THERMAL STRUCTURE OF A FOSSIL MANTLE DIAPIR INFERRED FROM THE DISTRIBUTION OF MAFIC CUMULATES

MAFIC lithologies constitute a minor but ubiquitous component of the m antle section of ophiolites; they are currently viewed as cumulates le ft by basaltic melts travelling from the melting region to the surface . These features may be used to constrain the mechanisms of melt migra tion in the mantle, provided their geophysical context of emplacement can be established in some detail. Here we report on the nature and di stribution of mafic cumulates in the harzburgites of the Oman ophiolit e, within and around a mantle diapir frozen beneath a spreading axis(1 -6). We show that their composition, texture and field characteristics are systematically related and display a concentric zoning centred on the diapir. We propose that this zoning reflects the thermal structur e of the diapir and of the associated melt plumbing system. Assuming a parental liquid with the composition of mid-ocean-ridge basalt(7-9), the cumulate compositions show that the melt temperature exceeded 1,23 0 degrees C in the innermost part of the diapir and decreased progress ively down to <1,100 degrees C in the surrounding lithospheric mantle. The transition between channelled porous flow and dyking is observed in an outer shell of the diapir, where the melt crystallized along the plagioclase-olivine cotectic (temperature ranging from 1,180 to 1,210 degrees C).