Exhumation of lower mantle inclusions in diamond: ATEM investigation of retrograde phase transitions, reactions and exsolution

doi:10.1016/S0012-821X(02)00514-9

Earth and Planetary Science Letters

Volume 198, Issues 1–2, 30 April 2002, Pages 1-9

https://doi.org/10.1016/S0012-821X(02)00514-9 Get rights and content

Abstract

A multiphase inclusion (KK-83) in a diamond from the Kankan deposits in Guinea, resulting from complex reactions between primary lower mantle phases, was examined in detail using analytical transmission electron microscopy. The inclusion consists of a large (300 μm) diopside crystal with a small symplectitic intergrowth in one corner, comprising olivine and tetragonal almandine pyrope phase (TAPP). The olivine part of the symplectite contains small exsolutions of Mg-Al-chromite and rare Ca-carbonate. A second inclusion of ferropericlase observed in the same diamond indicates a primary origin within the lower mantle. The clinopyroxene formed through a series of reactions at the expense of touching inclusions of CaSi- and MgSi-perovskite during convective ascent of the diamond through the transition zone. The proposed reaction sequence is: MgSiPvk+CaSiPvk→MgSiIlm+CaSiPvk→ringwoodite+stishovite+CaSiPvk→clinopyroxene+relict ringwoodite. The high Al-content of the bulk symplectite indicates ringwoodite as precursor phase coexisting with clinopyroxene. A slight deficiency of SiO₂ during the diopside forming reaction leads to a small amount of relict ringwoodite (<0.1 vol% of the total inclusion is occupied by ringwoodite) coexisting with diopside. The ringwoodite captures the observed high amount of Al. Subsequent breakdown of ringwoodite to wadsleyite+TAPP produces the observed symplectitic intergrowth. During the phase transformation of wadsleyite to olivine, exsolution of Mg-Al-chromite occurs further decreasing the original solubility of Al, Cr and Ti, in accordance with experimental data on the element partitioning between wadsleyite and olivine. Based on these observations, TAPP can form as a retrograde phase within the transition zone of the Earth’s mantle and is not restricted to the upper part of the lower mantle. High Fe³⁺-contents may favour its formation.

Introduction

A large number of inclusions in diamonds from the Kankan district in Guinea originate from the transition zone [1] and the lower mantle [2]. Some of the ultra-deep diamonds from this occurrence contain inclusions resulting from retrograde reactions between primary lower mantle phases. To gain a better understanding of the exhumation history of such inclusions and the general processes occurring in upwelling lower mantle, one of these reacted inclusions has been examined using analytical transmission electron microscopy (ATEM). An original lower mantle paragenesis for this inclusion is indicated by the coexistence of ferropericlase (MgO) with mineral phases having MgSiO₃ or CaSiO₃ chemistries, believed to have originally formed in the perovskite structure [3], [4]. In addition, inclusions of pure SiO₂ (primary stishovite) and tetragonal almandine pyrope phase (TAPP) were found to coexist with ferropericlase in lower mantle diamonds [4], [5], [6], [7]. The high Al³⁺ and Fe³⁺ in TAPP suggested that this phase was the principal host of these elements in the uppermost (<100 km) lower mantle [6]. Surprisingly, TAPP has never been reported from high pressure experiments which may indicate more oxidizing conditions than commonly assumed for the transition zone.

The lower mantle inclusion parageneses from Kankan, originally studied by [2] (sample KK-83), consists of two non-touching inclusions of ferropericlase and clinopyroxene. The clinopyroxene is believed to be a retrograde reaction product of a former bi-mineralic inclusion of MgSi-perovskite and CaSi-perovskite [2]. The low Na, Cr and Ni and high Sr in the clinopyroxene inclusion, are characteristics not shared by lherzolitic clinopyroxenes but which could result from a combination of observed MgSi- and CaSi-perovskite chemistries. In one corner of the main inclusion is a symplectitic intergrowth of olivine with a ‘garnet’-like composition, constituting about 0.1 vol% of the total inclusion [2]. Electron microprobe analyses (EMPA) of the garnet-like phase yield a composition of Cr-poor subcalcic pyrope without a majorite component and such a Ca-poor composition cannot coexist with clinopyroxene under equilibrium conditions [9]. The olivine part of the symplectite shows frequent small exsolutions which are spinels but which are too small (<1 μm) for accurate EMPA measurements. Microprobe analyses also indicated a rather high and inhomogeneously distributed Ca-content throughout the olivine.

