The data include:- sample description file: sample full names, site, hole, depth etc., a quick petrographic description of the sample and the embedding serpentinite mud, and information regarding sample selection for bulk rock chemistry - microprobe analyses of selected samples - bulk rock major and trace elements from 2 labs (SARM, Nancy and VUB, Brussels) - Oxygen Isotopes of both pore fluids and rock samples with estimated temperature equilibrium

Porewaters from IODP Expedition 366 were extracted from serpentinite mud volcano sediments onboard the RV JOIDES Resolution (see Fryer et al, 2017; 2018 for details). Selected samples were then analysed at the University of Southampton for 87Sr/86Sr and boron isotopes and SUERC for stable hydrogen and oxygen isotope data. The strontium isotope data has recently been accepted for publication (Albers et al., 2019 (In Press) Fluid–rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions, Solid Earth special issue "Exploring new frontiers in fluids processes in subduction zones").