The diamonds of South Australia

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May 19, 2009 - Diamonds in South Australia occur in kimberlites at Eurelia (Orroroo), ...... the diamonds formed in a lithosphere with an elevated geothermal.
Lithos 112S (2009) 806–821

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Lithos j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s

The diamonds of South Australia Ralf Tappert a,⁎, John Foden a, Thomas Stachel b, Karlis Muehlenbachs b, Michelle Tappert c, Kevin Wills d a

Geology and Geophysics, School of Earth and Environmental Sciences, University of Adelaide, Adelaide, 5005, South Australia, Australia Department of Earth and Atmospheric Sciences, 1-26 Earth Science Building, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 c Centre for Mineral Exploration Under Cover, School of Earth and Environmental Sciences, University of Adelaide, Adelaide, 5005, South Australia, Australia d Flinders Mines Ltd., Norwood, 5000, South Australia, Australia b

a r t i c l e

i n f o

Article history: Received 15 September 2008 Accepted 20 April 2009 Available online 19 May 2009 Keywords: Sublithospheric diamonds Ferropericlase Placer diamonds South Australia Kimberlites Permian glaciation

a b s t r a c t Diamonds in South Australia occur in kimberlites at Eurelia (Orroroo), and in placer deposits, which include the Springfield Basin and the historic Echunga goldfield. To identify the kimberlitic and mantle sources of the placer diamonds, and to determine any possible connections between the placer diamonds and the diamonds from the Eurelia kimberlites, we examined the physical and compositional characteristics, and the mineral inclusion content of 122 diamonds from the Springfield Basin and 43 diamonds from kimberlites at Eurelia. Additional morphological data for three Echunga diamonds are also given. Most of the diamonds from the Springfield Basin are similar to the diamonds from Eurelia with respect to their crystal shapes, surface textures, and colors. The diamond populations from both areas are characterized by a high abundance of lownitrogen (b 100 ppm) diamonds with variable nitrogen aggregation states. The stable carbon isotope compositions of the Springfield Basin diamonds are similar to the Eurelia diamonds with δ13C values in the range − 20.0 to − 2.5‰, and a mode at − 6.5‰. Ferropericlase inclusions in two diamonds from the Springfield Basin are consistent with ferropericlase-bearing mineral inclusion assemblages found in the Eurelia diamonds and indicate that part of the diamond population from both areas is of sublithospheric origin. One diamond from the Springfield Basin contained an inclusion of lherzolitic garnet. The overall similarities between the Springfield Basin and Eurelia diamonds indicates that the bulk of the Springfield Basin diamonds are derived from kimberlitic sources that are similar (or identical) to those at Eurelia. However, three diamonds from the Springfield Basin are markedly distinct. These have well-developed crystal shapes, large sizes, yellow body colorations, and brown irradiation spots. The brown irradiation spots and abrasion textures provide evidence that these diamonds are much older than the other diamonds in the Springfield Basin, and that they are derived from distal kimberlitic sources. The diamonds are most likely derived from Permian glacigene sediments and may ultimately be sourced from kimberlites on the East Antarctic craton. Abrasion textures and brown irradiation spots are also present on diamonds from Echunga. This provides a link to the three “old” Springfield Basin diamonds and other alluvial diamonds in Eastern Australia, and suggests that Permian glaciations caused a widespread distribution of diamonds over large parts of southern Australia, which at that time was part of the supercontinent Gondwana. © 2009 Elsevier B.V. All rights reserved.

1. Introduction In South Australia, diamonds occur in kimberlites, placer deposits, and as isolated grains from unknown sources in surface samples (Fig. 1). Most of the known kimberlites in South Australia are located in the Adelaide Fold Belt (Colchester, 1972; Ferguson and Sheraton, 1979; Stracke et al., 1979; Scott Smith et al., 1984), where they occur as a semicontinuous, northwest trending dyke–swarm (Fig. 1). Two additional kimberlite clusters are present on the adjacent Gawler Craton; these are located near Cleve and Elliston/Mount Hope (Atkinson et al., 1990;

⁎ Corresponding author. Tel.: +61 8 8303 5844; fax: +61 8 8303 4347. E-mail address: [email protected] (R. Tappert). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.04.029

Wyatt et al., 1994; Fig. 1). Although more than 150 individual kimberlite occurrences have been discovered within South Australia, the only marginally diamondiferous kimberlites, so far, are restricted to the Eurelia area in the Adelaide Fold Belt, ∼20 km north of Orroroo (Scott Smith et al., 1984). A notable feature of the diamonds from Eurelia is the presence of ferropericlase-bearing mineral inclusion assemblages, which suggests that part of the diamond population from Eurelia is of unusually deep, sublithospheric origin (Scott Smith et al., 1984; Tappert et al., 2009). Diamond placer deposits within South Australia are located in the Echunga area, (∼30 km SE Adelaide), and in the Springfield Basin (∼50 km NW Orroroo) (Fig. 1). Diamonds at Echunga were first discovered in 1859, as a rare byproduct during placer gold mining. Until around 1900, up to 50 diamonds were found at Echunga, with the largest stone weighing

R. Tappert et al. / Lithos 112S (2009) 806–821

807

Fig. 1. Locations of diamond and kimberlite occurrences in South Australia.

∼5.3 carats (Gommers,1988). Only five of the diamonds from the Echunga goldfield are still known to exist. At Echunga, the diamonds were recovered from auriferous Tertiary conglomerates, which are devoid of indicator minerals commonly associated with diamonds. The primary sources of the Echunga diamonds are unknown. The Springfield Basin is a small sedimentary basin (∼ 9 km2), which unconformably overlies folded sedimentary rocks of the Adelaidean supergroup. Within the Springfield Basin, diamonds occur exclusively in the basal conglomerate, which is considered to be Permian in age (Drexel and Preiss, 1995). The conglomerate is overlain by argillites and partially coal-bearing sediments of approximately Late Triassic age (Amtsberg, 1969). Within the basal conglomerate of the Springfield Basin, the diamonds occur together with

indicator minerals, including magnesiochromite, picroilmenite, pyrope, and chrome diopside. During bulk sampling in the Springfield Basin, around 200 diamonds were recovered from N2000 tons of conglomerate, with the largest diamond weighting 0.34 carat. Although the diamondiferous kimberlites at Eurelia are located less than 40 km southeast of the Springfield Basin, they have not been considered to be the source of the Springfield Basin diamonds, because of their apparently younger (Jurassic) ages. In order to gain information about possible kimberlitic sources and the mantle origins of the placer diamonds in South Australia, we analyzed the physical and compositional characteristics as well as the mineral inclusion content of placer diamonds from the Springfield Basin and the Echunga area. In order to determine possible links between the placer diamonds

808

Table 1 Physical, compositional, and isotopic characteristics of diamonds from the Springfield Basin and kimberlites in the Eurelia area, South Australia. Surface textures Sample

Weight [mg]

Color Ir. Spots.

Springfield Basin FLIN2-01 20.2 FLIN2-02 14.8 FLIN2-03 3.7 FLIN2-04 4.6

I D O I

YL YL CL YL

FLIN2-05 FLIN2-06

3.9 3.6

PsA Ps

FLIN2-07

3.4

OA

FLIN2-08 FLIN2-09

10.2 6.3

D I

FLIN2-10 FLIN2-11 FLIN2-12 FLIN2-13 FLIN2-15 FLIN2-16 FLIN2-17 FLIN2-18 FLIN2-19 FLIN3-01 FLIN3-02 FLIN3-03 FLIN3-04 FLIN3-05 FLIN3-06 FLIN3-07 FLIN3-08 FLIN3-09 FLIN3-10 FLIN3-11 FLIN3-12 FLIN3-13 FLIN3-14 FLIN3-15 FLIN3-16

6.5 3.9 6.1 3.2 4.8 9.0 4.3 14.7 7.8 1.4 2.2 3.2 11.1 7.8 1.9 4.3 6.7 7.3 9.0 5.0 3.3 4.2 3.9 3.8 5.0

