New study from a team of scientists led by Dr. Renbiao Tao from the Center for High Pressure Science and Technology Advanced Research (HPSTAR) delineated the mineral assemblages and elemental partitioning patterns within the eclogite under conditions mimicking those of the lower mantle by employing a series of high-temperature and high-pressure experimental simulations. Their focus was on unravelling the potassium (K) - sodium (Na) cycle within oceanic subduction zones. The study chose a typical phengite-rich metamorphic rock as the starting material, sourced from the western Tianshan orogenic belt in China, which has undergone subduction and ultra-high pressure metamorphism.  This endeavour culminated in the construction of a model depicting the K-Na cycle in ultra-deep oceanic subduction zones, effectively quelling prior debates regarding the capacity of davemaoite inclusions in natural ultra-deep diamonds to host a plethora of heteroatoms such as K and Na.The work is published in Earth and Planetary Science Letters.

The spatial distribution and prevalence of K and Na within Earth's interior exert a profound influence on its geophysical and geochemical properties (e.g., Mookherjee and Steinle Neumann, 2009; Massonne and Fockenberg, 2022). Notably, the presence of ⁴⁰K, a radioactive isotope, and its cycling dynamics within the deep Earth can significantly impact the mantle's thermal evolution over geological timescales (Wasserburg et al., 1964). Furthermore, the origin and material source of K- and Na-rich davemaoite inclusions, a recent discovery within ultra-deep diamonds, remain contentious debate (Tschauner et al. 2021, 2022; Walter et al. 2022), necessitating further investigation into the K-Na cycle and its distribution dynamics within Earth's interior.

Historical evidence from geophysical surveys and geochemical analyses of natural ultra-deep diamond inclusions suggests the subduction of numerous oceanic plates to the mantle transition zone and even the core-mantle boundary (Geos et al. 2017; Walter et al. 2022). With the subduction of oceanic plates, the copious reserves of K and Na deposited at the ocean floor may undergo transportation to the deep mantle, engaging in a myriad of physical and chemical transformations. While prior studies have extensively scrutinized the cycling of K and Na within the crust and upper mantle (Schmidt 1996), a knowledge gap persists regarding their cycling and distribution dynamics within subducted plates under the extreme conditions of the deep upper mantle and lower mantle.

In light of these gaps in understanding, the team conducted a series of experimental simulations on the K-Na deep cycle under high pressures and temperatures. Utilizing ultra-high pressure metamorphic eclogite samples (containing 1.47 wt.% K₂O and 3.04 wt.% Na₂O) sourced from the orogenic belt of the Western Tianshan Mountains (Figure 1), the study scrutinized the distribution behaviours of K-Na within the high-temperature and high-pressure phase transition products of phengite-rich eclogite systems at pressures ranging from 5 to 30 GPa and temperatures from 850 to 1550 ℃, with particular emphasis on the content of K and Na within davemaoite and bridgmanite.

Figure 1. TIMA diagram of mineral assemblage of the natural phengite-rich eclogite.

Analysis of backscattered electron imagery and energy spectrum element distributions revealed a compelling correlation between the presence of potassium (K) and sodium (Na) and aluminum (Al) within the experimental products subjected to lower-mantle conditions, notably at 30 GPa and 1550 ℃. Intriguingly, this correlation did not extend to calcium (Ca) within the same experimental conditions, as depicted in Figure 2. Such findings unequivocally suggest that K and Na predominantly localize within the aluminium-rich phase, with their presence in davemaoite being markedly minimal.

Figure 2. Backscattered electron image and energy spectrum element distributions of the high-pressure eclogite phase at 30 GPa and 1550 ℃. K-hol=K-hollandite; St=Stishovite; Al-Brg=Al-rich bridgmanite; Dvm=Davemaoite; NAL=NAL phase.

To delve deeper into the geochemical compositions of perovskite-structured minerals, including davemaoite and bridgmanite, in both synthetic high-pressure phases and natural ultra-deep diamonds, triangular diagrams featuring various end-member compositions were meticulously crafted (refer to Figure 3). Subsequent analysis unveiled a striking revelation: davemaoite and bridgmanite, whether synthesized under high-temperature and high-pressure conditions or found within ultra-deep diamonds, exhibit exceedingly scant traces of potassium (K) and sodium (Na), each registering at less than 1 mol.%, a stark contrast to prior reports by Tschauner et al. (2021). Consequently, it can be inferred that the K- and Na-rich davemaoite inclusions within ultra-deep diamonds are not remnants from the deep subduction of oceanic crust. This seminal finding effectively resolves previous debates concerning the capacity of davemaoite within ultra-deep diamonds to harbour substantial quantities of K, Na, and other heteroatoms, as posited by Tschauner et al. (2021, 2022) and Walter et al. (2022).

