Uniform “water” content in quartz phenocrysts from silicic pyroclastic fallout deposits – implications on pre-eruptive conditions

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.

Structural hydroxyl distribution in jadeite grains and the diagenesis mechanism of jadeitite in Myanmar, Guatemala and Russia

Carrying out jadeitite diagenesis in its basic state as well as in combination with the multiple fluid activities of water, we focused on jadeite grains in production areas such as Myanmar, Guatemala and Russia to determinethe essence of nominal anhydrous jadeite grains and the distribution of structural hydroxyl in individual jadeite grains via infrared microscope. This is a scientific problem that urgently needs to be solved. The results show that a microscale of structural water in the form of structural hydroxyl is widely found in the jadeite grains of primary jadeitite in Myanmar, Guatemala and Russia. There are certain differences in the three groups of characteristic infrared absorption bands generated by the stretching vibration of the structural hydroxyl in jadeite from these different producing areas. The structural hydroxyl content of the individual jadeite grain shows obvious differences according to the producing areas, and the structural hydroxyl content in jadeite grains in Myanmar is up to 2522.11 × 10-6. By contrast, jadeite grains in Guatemala and Russia yield lower results. In addition, the structural hydroxyl content in individual jadeite grains in the three producing areas are inhomogeneous, showing an increasing trend from interior to exterior. However, dynamic metamorphism of different intensities and multi-period fluid participation lead to differences in the increase, whereby an increase in structural hydroxyl content in jadeite grains from Guatemala and Russia is slower. The study of the distribution of structural hydroxyl in jadeite grain from different producing areas is helpful for exploring the interaction trajectory of fluids involved in jadeitite and the diagenesis mechanism of jadeitite, a fact that is of great scientific significance for revealing the formation process and conditions of jadeitite.

New FTIR data of calc-alkaline volcanic rocks from the Oas-Gutai Mts. and post eruption effects on the water content of phenocrysts

<p>We studied volcanic rocks from the Oas-Gutai Mts. (Transylvania, Romania) to measure the ‘structural hydroxyl’ content of the nominally anhydrous minerals (NAMs such as clinopyroxene, plagioclase, quartz), from which water content of the parental magma can be estimated.  The Neogene volcanic chain of the Carpathian-Pannonian region (CPR), due to petrologic variability, is an excellent area for such investigation.</p><p>Recent FTIR studies on the calc-alkaline rocks from CPR, showed that the ‘structural hydroxyl’ content of NAMs could be modified during and after volcanic eruptions [1], [2], [3]. However, transmission FTIR-microscopy is an adequate technique for recognizing this these changes because FTIR spectra of the NAMs indicate signs in the case of hydroxyl loss [4].</p><p>For studying the pre-eruptive water contents clinopyroxenes are the most promising mineral because it has one of the lowest diffusion rates for hydroxyl in NAMs [5]. With the detailed study of the clinopyroxenes FTIR spectra, conclusions can be drawn concerning the potential post-eruptive loss of hydroxyl [4].</p><p>We have examined 8 volcanic rock samples, four dacite samples from Oas and one basalt two andesite and one rhyolite sample from the Gutai Mts. The samples show diverse volcanic facies such as lava, ignimbrite and debris avalanche. The diversity of samples is important for future research because it will help to choose the most adequate volcanic facies to estimate the magmatic equilibrium water contents.</p><p>The studied clinopyroxenes contain 83-371 ppm ‘structural hydroxyl’ content,which can be considered as normal values compared to the work of [6] where ‘structural hydroxyl’ content in clinopyroxenes show a range from 75 to 390 ppm in the mafic calc-alkaline lavas from Salina, Italy.</p><p>[1] Lloyd, A.S., Ferriss, E., Ruprecht, P., Hauri, E.H., Jicha, B.R., & Plank, T. (2016): Journal of Petrology, 57, pp. 1865-1886</p><p>[2] Biró, T., I. Kovács, D. Karátson, R. Stalder, E. Király, G. Falus, T. Fancsik, J. & Sándorné Kovács (2017): American Mineralogist, 102, pp.</p><p>[3] Pálos, Z., Kovács, I. J., Karátson, D., Biró, T., Sándorné Kovács, J., Bertalan, É., & Wesztergom, V. (2019): Central European Geology, 62(1)</p><p>[4] Patkó, L., Liptai, N., Kovács, I., Aradi, L., Xia, Q.K., Ingrin, J., Mihály, J., O'Reilly, S.Y., Griffin, W.L., Wesztergom, V., & Szabó, C. (2019): Chemical Geology, 507, pp. 23-41.</p><p>[5] Farver, J.R. (2010): Reviews in Mineralogy and Geochemistry, 72 (1), pp. 447–507.</p><p>[6] Nazzareni, S., Skogby H., & Zanazzi, P.F. (2011): Contributions to Mineralogy and Petrology, 162, pp. 275–288.</p>

A new method to determine both water content and hydrogen isotope composition of two forms of water in nominally anhydrous minerals

<p>    A new continuous flow method, by combining high vacuum stepwise-heating (HVST) device with the thermal conversion elemental analyzer and gas isotope mass spectrometer (TC/EA-MS), is presented for determination of water contents and H isotope compositions for both structural hydroxyl and molecular water in garnet. By using the HVST device, molecular water and structural hydroxyl can be liberated step by step from garnet at different heating temperatures. By using the on-line quadrupole mass spectrometer in the HVST device, heating temperatures were determined for releasing the two forms of water from garnet from ultrahigh-pressure metamorphic eclogite in the Dabie orogen. Releasing temperatures of molecular water and structural hydroxyl from the garnet are 400°C and 1400°C, respectively. The garnet gives water of 228±39 ppm and a dD value of -110±10‰ for molecular water at dehydration temperature of 400°C for 1 hour, and water of 301±27 ppm and a dD value of -81±4‰ for structural OH at dehydration temperature of 1400°C for 1 hour. Therefore, the HVST-TC/EA-MS method can be used to analyze both water content and H isotope composition of the two forms of water in nominally anhydrous minerals.</p>

On the use of nominally anhydrous minerals as phenocrysts in volcanic rocks: A review including a case study from the Carpathian–Pannonian Region

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.