Water contents in nominally anhydrous minerals in oceanic peridotite
Final Report Abstract
Our new data expand the knowledge about the amount and distribution of water in the sub-oceanic mantle. Until recently, water concentration data were exclusively derived from sub-continental peridotite xenoliths, presumably because oceanic peridotites are partly serpentinized and, thus, are not very attractive measurements. The first data for oceanic peridotites were obtained in the present research project, published first by Gose et al. (2009) for samples from ODP Leg 153, followed by the study of equivalent rocks from ODP Leg 209. We are able to show that oceanic peridotite preserved information on water contents in its mantle source and that serpentinization did not modify the original water contents. Orthopyroxene of Mid-Atlantic Ridge peridotite contains water contents that consistently differ by one order of magnitude, from about 270 ppm in Leg 153 to low contents of c. 15 ppm in Leg 209. This shows that the water content of the sub-oceanic mantle strongly varies along the Mid-Atlantic Ridge, up to values indicative of water saturation. The water contents of Leg 209 samples are compatible with residual values after 18 % batch melting of a peridotitic source with 120-130 ppm bulk rock water and 300 ppm in the original orthopyroxene. Leg 153 samples have water contents to be expected for non-depleted rocks that cannot be reconciled with residual amounts after 12 % partial melting. Following melt depletion, the water content must have reequilibrated in the spinel-peridotite facies. This may have been facilitated by a longer mantle residence time of Leg 153 peridotite (compared to Leg 209) after partial melting and prior to tectonic exhumation from c. 60 km depth. An extended mantle residence time of Leg 153 peridotite is compatible with the observed episode of isobaric cooling from >1200 °C to 950-1000 °C (at c. 20 kbar pressure). This enables re-introduction of water from less depleted mantle regions due to the high diffusivity of H. Defect water is evenly distributed in orthopyroxene grains and unrelated to serpentinization. Water is not released from orthopyroxene due to partial re-equilibration on the ocean floor, nor is water, stemming from late-stage fluid infiltration, incorporated into orthopyroxene. Serpentinization preferably starts at grain margins and cracks but may also occur in optically clear parts mineral cores. Because the latter is microscopically invisible, caution is needed if other trace elements are analyzed. IR spectroscopy is very sensitive to detect even trace amounts of secondary alteration. Oceanic spinel peridotites may contain as much water as continental spinel peridotite xenoliths, up to values that equal the water storage capacity of orthopyroxene at spinel peridotite-facies pressure. This implies water-saturated conditions in parts of the mantle and strongly argues against decompressioninduced water loss during uplift. We suggest that c. 200-300 wt.-ppm H2O are representative for orthopyroxene in non-depleted (or re-fertilized) spinel peridotite. As documented here for the first time, sub-oceanic peridotite preserves its water content originally attained at mantle depth. Hence, the measured values in tectonically exhumed mantle rocks, such as oceanic and alpine-type peridotite, may be used to mirror the water contents of the mantle. Moreover, information on mantle water may even be preserved in partially serpentinized rocks.
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
(See online at https://doi.org/10.1007/s00710-010-0125-7) - (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 of spinel peridotite from the Mid-Atlantic Ridge - no visible change in defect water during alteration. European Journal of Mineralogy 23: 529-536
Gose J, Schmädicke E, Stalder R
