H2o Quantitative Analysis of Transition Zone Minerals Wadsleyite and Ringwoodite By Raman Spectroscopy

Abstract:

Liquid H₂O covers approximately 70% of the Earth’s surface but it can also be incorporated as OH^- groups in nominally anhydrous minerals (NAMs) that constitute the Earth’s mantle, as observed in peridotitic xenoliths. The presence of even trace amounts (ppm wt) of hydrogen in mantle minerals strongly affect the physical, chemical and rheological properties of the mantle. The Earth’s transition zone (410 to 660 km depth) is particularly important in this regard since it can store large amounts of H₂O (wt%) as shown by experiments and recently by a natural sample. Addressing the behavior of H₂O at high depths and its potential concentration in mantle NAMs is therefore fundamental to fully comprehend global-scale processes such as plate tectonics and magmatism.

We developed an innovative technique to measure the H₂O content of main transition zone NAMs wadsleyite and ringwoodite by Raman spectroscopy. This technique allows to use a beam of 1-3 µm size to measure small samples that are typical for high pressure natural and synthetic specimens. High pressure polyphasic samples are indeed very challenging to be measured in terms of H₂O content by the routinely used Fourier transform infra-red (FTIR) spectroscopy and ion probe mass spectroscopy analyses, making the Raman approach a valid alternative.

High quality crystals of wadsleyite and ringwoodite were synthesized at high pressure and temperature in a multi-anvil press and analyzed by Raman and FTIR spectroscopy as well as elastic recoil detection analyses (ERDA) which is an absolute, standard-free technique. We will present experimental data that allow to apply Raman spectroscopy to the determination of H₂O content of the most abundant minerals in the transition zone. The data gathered in this study will also permit to investigate the absorption coefficients of wadsleyite and ringwoodite that are employed in FTIR quantitative analyses.