Trace element signature in magmatic sulphides of oceanic crust gabbros

The chemical composition of magmatic sulphides can be used to comprehend the chalcophile element fractionation processes in magmatic systems that reached sulphide saturation [1-2]. We performed LA-ICP-MS analyses of sulphides hosted by gabbros of the Pacific (ODP147), Indian (ODP176, ODP179, IODP360), and Atlantic (OPD209 and IODP305) spreading centres, as well as from the Raoul Island and Troodos Ophiolite. Our results show that most gabbros contain large magmatic sulphides (mostly above 100 m to 1 mm). These sulphides can be classified as former sulphide liquids that crystallised as Fe- and Ni-rich monosulphide solid solutions, mainly consisting of pyrrhotite, and Cu-rich intermediate solid solutions. A few sulphides found in the Mid-Atlantic Ridge and South West Indian Ridge show lower Cu contents and consist only of monosulphide solid solutions.

Interestingly, sulphides of the Troodos Ophiolite and Hess Deep (Pacific Ocean) show a large variation in Ni and Ni/Cu ratios. We believe this indicates that these magma reservoirs possibly experienced an onset and offset of sulphide saturation, reflected by different Ni content. Furthermore, particular elements with intermediate solid solution affinity, including Cu, Se, Ag, Sb, Au, Te, and Bi, show a higher content in arc and the Pacific sulphides. This indicates that the initial Cu content of a magmatic system strongly influences the chalcophile element content of magmatic sulphide droplets.

[1] Wood, B. J. and Kiseeva, E. S. (2015), Earth and Planetary Science Letters, 424, 280-294. [2] Patten, C. et al. (2013), Chemical Geology, 358, 170–188.