Nanoscale amphibole nucleation and growth in the Earth's mantle is poorly understood despite its important role in the evolution and stability of the lithosphere, along with its influence on the coexisting fluid composition. Here, we focus on the nanoscale features of a clinopyroxene-amphibole-fluid inclusion system in an amphibole-bearing upper mantle xenolith and propose a new amphibole formation mechanism at mantle depth. Our approach includes crystallographic observations and interpretations of fluid inclusion formation and molecular properties of the entrapped H2O-bearing supercritical CO2-rich mantle fluids. First, the differentiation of H2O and CO2 along the supercritical fluid-clinopyroxene interface leads to nanometre-thick hydrous monolayer formation. The disruption of the supercritical fluid-clinopyroxene interface, including the hydrous monolayer, leads to amphibole formation. Hydrous fluid-mobile elements in the hydrous monolayers support amphibole growth via diffusion along the clinopyroxene-amphibole interface enhanced by various structural misfits. Our results help to clarify the driving forces of element exchange in a supercritical fluid-solid system and present the initial stage of hydrous mineral formation and growth. This process causes fixation of H2O and common lithophile elements with relative CO2 enrichment in the residual supercritical fluid. Nanochannels can contribute to element migration in clinopyroxene-amphibole-rich, non-porous domains in the lithospheric mantle.

Mantle metasomatism plays an important role in the element transport within the Earth’s mantle causing significant change in rheology and geochemistry of the infiltrated region. During volatile-rich fluid-mediated mantle metasomatism, new volatile-rich phases form at the micro- and nanoscale due to fluid-solid interaction. These micro- and nanoscale processes go hand-in-hand but evidence regarding the similarity or difference is scarce. We studied amphibole lamellae and here the connection should be mentioned nano-silicate melt inclusions on clinopyroxene from an amphibole-bearing mantle xenolith from the Persani Mountains Volcanic Field, southeastern Transylvania (Romania) with transmission electron microscopy (TEM).Based on petrography, post-entrapment interaction occurred between the host clinopyroxene and the trapped CO2-rich fluid in the inclusion, which all was essential to form the amphibole lamellae. According to our observation at the nanoscale, fluid escaped from the fluid inclusion followed by amphibole formation along the clinopyroxene-amphibole interface. The escaped fluid formed nano-silicate melt inclusions (NSMIs) that was studied by TEM. These NSMI consist of ~ 80 v% silicate glass and ~ 20 v% bubble. The composition of the silicate glass is as follows: high SiO2 (>60 wt.%) and Al2O3 (>20 wt.%), whereas low CaO, FeO and MgO (sum

We studied amphibole lamellae and associated hydrous nano-silicate melt inclusions (NSMI) in upper mantle xenoliths from the Persani Mountains Volcanic Field, southeastern Transylvania (Romania). Transmission Electron Microscope (TEM) results revealed submicron-scale pieces of evidence of escaped fluids in the form of nano-silicate melt inclusions along clinopyroxene-amphibole interfaces. These NSMIs consist of -80 vol% silicate glass and - 20 vol% volatile bubble from which the former has a high SiO2 (>60 wt%) and Al2O3 (>20 wt %) and low CaO, FeO and MgO (<8 wt%) content determined by SDD-EDS analyses. We calculated the original bulk composition of the nano-silicate melt inclusions with Monte Carlo simulation, using hydrated fluid molecules and suggest that originally the nano-silicate melt inclusions had most probably low SiO2 (- 43.6 wt%) and high Al2O3 (- 15.5 wt%), Na2O (-11.9 wt%) and H2O (- 30.3 wt%) contents. Using petrographic evidence, we propose that the amphibole lamellae formed as a result of post-entrapment reaction between clinopyroxene and the trapped fluid of the fluid inclusion. The presence of the hydrous nano-silicate melt inclusions suggest fluid migration along the clinopyroxene-amphibole interface, supporting the continuous growth of the studied amphibole. The association of the studied amphibole with hydrous nano-silicate melt inclusions suggests that H2O consumption from the fluid found in the micrometer-scale fluid inclusion continued at the nanoscale after a metasomatic event in the lithospheric mantle. This nanoscale process may also give information about the behavior of H2O globally in the lithospheric mantle where amphibole is stable, and along the lithosphereasthenosphere boundary in younger oceanic and continental plates.

