Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths

We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (delta Mg-25 = -0.14 +/- 0.06; delta Mg-26 = -0.27 +/- 0.12 parts per thousand, 2SD), but our enstatite chondrite samples have lighter delta Li-7 (by up to similar to 3 parts per thousand) than our mean carbonaceous and ordinary chondrites (3.0 +/- 1.5 parts per thousand, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of delta Li-7 = 3.5 +/- 0.5 parts per thousand, delta Mg-25 = -0.10 +/- 0.03 parts per thousand and delta Mg-26 = -0.21 +/- 0.07 parts per thousand. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk delta Li-7 and delta Mg-26, which vary from -3.7 parts per thousand to + 14.5 parts per thousand, and -0.36 parts per thousand to + 0.06 parts per thousand, respectively. Values of delta Li-7 and delta Mg-26 lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require similar to 5-10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High delta Li-7, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy delta Li-7 seen in some oceanic basalts. (C) 2011 Elsevier Ltd. All rights reserved.