Project Details
Projekt
Water in the sub-oceanic mantle: An infrared spectroscopic study of orthopyroxene from abyssal Atlantic and Pacific peridotite
Applicant
Professorin Dr. Esther Schmädicke
Subject Area
Palaeontology
Term
from 2014 to 2022
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 260734520
Water, commonly present in trace amounts in nominally anhydrous mantle minerals, significantly influences the physical properties and dynamics of the Earth mantle. Orthopyroxene is a very important container for mantle water. It may incorporate several hundred ppm H2O. Orthopyroxene contains considerably more water than olivine, plus it is able to preserve mantle water contents during exhumation better than olivine rendering it a useful proxy for mantle water levels. Previous studies were restricted to orthopyroxene from the sub-continental mantle. The first water data for sub-oceanic mantle orthopyroxene are the result of own investigations, financed by DFG. The recently published results provide the first direct information on the water content in oceanic peridotite. Our initial study (i) proved that water is present in sub-oceanic anhydrous minerals, (ii) yielded information on the content of water and its variation in the mantle, (iii) showed that decompression-induced water loss did not occur, and (iv) confirmed that partial serpentinization has no affect on structural water in orthopyroxene. The novel data are surprising because they document contrasting water contents in the sub-Atlantic mantle and comprise concentration levels typical for water saturation (ODP-Leg 153) as well as those that are one order of magnitude lower (Leg 209). Samples from the East Pacific Rise (unpublished data from ongoing project) show intermediate characteristics. The own previous study yielded various unexpected results that have far-reaching consequences for models concerning (i) the distribution and re-distribution of water in the upper mantle and (ii) the processes that were involved in the exhumation of mantle rocks along mid-ocean ridges. However, the new findings are difficult to explain because the available data set is still very limited. At present, the exhumation rate or, better, mantle residence time after melt depletion seems to be the most important variable. Hence, the main focus of the planned project is to test this hypothesis and to constrain the parameters that govern the water contents in the sub-oceanic mantle. For this, oceanic peridotite from various ODP sites will be investigated; the samples have to be selected to account for variability in spreading rate, degree of melt depletion, mineral composition, equilibrium temperature and pressure. The planned study will provide further information on the content and distribution of mantle water and, more importantly, put constraints concerning dynamic processes below spreading centers. To develop a more general model and to test the preliminary hypothesis, it is absolutely necessary to study peridotite from more sites along slow- and fast-spreading ridges. The project suggested is aimed at closing a fundamental gap of knowledge, and will also help to better understand the dynamic processes that take place beneath spreading centers and that are involved in the exhumation of mantle rocks.
DFG Programme
Infrastructure Priority Programmes
International Connection
Austria
