Caveats and challenges in geospeedometry: A reply to Faak et al.'s critique of the Mg-REE coupled geospeedometry

The commentary note from Faak et al. questioned the new Mg-REE coupled geospeedometry and its application to Hess Deep gabbros in the recent paper of Sun and Lissenberg (2018; denoted as S&L). Characterizing S&L's approach as “flawed”, Faak et al. mistakenly asserted that S&L's results regarding rapid hydrothermal cooling of the fast-spreading lower oceanic crust were “incorrect”. Instead, they claimed that the Mg-in-plagioclase geospeedometry used in Faak et al. (2015) showed “good precision as well as accuracy”, which produced slower cooling rates consistent with conductive cooling. We strongly disagree with Faak et al.'s claims but appreciate the opportunity here allowing us to further elaborate S&L's approach in greater detail. In this reply, we show that the erroneous claims of Faak et al. are derived from their misunderstandings on geospeedometry regarding the calibration of exchange thermometers and bulk-diffusion modeling for multi-mineral systems.