A mantle origin for sulfates in the unusual "salty" Udachnaya-East kimberlite from sulfur abundances, speciation and their relationship with groundmass carbonates

Abstract

The Udachnaya-East pipe in Yakutia in Siberia hosts a unique dry (serpentine-free) body of hypabyssal kimberlite ( O), associated with a less dry type of kimberlite and a serpentinized kimberlitic breccia. The dry kimberlite is anomalously rich in salts (Na2 O and Cl both up to 6wt%) whereas the slightly less dry and the breccia kimberlite are salt free. Yet the Udachnaya kimberlite is a group-I kimberlite, as is the archetypical kimberlite from Kimberley, South Africa. Samples were studied from the three different types of kimberlite (dry-salty, n=8, non-salty, n=5 and breccia, n=3) regarding their mineralogy, geochemistry, and more specifically their sulfur content. Our results show the salty kimberlite is unprecedentedly rich in sulfur (0.13-0.57wt%) compared to the non-salty kimberlite (0.04-0.12wt%) and the breccia (0.29-0.33wt%). In the salty kimberlite, most of the sulfur is present as sulfates (up to 97% of Stotal ) and is disseminated throughout the groundmass in close association with Na-K-bearing carbonates. Sulfates occur within the crystal structure of these Na-K-bearing carbonates as the replacement of (CO3 ) by (SO3 ) groups, or as Na- and K-rich sulfates (e.g. aphtitalite, (K,Na)3 Na(SO4 )2 ). The associated sulfides are djerfisherite; also Na- and K-rich species. The close association of sulfates and carbonates in these S-rich alkaline rocks suggests that the sulfates crystallized from a mantle-derived magma, a case that has strong implication for the oxygen fugacity of kimberlite magmatism and more generally for the global S budget of the mantle.