We studied a unique chrysotile–antigorite serpentinite, drilled on Deep Sea Drilling Project Leg 84 (Site 566) in the Guatemala forearc. Our in situ major and trace element data provide new constraints on possible reactions and associated trace element mobilisation during shallow serpentinite subduction. Chrysotile of the studied serpentinite, formed by the hydration of an upper mantle peridotite precursor, is partially replaced by antigorite (alone) which also occurs in 0.5 mm wide unoriented veins crosscutting the rock. Based on textural relationships and the P–T–X stability of the rock forming phases, the replacement of chrysotile by antigorite occurred at T < 300 °C, due to interaction between the chrysotile–serpentinite and an aqueous fluid. A comparison of the chemical compositions of reactant and product phases reveals that about 90% of the Cl, more than 80% of the B and about 50% of the Sr hosted originally by chrysotile was lost during fluid-assisted chrysotile-to-antigorite transformation and accompanying partial dehydration, and documents the much lower affinity of antigorite for trace element uptake than that of chrysotile. The fluid-assisted chrysotile-to-antigorite transformation and associated trace element loss documented here can occur in the shallow (< 30 km) region of subduction zones. This transformation decreases notably the Cl and B inventory of subducting serpentinites, which are regarded as one of the most important carriers of these elements into subduction zones. The evolution of serpentinites during initial subduction stages thus appears to be critical in the recycling of specific trace elements such as B or Cl from forearc to subarc depths. ► Antigorite replaced chrysotile in a serpentinite from the Guatemalan forearc. ► The mineral replacement was the result of aqueous fluid-rock interaction. ► Partial dehydration and loss of Cl, B and Sr accompanied the reaction. ► Transformation of chrysotile into antigorite may significantly reduce Cl, B and Sr inventories of serpentinites during early subduction. ► Serpentinite evolution during initial subduction stages can be critical in the recycling of B and Cl from forearc to subarc depths.