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1. Efficiency of eclogite removal from continental lithosphere andits implications for cratonic diamonds

   https://doi.org/10.1130/G48204.1  

   Luo, Yantao ; Korenaga, Jun ( December 2020 , Geology)

   null (Ed.)

   Continental lithospheric mantle (CLM) may have been built from subducted slabs, but the apparent lack of concurrent oceanic crust in CLM, known as the mass imbalance problem, remains unresolved. Here, we present a simple dynamic model to evaluate the likelihood of losing dense eclogitized oceanic crust from CLM by gravitational instability. Our model allowed us to assess the long-term evolution of such crust removal, based on how thermal and viscosity profiles change over time across the continental lithosphere. We found that the oceanic crust incorporated early into CLM can quickly escape to the asthenosphere, whereas that incorporated after a certain age would be preserved in CLM. This study provides a plausible explanation for the mass imbalance problem posed by the oceanic ridge origin hypothesis of CLM and points to the significance of preservation bias inherent to the studies of cratonic diamonds. 

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2. Kinetic Control on the Depth Distribution of Superdeep Diamonds

   https://doi.org/10.1029/2018GL080740  

   Zhu, Feng ; Li, Jie ; Liu, Jiachao ; Lai, Xiaojing ; Chen, Bin ; Meng, Yue ( February 2019 , Geophysical Research Letters)

   Superdeep diamonds contain unique information from the sublithospheric regions of Earth's interior. Recent studies suggest that reaction between subducted carbonate and iron metal in the mantle plays an important role in the production of superdeep diamonds. It is unknown if this reaction is kinetically feasible in cold slabs subducted into the deep mantle. Here we present experimental data on real‐time tracking of the magnesite‐iron reaction at high pressures and high temperatures to demonstrate the production of diamond at the surface conditions of cold slabs in the transition zone and lower mantle. Our data reveal that the diamond production rate has a positive temperature dependence and a negative pressure dependence, and along a slab geotherm it decreases by a factor of three at pressures from 14.4 to 18.4 GPa. This rate reduction provides an explanation for the rarity of superdeep diamonds from the interior of the mantle transition zone.

    

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3. Helium in diamonds unravels over a billion years of craton metasomatism

   https://doi.org/10.1038/s41467-021-22860-3  

   Weiss, Yaakov ; Kiro, Yael ; Class, Cornelia ; Winckler, Gisela ; Harris, Jeff W. ; Goldstein, Steven L. ( May 2021 , Nature Communications)

   Chemical events involving deep carbon- and water-rich fluids impact the continental lithosphere over its history. Diamonds are a by-product of such episodic fluid infiltrations, and entrapment of these fluids as microinclusions in lithospheric diamonds provide unique opportunities to investigate their nature. However, until now, direct constraints on the timing of such events have not been available. Here we report three alteration events in the southwest Kaapvaal lithosphere using U-Th-He geochronology of fluid-bearing diamonds, and constrain the upper limit of He diffusivity (toD ≈ 1.8 × 10⁻¹⁹cm²s⁻¹), thus providing a means to directly place both upper and lower age limits on these alteration episodes. The youngest, during the Cretaceous, involved highly saline fluids, indicating a relationship with late-Mesozoic kimberlite eruptions. Remnants of two preceding events, by a Paleozoic silicic fluid and a Proterozoic carbonatitic fluid, are also encapsulated in Kaapvaal diamonds and are likely coeval with major surface tectonic events (e.g. the Damara and Namaqua–Natal orogenies).

    

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4. Ringwoodite and zirconia inclusions indicate downward travel of super-deep diamonds

   https://doi.org/10.1130/G50111.1  

   Lorenzon, Sofia ; Novella, Davide ; Nimis, Paolo ; Jacobsen, Steven D. ; Thomassot, Emilie ; Pamato, Martha G. ; Prosperi, Loredana ; Lorenzetti, Alessandra ; Alvaro, Matteo ; Brenker, Frank ; et al ( May 2022 , Geology)

   Abstract Natural diamonds and their inclusions provide unique glimpses of mantle processes from as deep as ~800 km and dating back to 3.5 G.y. Once formed, diamonds are commonly interpreted to travel upward, either slowly within mantle upwellings or rapidly within explosive, carbonate-rich magmas erupting at the surface. Although global tectonics induce subduction of material from shallow depths into the deep mantle, mineralogical evidence for downward movements of diamonds has never been reported. We report the finding of an unusual composite inclusion consisting of ringwoodite (the second finding to date), tetragonal zirconia, and coesite within an alluvial super-deep diamond from the Central African Republic. We interpret zirconia + coesite and ringwoodite as prograde transformation products after zircon or reidite (ZrSiO4) and olivine or wadsleyite, respectively. This inclusion assemblage can be explained if the diamond traveled downward after entrapping olivine/wadsleyite + zircon/reidite, dragged down by a subducting slab, before being delivered to the surface. This indicates that the commonly assumed view that diamonds form at, and capture material from, a specific mantle level and then travel upward is probably too simplistic. 

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5. Experimental Constraints on the Fate of Subducted Sedimentary Nitrogen in the Reduced Mantle

   https://doi.org/10.1029/2022JB025169  

   Huang, Shengxuan ; Wu, Xiang ; Chariton, Stella ; Prakapenka, Vitali B. ; Liang, Lin ; Zhang, Dongzhou ; Yang, Yan ; Chen, Bin ; Qin, Shan ( November 2022 , Journal of Geophysical Research: Solid Earth)

   Nitrogen is considered to be transported from Earth′s surface to the top of the lower mantle through subduction. However, little is known on the transportation and fate of subducted nitrogen to the Earth′s interior during slab‐mantle interactions. In this study, the stability of subducted sedimentary nitrogen in the reduced mantle was investigated to 35 GPa and 1600 K by laser‐heated diamond anvil cell experiments and first‐principles calculations. Our results showed that subducted nitrogen‐bearing silicates and fluids could not coexist with the metallic iron or iron‐rich alloys, and reacted with them to form different products at high pressure‐temperature conditions. Combining our results with previous data, we re‐determined the relative stability of iron‐light element binary compounds to 35 GPa and 1600 K to be Fe‐O > Fe‐N > Fe‐S > Fe‐C. This stability sequence contributes to explaining the observation that iron nitrides are trapped as inclusions in sulfur‐depleted lower‐mantle diamonds and are absent in sulfur‐rich ones. The recycling efficiency of subducted sedimentary nitrogen is strongly related to the availability of the metallic iron of the reduced mantle. Hydration of the metallic iron limits the storage of nitrogen in it and contributes to recycling nitrogen to Earth′s surface. Therefore, unlike subducted continental sediments, subducted marine sediments are unlikely to transport a large amount of surficial nitrogen to the metallic iron of the reduced mantle in which nitrogen could reside over long geologic periods.

    

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