Researchers have recently uncovered notable nonlinear characteristics of the post-spinel transition, which plays a significant role at the boundary between the Earth's upper and lower mantle. This transition, occurring at around 660 kilometers beneath the Earth's surface, has long been assumed to exhibit consistent behavior; yet, groundbreaking findings now reveal the transition's variability significantly influences mantle dynamics and geological processes.
The post-spinel transition involves the transformation of ringwoodite, a form of olivine, which dissociates under high pressure and temperature conditions to form bridgmanite and ferropericlase. Understanding the nature of this transition is central to comprehending mantle convection—an Earth process driven by pressure and temperature changes affecting the movement of tectonic plates.
Recent experiments employing laser-heated diamond anvil cells and synchrotron X-ray diffraction have shown surprising findings: the Clapeyron slope, which determines the transition's influence on mantle flow, varies with temperature. Specifically, slopes ranged from –4 MPa/K at 2100 K to 0 MPa/K at 1600 K, marking the first strong evidence for the post-spinel transition's nonlinearity.
“The nonlinear nature of this boundary indicates the magnitude of its negative slope, and hence its impedance to mantle flow, varies with temperature,” said the authors of the article. These observations challenge previous assumptions where the transition was considered linear, implying all upwelling and downwelling mantle flows encountered the same boundary conditions regardless of local thermal variations.
Through their extensive experiments, the researchers gathered data from various pressures and temperatures, successfully building more accurate phase diagrams of key mantle minerals. The experiments illuminated how diverse temperatures across different regions lead to variable effects on mantle convection and the actions of tectonic plates.
More intriguingly, this nonlinear behavior has significant ramifications for the dynamics of subducting slabs and rising mantle plumes. Presently, half of the Earth’s subducting slabs are relatively cold, with temperatures around 1400–1600 K, exhibiting minimal resistance at the post-spinel transition. Conversely, warmer slabs have slopes averaging –1.6 MPa/K, meaning they encounter greater resistance as they descend through this transition zone.
On the other hand, certain mantle plumes, which are columns of hot rock rising from deep within the Earth, interact differently with the post-spinel boundary. For plumes operating under temperatures less than 2150 K, the study found these systems exhibiting significantly different dynamic behaviors. For example, some plumes remain trapped near the 660 km discontinuity, whereas others surge upwards, reshaping their morphology as they encounter changes along the post-spinel boundary.
“Our global map of lateral variations provides insights for studying slabs and plumes worldwide,” the researchers stated, underscoring the importance of contextualizing the post-spinel transition within broader mantle processes.
The ramifications of these findings could extend beyond immediate geological processes, with potential influences on our models of Earth's thermal evolution. Historically, mantle constituents from the early Earth might have experienced different Clapeyron slopes compared to present-day conditions, which could lead to diverse dynamics, composition, and structures within the mantle.
Looking forward, the researchers advocate for explorations of how this nonlinear post-spinel transition could be modeled within current geodynamic frameworks to comprehensively understand the evolution of mantle behavior through time. This could promise insights not just for the present state of Earth’s mantle, but also for its historical and future transformations.
This study illuminates the complex interactions within our planet’s interior, reshaping long-standing assumptions about fundamental processes governing geology. The knowledge gleaned from analyzing the post-spinel transition’s characteristics advances our scientific grasp of Earth’s inner workings, carving paths for future geological research and exploration.