DIAMOND-FORMING FLUIDS – THE IMPORTANCE OF BEING SALTY

Fluids are now thought to be the growth medium for most diamonds sampled from the base of the lithosphere. Fluids trapped in fast-growing, fluid-rich diamonds provide the only direct view of this growth medium and provide valuable information on the geochemistry of deep mantle fluids in general.

The most common fluids within fluid-rich diamonds are those belonging to the low- and high-Mg carbonatite affinity as well as more Si-rich variants. A sub-class of fluids that are very rich in alkalis and Cl, known as “saline” fluids, have been found but are generally scarce. At both Ekati and Diavik saline fluids appear much more common and provide a unique insight into their origin. We describe a novel sampling method that allows the analysis of the trace element and radiogenic isotope composition of diamonds (both gem and fluid-rich). Using these methods we analyzed 11 diamonds from the Fox kimberlite in the Ekati kimberlite cluster. The diamonds containing saline fluids are solely associated with peridotite on the basis of their micro-mineral inclusions. Silicic fluid compositions are related exclusively to eclogitic inclusions. Striking differences between the two fluid compositions are the positive Eu and Sr anomalies within saline fluids versus no anomalies in the silicic fluids. These characteristics are identical to previously studied fluids in fibrous diamonds from neighbouring kimberlites in Ekati and Diavik, which also contains diamonds carrying high- and low-Mg carbonatitic fluids. Combining the data, we show a clear chemical evolutionary trend, identifying for the first time saline fluids as parental to silicic and carbonatitic deep mantle melts, via fluid–rock interaction in the Slave CLM. Moreover, the trace-element and Sr isotopic fingerprints of subducting slabs and the timing of host diamond formation suggest that a subducting plate under western North America is the source of the saline fluids, which controlled metasomatism in the Slave lithosphere prior to Mesozoic kimberlite eruption. Saline fluids can be documented as a metasomatic product interacting with the lithosphere above shallow-subducting slabs such as the Farallon slab. As such they appear to be key players in the enrichment of the base of the lithosphere and the formation of diamonds.