D I I O I I I D I I I O Ps I I D I IA I O OA I Ps Ps OA

FLIN3-17 FLIN3-18 FLIN3-19 FLIN3-20 FLIN3-21 FLIN3-22 FLIN3-23 FLIN3-24

2.4 2.5 6.6 5.0 3.2 4.0 5.0 3.6

O O I I OA I I I

Fragment, broken (old) PBR D fragment, broken (old + new) YL D fragment, broken (old) BR CL O fragment, broken (new) CL Fragment, broken (old + new) BR Fragment, broken (old + new) CL Flattened YL Fragment, broken (old + new) PBR O fragment, broken (new) YL Fragment, broken (new) BR CL Distorted, broken (old) PBR Fragment, broken (old) CL Fragment, broken (new) PBR Broken (old) CL Fragment, broken (old), twinned PBR Fragment, broken (old) CL Fragment CL BR Broken (new) CL D fragment, broken (old) BR PBR Flattened PBR Partially twinned, stepped, PBR broken (old) Fragment, broken (old) PBR Broken (old), slightly stepped PBR D fragment, broken (old) CL Fragment, broken (old) CL Stepped PBR Fragment, broken (old) PBR Fragment, broken (old + new) BR D fragment, broken (old) BR

FLIN3-25 FLIN3-26 FLIN3-27

3.4 3.6 4.9

I I I

Fragment, broken (old + new) Fragment, broken (old + new) Fragment, broken (new)

DO fragment, broken (new) Broken (old) Broken (old) DO fragment, bloken (new + old) Distorted Multiply twinned, one side concave Fragment, broken (new) Fragment, broken (new)

Defor. Sh. Trigons Hillocks Terraces Frosting Corr. Lam. Sc. X

BR, GR

X

BR BR

X

X

CL BR BR

CL PBR BR

X X

X X X X

X X

X X

X

X

X

X

X

X

X X

X Inclusion cavity X X

X X

X X X X X

GR

X X

X

X X

X BR, GR

X X X

X

Triangular plates

X

X X

Etch Ruts Other

X X

X

X X

X

GR GR GR

X X X X

X X

X X X X X X

X

GR

X X X X

X

X X X X X X

X X

X X

X X

X X

X

X

X X

X

X X

X X

X

X

GR

X X X X

X X GR

X

X X X X

X

X X X X

X X X X X X X

X X

Inclusion cavity X

X X X X

X X

X X

X X X X

X

X X

Enhanced lustre

X X

UV-color

N-Conc. N-Agg. [atomic ppm] [%B]

Type Hydrogen δ13C [‰ PDB]

BL GR BL BL

820 472 25.2 1820

27 1.8 95 78

IaAB IaA IaB IaAB

− 7.13 − 6.39 − 6.64 − 5.59

YL–GR YL–GR (BL) BL

197 15.2

2.3 46

IaA IaAB

− 6.13 − 6.63

955

67

IaAB

− 3.71

297 73.1

35 12

IaAB IaAB

n.a. n.a.

1079 (785) 1050 161 20.2 27.9 0.0 1199 (1071) 594 727 (748) 9.9 209 909 24.1 680 0.0 409 14.8 1012 (1133) 1243 93.2 30.0 514 17.4 7.0 72.1

63 (59) 63 42 99 0.0

IaAB IaAB IaAB IaB IaA II IaAB IaA IaAB IaAB IaA IaAB IaAB IaAB II IaAB IaAB IaAB IaAB IaAB IaAB IaAB IaAB IaAB IaAB

n.a. n.a. n.a. n.a. − 5.37 n.a. − 6.79 − 4.71 n.a. − 6.10 n.a. − 6.91 n.a. − 5.51 n.a. − 7.13 − 11.47 − 5.25 n.a. n.a. n.a. n.a. n.a. − 6.40 n.a.

BL YL–GR (BL) BL BL YL–GR BL BL BL BL GR BL BL YL–GR BL YL–BL BL BL BL YL–BL BL BL YL–GR BL BL BL BL YL–BL

YL–BL BL Enhanced lustre BL BL YL–GR BL YL–GR YL–GR (BL) Enhanced lustre BL Circular micro pits BL YL–GR

63 (61) 0.0 50 (50) 63 3.4 16 16 26 52 16 63 (59) 67 26 39 39 56 64 19

69.3 (53.2) 17.8 0.0 1126 88.1 5.0 102 (344) 58.9

16 (21) 71

IaAB IaAB II 90 IaB Very high 0.0 IaA 100 IaB 22 (7.9) IaAB 47 IaAB

n.a. n.a. −20.01 − 10.97 − 4.83 − 6.68 n.a. n.a.

14.2 0.0 387

46

n.a. − 8.98 n.a.

0.6

IaAB II IaA

R. Tappert et al. / Lithos 112S (2009) 806–821

Shape Shape comment

3.3 4.0 0.8 1.8 1.3

PsA I I PsA Ps

FLIN4-04 FLIN4-06 FLIN4-07 FLIN4-08 FLIN4-09 FLIN4-10 FLIN4-11 FLIN4-12 FLIN4-13 FLIN4-14

0.9 7.2 3.2 1.3 1.7 1.3 3.2 0.5 0.8 3.6

O D D DA O I D PsA O D

FLIN4-15 FLIN4-16 FLIN4-17 FLIN4-18 FLIN4-19

2.4 2.0 2.2 4.7 6.5

PsA D Ps I I

FLIN4-20 FLIN4-21 FLIN4-22 FLIN4-23 FLIN4-24 FLIN4-25 FLIN4-26 FLIN5-01 FLIN5-02 FLIN5-03

4.3 4.2 2.6 4.4 1.2 2.2 1.1 2.3 3.1 1.8

I I D D OA Ps PsA I I Ps

FLIN5-04 FLIN5-05 FLIN5-06 FLIN5-07 FLIN5-08 FLIN5-09 FLIN5-10 FLIN5-11 FLIN5-12 FLIN5-13 FLIN5-14 FLIN5-15 FLIN5-16 FLIN5-17 FLIN5-18 FLIN5-19 FLIN5-20 FLIN5-21 FLIN5-22 FLIN5-23 FLIN5-24 FLIN5-25 FLIN5-26 FLIN5-27 FLIN5-28 FLIN5-29

2.5 1.0 4.0 1.7 0.6 0.8 0.9 0.9 4.3 4.9 3.6 3.4 1.8 2.5 1.1 2.8 1.0 1.1 1.1 1.4 0.4 1.3 0.5 1.0 0.7 1.0

D O OA O D D Ps DA OA I I PsA O Ps D D D I D OA I O O I Ps Ps

Fragment, broken (old + new) Fragment, broken (new)

Flattened, half, broken (old) One remnant O face Aggregate of two Flattened, distorted, stepped D fragment, broken (old) Broken (old + new)

Half, one side concave, broken (old) Fragment, broken (old + new) Flattened, twinned Fragment, broken old D fragment, broken (new + old) D fragment, broken (new + old), distorted Fragment, broken (new + old) Fragment, broken (new + old) D fragment, broken (old) D fragment, broken (new)

D fragment, broken (new) Fragment, broken (old) Distorted, small pertrusion Fragment, broken (old) Macle Broken (old + new) Twinned, broken (new) Distorted Distorted

PBR PBR BR PBR PBR

X X X X

PBR YL PBR CL BR PBR BR BR CL CL

X X X X

YL CL CL CL PBR PBR PBR Distorted CL Aggregate of two, broken (new) BR Fragment, broken (new + old) PBR Fragment, broken (old) YL Aggregate of two PBR Rounded macle twin PBR Broken (old) PBR Perfect PBR Broken (new) BR CL Fragment, broken (old) PBR Flattened, broken (old) BR Aggregate of two PBR PBR PBR Twinned, stepped (rounded) BR Fragment, broken (old + new) PBR Partly concave PBR Fragment, broken (old) PBR

X X

X X X X X

X X X X X

X

X

X X X X

X

X X

X X X

X

X X

X X X

BR CL PBR CL BR BR PBR YL CL PBR PBR CL YL YL BR

X X

X X

Triangular plates

X X X

X

X

X

X

X

X X X

X X X

X X X X X X

X

GR

GR GR

X X X X X X

X X

GR

X

X

X X X

X

X

X X X X X X

X X X X X X

X X

GR GR GR

X

X X X X X

X X X

X X X X X X X X X X

X X

X

X

X

X X

X X X X X X X

X

X X X

X X X

X

X

X X

X X

X X

X X X

X X X X X X X X X

X X X X X X X X X X X X X X X X X X X

X X

X

X X

X X

X X X X

X X

X

YL–GR BL BL BL YL–BL

406 62.9 0.0 51.8 (101) 625 (491)

BL BL BL BL YL–GR BL YL–GR BL BL BL

7.0 17.8 (32.6) 71.2 8.9 78.9 (268) 930 63.2 (79.4) 745 1054 519 (149)

BL BL BL BL YL–GR (BL) YL–GR Circular micro pits BL Enhanced lustre BL BL BL Inclusion cavity BL BL n.a. YL–BL YL–GR (BL) BL Triangular plates BL BL BL BL BL BL BL BL BL BL BL BL BL BL YL BL BL YL–GR BL BL BL YL–BL YL–BL BL BL

20.5 0.0 42.4 107 (96.7) 30.7

3.5 69 52 (49) 3.8 (5.9) 100 5.0 (31) 16 100 0.0 (15) 27 23 (11) 72 100 95 (77) 86 60 73 (89) 54

IaA IaAB II IaAB IaA

n.a. n.a. − 2.80 n.a. − 5.53

IaB IaA IaAB IaB IaA IaAB IaAB IaAB IaB IaB Very high

− 6.59 n.a. n.a. − 4.48 − 4.00 n.a. − 6.16 n.a. − 7.00 n.a.

