DISCUSSION Comment on ‘‘The beginnings of hydrousmantle wedge melting’’, CB Till, TL Grove, AC Withers, Contributions to Mineralogy and Petrology, DOI 10.1007/s00410-011-0692-6 David H. Green • Anja Rosenthal • Istva ´n Kova ´cs Received: 26 November 2011 / Accepted: 24 February 2012 / Published online: 14 September 2012 Ó Springer-Verlag 2012 The paper by Till et al. (2011) reports new experiments on high pressure melting in the system (lherzolite ? water). In comparison with earlier published work, several appropri- ate methods of phase characterisation and experimental techniques, have not been reported. A result is that the authors’ conclusions are not substantiated for two impor- tant issues: the position in pressure, temperature (P, T) space of the hydrous vapour-saturated solidus and the compositions of near-solidus melts. The application of the experimental results to discussion of melting in the mantle wedge overlying subduction zones is therefore questionable. With respect to the position of the vapour-saturated solidus, Grove et al. (2006) and Till et al. (2011) place the solidus at *820 °C at 3–5 GPa, which is *230 °C (3 GPa) to 500 °C (5 GPa) below the solidus inferred by Green et al. (2010, 2011) for the same HZ1 lherzolite composition, by Niida and Green (1999) on the composi- tionally near-identical MOR pyrolite composition and by Green (1972, 1973, 1976), Millhollen et al. (1974), Mengel and Green (1989), Wallace and Green (1991) and Fuma- galli et al. (2009) on other lherzolite compositions. Grove et al. (2006) and Till et al. (2011) base the position of the solidus on the appearance of glass among the quench products and on a marked change in texture of the exper- imental charge at *810 °C from ‘fine-grained, uniform in porosity and homogeneous in texture’ (inferred subsolidus) to ‘coarser grained, mineralogically zoned, exhibit com- paction of minerals in the bottom—and a gradient in porosity towards the top (hot) end of the charge’ at tem- peratures above 810 °C, 3–5 GPa (inferred above-solidus). To support their interpretation, Till et al. (2011) publish low-magnification scanning electron microscope (SEM) images (Fig. 2 in Till et al. 2011) of their charges and plots of Mg# versus Temperature for minerals (Fig. 3 in Till et al. 2011). In the latter figure and in Electronic Supple- mentary Material of Till et al. (2011) there are no analyses from \ 840 °C (i.e. no ‘subsolidus’ mineral data) at 3.2 GPa, and the higher-temperature data suggest a solidus between 1020 and 1060 °C. The SEM images presented show an absence of poikiloblastic garnet in runs interpreted as ‘subsolidus’ at other pressures. The mineral analyses presented from five ‘subsolidus’ experiments show garnet with 17.3–24.9 wt % Al 2 O 3 and 8–16.6 wt % CaO (Electronic Supplementary Material of Till et al. 2011). Compositions do not meet stoichiometric constraints, and natural lherzolite garnets are restricted to 4–6 wt % CaO. Similarly, orthopyroxene compositions are reported with 0.2–3.9 wt % CaO. Clinopyroxenes are reported with 12.5–20.7 wt % CaO and from below detection to This comment refers to the article available at 10.1007/s00410-011-0692-6. An author’s reply to this comment is available at 10.1007/s00410-012-0803-z. Communicated by J. Hoefs. D. H. Green (&) School of Earth Sciences and Centre for Ore Deposit Studies, University of Tasmania, Hobart, TAS 7001, Australia e-mail: david.h.green@utas.edu.au A. Rosenthal Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia e-mail: anja.rosenthal@anu.edu.au I. Kova ´cs Department of Data Processing, Eo ¨tvo ¨s Lora ´nd Geophysical Institute of Hungary, Columbus U. 17-23, Budapest 1145, Hungary e-mail: kovacs.istvan.janos@mfgi.hu 123 Contrib Mineral Petrol (2012) 164:1077–1081 DOI 10.1007/s00410-012-0802-0