No abstract

No abstract

Chemical reduction-oxidation mechanisms within mantle rocks\ud link to the terrestrial carbon cycle by influencing the depth at which\ud magmas can form, their composition, and ultimately the chemistry of\ud gases released into the atmosphere. The oxidation state of the uppermost\ud mantle has been widely accepted to be unchanged over the past\ud 3800 m.y., based on the abundance of redox-sensitive elements in\ud greenstone belt–associated samples of different ages. However, the\ud redox signal in those rocks may have been obscured by their complex\ud origins and emplacement on continental margins. In contrast, the\ud source and processes occurring during decompression melting at\ud spreading ridges are relatively well constrained. We retrieve primary\ud redox conditions from metamorphosed mid-oceanic ridge basalts\ud (MORBs) and picrites of various ages (ca. 3000–550 Ma), using V/Sc\ud as 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 ±\ud 1.6), whereas Archean suites have lower V/Sc (5.2 ± 0.4, n = 5). The\ud lower Archean V/Sc is interpreted to reflect both deeper melt extraction\ud from the uppermost mantle, which becomes more reduced with\ud depth, and an intrinsically lower redox state. The pressure-corrected\ud oxygen fugacity (expressed relative to the fayalite-magnetite-quartz\ud buffer, DFMQ, at 1 GPa) of Archean sample suites (DFMQ –1.19 ±\ud 0.33, 2s) is significantly lower than that of post-Archean sample suites,\ud including MORB (DFMQ –0.26 ± 0.44). Our results imply that the\ud reducing Archean atmosphere was in equilibrium with Earth’s mantle,\ud and further suggest that magmatic gases crossed the threshold that\ud allowed a build-up in atmospheric O2 levels ca. 3000 Ma, accompanied\ud by the first “whiffs” of oxygen in sediments of that age

No abstract