IaAB II IaAB IaAB IaAB

− 6.08 − 6.62 − 6.83 − 8.97 − 7.22 − 6.40 n.a. n.a. n.a. n.a. n.a. − 4.03 n.a. n.a. n.a.

118 (91.1) 808 1185 (1076) 573 (648) 8.6 29.6 227 0.0 288 (269) 76.9

12 (0.0) 51 62 (60) 45 (50) 92 25 100 18 (10) 50

IaAB IaAB IaAB IaAB IaB IaAB IaB II IaAB IaAB

758 (751) 19.6 35.0 22.9 9.6 1.9 9.0 951 (581) 1.3 68.0 (25.5) 572 9.0 4.2 9.9 404 25.1 175 36.8 353 78.9 27.7 5.7 79.1 68.0 548 75.9

40 (37) 71 100 86 100 100 83 87 (71) 46 47 (5.5) 100 66 90 100 87 49 98 49 0.9 47 90 98 18 2.5 26 74

IaAB n.a. IaAB n.a. IaB − 4.32 IaAB n.a. IaB − 3.26 IaB − 5.79 IaAB n.a. IaAB Very high − 9.65 IaAB − 6.53 IaAB n.a. IaB n.a. IaAB n.a. IaB n.a. IaB n.a. IaAB Very high − 9.57 IaAB n.a. IaB High − 12.09 IaAB − 6.50 IaA n.a. IaAB n.a. IaAB n.a. IaB n.a. IaAB n.a. IaA − 7.01 IaAB n.a. IaAB − 6.49

809

(continued on next page)

R. Tappert et al. / Lithos 112S (2009) 806–821

FLIN3-29 FLIN3-30 FLIN4-01 FLIN4-02 FLIN4-03

810

Table 1 (continued) Surface textures Sample

Weight [mg]

Shape Shape comment

Ps O PsA Ps D

FLIN6-05 FLIN6-06 FLIN6-07 FLIN6-08 FLIN6-09 PBS-01

1.0 0.5 0.7 2.2 2.0 68.3

D I D I I D

SL1-01 SL1-02 SL2-01 SL2-02 SL2-03

0.7 0.5 3.5 0.9 1.0

I I D Ps O

SL2-04 SL3-01 SL3-02 SL3-03 SL4-01

0.6 5.8 0.5 0.5 0.2

Ps PsA OA D D

One side flat Twinned Aggregate of two Aggregate of two One side remnant O face, broken (old) O fragment, broken (new) Fragment, broken (old) Fragment, broken (old) D fragment, broken (new) Macle, broken (new) Fragment, broken (new) Fragment, broken (new) Flattened, broken (old) D side flat, broken (new) Macle twin, stepped, broken (old) Stepped (rounded) D side flat Stepped Fragment, broken (old) Fragment, broken (old)

Eurelia K3 kimberlite FBS-03-01 6.6

I

FBS-03-02

1.7

I

FBS-03-03

5.0

I

FBS-03-04 FBS-03-05 FBS-03-06 FBS-03-07 FBS-03-08

3.0 5.0 4.3 2.7 7.9

D D DOA Ps OD

Aggregate of two

D

Eurelia K2 kimberlite FBS-04/1-01 3.2

DO fragment, broken (old + new) DO fragment, broken (old) DO fragment, broken (old + new)

PBR CL PBR PBR PBR CL PBR PBR PBR PBR YL CL CL YL PBR PBR PBR BR BR PBR PBR

Defor. Sh. Trigons Hillocks Terraces Frosting Corr. Lam. Sc.

GR

X X

BR

X X X X

GR

X X

X

X X X X X

X X X X X

X

X X

X X

X

X

X

Etch Ruts Other

UV-color

N-Conc. N-Agg. [atomic ppm] [%B]

Type Hydrogen δ13C [‰ PDB]

X

BL BL BL BL BL

41.4 641 26.1 26.6

33 1.9 49 94 98

IaAB IaA IaAB IaB IaB

n.a. n.a. n.a. n.a. − 6.14

BL BL PI–BL BL BL Network patterns, GR percussion marks BL BL BL YL–BL BL

14.1 13.6 117 41.0 14.2 417

77 93 100 100 87 0.0

IaAB IaB IaB IaB IaAB IaA

n.a. n.a. − 5.81 n.a. n.a. n.a.

625 1512 657 (646) 95.8 23.3

53 72 35 (48) 6.5 45

IaAB IaAB IaAB IaA IaAB

n.a. n.a. n.a. − 5.56 n.a.

GR YL–GR YL–GR BL BL

173 189 157 594 1015

88 13 28 34 35

IaAB High IaAB IaAB IaAB IaAB

n.a. − 4.83 − 2.75 − 4.22 n.a.

OR

16.6

71

IaAB

− 6.39

YL–GR (BL) OR

18.3

0.0

IaA

− 8.30

14.2

71

IaAB

− 6.32

BL BL BL BL PI–BL

7.1 181 8.6 5.5 238 (50.9)

97 56 73 69 100 (100)

IaB IaAB IaAB IaAB IaB

− 4.35 − 8.60 − 1.88 − 6.22 n.a.

BL

232

79

IaAB

− 9.40

X X X X

X

X X X X

X X

X X

X X X X X

X X X

X X X

X

X X X X

X X

X X

X X X

X X

X

BR

X

X

X

BR

X

X

X

X

X

X

X

X

X X X

X X X X X

X X X

X

X

BR

X X X X

Broken (old + new)

PBR PBR PBR PBR BR

Broken (old)

PBR

X

X

X X

X

X X

X X X X Triangular plates

X

X

Shallow depressions

R. Tappert et al. / Lithos 112S (2009) 806–821

Springfield Basin FLIN5-30 1.6 FLIN6-01 1.7 FLIN6-02 2.1 FLIN6-03 0.7 FLIN6-04 3.9

Color Ir. Spots.

FBS-04/1-02 FBS-04/2-01

6.8 3.1

DOA D

I DOA

FBS-05-03 FBS-05-04 FBS-05-05 FBS-05-06 FBS-05-07 FBS-05-08

5.0 13.0 3.7 2.8 11.6 17.6

DO DA OD I DA I

FBS-05-09 FBS-05-10 FBS-05-11 FBS-05-12 FBS-05-13

2.5 1.8 1.8 1.7 3.8

I O O O DO

FBS-05-14 FBS-05-15 FBS-05-16 FBS-05-17 FBS-05-18 FBS-05-19 FBS-05-20 FBS-05-21 FBS-05-22 FBS-05-23 FBS-05-24 FBS-05-25 FBS-05-26 FBS-05-27 FBS-05-28 FBS-05-29 FBS-05-30 FBS-05-31 FBS-05-32

1.8 1.7 0.9 1.0 1.5 0.9 1.0 0.7 0.9 0.6 1.5 0.5 0.8 0.4 0.8 0.8 1.6 0.1 6.2

DO Ps D D I PsA Ps O D DO D I OA I OA OA C I Ps

PBR CL

X X

X X

X X

X X

X X

X X

X

X

X X

X X

X X

X

X

X X X X X X

X X X

X

X

X

X X X X

X

PBR PBR

Slightly distorted Stepped

D fragment, broken (new) Partly twinned (macle) Twinned (macle) Twinned (macle) Elongated fragment, broken (old) Broken (old)

PBR CL PBR CL PBR CL

X X

BR CL CL CL CL

PBR PBR Broken (old) PBR Elongated CL D fragment, broken (old + new) BR CL PBR Broken (old) CL Flattened, distorted PBR Fragment, broken (new) PBR Broken (old) CL Skeletal fragment CL CL DO fragment, broken (new) BR CL CL Distorted GRY Fragment, broken (old) CL One side concave BR

X

X X

X X X X

X X X

X X X X

X

X X

X X

X X

X

X

X

X X

X

X X

BL BL

9.6 660

100 16

IaB IaAB

− 5.23 − 3.96

X X

YL YL–GR (BL) BL BL YL–GR YL–GR BL BL

31.2 32.4

40 42

IaAB IaAB

− 15.74 − 7.24

21.9 27.2 280 54.1 556 759 (823)

93 84 1.5 46 51 52 (53)

IaB IaAB IaA IaAB IaAB IaAB

− 4.54 − 6.46 − 5.45 − 15.75 − 6.96 n.a.

