Evidence for a reducing Archean ambient mantle and its effects on the carbon cycle

Chemical reduction-oxidation mechanisms within mantle rocks\udlink to the terrestrial carbon cycle by influencing the depth at which\udmagmas can form, their composition, and ultimately the chemistry of\udgases released into the atmosphere. The oxidation state of the uppermost\udmantle has been widely accepted to be unchanged over the past\ud3800 m.y., based on the abundance of redox-sensitive elements in\udgreenstone belt–associated samples of different ages. However, the\udredox signal in those rocks may have been obscured by their complex\udorigins and emplacement on continental margins. In contrast, the\udsource and processes occurring during decompression melting at\udspreading ridges are relatively well constrained. We retrieve primary\udredox conditions from metamorphosed mid-oceanic ridge basalts\ud(MORBs) and picrites of various ages (ca. 3000–550 Ma), using V/Sc\udas a broad redox proxy. Average V/Sc values for Proterozoic suites\ud(7.0 ± 1.4, 2s, n = 6) are similar to those of modern MORB (6.8 ±\ud1.6), whereas Archean suites have lower V/Sc (5.2 ± 0.4, n = 5). The\udlower Archean V/Sc is interpreted to reflect both deeper melt extraction\udfrom the uppermost mantle, which becomes more reduced with\uddepth, and an intrinsically lower redox state. The pressure-corrected\udoxygen fugacity (expressed relative to the fayalite-magnetite-quartz\udbuffer, DFMQ, at 1 GPa) of Archean sample suites (DFMQ –1.19 ±\ud0.33, 2s) is significantly lower than that of post-Archean sample suites,\udincluding MORB (DFMQ –0.26 ± 0.44). Our results imply that the\udreducing Archean atmosphere was in equilibrium with Earth’s mantle,\udand further suggest that magmatic gases crossed the threshold that\udallowed a build-up in atmospheric O2 levels ca. 3000 Ma, accompanied\udby the first “whiffs” of oxygen in sediments of that age