Low-δ18O silicic magmas (δ18O < 5.3±0.6 ‰) is very rare on the planet, as they require significant incorporation of rocks that were hydrothermally altered by meteoric water at high water/rock ratios in the shallow, permeable, and relatively cold upper crust (< 400 °C). However, what is the origin of the high-δ18O silicic magmas (δ18O ＞10 ‰)？
According Troch et al. (2020) (Low-δ18O silicic magmas on Earth: A review. Earth-Science Reviews) "Low-δ18O silicic magmas are generated by partial melting and assimilation of rocks that previously experienced hydrothermal alteration at high temperatures (> 300 °C) by isotopically light meteoric water, prevalent at high latitudes and altitudes. This process can occur in any magmatic-tectonic setting".
For the origin of high-δ18O silicic magmas there are several possible sources. One of them would be related to the partial melting of a sedimentary (metasedimentary) source (e.g., Seitz et al., 2018) or carbonate rocks. Also, the partial melting of an altered oceanic crust (e.g., Carmody et al., 2013) or the assimilation of rocks that previously experienced hydrothermal alteration at low temperatures (< 300 °C) (Valley et la., 2005) could produce high-δ18O silicic.
Most high-d18O silicic magmas seem to be related to assimilation or partial melting of sedimentary or metasedimentary rocks, particularly in thick supracrustal continental crust. For example, S-type granites commonly have d18O values >10 permil. Assimilation of carbonates or low-temperature hydrothermally altered rocks would also be a possibility, but is probably less significant in terms of producing larger volumes of high-d18O magma
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