Understanding the Rare Earth Element Budget of Low to Medium Grade Hydrothermal Chlorite

This study provides new insights into the distribution of rare earth elements (REE) in a range of chloritic minerals from hydrothermally altered mafic rocks. Low-to-medium grade ocean-floor metamorphic conditions give rise to unique geochemical signatures of analyzed chlorite through the interaction between hydrothermal fluids and surrounding lithologies. This research employs optical microscopy, X-ray diffraction, electron microscopy, and in situ laser ablation inductively coupled plasma mass spectrometry to support the codependency of major and trace element geochemistry on the prevailing hydrothermal conditions. Fe-chlorite is the dominant alteration phase and collectively presents a gradient that is defined by an octahedral Fe-Mg substitution depending on fluid chemistry. Additionally, numerous textural varieties such as amygdales, pseudomorphic, vein, and matrix chlorite have been identified and their phase chemistry largely suggests a common genetic link. The Tschermak substitution vector is used to estimate relative temperatures of formation and the capability to adsorb REE in outer-sphere complexes. Rare earth element chemistry of chlorite reflects the formation conditions such as redox state, ligand complexation, and the potential control on the REE budget of chlorite. Chlorite from non-ophiolite rocks exhibit LREE enrichments determined by the Cl- content of the hydrothermal solution and a negative Ce anomaly as a testament towards oxidizing conditions. Conversely, the REE abundances in chlorite from ophiolite rocks indicate a strong influence by primary mineral dissolution, which depends on the extent of clinopyroxene or plagioclase alteration. Mixed-layer phases tend to harbor greater REEtotal than discrete chlorite and represent a separate hydrothermal fluid regime. In total, this research advocates the use of chloritic minerals as a promising avenue towards understanding REE mobility in seafloor hydrothermal settings.