Structural hydroxyl content of volcanic quartz phenocrysts was investigated with unpolarized Fourier-transform infrared spectroscopy. The phenocrysts originated from five pyroclastic fallout deposits from the Bükk Foreland Volcanic Area (BFVA), Hungary, and two from the AD 1314 Kaharoa eruption (KH eruption), Okataina Volcanic Complex (Taupo Volcanic Zone), New Zealand. All investigated quartz populations contain structural hydroxyl content in a narrow range with an average of 9.3 (±1.7) wt ppm. The earlier correlated horizons in the BFVA had the same average structural hydroxyl content (within uncertainty). Thus, it can be concluded that the structural hydroxyl content does not depend on the geographical distance of outcrops of the same units or the temperature or type of the covering deposit. The rare outlier values and similar structural hydroxyl contents show that the fallout horizons cooled fast enough to retain their original structural hydroxyl content. The similarity of the structural hydroxyl contents may be the result of similar P, T, and x (most importantly H2O and the availability of other monovalent cations) conditions in the magmatic plumbing system just before eruption. Therefore, we envisage common physical–chemical conditions, which set the structural hydroxyl content in the quartz phenocrysts and, consequently, the water content of the host magma (∼ 5.5 wt %–7 wt % H2O) in a relatively narrow range close to water saturation.

Amphibole is one of the most abundant ’water’-bearing minerals in the Earth’s upper mantle. Amphiboles occur as interstitial grains, lamellae within pyroxenes or as daughter minerals within fluid inclusions. Most commonly amphibole formation is related to mantle metasomatism, where the agent has a subducted slab (e.g. Manning 2004) or an asthenospheric origin (e.g. Berkesi et al. 2019). After the formation of fluid inclusions, a subsolidus interaction can take place where the H2O content of fluid inclusions may crystallize pargasite (e.g. Plank et al. 2016). Here we present amphibole lamellae formation in mantle xenoliths from the Persani Mountains Volcanic Field that is interrelated to a reaction between fluid inclusions and host clinopyroxene. Newly formed amphibole lamellae occur only in the surroundings of the fluid inclusions and grow within the host clinopyroxene in a preferred crystallographic direction. Studied lamellae do not reach the rim of the host mineral implying that components needed for formation of amphibole lamellae in clinopyroxene could have only originated from the fluid inclusion itself. We measured the major element composition of amphibole lamellae and host clinopyroxene (1) and used Raman spectroscopy and FIB-SEM on fluid inclusion study situated next to the lamellae (2). Results support the hypothesis that chemical components (dominantly H+) migrated sub-solidus from the fluid inclusion into the host mineral after fluid entrapment via subsolidus interaction. Beyond the clinopyroxene-hosted fluid inclusions, fluid inclusions in orthopyroxenes were also studied as a reference. Our study shows that post-entrapment diffusion from a fluid inclusion into the host mineral changes the solid/fluid ratio of the mantle which could modify the rheology of the lithospheric mantle. Berkesi, M. et al. 2019. Chemical Geology, 508, 182-196. Kovács et al. (2017) Acta Geodaetica et Geophysica, 52(2), 183-204. Manning C. E. 2004. Earth and Planetary Science Letters, 223, 1-16. Plank, T. A. et al. 2016. In AGU Fall Meeting Abstracts.

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.

The past decade has seen a great number of studies dealing with magmatic water contents and how these could be retrieved by the nominally anhydrous minerals’ (NAMs) trace structural hydroxyl (water) contents. Constraints have been made to magmatic hygrometry with clinopyroxene and plagioclase. Although results suggest that the method is more flexible and reliable than melt inclusion studies, they also indicate that the trace hydroxyl contents could still be overprinted by syn- and post-eruptive processes. Clinopyroxenes can hold more structural hydroxyl than plagioclases. A comprehensive review is presented with the inclusion of all published results so far to compile the available pieces of information. As a case study, micro-FTIR measurements are made of a representative set of plagioclase phenocrysts from the Börzsöny Mts. (Carpathian–Pannonian Region). The samples were selected to represent the progress of the volcanic activity in time and space, considering the petrologic and geochemical evolution of volcanic products in well-defined volcanostratigraphic positions. The syn- and post-eruptive cooling rate seems to have the greatest effect on water retention. This means that the systematic investigation of water in volcanic phenocrysts can contribute to distinguish the slowly and rapidly cooling parts of the volcanostratigraphic units.