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Tracking the transition from magmatic to post-crystallization environment by coupling OH-defects and trace element analyses in quartz

Tumaini Giulia1, Tavazzani Lorenzo 2, Skogby Henrik3, Bernardi Francesco1, Sinigoi Silvano1 & Lenaz Davide1

  • Affiliations:  1Department of Mathematics, Informatics and Geosciences, University of Trieste, Italy; 2Institute of Geochemistry and Petrology, ETH Zürich, Switzerland; 3Department of Geosciences, Swedish Museum of Natural History, Sweden 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 247

  • Programme No: 1.1.33

  • Theme 1 > Session 1

Abstract

Quartz is a major constituent of felsic igneous rocks and can incorporate trace elements through isovalent and coupled substitutions. In this latter case, the involvement of hydrogen in the substitution mechanism can lead to the formation of hydroxyl dipoles (OH) with oxygen anions from the quartz lattice. Hydrogen can also be incorporated in interstitial, non-lattice-bound compounds. Physico-chemical parameters of the melt at the time of quartz crystallization regulate incorporation of trace elements and OH-defects formation, but post-crystallization processes might alter their budget and obscure the magmatic signal. We present the results of a coupled OH and trace elements systematic study in quartz from the silicic intrusive and eruptive products of a Permian transcrustal magmatic system (Sesia Magmatic System). Each grain was analysed using FTIR spectroscopy to investigate hydrous defects both qualitatively (OH species) and quantitatively (OH content), followed by the determination of trace elements content via LA-ICP-MS analysis. Results indicate systematic variations: (1) intrusive quartz shows decrease in Ti and increase in total water content (1-25 ppm H2O) with differentiation; (2) volcanic quartz displays variable Ti and water contents (2-14 ppm H2O) comparable to intrusive units; (3) in both intrusive and eruptive units, lattice-bound Al-specific defects dominate over non-lattice bound Li-specific defects, except in a porphyritic dike showing evidence of fast cooling. These observations suggest that, in absence of fast cooling, only slow-diffusing elements and molecules (Al, Ti, AlOH) can preserve the primary magmatic signal in quartz, suggesting caution in interpreting fast-diffusing elements (Li) and compounds (LiOH) as petrogenetic indicators.