Exhumation of lower mantle inclusions in diamond: ATEM investigation of retrograde phase transitions, reactions and exsolution
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:
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determine the structure of the garnet-like phase.
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to better resolve the small exsolutions and/or precipitations in the olivine.
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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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