H2O content of nominally anhydrous mineral inclusions in diamonds from the Udachnaya kimberlite

Abstract:

Nominally anhydrous minerals (such as olivine, pyroxene and garnet) present in mantle xenoliths have been found to contain up to hundreds of ppm wt H₂O, bonded as H to their mineral structure. However, it is not well understood whether these H₂O contents are representative for the hydrous state of the deep mantle where they formed, or if they are the result of interactions between the xenoliths and metasomatic fluids or magmas during their travel to the surface. Given the fact that trace amounts of H₂O can alter the physical and chemical properties of mantle materials and therefore affect Earth’s dynamics, it is important to accurately determine the H₂O content of deep mantle minerals.

Natural diamonds can contain mineral inclusions that formed at high depths (>5 GPa) and are representative for the deep and inaccessible portions of the mantle where they originated. This is because the strong and inert diamond prevents the inclusions to react with any fluid or melt that get in contact with it. Therefore, valuable information regarding the H₂O content of the deep mantle can be obtained by studying these minerals trapped in diamonds.

In this study we measured the H₂O contents of 10 olivine and garnet inclusions in diamonds from the Udachnaya kimberlite (Siberian craton) by Fourier Transform Infrared spectroscopy. Olivine crystals contain 1-5 ppm wt H₂O while garnets do not show absorption bands indicating the presence of detectable H in their structure and are therefore considered dry. The H₂O contents of olivine and garnet inclusions in diamonds presented here are considerably lower than those found in xenoliths or xenocrists from the same locality. Based on these new results, we discuss the presence of H₂O in the cratonic mantle and its importance in stabilizing these areas during geological time, as well as the volatile signature of diamond forming melts in the Siberian craton.