Granite geochemistry is not diagnostic of the role of water in the source
Introduction
Since the time continental crust has existed, partial melting under variable pressure, temperature and fluid (H2O) conditions has promoted its maturation (Sawyer et al., 2011). Fluid-absent melting has been the paradigm for more than 40 years (Brown, 2013; Thompson and Algor, 1977). However, regional-scale shear zones seem to be ideal loci for the infiltration of external water triggering melting (Carvalho et al., 2016; Sawyer, 2010; Weinberg and Hasalová, 2015a), and multiple and protracted water-fluxed melting events seem to occur in crustal roots of orogenic belts (Rubatto et al., 2009), of magmatic arcs (Collins et al., 2016, 2020) and in continental back-arcs (Wolfram et al., 2019). In particular, in the last years there has been renewed interest in trying to understand the real extent of water-fluxed melting and its role in crustal maturation (Collins et al., 2016; Weinberg and Hasalová, 2015a). This has added fuel to the fire, heating up the ongoing debate (Clemens and Stevens, 2015; Weinberg and Hasalová, 2015b), because water-fluxed melting may deeply impact on thermal structure, fertility and rheology of the orogenic crust (Weinberg and Hasalová, 2015a).
A major, as yet unresolved question is if the geochemical signature of water-fluxed melting is real or not. The results of experiments of Patiño Douce and Harris (1998) led to assume that water-fluxed melting of the metasedimentary crust produces trondhjemites, in contrast to peraluminous granites which, instead, are the products of fluid-absent reactions (e.g., Jiang and Zhu, 2017; Johnston et al., 2015; Nabelek, 2020; Wang et al., 2012; Yang et al., 2019; Zeng et al., 2005). On the other hand, the application of quantitative phase petrology (i.e. thermodynamic modeling or phase equilibria modeling) has raised doubts about the real impact of water-fluxed melting on melt composition, in particular at low-pressure (García-Arias et al., 2015; Schwindinger et al., 2019; Sola et al., 2017). However, the same tool has recently provided different results in the study of Mayne et al., 2020, supporting the inferences of Patiño Douce and Harris (1998). The fact that metasedimentary-derived trondhjemites are less abundant in the geological record compared to anatectic granites is largely used to minimize the role of water-fluxed melting, supporting the view of a dry continental crust (Clemens et al., 2020; Mayne et al., 2020) where water in crustal melts derives solely from the breakdown of hydrous minerals. In terms of trace elements, Harris and Inger (1992) and Inger and Harris (1993) proposed the use of LILE (Large Ion Lithophile Element; Rb, Sr and Ba) contents of anatectic granitoids to discriminate between dehydration and water-fluxed melting. Despite this approach has been largely adopted in last decades (e.g., Ferreira et al., 2020; Gao et al., 2017; Wang et al., 2012; Zeng et al., 2005), recent studies have questioned its reliability (Aikman et al., 2012; Schwindinger et al., 2019).
Here, I address these issues with a new approach that combines the three tools allowing the investigation of crustal melts in their source region (melting experiments, quantitative phase petrology and nanogranitoids). This study has three main objectives: (1) to evaluate the reliability of some experimental data; (2) to model the possible effect of pressure and bulk composition on the formation of melt compositions; and (3) to interrogate the current database of nanogranitoids (crystallized melt inclusions from deep crustal rocks) which provide us with the pristine composition of natural crustal melts produced under variable H2O conditions (Bartoli et al., 2016; Carvalho et al., 2019; Cesare et al., 2015). These objectives are being synthesized in this manuscript to address the larger question on the use and misuse of geochemical records to decipher fluid regime during crustal melting. For the comparative study between experiments and phase equilibria modeling, I focus specifically on the benchmark work of Patiño Douce and Harris (1998), having led to a link in the literature between fluid regime and melt chemistry and representing, therefore, a landmark study for many crustal petrologists (see above). It will be shown that water-fluxed crustal melting does not produce anatectic melts characterized by a specific geochemical signature and that the nature of the fluid regime during melting processes cannot be recovered considering the geochemistry of granitoid rocks. In this paper I use water-fluxed melting for anatectic systems characterized by the ingress of external water, opposed to water-present melting specifically used for melting occurring in the proximity of the wet solidus, where only a internally-derived, free aqueous fluid phase is present. Water refers to a supercritical H2O phase.
