New activity–composition (a–x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland & Powell (, Journal of Metamorphic Geology, 29, 333–383). The a–x relations include a broader consideration of Fe2O3 in minerals, changes to the formalism of several phases and order–disorder in all ferromagnesian minerals where Fe–Mg mixing occurs on multiple sites. The a–x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the first time, the entire set of a–x relations for the common ferromagnesian minerals in metapelitic rocks is parameterized simultaneously, with attention paid to ensuring that they can be used together to calculate phase diagrams of geologically appropriate topology. The a–x relations developed are for use in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system for both subsolidus and suprasolidus conditions. Petrogenetic grids in KFMASH and KFMASHTO are similar in topology to those produced with earlier end‐member datasets and a–x relations, but with some notable differences. In particular, in subsolidus equilibria, the FeO/(FeO + MgO) of garnet is now greater than in coexisting staurolite, bringing a number of key staurolite‐bearing equilibria into better agreement with inferences from field and petrographic observations. Furthermore, the addition of Fe3+ and Ti to a number of silicate phases allows more plausible equilibria to be calculated in relevant systems. Pseudosections calculated with the new a–x relations are also topologically similar to equivalent diagrams using earlier a–x relations, although with many low variance fields shifting in P–T space to somewhat lower pressure conditions.

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An extensive, thick MgO-rich primary crust underlain by highly residual mantle must have formed during the Archaean as a consequence of higher ambient mantle potential temperatures 1. However, the preserved volume of this crust is low suggesting much of it was recycled 2. Further, the tonalite-trondhjemite-granodiorites that dominate exposed Archaean crust cannot have been generated directly from MgO-rich primary crust since a hydrated low-MgO basalt source is required 3. Here we show that the thermodynamically stable mineral assemblages expected at the base of fully hydrated and anhydrous MgO-rich crust 45 km thick make it denser than the complementary underlying residual mantle. We use 2-D geodynamic models to explore the fate of this gravitationally unstable crust. Our results demonstrate that magmatically-overthickened MgO-rich crust, whether fully hydrated or anhydrous, could have delaminated by Rayleigh-Taylor instabilities for mantle potential temperatures > 1500-1550 °C, depending on rheology. The dripping instabilities generate return flow of asthenospheric mantle that melts adiabatically producing additional primary crust. Melting of overthickened and dripping MgO-rich crust and intracrustal fractionation of primary magmas both may produce the hydrated nature geoscience SUPPLEMENTARY INFORMATION

The a-x relations recently presented in White et al. (2014, Journal of Metamorphic Geology, 32, 261-286) are extended to include MnO. This provides a set of internally consistent a-x relations for metapelitic rocks in the MnO-The mixing parameters for the Mn-bearing minerals were estimated using the micro-/ approach of Powell et al. (2014, Journal of Metamorphic Geology, 32, 245-260). Then the Mn-end-member thermodynamic properties were calibrated using a database of co-existing minerals involving literature data from rocks and from experiments on natural materials. Mn-endmembers were calibrated for orthopyroxene, cordierite, staurolite, chloritoid, chlorite, biotite, ilmenite and hematite, assuming known properties for the garnet end-member spessartine. The addition of MnO to phase diagram calculations results in a marked expansion of the stability of garnetbearing assemblages. At greenschist facies conditions garnet stability is extended down temperature. At amphibolite facies conditions, the garnet-in boundary shifts to lower pressure. While the addition of MnO greatly influences the stability of garnet, it has relatively little effect on the stability of other common metapelitic minerals, with the resultant diagrams being topologically very similar to those calculated without MnO. Furthermore, the addition of MnO in the amounts measured in most metapelites has only a small effect on the mode of garnet, with calculated garnet modes remaining smaller than 1% in the P-T range outside its predicted Mn-free P-T range.

In this Letter we omitted to cite a paper 1 that also used recently developed thermodynamic models 2 to predict the melting process in Archaean metabasaltic rocks. Importantly, the average enriched Archaean tholeiite used by ref. 1 as a proposed source rock 3 for tonalite-trondhjemite-granodiorite rocks has a magnesium number (Mg#) of 57, significantly higher than the average value for the CF-2 basalts (with Mg# of 35) 4. This difference has profound implications for the results of these studies. We regret not citing ref. 1 to emphasize the clear distinction between their findings and those of our study. The original Letter has not been corrected.