 |
| Titel |
Diamond growth beneath Letlhakane established by Re-Os and Sm-Nd systematics of individual eclogitic sulphide, garnet and clinopyroxene inclusions |
| VerfasserIn |
Michael U. Gress, D. Graham Pearson, Suzette Timmerman, Ingrid L. Chinn, Janne M. Koornneef, Gareth R. Davies |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141975
|
| Publikation (Nr.) |
 EGU/EGU2017-5540.pdf |
|
|
|
|
|
| Zusammenfassung |
| The diamondiferous Letlhakane kimberlites are part of the Orapa kimberlite cluster (∼ 93.1
Ma) in north-eastern Botswana, located on the edge of the Zimbabwe Craton, close to the
Proterozoic Magondi Mobile Belt. Here we report the first Re-Os ages of six individual
eclogitic sulphide inclusions (3.0 to 35.7μg) from Letlhakane diamonds along with their
rhenium, osmium, iridium and platinum concentrations, and carbon isotope, nitrogen content
and N-aggregation data from the corresponding growth zones of the host diamonds. For the
first time, Re-Os data will be compared to Sm-Nd ages of individual eclogitic silicate
inclusions recovered from the same diamonds using a Triton Plus equipped with four 1013Ω
amplifiers.
The analysed inclusion set currently encompasses pairs of individual sulphides from two
diamonds (LK040 sf4 & 5, LK113 sf1 & 2) and two sulphide inclusions from separate
diamonds (LK048, LK362). Ongoing work will determine the Sm-Nd ages and element
composition of multiple individual eclogitic garnets (LK113/LK362, n=4) and an eclogitic
clinopyroxene (LK040) inclusion.
TMA ages of the six sulphides range from 1.06 to 2.38 Ga (± 0.1 to 0.54 Ga) with Re
and Os contents between 7 and 68 ppb and 0.03 and 0.3 ppb, respectively. The
host diamond growth zones have low nitrogen abundances (21 to 43 ppm N) and
high N-aggregation (53 to 90% IaB). Carbon isotope data suggests the involvement
of crustal carbon (δ13C between -19.3 to -22.7 ± 0.2 per mill) during diamond
precipitation.
Cathodoluminescence imaging of central plates from LK040 and LK113 displays
homogenous internal structure with no distinct zonation. The two sulphide inclusions from
LK040 define an ‘isochron’ of 0.92 ± 0.23 Ga (2SD) with initial 187Os/188Os =
1.31 ± 0.24. Sulphides from LK113 have clear imposed diamond morphology and
indicate diamond formation at 0.93 ± 0.36 Ga (2SD) with initial 187Os/188Os =
0.69 ± 0.44. The variation in the initial 187Os/188Os does not justify including
these inclusions (or any from other diamonds) on the same isochron and implies an
extremely heterogeneous diamond crystallisation environment that incorporated recycled
Os. C1-normalized osmium, iridium and platinum (PGE) compositions from the
analysed sulphide inclusions display enrichment in Ir (3.4 to 33) and Pt (2.3 to 28.1)
in comparison to eclogitic xenolith data from Orapa that are depleted relative to
chondrite.
The Re-Os isochrons determined in this study are within error of previously reported ages
from the adjacent (∼40km) Orapa diamond mine (1.0 to 2.9 Ga) based on sulphide inclusions
and a multi-point 990 ± 50 Ma (2SD) isochron for composite (n=730) silicate inclusions.
Together with additional new Sm-Nd isochron age determinations from individual silicate
inclusions from Letlhakane (2.3 ± 0.02 (n = 3); 1.0 ± 0.14 (n = 4) and 0.25 ± 0.04 Ga (n =
3), all 2SE) these data suggest a phase of Mesoproterozoic diamond formation as well as
Neoarchean/Paleoproterozoic and Mesozoic diamond growth, in punctuated events spanning
>2.0 Ga. |
|
|
|
|
|
|