Application of the triple oxygen isotope system to mantle materials

Oxygen isotope ratios (¹⁶O/¹⁸O) have been used in numerous studies of mantle lithologies to estimate 1) the protolith 2) the source of metasomatizing fluids, 3) temperature of formation, and 4) degree of equilibration between coexisting minerals. The δ¹⁸O value of mantle olivine comprises a very restricted range (Mattey et. al., 1994), so that even subtle deviations from this ‘canonical’ value indicate some form of contamination. Eclogites span a range of 2–8‰ and suggest an altered oceanic crust or sediment protolith (MacGregor and Manton, 1986). However, the ¹⁸O/¹⁶O information alone provide limited information. The addition of the novel ¹⁷O/¹⁶O ratios provide additional constraints on mantle minerals, expanding the isotope data from a one-dimensional line to a two-dimensional field.

Our preliminary results demonstrate the utility of triple oxygen isotope analyses of mantle materials. OPX and olivine mineral separates from spinel peridotites are generally well-behaved for ¹⁷O, with Δ' ¹⁷O values (see Sharp, 2013 for nomenclature) of -0.05 ± 0.01‰, the same as presumed mantle olivine. We see that CPX is out of δ¹⁸O equilibrium in all cases. The Δ¹⁸O values for olivine, garnet, OPX, CPX and phlogopite from a granular garnet peridotite (Kimberley, SA) suggest complete ¹⁸O/¹⁶O disequilibrium, but the Δ' ¹⁷O values of all samples are again typical of that of the average mantle. In contrast, a sheared peridotite from the same location has Δ18O values in equilibrium at ~950 °C, but Δ' ¹⁷O values that suggest mixing between a low Δ' ¹⁷O – low δ¹⁸O and a high Δ' ¹⁷O – high δ¹⁸O component. One mantle eclogite sample, a grospydite from Smyth and Hatton (1977), has a wide range of Δ' ¹⁷O values but δ¹⁸O values that appear in equilibrium. It has been suggested that the wide range in δ¹⁸O values could suggest an origin for the eclogites from subducted altered oceanic crust (MacGregor and Manton, 1986). We are measuring the triple oxygen isotope values of altered oceanic crust in order to identify potential sources for the ¹⁷O isotope disequilibrium seen. The Δ' ¹⁷O values of these samples can be used to support such an idea. In general, the Δ' ¹⁷O values of altered oceanic crust are higher than typical mantle. Low Δ' ¹⁷O values may be indicative of subducted sedimentary material.