A fossil melt lens in the Oman ophiolite: Implications for magma chamber processes at fast spreading ridges

From seismic evidence we know that a small body of magma is present at mid-crustal levels beneath most fast spreading ridges, and that this small sill or melt lens overlies a much broader area of hot but largely solid material (a 'crystal mush') that constitutes the lower ocean crust. Beyond this, however, we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens, nor the role the melt lens plays in the storage and aggregation of mid-ocean ridge basalts (MORB). We here offer constraints on the processes of melt transport, modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the Oman ophiolite, concentrating on the uppermost plutonic racks and transition into sheeted dykes. We find that almost the entire Oman plutonic suite - the layered gabbro, and most of the 'high-level gabbro' - is dominated by cumulate rocks containing very low proportions of trapped interstitial melt. The exception to this is the very uppermost part of the section, within similar to 150 metres of the base of the sheeted dyke complex, at which level the gabbros instead display a marked heterogeneity of structure, texture and composition, and plutonic rocks of generally basaltic composition occur for the first time. Associated pgematitic ferrogabbros formed by in situ fractionation of these liquids. This horizon passes up into a more homogeneous microgabbroic facies, also of basaltic composition: within which isolated asymmetric doleritic chilled margins are found, and thence into doleritic sheeted dykes. We argue that the thin heterogeneous gabbro horizon is a fossilised melt lens, containing pooled liquids of net basaltic composition (Mg# of similar to 65) expelled from the crystal mush beneath. Existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens, but this is not supported by the field relationships. Instead we propose that the fabrics record the buoyant ascent of magma through the mush pile (probably in part by porous flow) and reflect crystallisation essentially in situ, without significant vertical transport of the mush. Low residual melt porosities in the sub-melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust. (C) 2000 Published by Elsevier Science B.V. All rights reserved.