Salty kimberlites, magmatic or contaminated? Insights from sulfur isotopes on the source of alkalies in kimberlite magmas

English Abstract:

Kimberlites are volatile-rich rocks of alkaline composition containing abundant olivine crystals. Rare but widespread, they carry diamonds and mantle xenoliths from great depths to the surface, thus providing information on the composition of the mantle underneath cratons. However, the strong hydrous alteration of most kimberlites, as reflected by the common serpentinization of olivine, raises questions over the pristine composition of kimberlite magmas. Indeed, common characteristics of kimberlites such as low Na and high water content may relate to post-magmatic alteration involving external meteoritic or hydrothermal waters. The Udachnaya East kimberlite pipe of the Siberian craton preserves a diversity of kimberlite types ranging from a hydrous and serpentinized breccia, to a unique occurence of serpentine-free kimberlite with unusually salty (Na₂O and Cl both up to 6 wt.%) and dry compositions (<1 wt.% H₂O) [1]. These kimberlites were emplaced through a portion of crust saturated with brines, and could thus have acquired their ususual Na, K, Cl and S-rich chemistry by contamination [2]. Alternatively, a magmatic origin for the salts is supported by the similar compositions of alkali carbonate and chloride minerals throughout the salty kimberlites, from minerals in the groundmass, melt inclusions in olivine, to chloride-carbonate "nodules" [1]. In the groundmass of the Udachnaya kimberlites, sulfates and sulfides are both present and were characterized by electron microprobe. In this contribution we will present the sulfur isotope compositions (d³⁴S) of sulfides and sulfates present in 3 types of kimberlites that coexist at the Udachnaya locality, including dry salty kimberlites containing fresh olivine (n=5), wet non-salty kimberlites containing fresh olivine (n=4), and brecciated kimberlite containing serpentinized olivine (n=1). We will compare these results to the canonic S-isotopic composition of the mantle [3], the surrounding hydrothermal deposits (n=4) and siberian cambrian carbonaceous sediments (e.g. Chukuck suite). In light of our S-isotope measurements, we will discuss the magmatic or contaminated origin of the kimberlites from the Udachnaya pipe.

[1] Kamenetsky et al. (2014), Earth-Sci. Rev.; [2] Kopylova et al. (2013), Earth-Sci. Rev.; [3] Labidi et al. (2014), Nature.