YL–GR BL BL BL BL

240 21.8 24.9 39.0 13.5

2.8 55 92 84 100

IaA IaAB IaB IaAB IaB

− 5.87 − 4.35 − 3.75 − 2.45 − 6.05

BL YL–GR PI–BL BL YL–GR YL–GR YL–GR BL YL–GR YL–GR YL–GR BL BL YL–GR BL BL YL n.a. n.a.

14.8 116 856 19.6 71.8 446 218 51.0 23.6 37.8 249 1311 24.2 253 45.2 52.9 (66.9) 862 2568 (2853) 10.8

36 1.0 25 82 29 0.0 0.0 2.7 17 4.8 5.4 50 28 8.4 11.9 25 (27) 13 39 (40) 50

IaAB IaA IaAB IaAB IaAB IaA IaA IaA IaAB IaA IaA IaAB IaAB IaA IaAB IaAB IaAB IaAB IaAB

− 4.59 − 4.87 − 4.65 − 6.09 − 6.14 − 4.85 − 5.95 − 6.05 n.a. − 4.16 − 5.57 − 5.94 − 3.08 − 6.18 − 6.09 − 6.05 − 4.30 − 6.03 − 7.02

X X X

Triangular plates X

X

X

Corrosion sculptures X X

X X

Inclusion cavity

X

X

X

X X X X

X X X

X

X

X

X X

X

X X X

X

X

X

X X

X

X X

Inclusion cavity Triangular plates

Triangular plates

R. Tappert et al. / Lithos 112S (2009) 806–821

Eurelia K7 kimberlite FBS-05-01 13.8 FBS-05-02 6.6

One face remnant O

Abbreviations: O—Octahedral, D—Dodecahedral, I—Irregular, Ps—Pseudohemimorphic, A—Aggregate, YL—yellow, CL—colorless, BR—brown, PBR—pale brown, GRY—grey, GR—green, BL—blue, PI—pink. Ir.Spot.—Irradiation spots, Defor.—Derformation Lines, Sh.Lam.—Shield Laminae, Corr.Sc.—Corrosion Sculpture, N-Conc.—Nitrogen concentration, N-Agg.—Nitrogen Aggregation, ()—rim composition/color, n.a.—not analyzed.

811

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and known kimberlite derived diamonds, we also included a comparison to diamonds from Eurelia kimberlites. 2. Samples and analytical techniques One hundred and twenty-two diamonds from the Springfield Basin and 43 diamonds from three different kimberlite dykes in the Eurelia area (K2, K3, and K7) were examined (Table 1). The diamonds were recovered by Flinders Diamonds Ltd. during sampling programs between 1998 and 2007. An additional three diamonds from the Echunga goldfields were provided by the South Australian Museum and the Department of Primary Industries and Resources South Australia (PIRSA) for a morphological study. All diamonds were visually examined and their physical characteristics were documented (Table 1). UV fluorescence characteristics of the diamonds were analyzed using a Leica MZ16FA stereo microscope attached to a HBO mercury short arc lamp. Concentration and aggregation state of nitrogen impurities in the Springfield Basin and Eurelia diamonds were determined by Fourier transform infrared (FTIR) spectroscopy using a Thermo Nicolet Nexus 470 FTIR spectrometer equipped with a Nicolet Continuum infrared microscope at the University of Alberta. Spectra were collected in transmittance mode, between 650 and 4000 cm− 1 (2.5–15.4 µm), at a resolution of 4 cm− 1, and a spot size of 100 × 100 µm. After conversion to absorption coefficients, the spectra were de-convoluted into the A, B and D components. Concentrations of nitrogen in A- and B-centers were calculated using the absorption strength at 1282 cm− 1 (Boyd et al., 1994; Boyd et al., 1995). The detection limits depend strongly on the quality of the fragments, but are generally in the range 5–20 ppm. The relative errors for concentration and aggregation state for each measurement are in the range 10–20%. If a high-quality spectrum could not be collected through the whole diamond, the diamond was crushed, and clear, inclusion free fragments selected for analysis. Multiple analyses through different parts of the diamond (core–rim) have been performed whenever possible, in order to detect variations of the nitrogen content and the aggregation state (Table 1). The presence of atomic hydrogen as impurity in the diamonds, which causes a sharp absorption band at 3107 cm− 1 in the FTIR spectrum (Runciman and Carter, 1971), has been monitored. The carbon isotope compositions of selected samples from the Springfield Basin and Eurelia were determined with a Finnigan MAT 252 gas flow mass spectrometer at the University of Alberta, after combusting 0.5–1.5 mg of inclusion free diamond fragments together with ∼1 g CuO as an oxygen source in a sealed and evacuated quartz tube at 1000 °C for ∼ 12 h. The results are given relative to the V-PDB standard (Coplen et al., 1983). Instrumental precision and accuracy is on the order of ±0.02‰. Mineral inclusions within the diamonds were released from the host diamonds by crushing. Completely released inclusions were mounted in brass rings and then polished. Partially exposed inclusions were analyzed in-situ. The major and minor element compositions of the inclusions were determined with a CAMECA SX-51 electron microprobe at the University of Adelaide. The measurements were made using a 15 kV acceleration voltage and 20 nA probe current, with a detection limit for oxide species of ∼ 100 ppm (except Na2O: 200 ppm). Accuracy and precision were tested on secondary standards and are within ∼1.0% relative for the major elements. 3. Results 3.1. Diamond sizes and crystal shapes The diamonds from the Springfield Basin and the Eurelia kimberlites range in size from 0.5 to 5 mm in diameter and weigh between 0.1 and 68.3 mg (0.0005–0.34 carat), with an average weight of 3.9 mg (0.02 carat) (Table 1). Diamonds from both areas present a

Fig. 2. Distribution of crystal shapes of diamonds from the Springfield Basin and Eurelia kimberlites.

range of crystal shapes, but the relatively small number of diamonds may not fully represent the entire diamond population. Diamonds with predominantly octahedral shapes are present in similar proportions in the Springfield Basin and Eurelia kimberlites (Fig. 2). Dodecahedral diamonds are less abundant in the Springfield Basin (23%) than in the Eurelia kimberlites (40%). Diamonds classified as irregular (i.e., diamonds with less than half of their crystal faces developed) are more abundant among the Springfield Basin diamonds (36%) compared to the diamonds from the Eurelia kimberlites (26%). Irregular diamonds were further distinguished based on the presence or absence of etch marks on fracture surfaces (Table 1). Pseudohemimorphic diamonds are present at both locations (Springfield Basin: 24 diamond = 21%, Eurelia: 5 diamonds = 12%, see Fig. 3A,B). One diamond cube was recovered from the K7 kimberlite at Eurelia. Twenty-one diamonds from the Springfield Basin and eight diamonds from Eurelia formed aggregates of two or more individual crystals. Twinning was observed on eleven diamonds from the Springfield Basin and three diamonds from the Eurelia area, with most of the diamonds forming macle twins. The three diamonds from the Echunga goldfield weigh 1.0, 0.84, and 0.46 carat. The two larger diamonds have octahedral shapes with rounded edges; the smallest diamond is a rounded dodecahedron. 3.2. Diamond surface textures Most of the surface textures, which were classified based on Robinson (1979), are restricted to octahedral or dodecahedral crystal faces (Table 1). The presence of deep ruts and etch pits on many of the diamonds from the Springfield Basin (41%) and the Eurelia area (51%) causes some of the diamonds to appear brittle (Fig. 4A). The ruts and etch pits are likely caused by exposure of the diamonds to the oxidizing host kimberlite during transport to the Earth's surface. Deformation lamellae (Fig. 4B), which were observed exclusively on dodecahedral crystal faces and are caused by plastic deformation of the diamond within the Earth's mantle, were present on 46% of the diamonds from the Springfield Basin and 28% of the diamonds from Eurelia. Less commonly observed surface textures include, e.g., microdisc patterns (Fig. 4C). The largest diamond from the Springfield Basin, PBS-01 (“The Springfield”), and all three Echunga diamonds exhibit surface textures that are diagnostic for abrasion during transport on the Earth's surface. These textures include percussion marks (Fig. 3F) and network patterns (Fig. 4D). Percussion marks are the result of the impact of

