Transmission electron microscopy investigations of (hydrous) chain silicates from the lithospheric mantle beneath the Carpathian Pannonian Region

Transmission electron microscopy (TEM) is a powerful, yet scarcely used technique when it comes to investigating mantle minerals and fluid inclusions. It is capable to collect structural information of the studied mineral, its precise chemical composition, and makes nanofeatures visible, such as dislocations and nano-inclusions.

In this study TEM and STEM (scanning transmission electron microscopy) measurements were carried out on a set of ortho- and clinopyroxene samples from central and marginal localities of Carpathian Pannonian region (CPR), where Plio-Pleistocene alkaline basalt volcanism sampled the lithospheric mantle retrieving lithospheric mantle xenoliths. Objective of the study was to constrain the presence and formation mechanisms of sub-microscopic occurrence of pargasitic amphibole.

The detailed investigation of pargasite in the upper mantle is rather timely, because its presence may be the major cause for the rheologic contrast experienced between the lithosphere and the asthenosphere [1], [2]. The nominally anhydrous minerals’ (NAMs, as ortho- and clinopyroxene) structural hydroxyl [3] content or volatiles in fluid inclusions could lead to formation of pargasite [4]. In addition, pargasite could form interstitially during metasomatic intereactions.

Our observations so far suggest that hydrous silicate formation as sub-solidus exsolution in the central CPR may not have taken place. Ordering of the Ca forming Ca-rich and Ca-poor domains in an orthopyroxene grain was identified. Precursors of H⁺ diffusion were also recorded, such as dislocations and nanosized fluid inclusions. Diffusion of H⁺ could be active in the lattice scale through the disclinations along subgrain boundaries [3], [5] or dislocations in the host mineral along the boundary of nanoscale fluid inclusions [6], [7]. Clinopyroxene-amphibole phase boundary has been prepared by focused ion beam (FIB) milling technique from the marginal area of CPR. The chemical composition of the amphibole lamella provides evidence that the H₂O content of the nearby fluid inclusion migrated into the host clinopyroxene producing an amphibole lamella growing along the ‘c’ crystallographic axis [4].

Observations of the boundary of clinopyroxene and amphibole confirm that the amphibole octahedral layers penetrate the clinopyroxene structure. The precise nanoscale measurements (STEM mapping) of chemical composition of both the host and the lamellae can lead to profound implications on the original composition of the studied fluid inclusions.

[1] Green, D. H., Hibberson, W. O., Kovács, I. J., & Rosenthal, A. (2010). Nature, 467(7314), 448–451.

[2] Kovács, I. J., Lenkey, L., Green, D. H., Fancsik, T., Falus, G., Kiss, J., Orosz, L., Angyal, J., Vikor, Zs. (2017). Acta Geodaetica et Geophysica, 52, 183–204.

[3] Liptai, N., Kovács, I.J., Lange, T.P., Pálos, Zs., Berkesi, M., Szabó, Cs., Wesztergom, V. (2019). Goldschmidt Abstracts, 2019 1981.

[4] Lange, T.P., Liptai, N., Patkó, L., Berkesi, M., Kesjár, D., Szabó, Cs., Kovács, I. J. (2019). 25th European Current Research on Fluid Inclusions (ECROFI) , Abstract Series, 68.

[5] Demouchy, S., & Bolfan-Casanova, N. (2016). Lithos, 240–243, 402–425.

[6] Bakker, R. J., & Jansen, J. B. H. (1994). Contributions to Mineralogy and Petrology, 116, 7–20.

[7] Viti, C., & Frezzotti, M. L. (2000). American Mineralogist, 85(10), 1390–1396.