This proposal is to evaluate the sensitivity of sulfur isotopes to several geologic processes which contribute to the formation of mid-ocean ridge basalts (MORB) on a global scale, and to use sulfur isotope variations to constrain the processes of assimilation and recycling in MORB and their mantle sources. The stable isotopes of sulfur (mainly 32S and 34S) are fractionated by many low-temperature processes such as chemical alteration, degassing and rock-seawater interactions, but are not fractionated by high-temperature processes such as partial melting and fractional crystallization. As a result, sulfur isotopes in MORB can be used as sensitive tracers of (1) assimilation of seawater sulfur in mid-ocean ridge magma chambers, and (2) recycling of sediments and oceanic crust through the upper mantle and back into the source regions of MORB. We will use high-precision ion microprobe measurements of sulfur isotopes in primitive immiscible sulfides from MORB glasses worldwide to define the global variability of sulfur isotopes in MORB, and examine the relationships of these variations to assimilation, crustal thickness, spreading rate, and proximity to hotspots.