R. Tappert et al. / Lithos 112S (2009) 806–821

813

Fig. 3. A and B: Diamonds with pseudohemimorphic crystal shape from Springfield Basin (A) and K3 kimberlite, Eurelia (B). The diamond from the Springfield Basin exhibits a small green irradiation spot on its surface (arrow). C and D: Diamonds with octahedral crystal shape from Springfield Basin (C) and K7 kimberlite, Eurelia (D). E: Dodecahedral diamond from Springfield Basin (FLIN2-18) with brown and green irradiation spots. F: Crescent-shaped percussion marks on the surface of diamond PBS-1, Springfield Basin.

particles on the diamond surface. These impacts are restricted to high energy environments, such as fast flowing rivers, where transportation allows mineral grains to saltate. Network patterns are caused by the steady abrasion of the diamond surface and develop typically on plastically deformed dodecahedral diamonds during transport in fluvial environments (Robinson, 1979). 3.3. Diamond colors Only a small proportion of the diamonds from the Springfield Basin and the Eurelia area are characterized by intense body colors, with brown being the most common. Twenty-six diamonds from the Springfield Basin (21%) and eight diamonds (19%) from the Eurelia area have an intense brown coloration (Fig. 5). More common, however, are diamonds with a very pale brown tinge. A large portion of the Springfield Basin diamonds (52 diamonds = 42%) and of the diamonds from the Eurelia area (17 diamonds = 40%) fall in this

category. Thirty diamonds from the Springfield Basin (25%) and 17 diamonds (42%) from Eurelia were classified as colorless. Diamonds with a yellow body color are restricted to the Springfield Basin, where 14 diamonds (12%) have pale yellow colorations. Yellow diamonds are not present in the set of diamonds from Eurelia. One diamond from Eurelia is grey. This grey diamond is also the only cube shaped diamond within the present sample set. Nineteen diamonds from the Springfield Basin and the two smaller diamonds from Echunga had green or brown colored surface spots. None of the diamonds from Eurelia was found to have surface spots. Of the nineteen Springfield Basin diamonds, sixteen had only green spots, one had only brown spots, and two had green and brown spots. Green spots were present on both of the Echunga diamonds, but only the smallest diamond from Echunga had an additional brown spot. The spots on the diamonds ranged in intensity from pale to dark, and they are present as single spots and as clusters covering large areas of the diamond surface (Fig. 3E, Table 1). Green surface spots form as a

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R. Tappert et al. / Lithos 112S (2009) 806–821

Fig. 4. Secondary electron images of surface textures of diamonds from the Springfield Basin. A: Deep ruts (dotted line), etch marks, and trigons on the surface of diamond FLIN3-11. B: Deformation laminae on a dodecahedral crystal face of diamond FLIN5-18. C: Microdisc patterns on the surface of diamond FLIN4-21. D: Network patterns on a dodecahedral crystal face of diamond PBS-1.

result of damage to the diamond surface by α-particle irradiation (Meyer et al., 1965). Irradiation damage on diamonds can occur within the host kimberlite, particularly within the upper oxidized zones where radioactive-element enriched groundwater can infiltrate the kimberlite (Harris, 1992). Irradiation spots are also commonly observed on the surface of diamonds from placer deposits, where they have been attributed to the presence of radioactive minerals (Vance et al., 1973). In this case, irradiation results in the formation of

Fig. 5. Distribution of colors of diamonds from Springfield Basin and Eurelia kimberlites.

green surface spots or clusters of variable color intensities. Because irradiation spots are not present on the surface of any diamonds from the Eurelia kimberlites, it is likely that the Springfield Basin diamonds were exposed to radioactive minerals, such as detrital zircon, within the Springfield Basin itself. Green surface spots may become brown when heated to temperatures N500 °C (Vance et al., 1973), which can result from regional metamorphism or contact metamorphism in the vicinity of a magmatic intrusion. The presence of green and brown surface spots on the same diamond implies that after an initial irradiation and heating event the diamond was moved to a different location and subsequently received additional irradiation damage.

Fig. 6. Relative frequency of nitrogen concentrations in diamonds from the Springfield Basin (dashed line) and diamonds from Eurelia. The Eurelia diamond with the highest nitrogen content (2568 ppm) is not represented.

R. Tappert et al. / Lithos 112S (2009) 806–821

3.4. Nitrogen concentrations and aggregation states

815

diamonds are the three Type IaA diamonds that also have brown radiation spots. The fourth green-fluorescing diamond (SL2-04) contains nitrogen in a highly aggregated state. This diamond also contains a high amount of hydrogen.

The nitrogen concentrations of the diamonds from the Springfield Basin and Eurelia range from below the detection limit (Type II diamonds) to a maximum of 2853 ppm for a diamond from the K7 kimberlite at Eurelia. The concentrations of nitrogen in diamonds from the Springfield Basin do not exceed 1820 ppm. The majority of the diamonds from the Springfield Basin (56.6%) and Eurelia (60.5%) have nitrogen concentrations below 100 ppm (Fig. 6). This includes seven diamonds from the Springfield Basin, which have been classified as Type II diamonds (no detectable nitrogen). The distribution of nitrogen among the nitrogen-rich (N100 ppm) diamonds from the Springfield Basin is characterized by two discrete modes at 600– 700 ppm and at 1100–1200 ppm (Fig. 6). The aggregation state of the nitrogen impurities in the diamonds from the Springfield Basin and Eurelia is variable and ranges from low aggregation levels, with nitrogen mainly in A-centers (Type IaA diamonds) to highly aggregated nitrogen (Type IaB diamonds) (Fig. 7).

The stable carbon isotope compositions (δ13C) of 52 diamonds from the Springfield Basin range from −2.8 to −20.0‰ relative to VPDB (Fig. 9). Most of the Springfield Basin diamonds, however, fall between − 2 and −10‰. This is similar to the isotopic range of the diamonds from Eurelia (− 2.5 to −15.8‰, see Tappert et al., 2009). The distribution of carbon isotope values for diamonds from the Springfield Basin has a mode at −6.5‰, which is similar to the −6.0‰ mode of Eurelia diamonds (Fig. 9). Obvious correlations between the physical characteristics of the diamonds (crystal shape, surface textures, colors) and their carbon isotope composition were not observed.

3.5. UV fluorescence

3.7. Diamond inclusions

All diamonds from the Springfield Basin and from Eurelia fluoresce at various intensities in response to UV illumination. The observed UV fluorescence colors include blue, yellow, pink and orange, and green. Eighty-seven diamonds from the Springfield Basin (71%) and twentytwo diamonds from Eurelia (51%) have blue UV fluorescence colors of variable intensities and shades (Fig. 8A). The blue UV fluorescence color, in most cases, correlates with the presence of nitrogen in elevated aggregation states (Type IaAB and IaB diamonds). However, four blue fluorescing Type IaA diamonds are present (Table 1). Yellow UV fluorescence colors (Fig. 8B) were observed on twenty-nine diamonds from the Springfield Basin (24%) and fifteen diamonds from the Eurelia area (35%). The shades vary from yellow blue to yellow– green (Fig. 8C), and yellow–green UV colors are commonly associated with diamonds of brown body coloration. Most diamonds with yellow UV fluorescence colors have nitrogen in low aggregation states (Type IaA diamonds, Type IaAB diamonds with b30% B-centers). Five diamonds from the Springfield Basin and two diamonds from Eurelia have a yellow fluorescing rim around a blue fluorescing core. Three diamonds, one from the Springfield Basin and two from Eurelia, have pink fluorescing cores, which in all cases are enclosed by a blue fluorescing rim (Fig. 8D). Two diamond fragments from Eurelia (FBS301 and FBS3-03) exhibit orange UV fluorescence (Fig. 8E). Based on their distinct brown color and very similar nitrogen characteristics, it is likely that these fragments are part of the same diamond. A distinctive emerald green UV fluorescence color (Fig. 8F) was observed on four diamonds from the Springfield Basin. These

