Advances in mantle geochronology

Recent development of methods for precise in-situ analysis of isotope ratios (mainly by laser ablation microprobe multi-collector (MC-) ICP-MS analysis) has led to significant advances in mantle geochronology. This technique provides rapid and precise data, within a micro-structural framework and allows integration with datasets produced by other micro-analytical techniques. The integration of multiple datasets is crucial in constraining the origin of the sample and the processes leading to its formation, to provide a meaningful geological interpretation of determined ages.
A major advance in applying Re-Os to mantle-derived peridotites came with the recognition that their Os budget is controlled by sulfides. Mantle sulfides are time-capsules: just as zircons record events in crustal rocks, they record multiple events in the lithospheric mantle, represented by sulfides with different Os contents and Re/Os.. In-situ analysis of Re-Os isotopes in individual sulfides constrains the timing of these events. Apart from dating the depletion events, the sulfides provide constraints on a range of mantle processes such as the addition of metasomatic fluids during lithosphere reworking. The identification of multiple generations of sulfides in most mantle peridotites means that whole-rock Re-Os ages reflect a mix of these different sulfide populations. In many samples, sulfides enclosed in silicates are less radiogenic than the whole-rock and yield older ages for original lithospheric mantle stabilization. Most interstitial sulfides that occur along grain boundaries are mixtures and reflect the end-product of multiple melting and metasomatic events. Age-relative probability diagrams, using sulfides with low Re/Os ratios (¹⁸⁷Re/¹⁸⁸Os ≤ 0.08), represent residual phases from partial melting or crystallizarion from sulfide melts. These ’age’ spectra can be used to establish linkages with thermal and tectonic events in the overlying crust and further our understanding of large-scale geodynamic processes.
The in-situ Re-Os method has also been applied to sulfides in abyssal peridotites and to Ru-Os-Ir alloys from podiform chromites in ophiolites, thus relating to the evolution of the convective upper mantle. Two populations of magmatic sulfides occur in most abyssal peridotites: one with low ¹⁸⁷Os/¹⁸⁸Os (ca 0.110) indicating a long term evolution in a low Re/Os environment, such as depleted lithospheric mantle, and the other with more radiogenic compositions (¹⁸⁷Os/¹⁸⁸Os from 0.13 to 0.20) associated with percolating melts. Coexistence of two groups of alloys with different ¹⁸⁷Os/¹⁸⁸Os in ophiolite chromitites has also been interpreted as interaction of asthenosphere-derived melts and old depleted lithospheric mantle. Both findings suggest that significant volumes of the ’convective’ mantle are relicts of ancient and depleted sub-continental lithospheric mantle.