Petrography of Snap Lake Kimberlite Dyke (Northwest Territories, Canada) and its Interaction with Country Rock Granitoids

Comment on: ‘Petrography of the Snap Lake Kimberlite Dyke (Northwest Territories, Canada) and its Interaction with Country Rock Granitoids’ by Fulopet al.(2018), Journal of Petrology, doi: 10.1093/petrology/egy025

“…Such asymmetric effects are not reported in this and previ-ous studies of Snap Lake. Our detailed observations of actual sections (Field et al, 2009;Gernon et al, 2012) agree with the schematic sections (Figures 2 and 3) in Fulop et al (2018) that show the variations across the intrusion are symmetric. One possibility for generating symmetrical sections is if the magma intrusion had high fluid (gas) overpressures resulting in decompression and outflow of volcanic fluids.…”

“…However, some features of these rocks could have originated by reactions between melts, fluids, primary ig-neous crystals and entrained xenoliths during the initial stages of solidification. Fulop et al (2018) discredited our earlier studies making the false statement that Gernon et al (2012) "mapped [rock types] from underground photographs". However, field data and observations were collected underground at Snap Lake with De Beers Canada geologists over several months using well-established underground mapping and direct observations of rock faces.…”

“…Now consider the water budget needed for alteration. Table 6 in Fulop et al (2018) shows typical loss on ignition values of around 15 wt%. Given that much of the Snap Lake magma is highly porphyritic the water would need to be carried in the melt, so the requirement is for the magma to transport 25-30 wt% water-more than an order of magnitude greater than what is plausible for magmatic water content.…”

“…Note that olivine macrocrysts are visible in the OPK, but less abundant. Such features are hard to explain in the context of deuteric alteration, which invokes a gradational transition between ORK and OPK (Fulop et al, 2018). In addition, a deuteric alteration overprint of OPK would need to selectively exclude many olivine macrocrysts, which remain intact (Figs.…”

Polymineralic Inclusions in Megacrysts as Proxies for Kimberlite Melt Evolution—A Review

“…The origin of serpentine in kimberlites is still very much debated, as reflected in the recent comment and reply discussion by Fulop et al [59,60] and Gernon et al [61]. Whereas Fulop et al [59,60] maintained that serpentinization in the Snap Lake kimberlite, Slave craton (Canada) is dominantly driven by deuteric fluids, Gernon et al [61] argued that it must be essentially completely driven by the interaction with groundwater, because the amount of released volatiles from the magma would not be large enough to cause the pervasive alteration of kimberlite. A major open question in this debate, which the study of polymineralic inclusions may help to address, is how rich in volatiles the kimberlite magma was at mantle depths.…”