach linking phase equilibria modelling, with trace element and isotopic modelling, and how this may be used to explore the sensitivity of melt Lu/Hf to partial melting processes. Phase equilibria modelling is a powerful approach to investigate the processes of partial melting (Johnson et al., 2017, White and Powell, 2002), and here we use new thermodynamic solution models applicable to the melting of mafic rocks (Green et al., 2016). We first model melting of N-MORB. We then focus on the early Earth scenario where different geodynamic styles and magmatic sources may have prevailed, and where the production of felsic continental crust can be more directly tied to the melting of mafic source rocks (Bédard, 2006, Johnson et al., 2014, Martin, 1986, Martin et al., 2014, Nagel et al., 2012, Palin et al., 2016, Rapp and Watson, 1995, Sizova et al., 2015).
During the process of partial melting, Lu/Hf is fractionated between the source rocks and the melt (e.g., the mantle melting event M1 on Fig. 1). Primary (basaltic) crust is ultimately derived from partial melting of the mantle, and will have a lower Lu/Hf than mantle (Sun and McDonough, 1989; Workman and Hart, 2005). This lower Lu/Hf results in the crust evolving along a shallower evolution trajectory than mantle (Fig. 1). Melting of this basaltic crust (the M2 melting event in Fig. 1) will in turn yield melt which may form first generation felsic crust (TTGs in an early Earth scenario). This second melting event will also result in fractionation of Lu/Hf between the basaltic (crustal) source and TTGs. The felsic crust will then evolve along an evolution array defined by its bulk 176Lu/177Hf (Fig. 1). If this crustal package is again melted at a later stage (the M3 melting event in Fig. 1), the crystallization of zircon will reflect the bulk 176Hf/177Hf of its parental magma at this point in time. Thus, the effect of the M2 melting event resulting in different bulk 176Lu/177Hf ratios, will be manifested in differences in the analyzed M3 zircon Hf isotope record.
Section snippets
Approach
We assess the sensitivity of melt Lu/Hf to partial melting of a hydrated mafic source through a combination of phase equilibria, trace element, and isotopic, modelling. Phase equilibria modelling allows calculation of the composition and abundance of phases (stable minerals, melt, and volatile species) as a function of pressure, temperature and bulk rock composition, assuming thermodynamic equilibrium. Trace element modelling uses mineral/melt partition coefficients and the abundances of
Phase control on melt Lu/Hf
The significantly higher compatibility of Lu in garnet compared to Hf means that retention of garnet in the source rocks during anatexis will significantly fractionate melt Lu/Hf (Salters and Hart, 1989, Vervoort and Patchett, 1996). Our modelling reproduces this effect. Fig. 3 also shows the modal proportions of the residual mineral phases (excluding quartz) formed in equilibrium with melt for different degrees of melting along each modelled geothermal gradient. We calculate garnet to be
Conclusions
Variations in melt Lu/Hf produced from the anatexis of hydrated basalts is sensitive to both the degree of partial melt, and to the depth of melting. This variance is controlled to a significant degree by the presence of garnet, an important phase in the lower continental crust, and one whose stability is itself highly sensitive to source XMg. Predicted variations in melt Lu/Hf are mediated by the geological processes of melting and segregation, notably the fertility of the melt as crystallized
Acknowledgements
NJG, TEJ and CLK acknowledges Curtin University for financial support. RHS publishes with permission of the Executive Director, Geological Survey of Western Australia. We thanks Jesse Reimink and two anonymous reviewers for their comments which have greatly enhanced this paper. Randy Parrish is thanked for his editorial handling.
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2021, Earth and Planetary Science LettersCitation Excerpt :As such, when estimating SiO2 by the method described here, the most weight should be placed on Rb/Sr, which can then be used to check whether the 176Lu/177Hf ratio of the precursor chosen for model age determination is appropriate. In Hf isotope studies, this ratio is either inferred from the slope of reworking arrays in εHf space, or assumed, with values of 0.022 commonly used for mafic precursor crust (Amelin et al., 1999; Pietranik et al., 2008), 0.015 used for average continental crust (Griffin et al., 2002), and 0.005–0.009 for Archean TTGs (Blichert-Toft and Albarède, 2008; Gardiner et al., 2018). Because of the ability of triple isotope system models to link the Lu/Hf and SiO2 of the precursor, we can improve on the practice and compare modelled values directly to the average Lu/Hf and SiO2 composition of crustal rocks, using data extracted from a global geochemical compilation (Gard et al., 2019).
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