Titanium stable isotope investigation of magmatic processes on the Earth and Moon

We present titanium stable isotope measurements of terrestrial magmatic samples and lunar mare basalts with the aims of constraining the composition of the lunar and terrestrial mantles and evaluating the potential of Ti stable isotopes for understanding magmatic processes. Relative to the OL-Ti isotope standard, the delta Ti-49 values of terrestrial samples vary from -0.05 to +0.55 parts per thousand, whereas those of lunar mare basalts vary from -0.01 to +0.03 parts per thousand (the precisions of the double spike Ti isotope measurements are ca. +/- 0.02 parts per thousand at 95% confidence). The Ti stable isotope compositions of differentiated terrestrial magmas define a well-defined positive correlation with SiO2 content, which appears to result from the fractional crystallisation of Ti-bearing oxides with an inferred isotope fractionation factor of Delta Ti-49(oxide-melt) = -0.23 parts per thousand x 10(6)/T-2. Primitive terrestrial basalts show no resolvable Ti isotope variations and display similar values to mantle-derived samples (peridotite and serpentinites), indicating that partial melting does not fractionate Ti stable isotopes and that the Earth's mantle has a homogeneous delta Ti-49 composition of +0.005 +/- 0.005 (95% c.i., n = 29). Eclogites also display similar Ti stable isotope compositions, suggesting that Ti is immobile during dehydration of subducted oceanic lithosphere. Lunar basalts have variable delta Ti-49 values; low-Ti mare basalts have delta Ti-49 values similar to that of the bulk silicate Earth (BSE) while high-Ti lunar basalts display small enrichment in the heavy Ti isotopes. This is best interpreted in terms of source heterogeneity resulting from Ti stable isotope fractionation associated with ilmenite-melt equilibrium during the generation of the mantle source of high-Ti lunar mare basalts. The similarity in delta Ti-49 between terrestrial samples and low-Ti lunar basalts provides strong evidence that the Earth and Moon have identical stable Ti isotope compositions. (C) 2016 Elsevier B.V. All rights reserved.