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Fluid-mobile element (FME) systematics in serpentinites are key to unravel the environments of mantle rock hydration, dehydration, and element recycling in subduction zones. Here we compile serpentinite geochemical data and, for the first time, report discriminative FME enrichment trends for mid ocean ridge vs. forearc serpentinisation by applying alkali element-U ratios. Characteristic element fractionations are thereby governed by redox-dependent differential U mobility at mid ocean ridges and in forearcs, and by high Cs input in forearcs due to fluids equilibrated with sediments. Simple modelling reproduces the observed enrichment trends in serpentinites that range over several orders of magnitude. From these systematics, first constraints on potentially discriminative fractionation trends for unconventional fluid tracers such as B, As, and Sb can be deduced. Prominent W enrichments that correlate with FMEs suggest significant W mobility in low-temperature serpentinising environments. Application of the alkali element-U systematics to the subducted metaperidotites of Erro Tobbio (recording initial brucite + antigorite breakdown during subduction) and Almirez (recording final antigorite breakdown) reveal that pre-subduction FME enrichment signatures are retained in progressively subducted hydrous mantle rocks to beyond subarc levels. Associated dehydration veins and fluid inclusions reveal subordinate alkali element-U fractionation trends during dehydration. Subducted hydrous mantle rocks therefore may introduce characteristic element signatures and thus contribute towards mantle geochemical heterogeneities.