TY - JOUR

T1 - The uniquely high-temperature character of Cullinan diamonds: A signature of the Bushveld mantle plume?

AU - Korolev, N. M.

AU - Kopylova, M.

AU - Bussweiler, Y.

AU - Pearson, D. G.

AU - Gurney, J.

AU - Davidson, J.

PY - 2018/4/1

Y1 - 2018/4/1

N2 - The mantle beneath the Cullinan kimberlite (formerly known as “Premier”) is a unique occurrence of diamondiferous cratonic mantle where diamonds were generated contemporaneously and shortly following a mantle upwelling that led to the formation of a Large Igneous Province that produced the world's largest igneous intrusion – the 2056 Ma Bushveld Igneous Complex (BIC). We studied 332 diamond inclusions from 202 Cullinan diamonds to investigate mantle thermal effects imposed by the formation of the BIC. The overwhelming majority of diamonds come from three parageneses: (1) lithospheric eclogitic (69%), (2) lithospheric peridotitic (21%), and (3) sublithospheric mafic (9%). The lithospheric eclogitic paragenesis is represented by clinopyroxene, garnet, coesite and kyanite. Main minerals of the lithospheric peridotitic paragenesis are forsterite, enstatite, Cr-pyrope, Cr-augite and spinel; the sublithospheric mafic association includes majorite, CaSiO3 phases and omphacite. Diamond formation conditions were calculated using an Al-in-olivine thermometer, a garnet-clinopyroxene thermometer, as well as majorite and Raman barometers. The Cullinan diamonds may be unique on the global stage in recording a cold geotherm of 40 mW/m2 in cratonic lithosphere that was in contact with underlying convecting mantle at temperatures of 1450–1550 °C. The studied Cullinan diamonds contain a high proportion of inclusions equilibrated at temperatures exceeding the ambient 1327 °C adiabat, i.e. 54% of eclogitic diamonds and 41% of peridotitic diamonds. By contrast, ≤ 1% of peridotitic diamond inclusions globally yield equally high temperatures. We propose that the Cullinan diamond inclusions recorded transient, slow-dissipating thermal perturbations associated with the plume-related formation of the ~2 Ga Bushveld igneous province. The presence of inclusions in diamond from the mantle transition zone at 300–650 km supports this view. Cullinan xenoliths indicative of the thermal state of the cratonic lithosphere at ~1.2 Ga are equilibrated at the relatively low temperatures, not exceeding adiabatic. The ability of diamonds to record super-adiabatic temperatures may relate to their entrainment from the deeper, hotter parts of the upper mantle un-sampled by the kimberlite in the form of xenoliths or their equilibration in a younger lithosphere after a decay of the thermal disturbance.

AB - The mantle beneath the Cullinan kimberlite (formerly known as “Premier”) is a unique occurrence of diamondiferous cratonic mantle where diamonds were generated contemporaneously and shortly following a mantle upwelling that led to the formation of a Large Igneous Province that produced the world's largest igneous intrusion – the 2056 Ma Bushveld Igneous Complex (BIC). We studied 332 diamond inclusions from 202 Cullinan diamonds to investigate mantle thermal effects imposed by the formation of the BIC. The overwhelming majority of diamonds come from three parageneses: (1) lithospheric eclogitic (69%), (2) lithospheric peridotitic (21%), and (3) sublithospheric mafic (9%). The lithospheric eclogitic paragenesis is represented by clinopyroxene, garnet, coesite and kyanite. Main minerals of the lithospheric peridotitic paragenesis are forsterite, enstatite, Cr-pyrope, Cr-augite and spinel; the sublithospheric mafic association includes majorite, CaSiO3 phases and omphacite. Diamond formation conditions were calculated using an Al-in-olivine thermometer, a garnet-clinopyroxene thermometer, as well as majorite and Raman barometers. The Cullinan diamonds may be unique on the global stage in recording a cold geotherm of 40 mW/m2 in cratonic lithosphere that was in contact with underlying convecting mantle at temperatures of 1450–1550 °C. The studied Cullinan diamonds contain a high proportion of inclusions equilibrated at temperatures exceeding the ambient 1327 °C adiabat, i.e. 54% of eclogitic diamonds and 41% of peridotitic diamonds. By contrast, ≤ 1% of peridotitic diamond inclusions globally yield equally high temperatures. We propose that the Cullinan diamond inclusions recorded transient, slow-dissipating thermal perturbations associated with the plume-related formation of the ~2 Ga Bushveld igneous province. The presence of inclusions in diamond from the mantle transition zone at 300–650 km supports this view. Cullinan xenoliths indicative of the thermal state of the cratonic lithosphere at ~1.2 Ga are equilibrated at the relatively low temperatures, not exceeding adiabatic. The ability of diamonds to record super-adiabatic temperatures may relate to their entrainment from the deeper, hotter parts of the upper mantle un-sampled by the kimberlite in the form of xenoliths or their equilibration in a younger lithosphere after a decay of the thermal disturbance.

KW - Adiabat

KW - Bushveld Igneous Complex

KW - Cullinan kimberlite mine

KW - Diamond inclusions

KW - Plume

KW - Thermobarometry

KW - SOUTH-AFRICA

KW - LITHOSPHERIC MANTLE

KW - CONTINENTAL MANTLE

KW - PERIDOTITE XENOLITHS

KW - MINERAL INCLUSIONS

KW - KAAPVAAL CRATON

KW - 4-PHASE LHERZOLITES

KW - GARNET PERIDOTITES

KW - PHASE-RELATIONS

KW - PREMIER KIMBERLITE

UR - http://www.scopus.com/inward/record.url?scp=85042671624&partnerID=8YFLogxK

U2 - 10.1016/j.lithos.2018.02.011

DO - 10.1016/j.lithos.2018.02.011

M3 - Article

AN - SCOPUS:85042671624

VL - 304-307

SP - 362

EP - 373

JO - Lithos

JF - Lithos

SN - 0024-4937

ER -