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

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.