SignificanceOur research presents a method that more accurately determines the quantitative composition of ultrahigh-pressure (UHP) fluid [high H2O (~40 wt.%) and solute (~60 wt.%) contents] released by a slab during deep subduction compared with that detailed in previous studies. The data provide important information for understanding the characteristics of UHP fluids, especially supercritical fluids. Supercritical fluids with high dissolved contents of carbon (2 wt.%) and sulfur (3 wt.%) released during deep subduction could cause efficient migration of carbon and sulfur from slabs. This improves their recycling efficiency in the subduction zone, playing a vital role in the deep cycling of these elements. The contribution of supercritical fluids to the deep carbon and sulfur cycle may have been significantly underestimated previously.