Abstract
Amphibole compositions record high‐pressure‐temperature ( P‐T ) hydration processes during magmatism and metamorphism in the deep Earth. However, amphibole thermobarometers calibrated for basic–ultrabasic systems are scarce. To address this issue, we compiled Ca‐amphibole data from published equilibrium experiments and used machine‐learning models to identify temperature‐ and pressure‐sensitive compositional parameters. Guided by these machine‐learning‐derived insights and subsequent statistical analyses, we developed empirical amphibole barometers based on Al VI and B‐site Na, coupled with whole‐rock SiO 2 , and internally consistent thermometers based on TiO 2 , Al IV , and A‐site cation sum (Sum‐A) for basic‐ultrabasic rocks. We applied them to amphibole‐bearing metamorphic eclogites and metasomatic mantle xenoliths. For example, application to amphiboles in retrograde eclogites from the Thongmön region of the Himalaya consistently captured well‐defined paths of ultrahigh‐temperature (UHT) metamorphism, corroborating previous estimates derived from thermodynamic phase equilibria modeling and other established thermobarometers (Wang et al., 2021, https://doi.org/10.1016/j.epsl.2021.116760 ). Furthermore, we successfully constrained the P‐T conditions of an igneous basic‐ultrabasic xenolith from the Pripyat rift of the East European Craton and peridotite xenoliths from Avacha volcano, Kamchatka. In both cases, the resulting P‐T conditions are consistent with the inferred igneous and metasomatic processes in these geological settings. Moreover, the P‐T estimates obtained from our amphibole thermobarometers are in strong agreement with those derived from previously established methods for UHT metamorphism and metasomatism, confirming their reliability. Consequently, our calibrated amphibole thermobarometers provide a robust framework for interpreting metamorphic and metasomatic P‐T conditions in hydrated basic–ultrabasic systems, which may also be relevant to planetary bodies beyond Earth. Amphibole is a common water‐bearing mineral in rocks that contain water on Earth, and its chemical composition can record the P‐T conditions under which these rocks formed. However, it has been difficult to accurately estimate these conditions in basic and ultrabasic rocks because few suitable methods are available. In this study, we compiled a large data set from laboratory high P‐T experiments and used machine learning to develop new thermobarometers for estimating pressure and temperature from amphibole compositions. We tested these methods on natural samples, including high‐pressure metamorphic rocks and mantle‐derived xenoliths. The results successfully reconstructed the pressure‐temperature path histories of these rocks and revealed evidence for extremely high‐temperature conditions during rock evolution in some rock samples. Our estimates are consistent with those obtained from established techniques, supporting the reliability of the new approach. These results provide a new tool for understanding how water influences geological processes deep inside the Earth, such as crustal metamorphism and mantle metasomatism. The new thermobarometers can also be useful for studying similar processes on other planetary bodies. New amphibole thermobarometers (Al VI , B‐site Na, TiO 2 , Al IV , Sum A) are specifically developed for basic–ultrabasic systems The amphibole thermobarometers reveal exhumation P‐T paths of UHT eclogites and mantle xenoliths Amphibole thermobarometers robustly trace high‐ P‐T hydration during metamorphic and metasomatic processes