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

Orthopyroxene in oceanic spinel peridotite from the Mid-Atlantic Ridge near 15 degrees N, ODP-Leg 209, contains about 15 wt.-ppm water that is bound as hydroxyl in the mineral structure. This value is one order of magnitude lower than the water contents reported for (i) orthopyroxene in peridotite from the Mid-Atlantic Ridge at 23 degrees N (270 wt.-ppm), ODP-Leg 153, and (ii) sub-continental spinel peridotite xenoliths. Orthopyroxene from both latter occurrences contains the maximum amount of water that can be incorporated in enstatitic orthopyroxene at pressures of the spinel peridotite facies (c. 200-300 wt.-ppm). The water contents in orthopyroxene from the sub-oceanic mantle at the Mid-Atlantic Ridge, ranging from about 15 (= highly depleted) to nearly 300 wt.-ppm (= water saturation in orthopyroxene), compare well with the highly variable water contents of Atlantic MORB glass that also scatter over one order of magnitude. Mineral trace element data indicate different degrees of partial melting: 18% for peridotite samples of Leg 209 and 12% for those from Leg 153. The mantle equilibrium temperatures differ by 200 degrees C (Leg 209: 1150-1200 degrees C, Leg 153: 950-1000 degrees C). The orthopyroxene water contents of Leg 209 are compatible with residual values after c. 18% partial melting of a peridotitic source with a water content of c. 120-130 wt.-ppm in the bulk rock and of c. 300 wt.-ppm in the original orthopyroxene, respectively. In contrast, the water content of 270 wt.-ppm in orthopyroxene of Leg 153 peridotite cannot be reconciled with residual amounts to be expected after 12% partial melting. This is because an unrealistically high initial water content of c. 3500 wt.-ppm would be required for the original orthopyroxene - exceeding the water storage capacity of orthopyroxene at an ambient pressure of c. 20 kbar by one order of magnitude. We conclude that - following melt depletion - the water contents of Leg 153 peridotite re-equilibrated in the spincl-peridotite fades. 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 is compatible with isobaric cooling from >1200 degrees C to 950-1000 degrees C (at c. 20 kbar pressure) as documented by Leg 153 peridotite, and it facilitates re-introduction of water from less or non-depleted mantle regions due to the high diffusivity of H. (C) 2011 Elsevier B.V. All rights reserved.