Interpreting ages from Re–Os isotopes in peridotites

Abstract

Since the first application of the 187Re– 187Os isotope system to determine the timing of melt extraction from peridotites twenty years ago, a plethora of studies have demonstrated its usefulness in dating melt depletion, hence, lithosphere formation in continental settings. However, these studies have also highlighted issues that affect this system and that need to be considered in its application. These include serpentinization and alteration, refertilization, melt–rock reaction (including sulfide metasomatism) and the heterogeneity of the Os isotopic composition of the convecting upper mantle. Serpentinization appears to have little effect on the Re–Os system, possibly due to the reducing conditions under which it occurs. Seafloor weathering of abyssal peridotites and kimberlite incursion into mantle xenoliths can increase Re abundances, thus impacting T MA model ages, but generally leaving T RD model ages little modified. Sulfide breakdown is common in mantle xenoliths and may cause Os depletion, elevated Re/Os and leave the xenoliths more susceptible to overprinting. Refertilization of refractory peridotites by silicate melts appears to be a natural consequence of adiabatic melting, a process we refer to as “auto-refertilization”. Osmium model ages will not be impacted by auto-refertilization as it occurs at or shortly following melt extraction. In contrast, refertilization occurring long after partial melting, as documented in some cratonic peridotite xenoliths, can alter Os model ages. Fortunately, refertilization can normally be recognized on plots of Al 2O 3 vs. 187Os/ 188Os, as Al 2O 3 is more strongly affected than 187Os/ 188Os. The influence of melt–rock reaction on peridotites depends on the melt–rock ratio. At high melt–rock ratios typical of ocean spreading centers, melts dissolve primary sulfides, producing discordant dunites with low Os concentrations and high 187Os/ 188Os. At the lower melt–rock ratios more typical of continental lithosphere, secondary sulfides having more radiogenic Os than that found in residual peridotites precipitate from silicate melts (“sulfide metasomatism”). The degree to which these processes can shift bulk rock Os isotope compositions is a function of the amount of secondary sulfides precipitated and whether primary sulfides are removed, creating low Os contents. The sulfur content can be used to place constraints on the amount of secondary sulfide precipitated in a residual peridotite. While secondary sulfides are commonly observed in peridotites, in most cases sulfide metasomatism has only a minor effect on whole rock composition. Perhaps the most important factor influencing the ability to derive robust ages from relatively young peridotites and their sulfides is the fact that the 187Os/ 188Os in the convective upper mantle is heterogeneous, and the average composition is not well constrained. However, these uncertainties diminish with increasing age of the lithosphere. In addition, Archean sulfides are exceedingly rare in samples of convective upper mantle, so continental peridotite xenoliths having Archean model ages are unlikely to be recent samples of convecting upper mantle. The body of literature reviewed here shows that the Re–Os system is a blunt chronometer, but one that, applied judiciously to well-characterized peridotites, can provide important age constraints on lithosphere formation.