TY - JOUR
T1 - Sulphide inclusions in diamonds from the Koffiefontein kimberlite, S Africa
T2 - Constraints on diamond ages and mantle Re-Os systematics
AU - Pearson, D. G.
AU - Shirey, S. B.
AU - Harris, J. W.
AU - Carlson, R. W.
PY - 1998/8
Y1 - 1998/8
N2 - Re-Os isotope compositions of syngenetic sulphide inclusions in both eclogite suite (E-type) and peridotite suite (P-type) parageneses in diamonds from the Koffiefontein mine, South Africa have been analysed using a technique capable of analysing single inclusion grains, or, in some cases multiple inclusions from the same diamonds. Sulphide inclusion Ni contents broadly correlate with Os abundances in that low-Ni (6.8-8.7% Ni), E-type sulphides have 4.7 to 189 ppb Os whereas the two high-Ni (25%), P-type sulphides have 5986 and 6097 ppb Os. Two P-type sulphides from the same diamond define the first mineral isochron obtained for a single diamond which has an age of 69 ± 30 Ma with chondritic initial 187Os/188Os. This indicates that the sulphides, and hence the host diamond, crystallised close to the time of kimberlite emplacement (90 Ma), in the Mesozoic. This is supported by Pb isotopic measurements of a fragment from one of the sulphides, together with the absence of significant Type IaB nitrogen aggregation in the host diamond lattice. E-type sulphide inclusions have radiogenic Os isotopic compositions, 187Os/188Os 0.346 to 2.28, and Re-Os model ages from 1.1 to 2.9 Ga. They define an array on a Re-Os isochron diagram that may be interpreted as defining a single period of E-type sulphide growth at 1.05 ± 0.12 Ga, with an elevated initial 187Os/188Os. Alternatively, two episodes of sulphide crystallisation, from a chondritic reservoir, may be invoked in the Archaean and in the Proterozoic. The results for both P- and E-type diamonds point to a spectrum of diamond crystallisation ages. High contents of both Re and Os, and the similarity of Re/Os ratios of sulphide inclusions in diamonds to whole rock eclogite and peridotite xenoliths indicate that small amounts of sulphides can dominate the mantle budget of both these elements during melting. Recent addition to the lithospheric mantle of high-Os material similar to that from which the P-type sulphides crystallised may explain the variable, sometimes young Os model ages seen in whole rock xenolith Re-Os data.
AB - Re-Os isotope compositions of syngenetic sulphide inclusions in both eclogite suite (E-type) and peridotite suite (P-type) parageneses in diamonds from the Koffiefontein mine, South Africa have been analysed using a technique capable of analysing single inclusion grains, or, in some cases multiple inclusions from the same diamonds. Sulphide inclusion Ni contents broadly correlate with Os abundances in that low-Ni (6.8-8.7% Ni), E-type sulphides have 4.7 to 189 ppb Os whereas the two high-Ni (25%), P-type sulphides have 5986 and 6097 ppb Os. Two P-type sulphides from the same diamond define the first mineral isochron obtained for a single diamond which has an age of 69 ± 30 Ma with chondritic initial 187Os/188Os. This indicates that the sulphides, and hence the host diamond, crystallised close to the time of kimberlite emplacement (90 Ma), in the Mesozoic. This is supported by Pb isotopic measurements of a fragment from one of the sulphides, together with the absence of significant Type IaB nitrogen aggregation in the host diamond lattice. E-type sulphide inclusions have radiogenic Os isotopic compositions, 187Os/188Os 0.346 to 2.28, and Re-Os model ages from 1.1 to 2.9 Ga. They define an array on a Re-Os isochron diagram that may be interpreted as defining a single period of E-type sulphide growth at 1.05 ± 0.12 Ga, with an elevated initial 187Os/188Os. Alternatively, two episodes of sulphide crystallisation, from a chondritic reservoir, may be invoked in the Archaean and in the Proterozoic. The results for both P- and E-type diamonds point to a spectrum of diamond crystallisation ages. High contents of both Re and Os, and the similarity of Re/Os ratios of sulphide inclusions in diamonds to whole rock eclogite and peridotite xenoliths indicate that small amounts of sulphides can dominate the mantle budget of both these elements during melting. Recent addition to the lithospheric mantle of high-Os material similar to that from which the P-type sulphides crystallised may explain the variable, sometimes young Os model ages seen in whole rock xenolith Re-Os data.
KW - Diamond
KW - Geochronology
KW - Inclusions
KW - Isotopes
KW - Isotopic analysis
KW - Kimberlite
KW - Re/Os
KW - South Africa
KW - Sulfide
UR - http://www.scopus.com/inward/record.url?scp=0032457109&partnerID=8YFLogxK
U2 - 10.1016/S0012-821X(98)00092-2
DO - 10.1016/S0012-821X(98)00092-2
M3 - Article
AN - SCOPUS:0032457109
VL - 160
SP - 311
EP - 326
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
SN - 0012-821X
IS - 3-4
ER -