Sulfate recycling at subduction zones indicated by sulfur isotope systematics of Mesozoic ultramafic island arc cumulates in the North American Cordillera

Abstract

The isotopic composition of sulfur in convergent margin-related orthomagmatic sulfides is critical to the debate on: 1) petrological mechanisms of sulfide saturation in relatively oxidized magmatic systems, and 2) the relative contribution of subducted sulfate to the overall sulfur budget of the sub-arc mantle. Magmatic sulfides that crystallize in deep-seated plutonic rocks are less susceptible to isotopic fractionation arising from magmatic degassing and are thus better suited for studies of primary S-isotope compositions of arc magmas than their extensively degassed volcanic counterparts. Isotopic compositions of chalcopyrite and pyrrhotite, which crystallized from trapped immiscible sulfide melt, and secondary fibrous pyrite after pyrrhotite in the Polaris Alaskan-type ultramafic-mafic intrusion (Early Jurassic) of the North American Cordillera were determined by secondary ion mass spectrometry. Chalcopyrite grains from five olivine clinopyroxenites are fresh and homogeneous and they have uniform near-chondritic δ34S values (-0.19 +0.48/-0.32‰; n = 97). In contrast, pyrrhotite (δ34S = -1.6 +2.8/-0.6; n = 34) and secondary pyrite (δ34S = -2.7 +1.8/-4.4‰; n = 21) grains display subchondritic but highly variable δ34S. The marked difference in measured δ34S of different sulfide phases attests to the critical advantage of using in-situ microanalytical techniques over whole-rock isotopic analysis. Pyrite grains from hornfelsed volcanogenic country rocks have elevated S-isotope values (δ34S = +7.4 +1.3/-1.7‰; n = 11). The near- to subchondritic isotopic compositions of sulfides from the Polaris intrusion demonstrate that wall-rock assimilation played a minor role and that sulfur in the Polaris magmas is largely magmatic. Fibrous textures and variable, but negative, δ34S of pyrrhotite and pyrite indicate post-magmatic mobility of sulfur due to low-temperature alteration by hydrothermal fluids. The narrow range of chalcopyrite δ34S, however, indicates that it remained unaffected by hydrothermal processes and equilibrated under uniform magmatic physicochemical conditions. Assuming sulfide liquid immiscibility at T ≥ 800 °C, geochemical modeling of S-isotope fractionation indicates that the Polaris parental magmas had suprachondritic δ34S between +1‰ (at log fO2 = FMQ+1) and +5‰ (at log fO2 =FMQ+2), consistent with the contribution of subducted sulfate to the sub-arc mantle.