1. Major Element Trends in Zoned Peridotitic Garnets from Newlands and Bobbejaan Kimberlites, RSA Tim Ivanic 1, Ben Harte 1, Pete Hill 1, John Gurney (University of Cape Town) 1 1

2. Introduction I -Peridotitic diamond inclusions  garnet-chromite-harzburgite host rock -Knowledge of the paragenesis is limited to diamond inclusion studies of isolated minerals -Xenoliths of this type are found at: -Newlands (South Africa) -Dalnaya (Russia) -Arnie (Canada)

3. Introduction II Presence of diamondiferous harzburgites at Newlands –Garnet and chromite with depleted (diamond inclusion-like) compositions (Menzies, 2001) –Diamonds contain peridotitic mineral inclusions Discovery of micro-xenolith suites –Newlands (and Bellsbank) –with P-type diamond inclusion affinity As a result of recent collections:

4. Depletion Garnet –High Mg/(Mg+Fe) and HREE/LREE –Decreased Ca/(Ca+Mg) Chromite –Decreased TiO 2 –Increased Mg/(Mg+Fe) Olivine and pyroxene –Increased Mg/(Mg+Fe) Garnet –Increased TiO 2, LREE/HREE, Sr. Very important for the interpretation of REE profiles –Decreased Mg/(Mg+Fe) Chromite –Increased TiO 2 –Decreased Mg/(Mg+Fe) Olivine and pyroxene –Decreased Mg/Mg+Fe on extraction of melt - producing harzburgites Enrichment on interaction with melt - a reversal of depletion trends VS.

5. Modified from Dawson (1980) Lherzolite Line from Gurney (1984), DGC from Grütter & Menzies (2003)

6. Location

7. Newlands 3 mm 2 mm

8. Chromite- bearing garnet lherzolites also analysed to investigate full spectrum of garnet-chromite equilibria Internal Zonation to Inclusions External Zonation to Matrix

9. Bobbejaan Fissure 10 mm

10. Aims Identify types of major element changes within harzburgitic (G10) specimens Compare to lherzolitic (G9) bulk compositions Identify internal and external zonations

11. Analysis Cameca SX 100 electron microprobe, University of Edinburgh 11 elements analysed quantitatively Traverses perpendicular to inclusions Traverses across sample Qualitative X-ray maps of sample surface (Al, Ca, Cr, Ti, Mg) - little Fe vatiation

12. Traverses vs. Concentrate Lherzolite line (Gurney, 1984) Diamond-Graphite line (Grütter & Menzies, 2003)

13. Trends

14. Traverses New – Hzb New – Lhz Bob – Hzb Bob – Lhz

15. Ti vs. Ca

16. External Zonation

17. Trends

18. Internal Zonation

19. Inclusion

20. Trends

21. Diffusion to srp-chr inclusion only affects Cr2O3 Concentric zonation accompanied by TiO2 External and Internal Zonation

22. Kelyphite rim Chr inclusion Srp inclusion NEW074 Cr (Mg) TiCa Al

23. 1: External Zonation Equilibration with matrix original core composition, isolated inclusion 2: Internal Zonation Equilibration with inclusion original core composition Srp + chr CaO, TiO 2 increase Local Cr 2 O 3 decrease in harzburgites Low-Ca, Grt Core Ca,Ti Cr, Mg, -Al

24. 1d 1c 2 2 1s

25. Summary External Zonation result of: –Grt re-equilibration with Ca+Ti-rich matrix –Diffusion data (Ca) suggest 100s Ma timescale Internal P-T re-equilibration: –Harzburgites in Cr 2 O 3 / Al 2 O 3, MgO –Lherzolites CaO and Cr 2 O 3 / Al 2 O 3, MgO –Diffusion data (Mg) suggest 100s ky timescale –Grt-Cpx final P-T estimates at mineral interfaces Ellis & Green ’79, Fe-Mg ~ 920°C for 45 kb Nimis & Taylor ’00, T En-Cpx, P Cr-Cpx ~ 970°C, 41kb