In this work we re-examined both the symplectitic intergrowth and part of the larger clinopyroxene using ATEM to:

•
determine the structure of the garnet-like phase.
•
to better resolve the small exsolutions and/or precipitations in the olivine.
•
to characterise microstructures indicative of phase reactions or transformations from high pressure polymorphs.

Crushed fragments of a 20 μm large slice of the symplectitic intergrowth and of a 50 μm large slice of the diopside were mounted onto carbon coated copper grids. The ATEM work involves selective area electron diffraction (SAED) and energy dispersive X-ray analysis (EDX) techniques. Chemical compositions were corrected using experimental determined k-factors applying the method of van Capellen and Doukhan [10].

Section snippets

Textural relationships within the symplectite

Small spinel crystals are spread through the olivine part of the symplectite, except for inclusion free zones along the contacts with both TAPP and the diopside (Fig. 1). This observation is indicative of an exsolution process. At upper mantle temperatures diffusion is too slow for the exchange of cations across the whole olivine crystal. However, close to the contact with TAPP and diopside, trivalent cations could be expelled from the newly formed olivine via diffusive exchange, rather than by

Formation of the diopside as a retrograde reaction product

The origin of diopside as a reaction product of the two lower mantle phases CaSiPvk and MgSiPvk is supported both by high amounts of minor elements like Al, Fe³ and Cr (for a more detailed discussion see [2]), and the observed microtwinning, noted earlier.

Based on the compilation of Oguri et al. [13], shown in Fig. 6, and using a starting composition close to pure diopside, three retrograde reaction sequences are possible, although the minor elements considered earlier, may change slightly the

Conclusions and implications for the transition zone

Our study shows that TAPP is a retrograde phase within the upper part of the transition zone resulting from complex reactions between components originally derived from the lower mantle. Based on cation calculations, a high ferric-iron-content is present, a result consistent with earlier Mössbauer spectroscopy [14], [21]. Present evidence therefore, suggests that TAPP may be a common phase within oxidised regions of the transition zone. If high Fe³⁺/Fe²⁺ ratios indeed favour formation of TAPP

Acknowledgements

We are grateful to W.F. Müller (Darmstadt), T. Walther and W. Mader (Bonn) for access to TEM facilities for this project. Gerhard Brey (Frankfurt) is thanked for his input into the petrological interpretation of the exhumation history. Support by the Deutsche Forschungsgemeinschaft (DFG) and De Beers Consolidated Mines Ltd. is gratefully acknowledged.[AH]

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Exhumation of lower mantle inclusions in diamond: ATEM investigation of retrograde phase transitions, reactions and exsolution

Abstract

Introduction

Section snippets

Textural relationships within the symplectite

Formation of the diopside as a retrograde reaction product

Conclusions and implications for the transition zone

Acknowledgements

Phys. Earth Planet. Int.

Geochim. Cosmochim. Acta

Kankan diamonds (Guinea) I from the lithosphere down to the transition zone

Contrib. Mineral. Petrol.

Kankan diamonds (Guinea) II lower mantle inclusion parageneses

Contrib. Mineral. Petrol.

Lower mantle associations preserved in diamonds

Miner. Mag.

A new tetragonal silicate mineral occurring as inclusions in lower-mantle diamonds

Nature

The crystal structure of ‘teragonal almandine-pyrope phase’ (TAPP): a reexamination

Am. Mineral.

Chrome-rich garnets from the kimberlites of Yakutia and their paragenesis

Contrib. Mineral. Petrol.