Non-Destructive In Situ Study of Plastic Deformations in Diamonds: X-ray Diffraction Topography and µFTIR Mapping of Two Super Deep Diamond Crystals from São Luiz (Juina, Brazil)

Giovanna Agrosì,Mark Hutchison,Giancarlo Della Ventura,Mariangela Cestelli Guidi,Fabrizio Nestola,Gioacchino Tempesta,Paolo Nimis

doi:10.3390/cryst7080233

Abstract

Diamonds from Juina, Brazil, are well-known examples of superdeep diamond crystals formed under sublithospheric conditions and evidence would indicate their origins lie as deep as the Earth’s mantle transition zone and the Lower Mantle. Detailed characterization of these minerals and of inclusions trapped within them may thus provide precious minero-petrogenetic information on their growth history in these inaccessible environments. With the aim of studying non-destructively the structural defects in the entire crystalline volume, two diamond samples from this locality, labelled JUc4 and BZ270, respectively, were studied in transmission mode by means of X-ray Diffraction Topography (XRDT) and micro Fourier Transform InfraRed Spectroscopy (µFTIR). The combined use of these methods shows a good fit between the mapping of spatial distribution of extended defects observed on the topographic images and the µFTIR maps corresponding to the concentration of N and H point defects. The results obtained show that both samples are affected by plastic deformation. In particular, BZ270 shows a lower content of nitrogen and higher deformation, and actually consists of different, slightly misoriented grains that contain sub-grains with a rounded-elongated shape. These features are commonly associated with deformation processes by solid-state diffusion creep under high pressure and high temperature.

Highlights

Diamond shows brittle behaviour at low temperatures, but with increasing temperature it softens considerably and plastic flow arises
Crystals 2017, 7, 233 major difference between the plastic deformation of sub-lithospheric diamonds compared to that of lithospheric diamonds is that the former grow at much higher pressure and temperature, and in a mantle characterized by convection, whereas lithospheric diamonds grow in a stiffer mantle host that is not undergoing convection
6–7 mm maximum are from São Luiz (Juina, Brazil; JUc4 from WGS84 Zone 21S 261000; 8708000 and BZ270in from an unkownexhibiting alluvial location in the wider area); theyand are broken crystals up toThis

Summary

Introduction

Diamond shows brittle behaviour at low temperatures, but with increasing temperature it softens considerably and plastic flow arises. Most natural diamonds experience post-growth plastic deformation during their period of residence in the mantle, which can be observed, for example, in birefringence patterns. Several different features of deformation can be found in lithospheric and sub-lithospheric diamonds, depending on both extrinsic and intrinsic factors. Temperature, pressure, time of residence and applied stress are extrinsic factors, whereas the different growth defects such as dislocations, twins, stacking faults, point defects and inclusions represent the intrinsic factors. Crystals 2017, 7, 233 major difference between the plastic deformation of sub-lithospheric diamonds compared to that of lithospheric diamonds is that the former grow at much higher pressure and temperature, and in a mantle characterized by convection, whereas lithospheric diamonds grow in a stiffer mantle host that is not undergoing convection. The plastic deformation of diamonds originating from the Earth’s sub-lithospheric mantle is much more complex and can only be investigated in detail by combining different techniques

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Conclusion