The Lusi mud eruption in East Java continues unabated after more than a decade of activity as a pulsating geyser system bursting mud, clasts, water, and gas. The origin of the fluids and their properties at depth are not well constrained, thus hampering our understanding and modelling of the system. Previous geochemical and geophysical studies show that Lusi is a sediment-hosted hydrothermal system with two sources of mud at depths between 0.3-2 km and another between 3.5-4.75 km. Here, we present results of mineral thermometry using Raman spectroscopy and chlorite composition on clasts erupted at Lusi. The results show that some of these specimens originate from the two main fluid sources, and at depths consistent with the geochemical and geophysical interpretation. Two main clast lithotypes erupt at Lusi. The first lithotype is a light grey shale returned to the surface from the Upper Kalibeng Formation (the primary mud source) and equilibrated at temperatures below 200 °C. The second lithotype is a black shale, whose source is the Ngimbang Formation at >3800 m depth. Two distinct temperature clusters are recorded in these black shales: one at 179 ± 17 °C, consistent with the temperature estimate at 3800 m depth with the pre-eruption geothermal gradient, while younger minerals in the same specimen recorded substantial anomalous heating at ∼250 °C. These high temperatures indicate interaction with a heat source associated with the eruption through mixing with hydrothermal fluids. This temperature jump suggests rapid heating at ∼4000 m depth, consistent with a scenario of a magmatic intrusion and hydrothermal fluids circulation. This rapid and significant temperature increase would initiate devolatilisation reactions of hydrous minerals within the deep sedimentary package and generate substantial (highly overpressured) fluids at depths significantly below the main source of erupted mud. We infer that reactive and thermal pressurisation led to a deep and metastable system susceptible to e.g. seismic activity ultimately opening fluid pathways towards the surface along the Watukosek fault system. The two temperatures clusters recorded in the clasts show that conductive heat flow was the dominant heat transport prior to the eruption, while the system is now driven by large-scale convection and advective heat transport. The Lusi eruption occurred in a highly populated area forcing the evacuation of several tens of thousands of people. This system appears unstoppable and poses a real and present hazard for the settlements still surrounding the eruption site. Continuous research is necessary to understand this spectacular phenomenon, both for its important scientific relevance, and the societal impact of this natural disaster.