Hydrothermal alteration of volcaniclastic sediments: experimental insights into smectite authigenesis and its subsequent chloritization

Volcaniclastic sediments are a significant component of the sedimentary record and are increasingly recognized as important targets for hydrocarbon exploration and energy transition. This study investigates the hydrothermal alteration of modern volcaniclastic deposits from the Opak Estuary (Java, Indonesia) under controlled laboratory conditions. Sediments were reacted with natural Red Sea water at 80, 150, 200 and 250 °C for 28 days, and the reaction products were characterized using petrography, SEM–EDS, X-ray diffraction (XRD), wavelength-dispersive X-ray fluorescence (WD-XRF), and total reflection X-ray fluorescence (TXRF). The starting material is dominated by plagioclase and pyroxene and contains no detectable clay minerals. At 80 °C, alteration is limited to incipient dissolution of primary minerals without detectable clay authigenesis. At 150 °C, smectite nucleates as grain coatings and pore-lining aggregates following dissolution of reactive volcaniclastic minerals. With increasing temperature, the clay assemblage evolves through mixed-layer chlorite–smectite (C/S) at 200 °C toward chlorite-dominated assemblages at 250 °C, reflecting progressive chloritization driven by the availability of Fe and Mg released from dissolving mafic minerals. Fluid chemistry evolves systematically, with progressive acidification, increasing total dissolved solids, enrichment in dissolved Ca and Fe, and non-linear Mg behavior that quantitatively reflects the incorporation of seawater-derived Mg into authigenic clay minerals. Mass-balance analysis demonstrates that approximately 20% of the original solid mass was lost during alteration and provides a quantitative link between mineral dissolution, fluid evolution, and secondary clay formation. The experiments define a systematic reaction pathway from dissolution-dominated alteration to progressive chloritization under the present experimental conditions. Because the experiments were conducted over 28 days in a closed system, the reported temperatures should be regarded as experiment-specific reaction milestones rather than universal diagenetic thresholds. The principal outcome of this study is an experimentally constrained framework describing the sequence and mechanisms of clay authigenesis during hydrothermal alteration of volcaniclastic sediments, providing a basis for interpreting similar mineral assemblages in volcanic sedimentary systems.