Résumé
Recent tomographic images from the East Pacific Rise [1] indicate that nearly 50% of the melt is delivered off-axis instead of beneath the ridge axis. The consequences of this process can be studied on land in the Oman ophiolite, where structural mapping has shown the existence of both on- and off-axis melt delivery systems [2]. Off-axis diapirs impinge directly on hydrothermally-altered lithosphere and may be contaminated by this material. Petrologically, off-axis melt delivery results in the development of an unusually thick Moho Transition Zone (MTZ) composed largely of clinopyroxenite,whereas on-axis MTZ contains troctolite and gabbro lenses. We are analyzing the Sr and Nd isotopic compositions of pyroxenites, gabbros, diorites, and dunites, from an on-axis (Maqsad) and an off-axis (Mansah) diapir in the Semail massif of the Oman ophiolite to explore the geochemical consequences of these contrasting settings. Leached and unleached whole rocks, as well as clinopyroxene (cpx) separates are being studied, to allow us to investigate both magmatic and later hydrothermal processes. Initial Nd isotopic compositions from unleached whole rock powders are comparable for the two diapirs (HNd = 8.5-9.2 on-axis; HNd = 5.6-10.3 off-axis), although off-axis rocks display a larger range of variation. In contrast, whole rock Sr isotopic ratios differ markedly, with on-axis samples displaying a limited range of enriched MORB-like compositions (87Sr/86Sr =0.703010 - 0.703438) whereas off-axis samples have more radiogenic and highly heterogeneous compositions (87Sr/86Sr = 0.703277 - 0.706192). We attribute this difference to the presence of a magma chamber above the on-axis diapir, which protects the MTZ from the effects of hydrothermal circulation. Separated cpx and leached whole rocks from off-axis pyroxenites have less radiogenic compositions (87Sr/86Sr = 0.703218 - 0.703576) than the corresponding unleached whole rocks. Nevertheless, several of these samples have 87Sr/86Sr ratios that approach the upper limit of the Indian MORB field. In outcrop, a mixing zone is observed between pyroxenites and crustal gabbros in the MTZ overlying the off-axis diapir. Samples from this mixing zone have more radiogenic cpx compositions than samples collected further from the Moho, suggesting magmatic contamination of the pyroxenites by incorporation of hydrated material rich in seawater-derived Sr.