This dataset contains measured isotopic compositions of noble gases (helium and neon) of water samples collected during the METEOR expedition (M176/2) between Sep 1st and Oct 6th, 2021 on the Rainbow hydrothermal field, Mid-Atlantic Ridge. Seawater samples for noble gases were collected in a copper pipe, which was connected to the Niskin bottles, with water flowing until bubbles in the tube were removed. While the water was flowing, the pipe was closed using an electrical drill and a ratchet to ensure it is tight enough. Helium and neon isotopes were analyzed at the University of Bremen, using the methods described by Sültenfuß et al. (2009). In detail, helium and neon were separated from other gases with a cryogenic system at 25 K. A fraction (3 %) of separated gases was diverted into the quadrupole mass spectrometer (QMS) (Balzers QMG112a®) to measure helium and neon. Upon raising the trap temperature to 45 K, helium and neon were released into the main sector field mass spectrometer (SMS) to measure the concentrations of 3He, 4He, and 20Ne. The measured concentrations were in a unit of CCSTP (cubic centimeters at standard temperature and pressure). delta3He (δ3He, in ‰) was calculated as: [3He/4He(sample)/3He/4He(air)-1]100, and delta22Ne (δ22Ne, in ‰) was calculated as: [22Ne/20Ne(sample)/22Ne/20Ne(air)-1]100. The background 3He (2.38 fM) in the deep ocean was calculated from the background δ3He (~ -1.7%) and equilibrium helium concentrations (~ 1.75 nM) at the salinity (35 PSU) and temperature (~ 3.5 °C) ranges of the seawater samples. The excess of 3He (3Hexs) was then calculated by extracting the measured 3He concentrations with the background 3He level.