Section snippets
Phase equilibria modeling
In this study the more recent re-parameterized activity–composition () models for some minerals and melt presented by White et al. (2014a, 2014b) and the dataset of Holland and Powell (2011) were utilized. Phase equilibria modeling was performed in the ten-component MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 (MnNCKFMASHT) system, using Perple_X software (Connolly, 2009). Ferric iron was not considered because i) it was not quantified in the selected bulk rocks, and ii) magnetite and
Melting experiments vs. equilibrium thermodynamics
Fig. 1a-e show the calculated phase diagram sections obtained for the bulk compositions used by Patiño Douce and Harris (1998). Calculated phase relations are comparable with those of typical metapelites undergoing partial melting. The upper amphibolite-facies (≤750–775 °C) assemblages are generally characterized by the presence of one or two micas along with quartz, garnet, aluminosilicates, plagioclase and a Ti-bearing oxide. Under granulite-facies conditions, mineral assemblages commonly
The role of metastable muscovite
For the aim of this study, the most important compositional discrepancy to take into account is that shown by K2O (Fig. 2), which in turn affects the normative albite/orthoclase (Ab/Or) of melts (Fig. 3). Although electron microprobe analyses of hydrous felsic glasses may be affected by alkali migration (Morgan and London, 1996), this cannot be a significant cause of the observed discrepancy of K2O content (Fig. 2), because analytical strategies were adopted to minimize its effect (Patiño Douce
The role of pressure and bulk composition
The comparative study reported above demonstrates that equilibrium thermodynamic calculations cannot properly reproduce the trondhjemitic composition of experimental melts of Patiño Douce and Harris (1998), and that the presence or absence of muscovite seems to be the controlling factor in whether the melts are trondhjemitic or granitic. Models and experiments agree about the formation of trondhjemites solely at 700 °C and 10 kbar, (Fig. 3), when muscovite is present in both experiment and
Model predictions: why are they so different?
The results presented in this study are consistent with those of García-Arias et al. (2015), Sola et al. (2017) and Schwindinger et al. (2019), but differ significantly from those of Mayne et al. (2020). The latter would suggest that the fluid state of the system has a stronger control on melt composition with respect to the path and that melt compositions calculated under H2O in excess conditions have only few counterparts in the natural rock record.
However, very important differences do
Where does nature lie in all this?
Because experiments and models are a significant simplification of nature (Bartoli and Carvalho, 2021; White et al., 2011; Section 4), the constraints they provide need to be carefully tested against natural occurrences. Here the nanogranitoid database is interrogated for the first time to evaluate the potential impact of water-fluxed melting of fertile lithologies on melt composition. Nanogranitoid inclusions represent former droplets of natural anatectic melt trapped in peritectic minerals of
Implications
The results of this study question the common assumption that water-fluxed melting of the metasedimentary crust produces only trondhjemites. Rather, high-pressure conditions and protolith composition play an important role in the formation of these rocks and water-fluxed melting may produce peraluminous K2O-rich granitoids too (see also Sola et al., 2017 and Schwindinger et al., 2019). Similarly, it is not unambiguously possible to distinguish between fluid-present and fluid-absent melting from
CRediT authorship contribution statement
Omar Bartoli: conceptualization, methodology, visualization, writing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was funded by University of Padua (grant BART_SID19_01 to O. Bartoli). I thank B. Cesare, S. Poli, G. Stevens, R.W. White and R.F. Weinberg for critically reading an earlier version. The manuscript benefitted from reviews by W.J. Collins and an anonymous reviewer.
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2022, Chemical GeologyCitation Excerpt :Rubidium, Sr, and Ba concentrations, oxygen isotopes, and liquid temperatures of granitic magma have been widely used to infer water-fluxed melting (Weinberg and Hasalová, 2015 and references therein; Volante et al., 2020). However, Sola et al. (2017) suggested that the main change caused by the presence of aqueous fluids during anatexis is an increase in melt fraction, with minor effects on melt elemental composition (Schwindinger et al., 2019; Bartoli, 2021). Because oxygen is a major element, oxygen isotopes are not sensitive to the addition of external fluids into the source regions (Weinberg and Hasalová, 2015), whereas Ba isotopes are.