Abstract Authigenic carbonates and muds from six mud volcanoes in the eastern Mediterranean Sea were recovered during the French/Dutch MEDINAUT cruise utilizing the submersible Nautile in November 1998. The mud volcanoes are active seafloor vents in two areas at the plate boundary between the converging African and Eurasian Plates: the Mediterranean Ridge accretionary prism near Crete (Greece) and the Anaximander Mountains south of Turkey. B contents and δ 11 B signatures were measured with the aim of identifying the diagenetic processes and source depths of the material in the collision zone. B concentrations of the carbonate precipitates cover a range of 8–45 ppm and vary isotopically from +15.6 to +22.9‰ (corresponding to a parent solution of 34.9–42.2‰ at pH 7). Both the B-enrichment and a δ 11 B valve slightly lower than seawater suggest the mud domes originate from a moderately deep fluid source, with local admixture of seawater. B contents and δ 11 B of the mud show distinct differences between the areas: the Mediterranean Ridge mud domes have lower B contents and higher δ 11 B (average 3.9‰) compared to Anaximander Mountains mud volcanoes (δ 11 B average −0.6‰). These B results attest the release of structurally-bound B from clay mineral lattices, probably due to stronger deformation near Turkey. These mudstones, which had previously been affected by deep-seated thrusting beneath the Antalya Complex, may have been liquefied and remobilized in their present setting. By contrast, the mud on the Mediterranean Ridge represents offscraped clay-rich strata that was incorporated into the large accretionary wedge.