Build-up of multiple volatiles in Earth's continental keels: Implications for craton stability

Abstract

Elevated contents of structurally-bound hydrogen in olivine, introduced to cratonic mantle during metasomatism, are widely believed to decrease the effective viscosity so that it is similar to the underlying asthenosphere (1018–1019 Pa⋅s) and susceptible to basal erosion. Nevertheless, large variations exist in H contents of mantle olivines from the Kaapvaal, Siberia, Slave, Tanzania and Wyoming cratons (0 to 320 ppmw) and the role of water as the main driver of rheological weakening is contentious. While recent experimental studies have shown that olivine has the capacity to host large quantities of fluorine, as well as hydrogen, the magnitude of this has not yet been established for cratonic mantle.Our new dataset for Kaapvaal craton peridotites shows that the style of metasomatism influences the addition of H2O and F to cratonic mantle. Silicic fluids derived from subducted slabs, and responsible for the pervasive orthopyroxene enrichment observed in the Kaapvaal (and many of the world's other cratons), deliver significant quantities of H2O but lesser amounts of F, whereas proto-kimberlite melts transport high quantities of both H2O and F. Kaapvaal mantle olivines are major hosts of both H2O (up to 105 ppmw) and F (up to 180 ppm), and we propose that a major increases in the bulk H2O (∼30%) and F (∼65%) of the Kaapvaal craton occurred over a short (20 Ma) time interval between the two main pulses of Cretaceous kimberlite emplacement in southern Africa.By combining the thermal structure of individual cratons with corresponding H2O data for olivine, we show that the effective viscosity contrast at the base of the mechanical boundary layer and asthenosphere below the Kaapvaal, Slave and Siberia cratons varies from 2 to 14. Some of the lowest effective viscosities (3.5×1017 Pa⋅s) occur in H2O-rich olivine in Siberia sheared peridotites from the base of the craton and are consistent with highly-localised metasomatism. More viscous peridotites (up to 1.5×1021 Pa⋅s) were entrained from the thermal boundary layer beneath the Kaapvaal and Tanzania cratons but residence times in this region are short due to convective overturn (<100 Ma) and this dehydrated mantle would offer only temporary protection to cratonic keels. Our viscosity estimates are based on the H content of olivine and, while the additional effect of structurally bound F in olivine is currently uncertain, we anticipate that it will have a similar effect on mantle rheology to H, so that the base of the mechanical boundary layer beneath cratons at the time and location of kimberlite generation is close to the tipping point for instability.The rapid build-up of volatiles associated with pervasive kimberlite activity may have been the catalyst for lithospheric thinning on the southern margin of the Kaapvaal craton, but more localised pulses of kimberlite activity occurred over longer time intervals in the Siberia and Slave cratons and so had a less profound effect on their stability. A prolonged and widespread subduction-flux of H2O to the Wyoming and N. China cratons, and subsequent sub-solidus partitioning in olivine, may have been the driver for the rheological weakening that ultimately led to delamination.