Messengers from the deep: Fossil wadsleyite-chromite microstructures from the Mantle Transition Zone

Abstract

Investigations of the Mantle Transition Zone (MTZ; 410–660 km deep) by deformation experiments and geophysical methods suggest that the MTZ has distinct rheological properties, but their exact cause is still unclear due to the lack of natural samples. Here we present the first direct evidence for crystal-plastic deformation by dislocation creep in the MTZ using a chromitite from the Luobusa peridotite (E. Tibet). Chromite grains show exsolution of diopside and SiO2, suggesting previous equilibration in the MTZ. Electron backscattered diffraction (EBSD) analysis reveals that olivine grains co-existing with exsolved phases inside chromite grains and occurring on chromite grain boundaries have a single pronounced crystallographic preferred orientation (CPO). This suggests that olivine preserves the CPO of a high-pressure polymorph (wadsleyite) before the high-pressure polymorph of chromite began to invert and exsolve. Chromite also shows a significant CPO. Thus, the fine-grained high-pressure phases were deformed by dislocation creep in the MTZ. Grain growth in inverted chromite produced an equilibrated microstructure during exhumation to the surface, masking at first sight its MTZ deformation history. These unique observations provide a window into the deep Earth, and constraints for interpreting geophysical signals and their geodynamic implications in a geologically robust context.

An Alternative Scenario on the Origin of Ultra-High Pressure (UHP) and Super-Reduced (SuR) Minerals in Ophiolitic Chromitites: A Case Study from the Mercedita Deposit (Eastern Cuba)

“…According to the models proposed in the literature, such petrographic feature could correspond to (1) chromite that originated at low pressure in the shallow mantle, and then experienced recycling in the deep mantle, at which point they could dissolve a silicate component. Once returned to the surface, the new spinel reverted back to low-pressure chromite and released the dissolved silicate component in the form of pyroxene lamellae [12,14,15,61,63,65]. It is also possible (2) that a high-pressure polymorph of chromite already containing high amounts of SiO 2 and CaO in its structure had crystallized near the bottom of the upper mantle or at the mantle transition and was carried upwards by deep-seated mantle plumes or asthenospheric melts, that experienced decompression to low-pressure chromite while releasing clinopyroxene lamellae, e.g., [6,19].…”

Synthesis of inverse ringwoodite sheds light on the subduction history of Tibetan ophiolites

“…The compound was found to crystallize with a distorted orthorhombic calcium-titanate (CaTi 2 O 4 ) structure type, space group Cmc 2 1 , and shows six-fold coordinated silicon. We suggest that both natural iRgw and synthetic post-spinel phase 20 are representative of the podiform chromitites in the Luobusa ophiolite, which contain diamond and other former ultrahigh-pressure minerals 2,19,21,22 . Yamamoto et al .…”

“…The recent discovery of ringwoodite in chromitite bodies of the Luobusa peridotite in Tibet establishes, together with other evidence, that while the chromitites formed at shallow depths, they later were subducted to depths equivalent to the mantle transition zone (MTZ; 440–660 km) 1,2 . This ringwoodite, however, labeled “BWJ phase”, is dissimilar to ringwoodite as synthesized in the laboratory or observed in meteorites 3 : its structural and chemical characterization showed that this ringwoodite exhibits an inverse-spinel structure.…”

“…Evidence for metamorphism in or near the upper part of the MTZ includes (1) exsolution of pyroxenes and rare coesite from chromite, suggesting inversion from a high- P polymorph of chromite; (2) microstructures suggesting that the present chromitites recrystallized under static conditions from fine-grained, highly deformed mixtures of wadsleyite and an octahedral polymorph of chromite 2 ; (3) harzburgites with coarsely vermicular symplectites of opx + Cr–Al spinel ± cpx. Reconstructions suggest that these are the breakdown products of majoritic garnets, with estimated minimum pressures to >13 GPa 1 .…”

Two‐layered oceanic lithospheric mantle in a Tibetan ophiolite produced by episodic subduction of Tethyan slabs

“…Two sets of observations on the Yarlung Zangbo ophiolites demonstrate that they are different from conventional ophiolites [e.g., Dilek and Furnes , ]. (1) The unusual ultrahigh‐pressure (UHP) and super reduced phases and microstructures that are preserved within the spinel‐facies, relatively oxidized lithospheric mantle rocks [e.g., Bai et al ., ; Robinson et al ., ; Yang et al ., ; Dobrzhinetskaya et al ., ; Howell et al ., ; McGowan et al ., ; Satsukawa et al ., ; Griffin et al ., ]. (2) The very thin (if any) crustal sections are much less in volume than the theoretical crust, which corresponds to the production of the thick mantle sections, and are isotopically decoupled from these mantle sections [e.g., Göpel et al ., ; Shi et al ., ; Gong et al ., ].…”