Magma Differentiation, Phase Separation, and Volatiles: Factors Controlling Chemistry of Submarine Hydrothermal Fluids

Abstract Submarine hydrothermal systems are critical for global geochemical cycles. However, hydrothermal fluid chemistry is influenced by multiple overlapping processes, making it difficult to isolate the effects of individual factors. In this study, we applied independent component analysis (ICA) to a global database of hydrothermal fluids to extract the key factors controlling fluid chemistry. The ICA results identified magma differentiation and phase separation as the key controls for major elements, gases, and rare earth elements (REEs). With increasing magmatic differentiation, the CO 2 and F concentrations increase, whereas the La/Yb values and Eu anomalies decrease. Associated mineral compositional changes reduce Ca and H 2 while increasing Mn/Fe and the K, Li, Pb, Sb, Au, and Ag concentrations. During phase separation, volatiles partition into the vapor phase, whereas metals exhibit element‐specific partitioning. This leads to the vapor‐rich fluids being enriched in trivalent middle REEs and reduced Eu anomalies. pH exerts a strong control on Fe, Mn, Co, Cu, Zn, and REE mobility but has a limited influence on REE patterns. Sediment‐hosted systems show elevated CH 4 and NH 3 levels although the sediment interaction appears to minimally affect the major elements and REEs. Acid sulfate fluids, formed by reactions between mixed magmatic fluids and seawater with highly altered rocks at high water–rock ratios, exhibit distinct chemical compositions, such as flat REE patterns. These findings demonstrate the utility of ICA for resolving overlapping geochemical processes in hydrothermal systems. Expanding the hydrothermal fluid database will enhance future efforts to model the hydrothermal contributions to oceanic geochemical budgets.