Temporal variations in plume flux: characterizing pulsations from tilted plume conduits in a rheologically complex mantle

Abstract

Summary Along age-progressive hotspot volcano chains, the emplacement rate of igneous material varies through time. Time-series analysis of changing emplacement rates at a range of hotspots finds that these rates vary regularly at periods of a few to several tens of millions of years, indicative of changing melt production within underlying mantle plumes. Many hotspots exhibit at least one period between ∼2–10 Myr, consistent with several proposed mechanisms for changing near-surface plume flux, and thus melting rate, such as small-scale convection, solitary waves, and instability formation in tilted plume conduits. Here, we focus on quantifying instability growth within plumes tilted by overlying plate motion. Previous studies using fluids with constant or temperature-dependent viscosity suggest that such instabilities should not form under mantle conditions. To test this assertion, we use a modified version of the finite element code ASPECT to simulate 400 million years of evolution of a whole-depth mantle plume rising through the transition zone and spreading beneath a moving plate. In a two-dimensional spherical shell geometry, ASPECT solves the conservation equations for a compressible mantle with a thermodynamically consistent treatment of phase changes in the mantle transition zone and subject to either a temperature-, and depth-dependent linear rheology or a temperature-, depth-, and strain-rate dependent non-linear rheology. Additionally, we examine plume evolution in a mantle subject to a range of Clapeyron slopes for the 410 km (1 to 4 MPa/K) phase transitions. Results suggest that plume conduits tilted by > 67° become unstable and develop instabilities that lead to initial pulses in the transition zone followed by repeated plume pulsing in the uppermost mantle. In these cases, pulse size and frequency depend strongly on the viscosity ratio between the plume and ambient upper mantle. Based upon our results and comparison with other studies, we find that the range of statistically significant periods of plume pulsing in our models (∼2–6 Myr), the predicted increase in melt flux due to each pulse (5–15 × 10−4 km3 km−1 yr−1), and the time estimated for a plume to tilt beyond 67° in the upper mantle (10–50 Myr) are consistent with observations at numerous hotspot tracks across the globe. We suggest that pulsing due to destabilization of tilted plume conduits may be one of several mechanisms responsible for modulating the melting rate of mantle plumes as they spread beneath the moving lithosphere.

Solitary waves forming pulsating thermal plumes and their implications for multiple eruption events in large igneous provinces

“…In this section we explore the possible mechanism of periodic swelling in the tail to produce multiple regular pulses. Earlier studies on flood basalt, hotspot swells, mantle plume evolution and melt migration in shallow mantle have shown that vertical fluid conduits embedded in another fluid can sustain nonlinear solitary waves (Olson and Christensen, 1986;Scott et al, 1986;Schubert et al, 1989, Helfrich andWhitehead, 1990;Neuharth and Mittelstaedt, 2023). Propagation of such waves causes the fluid conduit column to undergo alternate thickening and thinning, a phenomenon similar to pipe instability (Scott et al, 1986).…”

“…Schubert et al (1989) proposed a theoretical model to show the time scale of such pulses as a function of mantle viscosity. Their model predicts a time scale of 9 Ma for viscosity ~10 22 Pa S, which decreases to 1 Ma when the viscosity is ~10 21 Pa S. Recent computational simulations by Neuharth and Mittelstaedt (2023) have reported that the pulsating nature of plumes, caused by the destabilization of plume conduits, leads to periodic magmatic events.…”

“…For example, viscosity ratio (R) determines the magnitude of viscous drag exerted by the ambient medium to the rising plumes and thereby largely dictates their ascent modes, leading to either balloon-or mushroom-like single heads trailing into narrow tails (Dutta et al, 2013(Dutta et al, , 2014. Some workers demonstrated from experiments a completely different ascent dynamics, where plumes form a train of heads of varying sizes, each of them migrates upward in the form of solitary waves (Olson and Christensen, 1986;Scott et al, 1986;Neuharth and Mittelstaedt, 2023). A range of theoretical models have been proposed to explain such pulsating ascent behavior, e.g.…”

“…On the other hand, numerical models suggest that these solitary waves are actually conduit waves, which are formed when small diapiric bodies are continuously generated from TBL at the base of the existing plume and rise along the plume stem (Schubert et al, 1989). According to a recent work by Neuharth and Mittelstaedt (2023), pulses can be generated in a plume, due to mantle shear flow acting on tilted plume conduit.…”

Effects of Composite Rheology on Plate‐Like Behavior in Global‐Scale Mantle Convection

“…While mantle composite rheology is typically considered in regional‐scale geodynamics models (e.g., Billen & Hirth, 2005; Garel et al., 2020; Neuharth & Mittelstaedt, 2023), it is often neglected in global‐scale models (e.g., Coltice et al., 2017; Li & Zhong, 2019; Stein et al., 2004), or simply mimicked by reduced activation energy in pure diffusion creep rheology (Christensen, 1983, 1984). However, this latter approximation causes differences in the planform of stagnant‐lid convection compared to using full composite rheology (e.g., Schulz et al., 2020).…”