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Water in the upper mantle: applications from the Carpathian-Pannonian region

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Water in the upper mantle: applications from the Carpathian-Pannonian region

Nóra Liptai

MTA Lendület Pannon LitH2Oscope Research Group

‘Water’ can be present in the lithospheric mantle in different forms, either as H2O in fluid or melt inclusions, or within mineral structures as structurally bound hydroxyl. In the latter case, the abundance can reach up to ~2 wt % in volatile bearing mantle minerals (most frequently pargasite), or tens to hundreds of ppm in nominally anhydrous minerals (NAMs; olivine and pyroxenes). Pargasite is generally stable up to 1050-1150°C in water-undersaturated conditions, and recently it was proposed that the upper limit of pargasite stability may correspond to the lithosphere-asthenosphere boundary in oceanic and young continental plates. This poses the question whether pargasite can form as a result of reaching stability conditions, for example via post-extensional cooling. Hyperspectral mapping of pargasite lamellae in pyroxenes of xenoliths from the Carpathian-Pannonian region suggests that to form pargasite lamellae on the micro-scale, external fluid source is needed (fluid inclusions, fluids or melts on the grain boundary).
Incorporation of water in the NAMs is known for the ‘hydrolytic weakening’ effect, which influences physical properties such as the strength and electrical resistivity of the rock. In the Carpathian-Pannonian region, upper mantle xenoliths can be found on the surface both at marginal and central regions of the basin system. The NAMs of xenoliths from the central localities have significantly lower water contents compared to those from the marginal ones. As a consequence, the lithospheric mantle in the central part of the basin has higher resistivity and effective viscosity, i.e., it is rheologically ‘stronger’ than at the marginal 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.