The Cumulate Sequence of the Oman Ophiolite consists of layered mafic and ultramafic cumulates that represent the products of a dynamic, long-lived, crustal magma chamber that underlay a late-Cretaceous spreading ridge, Regional variations in Cumulate-Sequence stratigraphy reflect not only contrasting degrees of mantle fractionation but also contrasting primary magma compositions. Microprobe data are presented from a 600 m section of modally layered olivine gabbros. Coherent cryptic cyclic variation is shown, with both 'normal' and 'reversed' geochemical gradients present; these are explained in terms of fractionation, mixing and eruption in an open-system magma chamber. The height of the liquid column required to precipitate the observed cyclic units is calculated to be about 100 m; the implications of this result are discussed. A contrast between the Fo-An co-variation between olivine and plagioclase in the Oman Cumulate Sequence and that observed for MORB votcanics is noted; this may reflect compositional stratification in spreading-ridge magma chambers or signifcant differences between the petrogenesis of the Oman ophiolite and MORB.

The Cumulate Sequence of the Oman Ophiolite consists of layered mafic and ultramafic cumulates that represent the products of a dynamic, long-lived, crustal magma chamber that underlay a late-Cretaceous spreading ridge, Regional variations in Cumulate-Sequence stratigraphy reflect not only contra...