Sulfur isotope budget (32S, 33S, 34S and 36S) in Pacific–Antarctic ridge basalts: A record of mantle source heterogeneity and hydrothermal sulfide assimilation

Abstract

To better address how Mid-Ocean Ridge Basalt (MORB) sulfur isotope composition can be modified by assimilation and/or by immiscible sulfide fractionation, we report sulfur (S), chlorine (Cl) and copper (Cu) abundances together with multiple sulfur isotope composition for 38 fresh basaltic glasses collected on the Pacific–Antarctic ridge. All the studied glasses – with the exception of 8 off-axis samples – exhibit relatively high Cl/K, as the result of pervasive Cl-rich fluid assimilation. This sample set hence offers an opportunity to document both the upper mantle S isotope composition and the effect of hydrothermal fluids assimilation on the S isotope composition of erupted basalts along segments that are devoid of plume influence.Δ33S and Δ36S yield homogenous values within error of Canyon Diablo Troilite (CDT), whereas δ34S are variable, ranging between −1.57±0.11‰ and +0.60±0.10‰ with a mean value of −0.64±0.40‰ (1σ, versus V-CDT). The geographic distribution of δ34S follows a spike-like pattern, with local 34S-enrichments by up to +1.30‰ compared to a low-δ34S baseline. As hydrothermal massive sulfides are characterized by relative 34S-enrichments, such first-order variability can be accounted for by hydrothermal sulfide assimilation, a process that would occur for a subset of samples (n=10). Excluding these particular samples, the mean δ34S is significantly less variable, averaging at −0.89±0.11‰ (1σ, n=28), a value that we suggest to be representative of the average MORB source value for Pacific–Antarctic basalts. Weak trends between δ34S and 206Pb/204Pb are displayed by such uncontaminated samples suggesting the recycled oceanic crust to have a modest impact on the S budget of the mantle. Their positive signs, however, suggest the depleted mantle to have a δ34S of −1.40±0.50‰. The sub-chondritic 34S/32S value that was previously observed for the South-Atlantic mantle is here extended to the Pacific–Antarctic domain. Such a feature cannot originate from oceanic crust recycling and substantiates the concept of a core–mantle fractionation relict.