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HAL Id: hal-02506335

https://hal-mines-paristech.archives-ouvertes.fr/hal-02506335

Submitted on 12 Mar 2020

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The Genesis of Blue Diamonds

Lucille Daver, Helene Bureau, Eloise Gaillou, E. Boulard, Benoît Baptiste, Oulfa Belhadj, Nicolas Guignot, Eddy Foy, Pierre Cartigny, Daniele L. Pinti

To cite this version:

Lucille Daver, Helene Bureau, Eloise Gaillou, E. Boulard, Benoît Baptiste, et al.. The Genesis of Blue Diamonds. 2019 Diamond Conference, Jul 2019, Warwick, United Kingdom. �hal-02506335�

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[1] Gaillou, E., Post, J. E., Rost, D., & Butler, J. E. Boron in natural type IIb blue diamonds: Chemical and spectroscopic measurements. American Mineralogist, 97 (1), 1-18 (2012). [2] Smith, E. M., Shirey, S. B., Richardson, S. H., Nestola, F., Bullock, E. S., Wang, J., & Wang, W. Blue boron-bearing diamonds from Earth's lower mantle. Nature, 560 (7716), 84-87 (2018).

Acknowledgment:

We thank GEOTOP, FRQNT international internship scholarship and the Diamond Conference Young Researcher Award for their funding. The SEM facility of IMPMC is supported by Région Ile de France Grant SESAME 2006 NOI-07-593/R, INSU-CNRS, INP-CNRS, UPMC, and by the French National Research Agency (ANR) Grant ANR-07-BLAN-0124-01. We thank M. Guillaumet for his assistance during FTIR measurements, and Christophe Sandt and Ferenc Borondics from the SMIS beamline at SOLEIL synchrotron for their support during Raman mapping. We thank Ludovic Delbes, Philippe Rosier and the cellule projet from IMPMC for their assistance during the preparation for XRD experiments.

References

Materials

Cullinan mine in South Africa

Boron: 0.22 to 0.38 ppm B

Four diamonds:

1 x3 mm for ~0.10 ct

- Two with primary inclusions

- Two with primary and secondary inclusions

B8-1 600 µm

B16-1 1000 µm

1200 µm

B18-1

800 µm

B20-1

Boron content measurement:

- Infrared spectroscopy (FTIR)

Mineral phase identification:

- µ-raman spectroscopy (532 nm)

- Synchrotron X-Ray diffraction

Methodology: in situ investigations

C B

Boron would be inherited from slab dehydration and carried to lower mantle (>660 km) in dense hydrous silicate minerals (DHMS).

It is proposed that blue diamonds are of ultra-deep origin, from the lower mantle, and exclusively formed in subduction settings [2] .

Boron cycle in the mantle is realtively unknown and the study of these boron-bearing diamonds brings new insights on this deep cycle.

Introduction:

Blue diamonds are among the rarest type of gems : <0.1% of the extracted diamonds. Mainly from South Africa (Cullinan mine) and India (Kollur mine), ±

Central Africa, South America and Borneo.Their blue color is due to trace amounts of boron in the lattice structure and the near absence of nitrogen, thus

defined as type IIb diamonds [1].

The Genesis of Blue Diamonds

4

IRAMIS

6

IRAMIS

7

1

2

1

3

5

2

1, * 2 3 2 2

Lucille Daver , Hélène Bureau , Eloıs̈e Gaillou , Eglantine Boulard , Benoit Baptiste , Oulfa

4 5 6 7 1

Belhadj , Nicolas Guignot , Eddy Foy , Pierre Cartigny and Daniele L. Pinti

¹GEOTOP, University of Quebec in Montreal, Canada, ²IMPMC, Sorbonne Université, France, ³Mines ParisTech, PSL Research University, France, ⁴Center for Research on the Preservation of Collection, National

Natural History Museum, France, Synchrotron Soleil, France, ⁶LAPA-IRAMAT, University of Paris-Saclay, France, ⁷Stable Isotope Geochemistry Laboratory, Institut de Physique du Globe de Paris, France.

5

*Correspondence: daver.lucille@uqam.ca

Discussion

Mineral assemblage: Inclusion of Ilmenite : Eclogitic paragenesis at lithospheric depth

Forming fluid: Primary and secondary inclusions may be the witness of a H 0-C

2 graphite

parent fluid

Inclusion of walstromite (retrogressed Ca-Perovskite (CaTiO3) ; >9 Gpa): Sub-lithospheric depth

Boron: inherited from sea water through the subduction zone : H 0 + C

2 organic

+ boron, available in the

lithosphere after slab dehydration: Data suggest a deep recycling of marine fluids that may be the the

parents of blue diamonds

We suggest that blue diamonds are not exclusively ultra-deep and may form at any depth in the

mantle, from lithosphere (>150km) down to the lower mantle (~750 km), in subduction-related B-C-

H O-rich fluids.

2

3

2

1

4

Oceanic lithosphere Continental

lithosphere

410 km

660 km Transition zone

Lower mantle 2.5

3

or serpentinite to DHMS and 2.5 DHMS breakdown and realease boron [2]

4: bring to surface throught kimberlite eruption

2: Water and boron release during slab dehydration

1: Seawater (H2O + C + B) throught subduction

3: Growth of B-bearing diamond

A unique water-C- rich fluid present in both primary and secondary inclusions

Two-phase inclusions

+ Inclusions of CaSiO3-walstromite

3200 3600 3800

3000 3400 4000

2800

B8-1-inclu07-013355 2900

2967 3247 3450 B8-1-inclu-01-02

B8-1-inclu03-02

3185 3480 2930

Water area Methane

3630 3000

10µm 15µm

10 µm

2905

Inclusions II

Raman shift (cm )-1

2000

1600 1800 2200 2400 2600 2800 3000

Intensity

Graphite ref.

Diamond ref.

1580 2725

2450 2660

C=O 1840

2450 CO 2660

2190

10 µm

10 µm

Central Hexagonal Mineral Outer Colorless Halo

Inclusions I

Results

+ Ilmenite (FeTiO )

3

50µm B20-1-2

50µm

B18-1

Diamond ref.

2000

1500 2500 3000 3500

Raman shift (cm )

-1

Intensity

Graphite ref.

Water area

1584 2727

3253 1587

3258 2733

B18-1 B20-1-2

Contemporaneous to the diamond

growth or at least trapped during the

growth

Healed fractures: Post-growth event

150µm

15µm

20µm

30µm

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