Major Element Trends in Zoned Peridotitic Garnets from Newlands and

Bobbejaan Kimberlites, RSA

Tim Ivanic1,

Ben Harte1,

Pete Hill1,

John Gurney (University of Cape Town)

11

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)

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:

Depletion

• Garnet– High Mg/(Mg+Fe) and

HREE/LREE– Decreased Ca/(Ca+Mg)

• Chromite– Decreased TiO2

– Increased Mg/(Mg+Fe)

• Olivine and pyroxene– Increased Mg/(Mg+Fe)

• Garnet– Increased TiO2, LREE/HREE,

Sr. Very important for the interpretation of REE profiles

– Decreased Mg/(Mg+Fe)

• Chromite– Increased TiO2

– Decreased Mg/(Mg+Fe)

• Olivine and pyroxene– Decreased Mg/Mg+Fe

on extraction of melt- producing harzburgites

Enrichmenton interaction with melt

- a reversal of depletion trends

VS.

Modified from Dawson (1980)

Lherzolite Line from Gurney (1984), DGC from Grütter & Menzies (2003)

Location

Newlands

3 mm2 mm

Chromite-bearing garnet lherzolites also analysed to investigate full spectrum of garnet-chromite equilibria

Internal Zonation to Inclusions

External Zonation to Matrix

Bobbejaan Fissure

10 mm

Aims

• Identify types of major element changes within harzburgitic (G10) specimens

• Compare to lherzolitic (G9) bulk compositions

• Identify internal and external zonations

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

Traverses vs. Concentrate

Lherzolite line (Gurney, 1984)

Diamond-Graphite line (Grütter & Menzies, 2003)

Trends

Traverses

New – HzbNew – LhzBob – HzbBob – Lhz

Ti vs. Ca

B48 Whole Sample Traverse

0

1

2

3

4

5

6

0

2339

.8

4680

.0

7020

.0

9359

.8

1169

9.7

1403

9.8

1637

9.7

1871

9.6

Distance (um)

Wt.

% O

xide

Cr2O3

CaO

External Zonation

Trends

Internal Zonation

6

8

10

12

14

16

18

20

0

476.

1

952.

4

1428

.5

1904

.9

2381

.4

2860

.6

3337

.9

3814

.1Distance (um)

Wt.

% O

xid

eAl2O3

Cr2O3

FeO

MgO

CaO

Internal Zonation

Inclusion

Trends

NEW074 Inclusion Traverse

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.0

778.

0

1555

.8

2333

.6

3111

.3

3889

.1

4667

.1

5444

.9

6222

.6

7000

.3

Distance (um)

TiO

2 W

t. %

Oxi

de

NEW074 Inclusion Traverse

0

2

4

6

8

10

120.

0

778.

0

1555

.8

2333

.6

3111

.3

3889

.1

4667

.1

5444

.9

6222

.6

7000

.3

By DataSet/Point

Oxi

de

Cr2O3

CaO

Diffusion to srp-chr inclusion only affects Cr2O3

Concentric zonation

accompanied by TiO2

External and Internal Zonation

Kelyphite rim

Chr inclusion

Srp inclusion

NEW074

Cr (Mg)

Ti Ca

Al

1: External Zonation

Equilibration with matrix

original core composition, isolated inclusion

2: Internal Zonation

Equilibration with inclusion

original core composition

Srp + chr

CaO, TiO2 increase

Local Cr2O3 decrease in harzburgites

Low-Ca, Grt Core

Ca,Ti

Cr, Mg, -Al

1d

1d

1d

1c1c

2 2

1s

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 Cr2O3 / Al2O3, MgO

– Lherzolites CaO and Cr2O3 / Al2O3, 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, TEn-Cpx, PCr-Cpx ~ 970°C, 41kb