The Distribution of Water in the Lunar Magmatic Ocean
by Matthias Christian Alexander Nieuwenhuis
Date of Examination:2023-05-10
Date of issue:2023-07-17
Advisor:Prof. Dr. Sharon Webb
Referee:Prof. Dr. Sharon Webb
Referee:Prof. Dr. Andreas Pack
Persistent Address:
http://dx.doi.org/10.53846/goediss-9963
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Abstract
English
Water plays an important role for many geological and organic processes. Nevertheless, it is still unclear why our Earth has such large quantities of it. Either it has been part of the building blocks of our planet or was delivered to it very early after its formation. The lunar formation took place simultaneously and therefore offers unaltered insights into the early history of the Earth where information was constantly lost by active processes like weathering for billions of years. The assumed Moon forming giant impact caused magma oceans on Earth and Moon. In this study, experiments have been made which simulate the conditions of the lunar magma ocean in the pressure range between 3 and 0.2 GPa. One of the goals was the investigation of the crystallisation process to derive the partitioning of water between melt and minerals at realistic lunar conditions. This results in a model distribution of the lunar interior water prior a possible mantle inversion. The experimental results predict olivine and orthopyroxenes as mantle phases followed by clinopyroxene at 81% solidification. The onset of crustal formation by precipitation of plagioclase has been found for the last 9% of solidification accompanied by metal oxides and quartz. The performed runs have shown the challenges of generating reliably the dry and reducing lunar conditions two log-units below the iron-wüstite buffer. We noticed precipitation of elemental iron in our series considered to be indicative of reducing conditions. Nevertheless, it has not been clearly demonstrated whether the Fe-nuggets found are a result of the conditions or an artifact caused by the use of iron capsules. To investigate these experimental findings, additional thermodynamic simulations were performed with MELTS. These predict a strong loss of iron in the deep Moon for the envisaged oxygen fugacity indicating ongoing core formation. The results of the simulations were combined with partitioning coefficients that have since been published for lunar conditions by another group to model the distribution of water in the interior of the Moon. The results predict a minor role of the actual mineral abundances for the water content of the residual melt. With an initial water content of 100 ppm in the LMO composition, the minerals show an increase in water content above 10 ppm only from a depth of 60-90 km. This predicts very dry mantle conditions for the previous 1300 km depth and a significant increase in water content of the uppermost clinopyroxene and plagioclase layers. To match the measured water content of lunar samples to our results, extensive degassing must have taken place for the late stages of the LMO crystallisation.
Keywords: Moon; Lunar Magmatic Ocean; Water; Piston Cylinder Press
