In the western median valley wall of the Mid Atlantic Ridge between 23°18' and 23°37'N serpentinized mantle peridotites are exposed. They are part of ODP Leg 153 and were drilled by two holes of 126.4 and 200.8 m depth below sea floor.
The original rocks were olivine-rich lherzolites with porphyroclastic texture (orthopyroxene porphyroclasts). Their modal ratios of 70-85% olivine, 15-22% orthopyroxene, 0-8% clinopyroxene and 0.5-1.5% chromium-bearing spinel are close to those of refractory mantle peridotites in ophiolites. Strong tectonic strain caused mylonitisation in most of the samples.
All primary minerals, i.e. olivine, orthopyroxene, clinopyroxene, and spinel, are present in pre- syn- and postkinematic morphological varieties. In case of orthopyroxene the large clasts were the first formation and were dismembered to mosaics of syn- to postkinematic grains with interstitial clinopyroxene and spinel. The orthopyroxene in mosaic arrangement is lower in Ca, Ti, Ni, and Cr. We explain these relations by a combined process of clast dismembering, recrystallisation, and exsolution and neoformation of clinopyroxene and spinel. These observations are in contradiction to current ideas, that (1) orthopyroxene clasts have formed via integration of mosaics, and (2) that all intergranular diopside has formed from magmatic percolation through the peridotites.
The bulk chemical composition of the serpentinized peridotites reveals changes due to alteration. Heavy rare earth elements together with Sc, Al, Si, Ti, and Cr are the only elements which do not indicate mobility. All other determined elements, including light rare earth elements (LREE), were more or less mobile during alteration. Among the mobile LREE, Eu was obviously of conspiciously strong mobility. This can be explained with hot and strongly reducing conditions during alteration, which caused the reduction of Eu+3 to significantly more mobile Eu+2.
The platinum group elements were also determined and reveal some mobility in course of alteration. The enrichment of Pd due to formation of secondary sulphides is indicated. Elevated Os-contents occur in samples with more Cr-spinel.
Several gabbroic injections occur in the investigated mantle sections. These are not products of partial mantle melting in situ, but have obviously been intruded from deeper parts of the mantle sections. The factors of enrichment or depletion of elements in these partial mantle melts were calculated, and they correspond to those gained by experimental partial melting of mantle at temperature of 1200°C. This shows a general validity of chemical partition between upper mantle and crust.