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

摘要

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 hydrother-mal fluids assimilation on the S isotope composition of erupted basalts along segments that are devoid of plume influence. Δ~(33)S and Δ~(36)S yield homogenous values within error of Canyon Diablo Troilite (CDT), whereas δ~(34)S 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 δ~(34)S follows a spike-like pattern, with local ~(34)S-enrichments by up to +1.30‰ compared to a low-δ~(34)S baseline. As hydrothermal massive sulfides are characterized by relative ~(34)S-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 δ~(34)S 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 δ~(34)S and ~(206)Pb/~(204)Pb 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 δ~(34)S of -1.40 ± 0.50‰. The sub-chondritic ~(34)S/~(32)S 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.