The most common inclusion in diamonds from the Springfield Basin and from Eurelia is graphite, which occurs as small flakes, often along fractures. One lherzolitic garnet, ∼ 250 mm in diameter, was recovered from diamond FLIN4-11. This garnet has a Cr2O3 content of 9.04 wt.%, a CaO content of 7.39 wt.%, and a TiO2 content of 0.26 wt.% (see Table 2). It is compositionally similar to garnet xenocrysts from kimberlites at Eurelia, which are also predominantly lherzolitic (Scott Smith et al., 1984). Diamond FLIN2-18 contained a ∼ 50 µm purplishred inclusion, which was lost during the crushing of the host diamond. This inclusion, which was likely a peridotitic garnet, was the only inclusion within any of the brown spotted diamonds from the Springfield Basin. Ferropericlase inclusions were found in two diamonds from the Springfield Basin (FLIN5-7, FLIN6-4). In both cases, multiple individual crystals were recovered, ranging in size from around 30 to 100 µm. The ferropericlase inclusions in both diamonds are compositionally similar, with Mg-numbers in the range 85–86, and NiO contents of 1.25–1.47 wt.% (Table 2). An additional crystal with olivine stoichiometry has been identified in diamond FLIN5-07 as a small (b20 µm) inclusion fragment, which was exposed on a diamond cleavage surface after crushing. Because of its small size it was not possible to recover the inclusion for polishing. Therefore, only a poor quality in-situ analysis could be attained (Table 2). The composition of the ferropericlase inclusions from the Springfield Basin diamonds is almost identical to the ferropericlase inclusions in diamonds from the Eurelia kimberlites, for which a sublithospheric, possibly lower mantle origin has been established based on their association with MgSiPvk (Tappert et al., 2009; Table 2). An inclusion with olivine stoichiometry, similar to the one in the Springfield Basin diamond FLIN5-07, was also found in diamond FBS5-12 from Eurelia (Table 2). Even though the olivine-bearing diamond from Eurelia did not contain additional ferropericlase, a possible sublithospheric origin for this diamond has been inferred based on its low-nitrogen concentration and high nitrogen aggregation states (Tappert et al., 2009).

3.6. Carbon isotopes

4. Discussion 4.1. Springfield Basin and Eurelia: a comparison

Fig. 7. Distribution of nitrogen aggregation states for Springfield Basin and Eurelia diamonds.

The physical characteristics (crystal shape, color, and surface textures) of the placer diamonds from the Springfield Basin resemble those of the diamonds from the Eurelia kimberlites (Figs. 3 and 5). Both have a high abundance of diamonds with pseudohemimorphic shapes, pale brown coloration, and deep ruts or etch pits (Table 1). Diamonds from both deposits also have similar nitrogen characteristics. They have a high abundance of low-nitrogen (b100 ppm)

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R. Tappert et al. / Lithos 112S (2009) 806–821

Fig. 8. Fluorescence colors of diamonds from the Springfield Basin and Eurelia kimberlites. A: Blue fluorescence (FLIN4-13, Springfield Basin) B: Yellow fluorescence (FBS5-01, K7 kimberlite, Eurelia) C: Yellow–green fluorescence (FBS5-09, K7 kimberlite, Eurelia) D: Blue fluorescing rim with pink fluorescing core (FBS3-08, 41C kimberlite, Eurelia) E: Orange fluorescence (FBS3-01, 41C kimberlite, Eurelia) F: Green fluorescence (FLIN2-02, Springfield Basin).

diamonds, which have variable nitrogen aggregation states (Fig. 10). The carbon stable isotope compositions of the diamonds from the Springfield Basin and Eurelia have similar ranges, and a similar distribution of isotope values (Fig. 9). The modes of δ13C values of around −6.0‰ in both deposits distinguishes the South Australian diamonds from other diamond populations worldwide, which commonly have δ13C modes of around − 5.0 to −4.0‰ (Deines, 1980; Cartigny et al., 1998). Diamonds with isotopic modes of around −5.0 to −4.0‰ are usually linked to primordial carbon reservoirs within the upper mantle (Deines, 2002). A small portion of the diamonds from the Springfield Basin and from Eurelia has an isotopically light composition, with δ13C values ranging from −8.5 to −20‰ (Fig. 9). Diamonds with similarly light compositions have been identified in numerous other deposits worldwide,

where they are almost exclusively associated with eclogitic inclusion assemblages (Sobolev et al., 1979; Kirkley et al., 1991). It is possible that eclogitic sources were also involved in the formation of the isotopically light diamonds from South Australia, but at this point it cannot be proven, because of the lack of mineral inclusions in these diamonds. The nature of the isotopically light carbon reservoirs for eclogitic diamonds is controversial; it has been proposed that low δ13C values of diamonds reflect fractionated mantle carbon (Cartigny et al., 2001), but other models favor the involvement of subducted organic matter, which is also isotopically light (Frank, 1969; Sobolev and Sobolev, 1980). Strong evidence for a close relationship between the diamond populations from the Springfield Basin and Eurelia comes from the presence of ferropericlase-bearing mineral inclusion assemblages in diamonds from each area. Ferropericlase has been identified as an

R. Tappert et al. / Lithos 112S (2009) 806–821

Fig. 9. Carbon stable isotope composition (δ13C) of diamonds from the Springfield Basin (A) and Eurelia kimberlites (B).

inclusion in diamonds from several deposits in the world, but its occurrence is generally restricted to a small number (b5) of diamonds (Liu, 2002). Only some diamond deposits in western Brazil, including the São Luiz/Juina area, and in the Kankan district in Guinea have yielded a larger number of diamonds (N15) with ferropericlase inclusions (Wilding, 1990; Harte and Harris, 1994; Hutchison, 1997; Harte et al., 1999; Stachel et al., 2000; Kaminsky et al., 2001; Bulanova et al., 2008; Kaminsky et al., this issue). The presence of ferropericlase

817

inclusions is an indicator of an unusual deep origin for a diamond, because ferropericlase, in a typical peridotitic mantle environment (pyroxene saturated under upper mantle conditions), is only stable at pressures greater than around 25 Gpa, which corresponds to a depth of N650 km (Liu, 1975, 1976b). This is considerably deeper than the estimated depth of origin for the vast majority of diamonds worldwide, which is in the range 140–250 km (e.g. Boyd and Gurney, 1986; Meyer, 1987; Stachel and Harris, 1997). Ferropericlase is one of the main mineral constituents of the lower mantle, along with magnesium silicon oxide (MgSiO3), which is compositionally equivalent with enstatite, but at pressure conditions of the lower mantle has perovskite structure (MgSiPvk) (Liu, 1976a). Unlike the presence of ferropericlase as a single inclusion mineral, which theoretically can form in chemically unusual environments in the upper mantle (Brey et al., 2004), the occurrence of ferropericlase together with MgSiO3 (MgSiPvk or retrograde enstatite) in the same diamond, is generally regarded as proof for their lower mantle origin. The possibility of a lower mantle origin for diamonds from Eurelia (also referred to as “Orroroo”) has previously been suggested by Scott Smith et al. (1984), based on findings of two diamonds with ferropericlase inclusions, and one diamond with an inclusion of MgSiO3-stoichiometry. Because the ferropericlase and MgSiO3 inclusions, in this case, were not only recovered from different diamonds, but also from different kimberlite dykes, it could not be established that the inclusions formed in equilibrium, and hence were of lower mantle origin. In addition, the relatively high nickel content of the MgSiO3 inclusion (0.13 wt.% NiO) suggests an upper mantle origin, since MgSiO3 inclusions from the lower mantle generally have nickel contents of b0.05 wt.% NiO (Stachel et al., 2000). More robust evidence for a lower mantle origin of the Eurelia diamonds was presented by Tappert et al. (2009), who recovered the diagnostic ferropericlase–MgSiO3 assemblage within a diamond from the K7 kimberlite at Eurelia. In this case, the low-nitrogen concentration and the high aggregation state of nitrogen of the host diamond supported the inclusions evidence for a lower mantle origin. The ferropericlase inclusions from the two Springfield Basin diamonds FLIN5-7 and FLIN6-4 are compositionally (Mg#: 85–86, NiO: 1.17–1.47 wt.%, Cr2O3: 0.11–0.62 wt.%) indistinguishable from the ferropericlase inclusions in diamonds from the Eurelia area (Table 2). In addition, both diamonds have low concentrations of nitrogen impurities, which are present in highly aggregated states (FLIN5-7: 22.9 ppm, 86%B; FLIN6-4: 8.2 ppm N, 98%B; Table 1). The nitrogen characteristics of the two Springfield Basin diamonds is consistent with the nitrogen data for the ferropericlase-bearing diamonds from

