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An eclogitic diamond from Mir pipe (Yakutia), recording two growth events from different isotopic sources

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Author(s)

Galina Bulanova, Daphne Wiggers de Vries, D Graham Pearson, Andrew Beard, Sami Mikhail, Alexander Smelov, Gareth Davies

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Abstract

An eclogitic octahedral macrodiamond from the Mir kimberlite (Yakutia) has a complex growth structure with distinctive core, intermediate and rim zones. Carbon isotope ratios change abruptly from depleted δ13C values of − 16.6‰ in the core to a mean mantle-like signature of − 6.8‰ in the intermediate and − 7.5‰ in the rim areas, establishing that two main stages of diamond formation took place from fluids/melts derived from different carbon isotope reservoirs. The core and intermediate growth areas are separated by a zone with oscillatory growth that records an episode of diamond dissolution and regrowth. The Mir kimberlite pipe is known to contain two populations of eclogitic diamonds, with light and heavy carbon isotopes, but this is the first case where both populations have been found in a single diamond monocrystal. Multiple sulphides and two silicate inclusions occur at the boundary of the oscillatory zone with the intermediate area of the diamond. Silicate inclusions are composite and there is textural and compositional disequilibrium between the mineral phases. A primary omphacite phase has Mg# 66, a high jadeite content, 0.3 wt.% K2O and contains no Cr. Texturally later omphacites in the composite inclusions have Mg# 70, lower jadeite, no potassium, 0.3 wt.% Cr2O3 and occur together with interstitial albite and single grains of phlogopite. Late omphacites are 1.6–3 times higher in Ba, Ti, and Yb. This later mineral assemblage was formed as the result of decompression of primary omphacite in a partly open system in the presence of a fluid/melt enriched in Ba, K, Ti, Yb, Cr, and possibly water. Estimated formation pressure for the primary omphacite based on experimental data is 6 GPa and significantly less than 6 GPa for the later omphacites. A mantle residence time of 1.7 Ga is implied for the core formation of the studied diamond, using published results that couple a 2.1 Ga Re–Os sulphide inclusion age for Mir eclogitic diamonds with δ13C values of <− 10‰, relative to the 0.36 Ga emplacement age of the kimberlite. The re–growth and formation of the intermediate and rim areas took place at 0.9 Ga based on a Re–Os isochron age determined from the sulphide inclusions of these zones, from a carbon source with mean mantle δ13C values. It is proposed that an early eclogitic diamond population characterised by low N content, negative δ15N and low δ13C values formed at a mantle depth of ~ 180 km during the final assembly of the Siberian Craton associated with active subduction at ~ 2.1 Ga. Formation of the second, 0.9 Ga aged population of eclogitic diamonds characterised by moderate N content, less negative δ15N and typical mean-mantle δ13C values (− 5‰) took place at a shallower depth of ~ 120 km and is associated with rift-related magmatism at the end of the Proterozoic. Evidence for both episodes of Mir eclogitic diamond growth is found recording its exhumation from lower to shallower upper mantle depths and formation in two distinct geochemical environments.
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Original languageEnglish
Pages (from-to)40-54
JournalChemical Geology
Volume381
DOIs
Publication statusPublished - 14 Aug 2014

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