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1 Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK
School of Earth Sciences, University of Greenwich, Medway Campus, Pembroke, Chatham Maritime, Kent ME4 4AW, UK
2 Department of Geology, University of Tasmania, GPO Box 252C, Hobart, Tasmania 7001, Australia
3 Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong
4 Geologisk Institutt, Universitetet i Bergen, Allegaten 41, N-5007 Bergen, Norway
In recent years a unique set of samples of uppermost mantle at the mantle-crust transition zone have been collected from two different environments along the fast-spreading East Pacific Rise (EPR): a ‘normal’ spreading segment (represented by samples from Hess Deep) and the end of a spreading segment where the EPR meets the Garrett transform fault (represented by samples from Garrett Deep). A review of the petrology of harzburgites from the two sites demonstrates that these rocks were produced by partial melting of adiabatically upwelling mantle and, subsequently, at the top of the mantle, they were impregnated by reactive and crystallizing mid-ocean ridge basaltic (MORB) melts. Despite this similar history, non-impregnated harzburgites at Garrett Deep have a more fertile spinel chemistry than those at Hess Deep, which is consistent with reduced partial melting of shallow mantle as a transform fault is approached — the ‘transform fault effect’.
The extent of reaction between melt and harzburgite during the impregnation event suggests that melt arrived in the uppermost mantle in a highly reactive state because along the adiabatic path it had been highly channelled in spatially restricted conduits. This implies that mantle upwelling below the EPR was, and presumably still is, dominantly two dimensional (sheet-like). Within this framework, the chemical evolution of MORB melt below fast-spreading ridges will be significantly affected by melt-periodotite reaction only when melt reaches the uppermost mantle and mantle-crust transition zone, where along-axis transport of melt may also be important.
Although the harzburgites from Hess Deep and Garrett Deep formed and evolved beneath different parts of different first-order segments of the EPR, the petrology of these rocks presents the best analogue available for defining real variations in mantle processes along a single first-order ridge segment in a fast-spreading environment.
