Depletion of Titanium in the mantle of the Earth

Many authors have used the chemical composition of peridotitic rocks from a variety of geological settings to estimate the composition of the upper mantle of the Earth, (PUM-primitive upper mantle). This composition should represent the bulk Earth mantle after core formation and before the onset of mantle melting. The basis for this estimate are linear negative correlations of refractory element such as Al and Ca with MgO and positive correlations with Ni and Co in peridotitic rocks (see [1] and references therein). One implication is that refractory lithophile elements occur in chondritic ratios in PUM. A close inspection of recent data on mantle rocks (e.g., [2]) showed, however, that Al/Ti ratios in these rocks are generally above 20, compared to the chondritic ratio of about 18. This signature is present in many suites of mantle rocks, even considering the often poor quality of Ti analyses in chondrites and mantle rocks. It appears that Ti is depleted in PUM by 10 to 15%.

The depletion of Ti in the mantle of the Earth may either reflect very reducing conditions during early Earth accretion or strong metal-silicate partitioning of Ti under extreme conditions of temperature and/or pressure. Recently metal-silicate partition coefficients at high pressures and temperatures were experimentally determined. Preliminary results indicate that some Ti may indeed partition into metal during core formation.

Lit.: [1] Palme H. and O’Neill H.St.C. in Treatise on Geochemistry (2nd ed., vol.3, 1-39. [2] Carlson R.W. and Ionov D.A. (2019) GCA 257, 206-223.