How scientific ocean drilling helps to decode chalcophile trace element behaviour in mid-ocean ridge magmatic systems

Immiscible sulphide liquids, preserved as magmatic sulphide droplets, are believed to strongly control the partitioning behaviour of chalcophile trace elements [1-2]. Hence, the chemical composition of sulphide droplets can be used to understand the fractionation processes of chalcophile elements in magmatic systems that reached sulphide saturation. We carried out LA-ICP-MS analysis of sulphide droplets from gabbros of the lower oceanic crust recovered by deep ocean drilling from mid-ocean ridge spreading centres in the Pacific (ODP147), Indian (ODP176, ODP179, IODP360) and the Atlantic (OPD209 and IODP305) Oceans. For comparison, sulphide droplets from mid-ocean ridge basalts from the East Pacific Rise, Mid-Atlantic Ridge and Southwest Indian Ridge were analysed. Our results show that most gabbros host abundant large magmatic sulphide droplets (mostly above 100 µm up to 1 mm) significantly exceeding those from the related lava units [2-4]. The droplets are commonly associated with or incorporated in olivine or clinopyroxene suggesting an early-stage sulphide saturation but are locally also incorporated in Fe-oxides indicating a later-stage formation during magma cooling [4-5]. The Ni contents of sulphide droplets hosted in gabbros from Hess Deep (Pacific Ocean) are highly variable ranging from ~10 µg/g to weight % levels. Nickel is also strongly controlled by olivine fractionation, and thus can be seen as a parameter indicating whether sulphide saturation was reached before or after the onset of olivine crystallisation. Due to the highly variable Ni contents and in combination with petrographic observations, we suggest that the magma reached early sulphide saturation at Hess Deep, as typically seen in mid-ocean ridge magmatic systems. However, the variable Ni contents in the sulphide droplets indicate that the magma was sulphide-saturated over a longer time span. Alternatively, the magma may frequently switch between being sulphide undersaturated and saturated, due to decreasing pressure during magma ascent accompanied by crystal fractionation at different levels in the crust. Generally, the trace element composition of the sulphide droplets hosted by gabbros from the different drill sites overlap. However, there are significant differences in the compositions of sulphide droplets from lava samples and from associated gabbroic xenoliths [4]. Thus, analysis of droplets from lavas alone provide an incomplete picture of the chalcophile element evolution of the magmatic system. We find no clear differences in sulphide composition with spreading rate or degree of melting as suggested for the silicate melt portion. Instead, the composition of sulphide droplets indicates that fractionation during magma ascent in the crust is the main driver that causes the observed chemical variations, which is part of ongoing investigations.

[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. [3] Peach et al. (1990), Geochimica et Cosmochimica Acta, 12, 3379-3389. [4] Keith, M. et al. (2017), Chemical Geology, 451, 67–77. [5] Jenner, F. E. et al. (2010), Journal of Petrology, 51, 2445-2464.