Wehrlites from continental mantle monitor the passage and degassing of carbonated melts

Abstract

Continental rifting has been linked to the thinning and destruction of cratonic lithosphere and to the release of enough CO 2 to impact the global climate. This fundamental plate tectonic process facilitates the infiltration and mobilisation of smallvolume carbonated melts, which may interact with mantle peridotite to form wehrlite through the reaction: enstatite þ dolomite (melt) = forsterite þ diopside þ CO 2 (vapour). Application to mantle xenolith suites from various rifts and basins shows that 2.9 to 10.2 kg CO 2 are released per 100 kg of wehrlite formed. For the Eastern Rift (Africa), this results in estimated CO 2 fluxes of 6.5 ± 4.1 Mt yr −1 , similar to estimates of mantle contributions based on surficial CO 2 surveys. Thus, wehrlite-bearing xenolith suites can be used to monitor present and past CO 2 mobility through the continental lithosphere, ultimately with diffuse degassing to the atmosphere. They may also reveal the CO 2 flux in lithospheric provinces where carbonated melts or continent-scale rifts are not observed at the surface.

Linking deep CO2 outgassing to cratonic destruction

“…1 ), record carbonate metasomatism of the CLM beneath the NCC. Wehrlites represent rocks where all, or most, orthopyroxene has been consumed through metasomatic reactions and are considered to be one of the end products of carbonate metasomatism in the CLM [ 8 , 31 ]. Pyroxenites and garnet pyroxenites represent rocks where all olivine and orthopyroxene have been consumed through metasomatic reactions with SiO 2 carried by supercritical fluid or silica-rich melt and are therefore considered to be other end products of carbonate metasomatism in the CLM.…”

“…A reduced CLM beneath the thick NCC (>200 km) is indicated by the Paleozoic diamond-bearing kimberlites. However, it can become oxidized as the diamondiferous CLM is exhumed to shallower depths due to lithospheric thinning, extension and mantle upwelling [ 8 ]. During this process, ‘redox melting’ would occur and carbon (diamond or graphite) in the CLM would become unstable and be oxidized by the reduction of Fe 3+ at depths of <170 km [ 40 ].…”

“…A recent study by Aulbach et al. [ 8 ] quantified the CO 2 flux related to the reaction of the CLM with silica-undersaturated (carbonated) melt, referred to as wehrlitization, and linked this flux to surficial degassing in rifts and basins. As discussed above, abundant wehrlite and pyroxenite xenoliths are found in Mesozoic mantle-derived rocks in the eastern NCC, and many of these xenoliths have light Mg isotopic compositions and high Ca/Al ratios (Fig.…”

“…A quantitative estimate suggests that 2.9 to 10.2 kg CO 2 can be released per 100 kg of wehrlite formed [ 8 ]. Extensive CO 2 release is suggested to have occurred during the carbonated melt–CLM reaction process in the course of this destruction of the eastern NCC, along with the Tan-Lu Fault Belt, which was most active in the early Cretaceous [ 33 ].…”

“…For example, up to 28 to 34 Mt of carbon per year (expressed as Mt C yr –1 ) may be released by continental rifting [ 1 ]. A quantitative flux estimate for the CO 2 outgassing along with the massive Tan-Lu Fault Belt in eastern China gave 70 ± 58 Mt C yr –1 [ 8 ]. Extensive CO 2 degassing (71 ± 33 Mt C yr –1 ) has been estimated through extensional faults along the entire East African Rift [ 3 ], which is even comparable to the estimates for CO 2 degassing in island arcs (18–43 Mt C yr –1 ) and mid-ocean ridges (8–42 Mt C yr –1 ) [ 9–11 ].…”