New insights into the crust-mantle transition zone beneath fast-spreading mid-ocean ridges: the role of Cr-spinel composition.

The formation of oceanic crust at mid-ocean ridges is one of the dominant processes in the chemical differentiation of our planet. Oceanic crust formed at fast-spreading ridges exhibits a relatively uniform seismic stratigraphy and is regarded as layered and relatively homogeneous in mineralogy. Because of the lack of natural in-situ exposures at the base of the crust of recent oceans, existing models on the geodynamics of the deep processes during crustal accretion have never been tested directly using natural samples. Using the CM1 and CM2 drill cores penetrating the Oman ophiolite sequence, the best analogue for fast-spreading crust on land, recovered by the ICDP Oman Drilling Project, we started a study combining experimental work and investigations on samples of the drill cores in order to shed light on the nature of this the poorly understood crust-mantle transition. The drill cores CM1 and CM2 cover the upper mantle harzburgites at the bottom, followed by a 90 m thick massive dunite layer with layered gabbros at the top. Ni and Mg# in olivine as well as Cr#, Mg# and trace elements in chrome spinel were analyzed by EPMA and fs-LA-ICP-MS. The data reveals a homogeneous harzburgite composition, followed by homogeneous dunite in the lower part and an upper dunite section showing decreasing Mg#, and therefore more differentiated compositions towards the top. We conclude that only the upper half of the 90 m thick dunite has a cumulative origin. The lower dunite section may be formed as a result of reactions with harzburgite.