Developing New Amphibole Thermobarometers to Constrain High P‐T Hydration Processes of Basic‐Ultrabasic Rocks

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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.

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