The A B C's of metasomatism in the North Atlantic Craton during Pangea breakup; characterized by fluid inclusions in Chidliak diamonds

Abstract

Here we report the nitrogen characteristics and composition of high-density fluid (HDF) trapped in microinclusions in a suite of fibrous diamonds from the ~142 Ma Chidliak CH-7 kimberlite pipe, the Hall Peninsula, southern Baffin Island, Nunavut. Within these diamonds, we observe three populations based on the chemistry of the encapsulated HDFs, the diamond's nitrogen aggregation states, and the diamond color. ‘Chidliak C' diamonds contain highly silicic HDFs, have nitrogen in A- and C-centers (with 5–20% in C-centers), and a characteristic intense yellow color. ‘Chidliak A' diamonds contain silicic to low-Mg carbonatitic HDFs, carry nitrogen solely in A-centers, and are mostly colorless. A third population, ‘Chidliak B', has grey color and distinctive low-K2O silicic to low-Mg carbonatitic HDF compositions and overall smoother and less fractionated trace element pattern relative to ‘Chidliak C' and ‘Chidliak A' diamonds; they carry nitrogen in A- and B-centers (with ~15% in B-centers) and are characterized by a grey hue. An eclogitic paragenesis of all diamonds is evident by the HDF compositional variation as well as the presence of omphacitic clinopyroxene inclusions. The appearance of a diamond with A- and B-centers in its octahedral core and A- and C-centers in its coat suggests formation at two distinct events at a similar depth. Combined with pressure and mantle residence estimates based on nitrogen aggregation considerations, we argue that the three diamond populations formed at the relatively shallow region of the lithosphere (likely <180 km) during distinct metasomatic events in the North Atlantic Craton (NAC) since the Proterozoic. The youngest event by silicic HDFs took place close in time to kimberlite activity at 142–157 Ma, as evident by the preservation of nitrogen C-centers in ‘Chidliak C' diamonds. A link between this event and the mid-lithosphere discontinuity (MLD) in eclogitic portions of the cratonic lithosphere in Chidliak is plausible. The timing of ‘Chidliak A' diamonds formation by more carbonatitic HDFs is less well constrained, but can be related to Ca-rich metasomatism observed in local peridotite xenoliths and/or alkaline magmatism between 610 and 550 Ma. A possible link between the formation of ‘Chidliak B' diamonds and the timing of Mesoproterozoic olivine lamproite magmatism ca. 1400 Ma is suggested based on the HDF trace element composition and the aggregated nature of nitrogen in these diamonds. The nitrogen systematics and eclogitic source of the fibrous diamonds are comparable with those observed for previously studied gem-quality diamonds from Chidliak. We suggest that these similarities show a temporal connection and mutual crystallization of the two diamond types. This strengthens the involvement of HDFs in the formation of gem-quality diamonds.