On multi-phase mineral inclusions associated with microdiamond formation in mantle-derived peridotite lens at Bardane on Fjortoft, west Norway

Abstract

This paper reports a study of the microstructures and mineral inclusions associated with microdiamond-formation in samples of a garnet websterite pod exposed within a mantle-derived peridotite body at Bardane on Fjortoft, western Norway. This pod is interpreted to have originally comprised a megacrystic assemblage of orthopyroxene + clinopyroxene + garnet + Cr-spinel (Cr * ≈ 75) + minor olivine that probably crystallised within a mid-Proterozoic (1651±47 Ma) mantle diapir/plume at conditions of around 1410°C and 3.2 GPa. The observed microdiamonds are clearly not part of this early (M 1 ) paragenesis but formed later, following a deformation-induced fluid infiltration event. The diamond and other carbon-phase mineral inclusions rarely occur in isolation, instead in composite aggregates with other, mostly micro-scale, mineral inclusions that include phlogopite, kalsilite, Cr-spinel (Cr * ≈ 55), magnesite, Ba-Mg carbonate, Fe-Ni sulphide, Cl-apatite, rutile, zircon and monazite. Thus a metasomatic fluid has clearly introduced other elements (such as K, Ba, Ti, Fe, S, P, Cl, Zr, Cl, Th, and Nb) into this peridotite body, in addition to carbon. Back-scattered electron SEM images demonstrate that the introduced suite of mineral inclusions is spatially intimately associated with M 2 generation grains of orthopyroxene, clinopyroxene, garnet and Cr-spinel (Cr * ∼ 55) that occur both as internally-exsolved phases within deformed and highly-strained M 1 megacrysts and as a coronitic boundary network of grains precipitated between physically-fragmented M 1 megacrysts. To date the microdiamonds have been found to be only rarely preserved as a component of multi-phase mineral inclusions that are doubly armoured in M 2 Cr-spinel (Cr * ≈ 55) in turn enclosed within M 2 generation garnet. Most carbon phase inclusions are now composed of disordered graphite thought most likely to have replaced pre-existing diamond. The M 2 mineral assemblage, with which the introduced microdiamond-bearing mineral inclusion suite is intimately associated, is considered to have formed during the Palaeozoic Caledonian orogeny on the basis of a Sm-Nd errorchron age of 518±78 Ma for the internally exsolved M 2 mineral assemblage at Bardane. This determined age is most likely a mixed age between the apparent age of the M 1 assemblage and the time of M 2 deformation-triggered exsolution and fluid infiltration. Thus microdiamond formation is interpreted to have probably occurred during the Scandian phase of the Caledonian orogenic cycle when this fragment of lithospheric mantle was introduced into a slab of Baltica margin continental crust that experienced short-lived deep-level subduction and attendant ultra-high pressure eclogite-facies metamorphism. The supercritical fluid responsible for the introduction of carbon and other elements into this peridotite body most likely originated from prograde dehydration and decarbonation reactions within the enclosing slab of continental crust gneisses. The microdiamond formation is considered to have occurred at the P-T conditions of ca. 875±25°C and 4.1±0.2 GPa estimated for the formation of the M 2 mineral assemblage within the Bardane peridotite body.