Wasser im ozeanischem Mantel am schnell spreizenden ostpazifischen Rücken; Infrarot-Spektroskopie an nominell wasserfreiem Orthopyroxen
Final Report Abstract
Trace amounts of water in the Earth’s mantle have a significant influence on mantle convection and magma genesis. Hence, it is important to know how much water is present in the mantle. For the sub-oceanic mantle, direct measurements did not exist until recently. This lack of data is due to the technical difficulty connected with measuring oceanic mantle rocks. A precursor project, conducted by the principal investigator and collaborators, on mantle rocks from below the Atlantic Ocean allowed - for the first time - to analyze water directly. The present study focused on equivalent rocks from the sub-Pacific mantle. We ware able to measure the water content of orthopyroxene, which is an important, if not the most important, mineral for water storage in the uppermost mantle. Based on these data, the water contents for the bulk rocks and the corresponding mantle regions can easily be calculated. The obtained water concentrations in orthopyroxene (up to 233 ppm) and in the bulk rock (up to 100 ppm) are astonishingly high. Considerably less water should the present in the sub-oceanic mantle having been served as the source region for more than 90 % of primary magmas. In contrast, other incompatible trace elements, such as the rare Earth elements, have low concentrations - as expected for melt-depleted rocks. This unexpected result and the mismatch between the contents of water and other trace elements needs an explanation. One possibility could be that water, due to its greater diffusivity, was re-enriched in the samples in contrast to elements with sluggish diffusion. However, such water re-introduction, following the removal of melts, is only possible if the rocks remain in the mantle after partial melting. This is because the high amounts of water determined can only be stored in mantle minerals at high pressures of ≥ 1.5 GPa corresponding to depths of ≥45 km. We, thus, calculated the equilibrium pressure and temperature for all samples with the result that pressure agrees with the expectation (1.5 GPa), but temperature is much too low (1000°C). This low temperature is in contrast to the melting temperature and the ambient thermal conditions below mid-ocean ridges (≥1300°C). An explanation for this mismatch is that the rocks cooled at mantle depths after they experienced partial melting and served as the source of basaltic melts. Hence, the rocks must have remained at mantle depth for an extended (geological) time span between partial melting and exhumation to the ocean floor. For this unexpected result gave rise to a new model for which the term ‘mantle residence time’ was coined.
Publications
- (2009) Water in enstatite from Mid-Atlantic Ridge peridotite: Evidence for the water content of sub-oceanic mantle? Geology 37: 543-546
Gose J, Schmädicke E, Beran A
- (2010) Deep solid-state equilibration and deep melting of plagioclase-free spinel peridotite from the slow-spreading Mid-Atlantic Ridge, ODP Leg 153. Mineralogy & Petrology 100: 185-200
Will TM, Schmädicke E, Frimmel HE
- (2011) Heterogeneous mantle underneath the North Atlantic: Evidence from water in orthopyroxene, mineral composition and equilibrium conditions of spinel peridotite from different locations at the Mid-Atlantic Ridge. Lithos 125: 308-320
Schmädicke E, Gose J, Will T
- (2011) Water in mantle orthopyroxene - no visible change in defect water during serpentinization. European Journal of Mineralogy 23: 529- 536
Gose J, Schmädicke E, Stalder R
- (2015) Water in orthopyroxene from abyssal spinel peridotites of the East Pacific Rise (ODP Leg 147: Hess Deep). Lithos 232: 23-34
Hesse KT, Gose J, Stalder R, Schmädicke E
(See online at https://doi.org/10.1016/j.lithos.2015.06.011)
