Graphitization of Neoproterozoic sedimentary marbles in The Aird, Scottish Highlands

Graphite occurs in Neoproterozoic (probable Loch Ness Supergroup) marbles of The Aird, in the Northern Highland Terrane, Scotland. The graphite occurs particularly in association with phlogopite mica, and also with other micas and Mg-chlorite. Although the graphite–phlogopite association is recorded widely elsewhere in mantle-derived rocks, our data suggest graphite at The Aird does not have a mantle origin. The carbon isotopic composition of the graphite (𝛿 13C: −1.6 to 0.4‰) indicates that graphitization occurred from a CO2-rich fluid associated with decarbonation or devolatilization reactions of a carbonate–silicate protolith. Graphite–phlogopite-bearing marbles in The Aird underwent extensive brecciation and hematite deposition that preceded carbon-rich, mantle-derived (carbonatite) fluids. Pyrite in veins within The Aird marble has a sulfur isotope composition depleted in 34S (−16.6 to −15.5‰), suggesting a biogenic origin. Elsewhere in The Aird and in surrounding fenitized rocks, 34S-enriched pyrite has sulfur isotope compositions between 6.1 and 7.7‰, outside the sulfur isotopic composition range of most carbonatite-hosted pyrite, suggesting that pyrite veining is likely to have been influenced by crustal fluid–rock interactions. The observations show that if the protolith has a carbonate–silicate composition, a graphite–phlogopite association can form without the need for mantle-derived fluids.