Graphitisation of Neoproterozoic sedimentary marbles in The Aird, Scottish Highlands

Abstract

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 suggests graphite at The Aird does not have a mantle origin. The carbon isotopic composition of the graphite ( δ 13 C, 0.4 to -1.6 ‰) indicates graphitisation occurred from a CO 2 -rich fluid associated with decarbonation or devolatilisation reactions of a carbonate-silicate protolith. Graphite-phlogopite bearing marbles in the Aird underwent extensive brecciation and haematite deposition that preceded carbon-rich, mantle-derived (carbonatite) fluids. Pyrite in veins within The Aird marble has a sulphur isotope composition depleted in 34 S (-15.5 to -16.6 ‰), suggesting a biogenic origin. Elsewhere in The Aird and in surrounding fenitised rocks 34 S -enriched pyrite has sulphur isotope compositions between 7.7 to 6.1 ‰, outside the sulphur isotopic composition range of most carbonatite-hosted pyrite, suggesting pyrite veining was likely 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. Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/topic/collections/early-career-research