Trace element and isotopic characterization of mafic cumulates in a fossil mantle diapir (Oman ophiolite)

This paper is devoted to an exploratory geochemical study (trace elements, Sr-Nd isotopes) of a suite of cumulate features cropping out in the mantle harzburgites of Oman. The cumulates are concentrically distributed around a fossil mantle diapir, From the centre to the periphery of the diapir, their mineralogy becomes more and more evolved, from troctolite to olivine gabbro and gabbronorite, and their textural and field characteristics point to injection in a cooler environment. Nd isotopic data are consistent with a mantle origin for all these cumulates. The mantle source is heterogeneous on a small scale (6.09 < epsilon(Nd) < 10.15) but this heterogeneity and the average epsilon(Nd) (approximate to 8) are comparable to those of Indian Ocean mid-ocean ridge basalts (MORB's). An origin of this cumulate suite, largely by fractional crystallization from liquids with MORE characteristics, is supported by major and compatible trace element variations. However compatible elements show that it is unlikely that all these cumulates are derived by different degrees of fractional crystallization from the same liquid. This conclusion is corroborated by incompatible trace element data. The calculated liquids in equilibrium with these cumulates have MORE-like REE patterns. However, variations in REE patterns from one lithological group to the next, and within each lithological group, imply a pronounced heterogeneity in the REE content of the equilibrium liquids that clearly does not result from a simple fractional crystallization process. Liquids in equilibrium with the troctolites have a higher range in [La](n)/[Yb](n) than liquids in equilibrium with the olivine gabbros. The most evolved cumulates (gabbronorites) have REE patterns that might imply a more complex origin for their parental melt involving mixing of MORE-like liquids with melts from a LREE-depleted source.