Effect of water on the rheology of the lithospheric mantle in young extensional basin systems as shown by xenoliths from the Carpathian-Pannonian region

Abstract

Incorporation of hydrogen as structural hydroxyl (commonly referred to as water) in nominally anhydrous mantle minerals is known for the ‘hydrolytic weakening’ effect, which decreases the strength and electrical resistivity of the rock. Recent models have provided means of calculating rheological properties from geochemical data of upper mantle xenoliths. In the Carpathian-Pannonian region, upper mantle xenoliths can be found on the surface at five locations, both at marginal and central regions of the basin system. In this study we present a comprehensive overview of water contents in these xenoliths, including previously lacking data from two outcrops (Füzes-tó and Tihany) from the Bakony-Balaton Highland Volcanic Field for the first time. The Tihany xenoliths have significantly higher water contents (< 1–5, 116–353 and 327–1394 ppm in olivine, orthopyroxene and clinopyroxene, respectively) than the Füzes-tó xenoliths (0–2.7, 6.2–114 and 3.1–213 ppm in olivine, orthopyroxene and clinopyroxene, respectively). This can be explained with the older eruption age of their host basalt and greater depth of origin, as they likely represent an asthenospheric layer that became part of the lower lithosphere during thermal relaxation. In contrast, the Füzes-tó xenoliths represent a lot dryer mantle portion and are assumed to have been more affected by decompression-induced water loss resulting from decreased water activity during the extension. In general, the marginal xenolith locations of the Carpathian-Pannonian region, associated to prior supra-subduction environment, contain more water than xenoliths from central locations (with the exception of Tihany locality) which were significantly affected by extension-related lithospheric thinning. To reveal the differences in rheology inferred from the different water contents, we calculated effective viscosities and electrical resistivities for the xenoliths of the Bakony-Balaton Highland and other locationss in the Carpathian-Pannonian region using previously published data for input. Based on the results, the central locations have higher effective viscosities (1.4∙10 20 –2.2∙10 21 Pa s) and electrical resistivities (48–913 Ωm) compared to the marginal locations (9.3∙10 19 –6.8∙10 20 Pa s and 36–182 Ωm, respectively), suggesting that the lithospheric mantle is more rigid in the former than in the latter areas. This may be a common feature for extensional basins, as the extension leads to the ‘drying’ of the upper mantle, whereas subduction zones keep hydrating their overlying mantle wedge. However, for more accurate estimations, other factors such as regional differences in strain rate or the potential presence of melts or fluids in the mantle need to be considered as well. • Water lowers effective viscosity and electrical resistivity in mantle xenoliths • Water contents of xenoliths from different tectonic environments were compared • Central regions of the back-arc basin are drier compared to marginal areas • Rheological differences are present in distinct parts of extension-subduction systems • Decompression related water loss results in strengthening of the lithosphere