Hydrous peridotitic fragments of Earth’s mantle 660 km discontinuity sampled by a diamond

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Marco E. Ciriotti
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Hydrous peridotitic fragments of Earth’s mantle 660 km discontinuity sampled by a diamond

Messaggio da Marco E. Ciriotti » lun 26 set, 2022 20:31

Referenza:
▪ Gu, T., Pamato, M.G., Novella, D., Alvaro, M., Fournelle, J., Brenker, F.E., Wang, W., Nestola, F. (2022): Hydro peridotitic fragments of Earth’s mantle 660 km discontinuity sampled by a diamond. Nature Geoscience, (in press).

Abstract:
T he 660 km discontinuity, demarcated by the abrupt changes in density and seismic wave velocities, is one of the global structures of our planet that controls heat and mass exchange between the surface and the deep interior. It is commonly accepted, on the basis of petrological evidence and laboratory observations, that the mantle transition zone (TZ), starting from ~410 km, is dominated by wadsleyite and ringwoodite, which have high water-storage capacities of 0.8-1.5 wt% H 2 O 1,2. At ~660 km depth, ringwoodite, (Mg,Fe) 2 SiO 4 , the dominant TZ mineral between 520 and 660 km depth, disproportionates into bridgmanite (Mg,Fe) (Si,Al)O 3 and ferropericlase (Mg,Fe)O 3. The water-storage capacity of these lower-mantle (LM) minerals is much lower than that of wadsleyite or ringwoodite and shows a strong correlation with chemical compositions ranging from 100 ppm at ~25 GPa for pure Mg-bridgmanite 4,5 to 2,000 ppm for Al-and Fe-bearing bridgman-ite at similar conditions 5,6. The transportation of water beyond this region is critical to understanding of the global water cycle, as well as the compositional nature of the 660 km discontinuity. However, it remains uncertain whether the chemical composition through the TZ-LM boundary is homogeneous or distinctly stratified 7-9. Geochemical analyses of the basaltic products of mantle melting in the upper mantle 7 and elasticity measurements on Al-bearing bridgmanite at LM conditions 9 suggest composi-tionally distinct upper and lower mantles. The compositional difference would not only affect partitioning of water and trace elements between the LM mineral assemblage 10 , but also play a role in the thermal stability of ringwoodite 11 and associated minerals 12. Recent small-scale seismic tomography data reveal rugged structures at the TZ-LM boundary 13 , suggesting regional chemical differences between the upper and lower mantles. Therefore, a partially restricted mantle circulation, alternating between lay-ered and whole-mantle convection, raised questions about the compositional nature as well as the role that water plays at the 660 km discontinuity. Direct samples from Earth's TZ and LM are rather scarce. Diamonds are one of the main sources that deliver samples from the most inaccessible regions to the surface. Inclusions enclosed by diamonds such as ice VII 14 , the hydrous phase EGG (AlSiO 3 (OH) 15 and ringwoodite with 1.5 wt% water 16 indicate water saturation in the TZ around where diamonds form. However, it is unclear whether these samples represent a local water-saturated condition or an environment throughout the region. Although a few diamonds show ferropericlase, enstatite (former bridgmanite) and olivine (former ringwoodite) as isolated inclusions or in composite pairs 17 (Extended Data Fig. 1), these phases have never been reported all together as one single inclusion in assemblage, and whether these retrogressed mantle phases have a direct link with a hydrous environment was ambiguous. Unearth a rare polyphasic mineral inclusion In this Article, we report an intimate mineral assemblage found in a rare type IaB, 1.5 carat, D colour, polished gem diamond from the Karowe mine, Botswana (Methods and Extended Data Fig. 2). Within the diamond, except for a milky cluster (inclusion 3) composed of sub-micro-sized inclusions, a total of 12 composite and single-phase inclusions are visible (Supplementary Fig. 1), which were examined by micro-Raman spectroscopy and X-ray diffrac-tion, including several that were exposed using a polishing wheel and Ar milling polisher at the surface for electron microprobe analysis (Supplementary Table 1). Strikingly, among these inclusions, the diamond shows a mineral assemblage with ringwoodite in contact with ferropericlase and enstatite (probably a polymorphic inversion from bridgmanite), which marks a scenario when ringwoodite began to break down into ferropericlase and bridgmanite at the TZ-LM 660 km boundary. This assemblage, observed within inclusion 5 (Fig. 1), displays an opaque centre (~30 µm; Fig. 1) on the up-front side with an iridescent blue colour, which is often observed on ferropericlase, The internal structure and dynamics of Earth have been shaped by the 660 km boundary between the mantle transition zone and lower mantle. However, due to the paucity of natural samples from this depth, the nature of this boundary-its composition and volatile fluxes across it-remain debated. Here we analyse the mineral inclusions in a rare type IaB gem diamond from the Karowe mine (Botswana). We discovered recovered lower-mantle minerals ringwoodite + ferropericlase + low-Ni enstatite (MgSiO 3) in a polyphase inclusion, together with other principal lower-mantle minerals and hydrous phases, place its origin at ~23.5 GPa and ~1,650 °C, corresponding to the depth at the 660 km discontinuity. The petrological character of the inclusions indicates that ringwoodite (∼Mg 1.84 Fe 0.15 SiO 4) breaks down into bridgmanite (∼Mg 0.93 Fe 0.07 SiO 3) and ferropericlase (∼Mg 0.84 Fe 0.16 O) in a water-saturated environment at the 660 km discontinuity and reveals that the peridotitic composition and hydrous conditions extend at least across the transition zone and into the lower mantle. NATUrE GEosciENcE | www.nature.com/naturegeoscience.
Marco E. Ciriotti

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