Figure 3. Chemical compositions of bridgmanite and davemaoite in high-temperature and high-pressure synthesized rock phases and ultra deep diamonds: (a) Triangle diagram of NK (Na₂O + K₂O) - MFC (MgO + FeO + CaO) - SA (SiO₂ + Al₂O₃); (b) Triangle diagram of CaSiO₃-MgSiO₃-FeSiO₃ and MgSiO₃-CaSiO₃-(Al, Fe)₂O₃. The color regions represent the chemical-composition range of bridgmanite and davemaoite.

Drawing upon prior investigations into the high-temperature and high-pressure phase transitions of the upper continental crust (UCC), oceanic basalt (MORB), and pyrolite rock systems, we developed a comprehensive geodynamic model depicting the K-Na cycle within oceanic subduction zones, extending up to the uppermost lower mantle (refer to Figure 4). In the oceanic crust, potassium (K) and sodium (Na) primarily accumulate within surface clay minerals and feldspar minerals. As the oceanic crust undergoes subduction, K and Na enrichment shifts towards amphibole and mica within the amphibolite phase. Upon further transformation into eclogite at greater depths, phengite and omphacite emerge as the primary reservoirs for K and Na, respectively. Within the cold subduction zones of the deep upper mantle (approximately ~300 km depth), phengite transitions into K-hollandite, predominantly retaining K with a minor Na content. Conversely, in the mantle transition zone, omphacite and garnet metamorphose into majorite, assimilating the majority of Na. Near the summit of the lower mantle, Na-rich majorite decomposes into bridgmanite, davemaoite, and an aluminium-rich phase (e.g., NAL). The NAL phase predominantly harbours Na with a minor presence of K, while bridgmanite and davemaoite exhibit extremely low concentrations of both elements. At the uppermost lower mantle boundary, K-hollandite and the NAL phase may coexist. As the oceanic crust further subducts into the lower mantle, dynamic distribution behaviour of K and Na may occur within the K-hollandite and NAL phases, underscoring the intricate interplay of mineralogical transformations and element partitioning throughout the depths of the Earth's interior.

Figure 4. (a) Model diagram of K and Na cycles during the subduction of oceanic plates into the lower mantle; (b) The phase transition process and K and Na distribution behaviours of the main silicate minerals in the high-pressure phase of subducting oceanic crust.

This study was greatly supported by Associate Professor Takayuki Ishii from Okayama University in Japan and Assistant Researcher Yunxiu Li from the Second Institute of Oceanography, Ministry of Natural Resources. Dr. Chao Wang and Dr. Jintao Zhu, joint training students from HPSTAR and Peking University, made invaluable contributions to the experimental work and discussions presented in this paper. Additionally, Professor Liang Liu from Northwestern University provided insightful suggestions, for which we are deeply appreciative. The culmination of our research efforts resulted in the publication of our findings in the prestigious journal. Notably, the paper garnered high praise from one of its reviewers, Dr. Michael Walter, Director of the Earth and Planetary Laboratory at the Carnegie Institute for Science in the United States. Dr. Michael Walter lauded the article, asserting that its experimental findings effectively addressed the ongoing debate surrounding K- and Na-rich davemaoite in ultra-deep diamonds (as discussed by Tschauner et al., 2022; Walter et al., 2022). He further recommended that we highlight this pivotal conclusion as a significant contribution to our paper. This work is supported by the Key Project of the Original Exploration Program from the National Natural Science Foundation of China, titled "The Cycle Processes of Key Elements into the Deep Lower Mantle and their Surface Effects ".

近日，北京高压科学研究中心陶仁彪研究员课题组在俯冲带关键元素钾 (K) - 钠 (Na) 循环方面取得了重要进展。该研究选用典型的天然富钾钠洋壳变质岩石：来自中国西天山造山带中经历了俯冲超高压变质的榴辉岩作为起始样品，通过一系列高温高压实验模拟确定了深下地幔温度-压力条件下富K-Na榴辉岩体系的岩石矿物组合以及元素分配行为，并构建了超深大洋俯冲带K-Na循环模型。该研究也平息了对天然超深金刚石中毛钙硅石能否容纳大量K-Na等杂原子的争论 。相关工作以“K- and Na-rich davemaoite inclusion in diamond is not inherited from deeply subducted oceanic crusts”为题发表于EPSL《Earth and Planetary Science Letters》上，第一作者是孙文清博士。