High Precision Ti stable Isotope Measurement of Terrestrial Rocks

Thursday, 18 December 2014
Marc-Alban Millet1, Nicolas Dauphas2, Helen M Williams3, Kevin W Burton4 and Geoffrey M Nowell3, (1)University of Durham, Durham, DH1, United Kingdom, (2)University of Chicago, Chicago, IL, United States, (3)Durham University, Durham, United Kingdom, (4)University of Durham, Durham, United Kingdom
Advances in multi-collection plasma source mass spectrometry have allowed the determination of stable isotope composition of transition metals to address questions relevant to both high and low temperature geochemistry. However, titanium has received only very limited attention. Here we present a new technique allowing the determination of the stable isotope composition of titanium in geological samples (d49Ti or deviation of the 49Ti/47Ti ratio from the OL-Ti in-house standard of reference) using double-spike methodology and high-resolution MC-ICP-MS. We have carried out a range analytical tests for a wide spectrum of samples matrices to demonstrate a external reproducibility of ±0.02‰ on the d49Ti while using as little as 150ng of natural Ti for a single analysis.

We have analysed a comprehensive selection of mantle-derived samples covering a range of geodynamic contexts (MORB, IAB, OIB, adakites, eclogites, serpentines) and geographical distribution (MORB: Mid-Atlantic Ridge, Southwest Indian Ridge and Eastern Pacific Ridge; IAB: New Britain reference suite and Marianas Arc). The samples show a very limited range from -0.06‰ to +0.04‰ with a main mode at +0.004‰ relative to the OL-Ti standard. Average values for MORB, IAB and eclogites are similar within uncertainty and thus argue for limited mobility of Ti during subduction zone processes and homogeneity of the Ti stable isotope composition of the upper mantle. However, preliminary data for more evolved igneous rocks suggest that they display heavier Ti stable isotope compositions, which may reflect the removal of isotopically light Ti as a function of Fe-Ti oxide crystallisation. This is in good agreement with Ti being present in 5-fold and 6-fold coordination in basaltic melts and preferential uptake of 6-folded Ti by Ti-bearing oxides [1].

This dataset will be complemented by analysis of abyssal peridotites to confirm the homogeneity of the mantle as well as data for a range of ferromanganese crusts in order to explore the potential applications of Ti stable isotopes as a new tracer in paleoceanography.

[1] Farges F. and Brown G.E., 1997, GCA, 1863–1870