Mantle-derived 40Ar in mid-ocean ridge hydrothermal fluids: implications for the source of volatiles and mantle degassing rates

40Ar excesses of up to 2.6% relative to atmospheric Ar are present in fluids in hydrothermal sulfides from 13°N, East Pacific Rise. Fluid inclusion 3He/4He are typical of mid-ocean ridge vent fluids implying a mantle origin for the 40Ar. Helium concentrations, calculated from 4He/36Ar, are similar to high temperature vent fluids and cannot be derived by leaching the oceanic crust during hydrothermal seawater convection. It is proposed that volatiles are acquired during fluid flow through a narrow, highly permeable zone close to the crystallising magma chamber. The ratio of mantle-derived 4He to 40Ar in the fluids, denoted (4He/40Ar)mantle, range from 3.4 to 36.4. The data displays a bimodality similar to mid-ocean ridge basalt ratios. The low (4He/40Ar)mantle volatiles are similar to bulk mantle melts and probably represent volatiles exsolved during an early stage of degassing. The high (4He/40Ar)mantle volatiles are derived from an already degassed magma. The mantle 40Ar flux calculated from 40Armantle in the hydrothermal fluids ranges from 1 to 2.4×107 mol a−1. This is at least twice the previous estimates and overlaps the flux calculated from highly vesiculated mid-ocean ridge basalt (1.6–4.2×107 mol a−1). This cannot be derived from the decay of 40K in an upper mantle above 670 km which produces less than 1×107 mol a−1. If the measured 40Ar flux reflects steady-state degassing of the upper mantle, then a significant proportion of the rare gases must be transferred from a deeper reservoir.