Rhenium abundances and systematics in oceanic basalts

Abstract

We have examined the systematics of Re abundances in concert with new and published data on major and trace elements in oceanic basalts. Re abundances in MORB average 927 ppt (parts per trillion), a factor of 2.6 higher than the OIB average (350 ppt). The variability of Re abundances in both MORB and 01B is only 45%, similar to moderately incompatible major and trace elements such as Ca, Al, and the heavy rare earths. The inferred mantle/melt partitioning behavior of Re is similar to Yb, and average Re/Yb ratios in MORB, O1B, and the continental crust are within 50% of each other. The Re/Yb ratios of the depleted mantle and continental crust are both lower than the primitive mantle. The Re/Yb systematics place constraints on core-mantle exchange. The incorporation of material from the outer core into mantle plumes, and isolation of the upper mantle from the core, would result in Re/Yb ratios in OIB elevated by factors of 5–24 over MORB. Although the scatter of the OIB Re/Yb data may allow core addition up to the 1.2% level, the lack of a correlation between Re/Yb and Os isotopes in OIB, and the small difference in average Re/Yb ratios of MORB and OIB suggests an upper limit of 0.2 wt% core material in mantle plumes. A mass balance calculation between continental crust, depleted mantle, and primitive mantle indicates that the crustal abundance of Re is too low (by a factor of ten) to balance the Re depletion of the depleted MORB mantle (DMM). Anoxic sediments and black scales are an important crustal reservoir for Re, but the estimated abundance of these rocks in the continental crust cannot be increased substantially without upsetting the crustal U budget and ratios of Th/U, Nb/U, and U/Pb in the depleted mantle. The Re missing from DMM must therefore reside in recycled oceanic crust stored in the deep mantle. We propose that elevated Os isotope ratios in OIB are a direct measure of a ubiquitous component of recycled MORB crust in mantle plumes. The Re-Os mass balance allows a simple estimate of the total size of the OIB reservoirs to be made. A mass of recycled MORB equivalent to 4.4% of the mantle is required to balance the Re depletion of DMM. The total mass of the OIB reservoirs is estimated at 4.4% to 22% of the mantle mass, depending on whether the low- 187Os/ 188Os component(s) in OIB sources are external or intrinsic, respectively. Using an estimate of the present-day plume flux, the average residence time of subducted slabs in the convecting mantle is estimated at 700–3500 Ma; however, the slab residence time scales linearly with the size of the OIB reservoir (and thus the proportion of recycled crust in mantle plumes). We prefer a model in which individual mantle plumes incorporate slabs with distinct ages. Consideration of Os and He isotope covariations in OIB suggest a model in which subducted slabs are recycled through the whole mantle on timescales of < 1.8 Ga.