Table 2 Composition of mineral inclusions in diamonds from South Australia (given in weight percent). Deposit

Springfield Basin

Diamond ID

FLIN411

FLIN507

FLIN507

FLIN507

FLIN507

FLIN507

FLIN507

FLIN604

FLIN604

Eurelia kimberlite K7 FBS5-11

Mineral

Garnet

Fper

Fper

Fper

Fper

Fper

Olivinea

Fper

Fper

MgSiPer Fper

P2O5 SiO2 TiO2 Al2O3 Cr2O3 MgO CaO MnO FeO NiO Na2O K2O Total Mg#

0.03 40.44 0.26 16.09 9.04 19.15 7.39 0.39 6.79 0.02 ≤0.02 ≤0.01 99.6 83.4

≤0.01 ≤0.01 ≤0.01 0.04 0.31 73.74 ≤0.01 0.17 23.60 1.47 0.03 ≤0.01 99.4 84.8

≤0.01 ≤0.01 ≤0.01 ≤0.01 0.16 75.19 ≤0.01 0.17 22.28 1.29 ≤0.02 ≤0.01 99.1 85.7

0.02 0.03 ≤0.01 ≤0.01 0.15 76.67 ≤0.01 0.17 22.75 1.25 0.03 ≤0.01 101.1 85.7

≤0.01 0.02 ≤0.01 ≤0.01 0.19 73.16 ≤0.01 0.13 23.25 1.25 0.00 0.01 98.0 84.9

0.02 0.02 ≤ 0.01 ≤ 0.01 0.14 74.97 ≤ 0.01 0.18 23.03 1.45 ≤ 0.02 ≤ 0.01 99.8 85.3

0.01 44.85 0.02 0.04 0.00 55.76 0.04 0.13 4.91 0.12 0.00 0.00 105.9 95.3

≤ 0.01 0.03 ≤ 0.01 0.03 0.50 74.81 ≤ 0.01 0.17 22.02 1.21 0.03 ≤ 0.01 98.8 85.8

≤0.01 0.03 0.02 ≤0.01 0.62 75.81 ≤0.01 0.17 22.16 1.17 0.04 ≤0.01 100.1 85.9

≤0.01 60.02 0.03 0.25 0.38 36.58 0.05 0.15 2.93 0.09 ≤0.02 ≤0.01 100.5 95.7

Abbreviations: Fper—ferropericlase, MgSiPer—MgSi perovskite. a Poor quality in-situ analysis.

FBS511 ≤0.01 0.02 0.02 ≤0.01 0.14 73.96 0.01 0.14 23.78 1.16 ≤0.02 ≤0.01 99.3 84.7

FBS511

FBS511

FBS511

FBS511

FBS511

Fper

Fper

≤0.01 0.02 0.03 ≤0.01 0.26 74.04 ≤0.01 0.17 23.57 1.26 0.03 ≤0.01 99.4 84.8

≤ 0.01 ≤ 0.01 0.02 ≤ 0.01 0.18 73.88 ≤ 0.01 0.20 23.12 1.16 0.03 ≤ 0.01 98.6 85.1

FBS512

Fper

Fper

Fper

Olivine

0.02 ≤0.01 ≤0.01 0.02 0.21 73.57 ≤0.01 0.21 23.59 1.19 ≤0.02 ≤0.01 98.8 84.8

≤0.01 0.02 ≤0.01 ≤0.01 0.19 73.62 ≤0.01 0.19 23.11 1.40 ≤0.02 ≤0.01 98.6 85.0

0.02 ≤0.01 ≤0.01 ≤0.01 0.18 75.14 ≤0.01 0.21 23.33 1.12 ≤0.02 ≤0.01 100.0 85.2

≤ 0.01 40.78 0.02 ≤ 0.01 0.07 48.81 0.06 0.11 8.45 0.47 ≤ 0.02 ≤ 0.01 98.8 91.1

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age of the conglomerates in the Springfield Basin. Contrary to the notion of proximal kimberlitic sources, this would indicate that the Eurelia kimberlites are not the direct source of the diamonds in the Springfield Basin. However, since age determinations are restricted to only two of the kimberlite dykes at Eurelia, it cannot be excluded that an older kimberlite generation at Eurelia exists. Differences in the distribution of mantle-derived xenocrysts in kimberlites from Eurelia, noted by Scott Smith et al. (1984), may reflect such different kimberlite age groups. Alternatively, the Springfield Basin diamonds may be derived from undiscovered Permian or pre-Permian kimberlites outside the Eurelia area. Inaccuracies in the estimates of the depositional ages of the conglomerates in the Springfield Basin may be another explanation for the discrepancy to the emplacement ages of the known kimberlites. If the conglomerates in the Springfield Basin were ∼ 80–100 million years younger than previously suggested, the Eurelia kimberlites could be the direct source of the diamonds within these conglomerates. Additional information about the emplacement ages of the kimberlites at Eurelia, and further constraints on the deposition ages of the conglomerates in the Springfield Basin may resolve the current age discrepancy. 4.2. The old diamonds of the Springfield Basin

Fig. 10. Concentration of nitrogen impurities versus aggregation states of nitrogen (given as percentage of higher aggregated B-centers relative to A-centers) in diamonds from the Springfield Basin and from Eurelia kimberlites. Dotted lines represent isochrones for an assumed residence temperature of 1200 °C; calculated after Taylor et al. (1990).

the Eurelia area, which provides evidence for the sublithospheric origin of the Springfield Basin diamonds and reinforces the observed similarities between the diamond populations of the two areas. The physical and compositional similarities between the bulk of the placer diamonds from the Springfield Basin and the diamonds from kimberlites at Eurelia suggests that the Springfield Basin diamonds are derived from kimberlitic sources that resemble or are identical with the Eurelia kimberlites. Additional support for this interpretation comes from the presence of abundant diamond indicator minerals in the Springfield Basin, which suggests proximal kimberlitic sources. However, the emplacement age of the kimberlites at Eurelia is generally considered to be Jurassic, based on two identical U–Pb ages for zircons of 170 Ma from the K3 and K5 kimberlites (Scott Smith et al., 1984), and therefore younger than the proposed Permian

Three diamonds from the Springfield Basin (FLIN2-02, FLIN2-18, PBS-1) are distinct from all other diamonds at this location and from the Eurelia diamonds. These diamonds have brown irradiation spots on their surfaces. They also have pale yellow colors in combination with an emerald green UV fluorescence, which is possibly caused by the presence of nitrogen as H3 centers (Davies, 1977). All three diamonds contain similar concentrations of nitrogen (417–594 ppm), with nitrogen in low aggregation states (Type IaA diamonds). These diamonds have well-developed dodecahedral crystal shapes (Fig. 2E), a small number of inclusions, and are relatively large in size. They account for ∼20% of the weight of the entire sample set from the Springfield Basin. The presence of abrasion textures (percussion marks, network patterns) on the surface of diamond PBS-1 (Fig. 4D) indicates that the diamonds are derived from distal kimberlitic sources. These kimberlitic sources must be much older than depositional ages of the conglomerates in the Springfield Basin, because the presence of brown irradiation spots provides evidence that these diamonds experienced at least one cycle of irradiation, followed by a metamorphic overprint at temperature N500 °C (Vance et al., 1973), before being deposited in the Springfield Basin. Based on the observed purple-reddish, likely peridotitic garnet inclusion in one of these diamonds (FLIN2-18), these older diamonds probably formed in a peridotitic environment within the lithospheric upper mantle, which is the source for the majority of diamonds worldwide (Sobolev, 1977; Gurney, 1984; Meyer, 1987). In accordance with that, the carbon stable isotope values of two of the “old” diamonds (− 4.7 and −6.4‰) are in the range of typical peridotitic diamonds worldwide (Sobolev et al., 1979; Cartigny et al., 1998). The low-nitrogen aggregation states (≤1.8% B-centers) of all three diamonds also indicate that they formed in an environment with a low geothermal gradient, which is typical for ancient cratonic lithosphere (c.f., Pollack and Chapman, 1977). If the diamonds formed in a lithosphere with an elevated geothermal gradient, such as the one beneath the Adelaide Fold Belt (Tappert et al., 2007), their mantle residence would have had to be rather short, in order to preserve their low-nitrogen aggregation states. However, the high quality of these diamonds is not consistent with rapid growth and subsequent exhumation during initiation of a kimberlite event. The “old” diamonds from the Springfield Basin are morphologically similar to the placer diamonds from the Echunga goldfield. Both groups of diamonds are well crystallized, are of relatively large size and, more importantly, exhibit brown irradiation spots and abrasion textures. These physical similarities suggest that the old Springfield Basin diamonds and the Echunga diamonds may not only be derived

