The role of pargasitic amphibole in the formation of major geophysical discontinuities in the shallow upper mantle

Kovács, István János and Lenkey, László and Green, David H. and Fancsik, Tamás and Falus, György and Kiss, János and Orosz, László and Vikor, Zsuzsanna and Angyal, Jolán (2017) The role of pargasitic amphibole in the formation of major geophysical discontinuities in the shallow upper mantle. Acta Geodaetica et Geophysica Hungarica, 52 (2). pp. 183-204. ISSN 1217-8977

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Abstract

Several explanations have been proposed for variation in geophysical properties and depths to the lithosphere-asthenosphere boundary (LAB) and mid-lithospheric discontinuities (MLD). Here we investigate the proposal that the dehydration solidus of pargasitic amphibole-bearing upper mantle with very low bulk water (hundreds ppm) may be one of the main reasons for the observed geophysical anomalies. The pargasite dehydration solidus may be associated with a very small degree of partial melting in the upper mantle at temperatures and pressures in excess of 1050 °C (for geochemically more depleted) or 1100 °C (for geochemically more fertile upper mantle) and from 1 to 3 GPa (~ 30 to 90 km) respectively. This small amount of partial melt may be responsible for changes in geophysical properties (e.g. lower seismic velocity, higher attenuation of seismic waves, higher electrical conductivity) in association with the LAB and MLD. This simple petrologic model is tested on the abundant geophysical data of the Carpathian-Pannonian region (CPR), central Europe. The high resolution heat flow data available in the CPR allows us to estimate the depths to intersection of area specific depth-temperature curves with the dehydration solidus temperatures (1050 and 1100 °C isotherms). There is relatively small mismatch (< 5 km) between the position of these intersections and the geophysically determined LAB in the central area of the CPR. These observations lend support for the proposition that the dehydration solidus may be largely responsible for depth variation of the LAB in young continental rift areas. Towards the margins of the CPR, however, where the heat flow is lower (< ~ 70 mW/m2), the predictive capability of the dehydration solidus model deteriorates. This is because, for lower geothermal gradients, pargasitic amphibole breaks down at ~90 km (or ~ 3 GPa) before temperature exceeds the dehydration solidus temperatures. Consequently we should expect changes in geophysical properties attributable to hydrous silicate melt at ~90 km depth in areas where surface heat flow is lower (i.e. Precambrian cratonic shields, Phanerozoic continental lithospheres or, possibly older oceanic plates). Alternatively, in these areas, the intersection of the geotherm with pargasite breakdown may correlate with the MLD rather than the LAB, which is at deeper levels. A global review is also included to assess whether: 1) the position of the dehydration solidus temperatures in the upper mantle (~1050 and 1100 °C isotherms) could explain the origin of the LAB under younger oceanic plates with higher surface heat flow; 2) there is indeed global anomalies at ~90 km depth, which are often interpreted as MLDs, in older continental areas, older oceanic crust and cratons with lower surface heat flow.

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