Estimates of the Temperature and Melting Conditions of the Carpathian‐Pannonian Upper Mantle From Volcanism and Seismology

Abstract

What drives the formation of basaltic melts beneath intraplate volcanoes not associated with extensive thermal anomalies or lithospheric extension? Detailed constraints on the melting conditions and source region are imperative to resolve this question. Here we model the geochemistry of alkali basalts and mantle nodules brought up by young (12-0.1 Ma) intraplate volcanoes distributed across the Carpathian-Pannonian region and combine the results with geophysical observations. Rare earth element inversion and forward calculation of elemental concentrations show that the basalts require the mantle to have undergone less than 1% melting in the garnet-spinel transition zone, at depths of about 63-72 km. The calculated melt distributions correspond to a mantle potential temperature of ∼1257°C, equivalent to a real temperature of 1290°C at 65 km beneath the Pannonian Basin. The composition, modal mineralogy, and clinopyroxene geochemistry of some of the entrained mantle nodules closely resemble the basalt source, though the latter equilibrated at greater depths. The gravity anomalies and topography of the Basin reveal no large-scale features that can account for the post-extensional volcanism. Instead, the lithospheric thickness and geotherm show that melting occurs because the base of the lithosphere, at ∼50-km depth, is close to or at the solidus temperature over a large part of the Basin. Hence, only a small amount of upwelling is required to produce minor volumes (up to a few cubic kilometers) of melt. We conclude that the Pannonian volcanism originates from upwelling in the asthenosphere just below thinned lithosphere, which is likely to be driven by thermal buoyancy. Plain Language Summary On Earth's continents, there are surprising instances of basaltic volcanism that are not related to plate stretching or hotspots. To understand just such activity in the Carpathian-Pannonian region, in eastern central Europe, during the last 12 million years, we studied the chemical composition of the erupted lavas. Using a computer code that takes as input the concentrations of elements in basaltic rocks, we calculated the fraction of melt and the depth at which it formed within Earth's mantle. We find that the mantle has undergone less than 1% melting in the garnet-spinel transition zone-a region at about 63-to 72-km depth where these minerals both occur. Using a model of mantle melting, we find that the temperature of the mantle at 65 km below the Pannonian Basin is about 1290°C. The plate's thickness and the temperature variations with depth tell us that the magma source region is at a temperature close to that at which melting begins (the solidus). This means that if mantle material is moved upwards just slightly, it will produce small volumes of melt that can rise to the surface. The most likely cause of such upward movement is convection in the mantle below the rigid plate.

Geophysical and Geochemical Constraints on Neogene‐Recent Volcanism in the North American Cordillera

“…These areas have thin 60–70 km lithospheres and uniform high asthenosphere temperatures of about 1,350°C at 100 km, based on tomography velocities and geochemical studies of volcanics (e.g., An & Shi, 2007; Chen, 2010; Huang et al., 2003; Kimura & Nakajima, 2014; Niu, 2005; Zhang et al., 2019; Zhu et al., 2012). Another example is the Carpathian‐Pannonian region which is in a geologically recent former backarc that has temperatures, LAB depths, and extensive volcanism similar to the North American Cordillera (e.g., Bracco Gartner & McKenzie, 2020; Horváth et al., 2006).…”

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

“…The Fe 3+ /ΣFe of lavas was assumed to be either 0.15 to 0.25 based on the mean of 0.15 ± 0.05 and 0.24 ± 0.08 for intraplate tholeiitic and alkaline lavas, respectively, determined in 271 olivine‐hosted melt inclusions (Supporting Information). Some other lava inversion studies assume anhydrous conditions (Ball et al., 2019; BraccoGartner & McKenzie, 2020; Klöcking et al., 2018). We assumed either 1 or 2 wt% based on previous work for these and other intraplate lavas (Anderson et al., 2001; Brehm & Lange, 2020; Plank & Forsyth, 2016).…”

“… (a) Compilation of the mean depth (±1σ) of equilibration for alkaline lavas from four regions (Yukon, northern, central and southern British Columbia) in the Canadian Cordillera combined with similar data binned by individual centers in the western USA, Carpathians and NW Africa (Ball et al., 2019; BraccoGartner & McKenzie, 2020; Klöcking et al., 2018; Plank & Forsyth, 2016). (b) Frequency plot showing dominant mode (green shaded area) in the depths to the LAB from seismic receiver functions in the western USA across US Array (Hansen et al., 2015).…”

Carpathian‐Pannonian Magmatism Database

“…Regional review papers (Pécskay, Lexa, & Kovacs, 1995; Seghedi et al., 2004) are not accompanied by such databases. The recent paper by Bracco Gartner and McKenzie (2020) does contain a thorough database for the Carpathian‐Pannonian region, being the first such effort that resembles a modern geochemical database anchored by rock age and location; however, that paper only discusses (and compiles) Quaternary basalts.…”