Hygrometric Control on the Lithosphere‐Asthenosphere Boundary: A 28 Million Year Record From the Canadian Cordillera

Abstract

The depth to the lithosphere‐asthenopshere boundary (LAB) provides a strong control on the temperature and strength of the lithosphere and its susceptibility to tectonic deformation. We probe the nature of the LAB by deriving the depth and source of mantle‐xenolith bearing alkaline lavas aged 28 Ma to 8 ka in the Canadian Cordillera (CC). We document a striking coincidence of the average depth of equilibration of these lavas (65 ± 5 km) with the seismically observed LAB both in the CC and in western USA, and the change in H2O storage capacity at the spinel‐garnet transition of fertile peridotite under the geothermal gradient of hot thin back arc regions. The convergence of these factors show the LAB of hot back arcs is hygrometrically controlled at this phase boundary, limiting the H2O to <150 ppm in the mantle lithosphere. Intraplate regions with a shallower LAB require more H2O and/or less fertile lithosphere.

The Structure and Dynamics of the Uppermost Mantle of Southwestern Canada From a Joint Analysis of Geophysical Observations

“…Our results with the Q F correction suggest a relatively hydrated cordilleran mantle at depths greater than 75 km, in agreement with Hyndman and Canil (2021) and Canil et al. (2021). However, the temperatures for the Q F correction are ∼200°C lower at ∼100 km depth (Figure 9).…”

“…In addition, there is a greater amount of mantle‐derived magmatism close to this profile (Figure 1) and therefore the heterogeneous mantle structure may reflect recent episodes of magma extraction. However, the volcanic centers exhibit small‐volume magmatism (e.g., Canil et al., 2021), and it is unclear if this would affect the mantle at the scales resolved by the geophysical data.…”

“…There are trade‐offs between the temperature and water content needed to explain the geophysical observations. If the petrological constraints on mantle temperature are correct (∼1,400°C; Canil et al., 2021), the mantle may only have an olivine water content of ∼600 ppm H/Si; in contrast, a more hydrated mantle would have a lower temperature. In either case, the results give viscosities <10 21 Pa s for the cordilleran mantle (Figure 12).…”

“…The cordilleran region is characterized by basaltic magmatism that originates in the shallow mantle. A study of recent alkaline lavas inferred temperatures of 1,300–1,475°C at 45–100 km depth (Canil et al., 2021). These temperatures are similar to those obtained farther south where analyses of magmas from the Basin and Range and Rio Grande Rift (western United States) indicate temperatures of 1,336 ± 67°C (Klöcking et al., 2018; Plank & Forsyth, 2016).…”

“…The thermal structure of continental lithosphere is important for understanding its thickness and strength, as well as the dynamics of the underlying asthenosphere. Direct measurements of temperatures in the continental crust and uppermost mantle come from studies of xenoliths and erupted magmas (e.g., Canil et al., 2021; Harder & Russell, 2006; Saruwatari et al., 2001), which provide detailed local information. In contrast, regional temperature estimates can be obtained indirectly using geophysical data, such as surface heat flow, effective elastic thickness, and mantle seismic velocity, and experimental petrology studies that allow links to be made between the geophysical observations and subsurface temperatures (e.g., Currie & Hyndman, 2006; Goes & van der Lee, 2002; Hyndman & Lewis, 1999; Hyndman et al., 2009; Klöcking et al., 2018; Schutt et al., 2018; Tesauro et al., 2014).…”