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from a common, intermediate, sedimentary source, but even from the same distal kimberlitic sources. 4.3. Sources of the “old” Springfield Basin and Echunga diamonds The conglomerates in the Springfield Basin are interpreted to be remnants of Permian glacial deposits, or slightly younger deposits of locally derived, reworked Permian glacial sediments (Drexel and Preiss, 1995). The diamond indicator minerals within the conglomerates are most likely derived from local kimberlitic sources and linked to the abundant “younger” diamonds of the Springfield Basin. It seems unlikely that any of the indicator minerals in the Springfield Basin are related to the “old” diamonds, considering their long history of transport and metamorphism. These three diamonds may solely represent rare and exotic components of the Permian glacial detritus. Although the diamonds from Echunga were recovered from conglomerates of Tertiary age (Ludbrook, 1980), these conglomerates are likely to contain components of reworked Permian glacigene sediments, which as remnants are present in many parts of the Southern Mount Lofty Ranges, just outside the Echunga area. Like the “old” diamonds from the Springfield Basin, the Echunga diamonds, therefore, may simply be part of the glacial detritus, transported to South Australia by Permian glaciers. Although much of the Permian sedimentary record in the Adelaide Fold Belt and in the adjacent Mount Lofty Ranges has been erased by erosion, as a result of Neogene uplift (Sprigg, 1945; Sandiford, 2003 and references therein), equivalent Permian glacial deposits are preserved in many other parts of southern Australia (Crowell and Frakes, 1971). These Permian glacial sequences reflect the globally cool climatic conditions during the Early Permian (Frakes et al., 1992), and the high latitude position of Australia, which at that time formed part of the supercontinent Gondwana (Irving and Green, 1958) (Fig. 11).

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Equivalents of the Permian glacigene sediments of southern Australia are also widespread in other parts of the former Gondwana supercontinent, e.g., the Dwyka-Group in Southern Africa (Du Toit, 1921; Visser, 1983), and the Itararé Subgroup in South America (RochaCampos and Santos, 1981). In some regions, these sediments are recognized as intermediate sources for alluvial diamond deposits (Harger, 1909; Tompkins and Gonzaga, 1989; Gonzaga et al., 1994). Within Australia, some of the alluvial diamonds from New South Wales were also interpreted to be derived from Permian glacigene sequences (Davies et al., 1999; Davies et al., 2002). Like the “old” Springfield Basin and Echunga diamonds, some of the New South Wales diamonds also show abrasion textures and brown irradiation spots on their surfaces (Davies et al., 1999), which suggests a direct connection between these diamonds, and similar, if not identical primary sources. Based on indicators for the direction of ice movement during the Permian glaciations in southern Australia (Crowell and Frakes, 1971), much of the sedimentary material, including the diamonds, may be derived from sources in the eastern part of Antarctica, which during the Permian was directly connected to southern Australia (Fig. 11). In this case, the primary kimberlitic sources of the diamonds may be located on the East Antarctic Craton. However, the diamonds may also be derived from intermediate sources, such as metamorphosed sedimentary rocks. 5. Conclusions The bulk of the placer diamonds from the Springfield Basin resembles the diamonds from kimberlites at Eurelia. Diamonds from both areas have a similar distribution of crystal shapes, body colors, and surface textures. Particularly prominent are the high abundance of diamonds with pseudohemimorphic shape, diamonds with pale brown

Fig. 11. Reconstruction of eastern Gondwana during Permo-Carboniferous glaciation, showing uplands as source regions of glacial sediments, and sedimentary transport directions; modified from Crowell and Frakes (1971) and Veevers et al. (1994). Locations of additional alluvial diamond occurrences in New South Wales from Davies et al. (1999) and Davies et al. (2002).

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body coloration, and the common presence of deep ruts and etch pits on diamond surfaces. In both areas, the majority of diamonds are nitrogenpoor, with nitrogen contents b100 ppm. Similarities also exist in the stable carbon isotope composition of the diamonds; with both diamond populations having a similar distribution of δ13C values and a marked δ13C mode at around −6‰. The presence of ferropericlase as inclusions in diamonds from the Springfield Basin provides another link to the diamonds from the Eurelia area, where similar inclusions in diamonds occur, and provides evidence for the sublithospheric origin of at least part of the Springfield Basin diamond population. The overall similarities between the Springfield Basin and Eurelia diamond populations suggest that most of the Springfield Basin diamonds are derived from nearby primary sources, which closely resemble the kimberlite dykes at Eurelia. Jurassic ages determined for two Eurelia kimberlites appear to contradict an origin of Permian Springfield Basin diamond deposits from these primary sources. However, the distinct possibility that protracted kimberlite activity in the area overlapped with Permian sedimentation exists, and the strong similarity with the younger Eurelia diamonds then would simply reflect repeated tapping of similar/identical diamond sources in the lithospheric and sublithospheric mantle over time. Despite their overall similarities, three diamonds from the Springfield Basin are distinct from the Eurelia diamonds and all other Springfield Basin diamonds. These diamonds are characterized by relatively large sizes, well-developed crystal shapes, yellow body colorations, and the presence of brown irradiation spots on their surfaces, which indicate that the diamonds are much older than other diamonds in the Springfield Basin. The presence of abrasion textures on the surface of one of the diamonds is consistent with that interpretation, and indicates that these diamonds have been transported considerable distances away from their original kimberlitic sources. Similarities in their physical characteristics possibly link these three “old” Springfield Basin diamonds with placer diamonds from the Echunga area, around 350 km south. In both cases, the local sources of the diamonds were probably fluvio-glacial sedimentary rocks of Permian age, which in part were reworked into younger sediments. Based on palaeo-transport directions of the Permian glacial sediments, the diamonds were most likely derived from intermediate sedimentary, or primary kimberlitic sources on the East Antarctic craton. A similar origin may also apply to part of the placer diamonds from New South Wales. Acknowledgements We are grateful to Allan Pring (South Australian Museum) for providing access to diamond from the Echunga goldfield. Funding was provided by the Australian Research Council (ARC), the Department of Primary Industries and Resources South Australia (PIRSA), and Flinders Diamonds Ltd. Reviews by Lynton Jaques and an anonymous reviewer are gratefully acknowledged. References Amtsberg, H., 1969. A contribution to the mesophytic flora of South Australia. Transactions of the Royal Society of South Australia 93, 79–85. Atkinson, W.J., Smith, C.B., Danchin, R.V., Janse, A.J.A., 1990. Diamond deposits of Australia. In: Hughes, F.E. (Ed.), Geology of the Mineral Deposits of Australia and Papua New Guinea. Australasian Institute of Mining and Metallurgy, Melbourne, pp. 69–76. Boyd, F.R., Gurney, J.J., 1986. Diamonds and the African lithosphere. Science 232, 472–477. Boyd, S.R., Kiflawi, I., Woods, G.S., 1994. The relationship between infrared absorption and the A defect concentration in diamond. Philosophical Magazine Part B 69, 1149–1153. Boyd, S.R., Kiflawi, I., Woods, G.S., 1995. Infrared absorption by the B nitrogen aggregate in diamond. Philosophical Magazine Part B 72, 351–361. Brey, G.P., Bulatov, V., Girnis, A., Harris, J.W., Stachel, T., 2004. Ferropericlase — a lower mantle phase in the upper mantle. Lithos 77, 655–663. Bulanova, G.P., Smith, C.B., Kohn, S.C., Walter, M.J., Gobbo, L., Kearns, S., 2008. Machado River, Brazil — a newly recognised ultradeep diamond occurrence. 9th International Kimberlite Conference. Extended Abstract No. 9IKC-A-00233. Cartigny, P., Harris, J.W., Phillips, D., Girard, M., Javoy, M., 1998. Subduction-related diamonds? The evidence for a mantle-derived origin from coupled 13C–15N determinations. Chemical Geology 147, 147–159.

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