Implications of radiogenic initial osmium isotopes in lower crust from the CM-1A drill site of the Oman Drilling Project (Oman ophiolite)

One of the goals of the Oman Drilling Project has been to advance our understanding of the processes that formed and modified the oceanic crust in the Wadi Tayin massif of the Oman ophiolite. In a reconnaissance study of samples from the CM-1A drill core we show that both the lower layered gabbros with dunite-troctolite and dunite-wehrlite layers and the 150 m thick dunite units of the crust-mantle transition zone (CMTZ) mostly display suprachondritic initial γOs up to +54 (γOs represents the deviation of ¹⁸⁷Os/¹⁸⁸Os from a mean chondritic value) at the time of their formation around 97-95 Ma. In contrast, underlying mantle harzburgites and dunites at the drill site display mantle-like γOs (95 Ma) of -8 to +2, implying that most magmas that crystallized the lower layered gabbros and the CMTZ dunites never equilibrated with the local mantle rocks underneath. Other data on gabbros further up in the lower crust of the Wadi Tayin massif also indicate positive γOs (95 Ma), however, because of the high Re/Os of some samples, it is difficult to calculate reliable initial γOs (95 Ma)-values (Peucker-Ehrenbrink et al. 2012, Geology 40, 199-202). Nevertheless, from the limited data it appears that most of the lower oceanic crust in the Wadi Tayin massif may have been affected by magma or fluid with suprachondritic γOs (95 Ma). We will discuss hypotheses concerning the origin of the radiogenic osmium in the lower crust and propose ways of testing these ideas.