Graphite in Ordovician ultramafic intrusions through Neoproterozoic country rocks in West Aberdeenshire, NE Scotland attests to the mobility of carbon into subduction-related magmas. Pelites in the country rock contain up to 5% organic carbon, and are consequently also sulphide-rich. This occurrence is one of many examples of graphite in ultramafic intrusions. The sedimentary origin of the graphite carbon is indicated by carbon isotope compositions (mean - 19.7%.) in the same range as in the country rocks (mean - 21.6%0. The graphite is spatially and temporally associated with nickel-bearing sulphides, whose precipitation was strongly influenced by incorporation of the carbonaceous, sulphidic country rocks into the intrusions. Raman spectra for graphite in the intrusions show it is fully ordered, deposited from fluid in the magma. Preferential intrusion into pelitic country rocks facilitated the ready assimilation of carbon. In this case reworking of carbon from the crust into intrusions occurred 200 million years after sedimentation, a much longer timescale than most cycling of carbon in subduction zones. (C) 2020 The Author(s). Published by Elsevier B.V.
Bibliographical noteWe are grateful to J. Johnston, J. Bowie and A. Boyce for skilled technical support. This work was partly supported by NERC grant NE/M010953/1. Electron Microscopy was performed with the help of J. Still in the ACEMAC Facility at the University of Aberdeen. Samples were donated from the University of Aberdeen (Cairnie sample 6932, Coulins Burn 5524, Allt Nathrach 6992, Dunbennan 11876). We are very grateful for two reviews which helped to clarify the manuscript.
- DALRADIAN SUPERGROUP
- SULFUR ISOTOPE
- GRAPHITE DEPOSIT
- SULFIDE DEPOSITS
- ACHANKOVIL SHEAR ZONE
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