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International audience; In this study, three clinopyroxene-enriched upper mantle xenoliths, petrographically classified as wehrlite,were investigated from the Nógrád–Gömör Volcanic Field with the use of X-ray microtomography. Our main goal was toquantify the volume of the glass phase and the vesicles to reveal their three-dimensional distribution. Among the studiedwehrlite xenoliths, one is weakly and two are strongly metasomatized. The two latter wehrlite xenoliths are characterizedby higher modal amount of glass and vesicles, which suggests a genetic connection between glass and concomitantvesicles, and the metasomatic agent. The glass, which was a melt at mantle conditions, forms an interconnected network.This may explain the presence of the electromagnetic anomaly with high electrical conductivity beneath the study area.Our study contributes to the better understanding of melt migration and its metasomatic effect in the lithospheric mantlebeneath monogenetic volcanic fields

International audience; The structural hydroxyl content of the nominally anhydrous minerals (olivine and pyroxenes) in the upper mantle is among the important attributes that influence the physical and chemical features of the upper mantle. In this study, we provide detailed Fourier-transform infrared (FTIR) data on 63 petrographically and geochemically well-defined upper mantle xenoliths from the Nograd-Gomor Volcanic Field (Pannonian Basin, Central Europe). These xenoliths show extremely low average structural hydroxyl contents (similar to 0, 31 and 185 ppm for olivine, orthopyroxene and clinopyroxene, respectively) compared to values reported regionally and worldwide. The studied xenoliths have anomalous types of FTIR spectra and high structural hydroxyl ratios between clinopyroxenes and orthopyroxenes (an average of similar to 8). Furthermore, there is usually no correlation between the structural hydroxyl content and other physical or chemical properties of the xenoliths. These specific FTIR characteristics suggest that the Nograd-Gomor upper mantle xenoliths were exposed to significant modification of their structural hydroxyl contents, which may be linked to pre- and post-eruptive processes. Decompression during extension leads to lower 'water' activity, which is most likely to have played a key role. However, pre-eruptive mantle metasomatism with an agent having low water activity cannot be excluded either. The post-eruptive cooling can be significant as well, as suggested by the higher structural hydroxyl content in xenoliths hosted in more rapidly cooled volcanic facies (i.e. pyroclastics). Our study reveals how FTIR characteristics may evolve in continental rift settings in young extensional basins. Furthermore, novel applications of our study are the diagnostic features that indicate significant changes in structural hydroxyl properties. This contributes to distinguishing low structural hydroxyl contents linked to the pre-eruptive (i.e., low structural hydroxyl content in pyroxenes, anomalous partitioning and anomalous band characteristic in pyroxenes) or the post-eruptive (completely 'dry' olivines) periods.

Peridotite xenoliths from the Nógrád–Gömör Volcanic Field (NGVF) record the geochemical evolution of the subcontinental lithospheric mantle beneath the northern margin of the Pannonian Basin. This study is focused on spinel lherzolites and presents petrography, and major and trace element geochemistry for 51 xenoliths selected from all xenolith-bearing localities of the NGVF. The xenoliths consist of olivine, orthopyroxene, clinopyroxene and spinel ± amphibole. No correlations between modal composition and textures were recognized; however, major and trace element geochemistry reveals several processes, which allow the distinction of xenolith groups with different geochemical evolution. The xenoliths have undergone varying degrees (∼7–25%) of partial melting with overprinting by different metasomatic processes. Based on their Mg#, the xenoliths can be subdivided into two major groups. Group I has olivine Mg# between 89 and 91, whereas Group II has Mg#, Journal of Petrology, 58 (6), ISSN:0022-3530, ISSN:1460-2415

The Nograd-Gomor Volcanic Field (NGVF) is one of the five mantle xenolith bearing alkaline basalt locations in the Carpathian Pannonian Region. This allows us to constrain the structure and properties (e.g. composition, current deformation state, seismic anisotropy, electrical conductivity) of the upper mantle, including the lithosphere-asthenosphere boundary (LAB) using not only geophysical, but also petrologic and geochemical methods. For this pilot study, eight upper mantle xenoliths have been chosen from Barna-Nagykő, the southernmost location of the NGVF. The aim of this study is estimating the average seismic properties of the underlying mantle. Based on these estimations, the thickness of the anisotropic layer causing the observed average SKS delay time in the area was modelled considering five lineation and foliation end-member orientations. We conclude that a 142–333 km thick layer is required to explain the observed SKS anisotropy, assuming seismic properties calculated by averaging the properties of the eight xenoliths. It is larger than the thickness of the lithospheric mantle. Therefore, the majority of the delay time accumulates in the sublithospheric mantle. However, it is still in question whether a single anisotropic layer, represented by the studied xenoliths, is responsible for the observed SKS anisotropy, as it is assumed beneath the Bakony–Balaton Highland Volcanic Field (Kovacs et al. 2012), or the sublithospheric mantle has different layers. In addition, the depths of the Moho and the LAB ( $$25\,\pm \,5, 65\,\pm \,10\,\hbox {km}$$ , respectively) were estimated based on S receiver function analyses of data from three nearby permanent seismological stations.