Infrared spectroscopy on garnet of eclogite and associated ultramafic rocks: Importance of garnet for water transport into the mantle

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This project is aimed at deciphering the water amount hosted in eclogite and transported into the mantle by subduction of oceanic plates, facilitating magma generation. At >100 km depth, water in eclogite is mainly stored in garnet and omphacite. Unfortunately, little is known about H2O in orogenic eclogite. To close this gap of knowledge, garnet was studied first to define its H2O-amount, the substitution mechanisms, and the influencing parameters (pressure, temperature, mineral composition). The initial, successful project stage revealed that garnet of orogenic eclogite is able to incorporate as much H2O as c. 3 vol.% amphibole and that H2O is positively correlated to Ca. The spurious correlation of H2O and pressure is ascribed to the pressure-enhanced Ca-content of garnet. Notably, the co-stability of OH-minerals is an important variable governing H2O in garnet. If OH-minerals are present, garnet is free of water. Another novel finding is that garnet contains up to three H2O species, two of them secondary. Thus, the quantification of primary H2O is challenging, but this problem was successfully solved by detailed analysis of infrared (IR) spectra. Unrecognized secondary water results in unrealistically high H2O contents (as in some literature examples). The most unexpected result of the second project stage was that the omphacite H2O content in quartz eclogite and coesite eclogite is identical. This implies that H2O incorporation does not depend on the PT-peak conditions, at least not up to 30 kbar and 900 ºC. Since no more H2O can be released from hydrous minerals beyond these conditions, the obtained H2O contents should reflect the amount being transported to >100 km depth. Despite this progress, it remains difficult to reliably decipher the H2O content in omphacite because, for omphacite, the H2O contents determined by IR spectroscopy strongly depend on the choice of the IR absorption coefficient. Therefore, an additional, independent method for H2O analysis has to be applied. The proponent is optimistic to solve this problem in a final project stage by utilizing a nuclear method, precisely a combination of Elastic Recoil Detection Analysis (ERDA) und Proton-Proton Scattering Analysis (PPSA). Our working group successfully applied PPSA for the analysis of H2O in olivine. The preliminary result of constant water storage in omphacite up to 30 kbar and 900 ºC, if confirmed, has far-reaching implications. Given that beyond these conditions no further dehydration reactions are expected, eclogite should preserve most of the water liberated by eclogite-facies hydrous minerals up to 100 km depth in its NAMs. During further subduction, this H2O volume should remain in the system and not be released to the mantle wedge, which, consequently, should apply also to fluid-mobile elements.