Evidence for a reducing Archean ambient mantle and its effects on the carbon cycle
Evidence for a reducing Archean ambient mantle and its effects on the carbon cycle
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DOI：
10.1130/g38070.1
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发表时间：
2016-09-01
期刊：
影响因子：
5.8
通讯作者：
Stagno, Vincenzo
中科院分区：
文献类型：
--
作者：
Aulbach, Sonja;Stagno, Vincenzo
地幔岩石中的化学还原-氧化机制通过影响岩浆形成的深度、岩浆的成分以及最终释放到大气中的气体的化学性质，与陆地碳循环联系在一起。人们普遍认为，在过去的38亿年里，最上层地幔的氧化态没有变化，根据不同年代绿岩带伴生样品中氧化还原敏感元素的丰度，然而，这些岩石中的氧化还原信号可能因其复杂的起源和在大陆边缘的就位而被掩盖。相比之下，在扩张脊的减压熔融过程中发生的源和过程相对较好地受到限制。我们检索原始氧化还原条件从变质洋中脊玄武岩（MORB）和苦橄岩的各种年龄（约。3000-550 Ma），使用V/Sc作为广泛的氧化还原代理。元古宙岩套的平均V/Sc值（7.0 +/- 1.4，2 sigma，n = 6）与现代MORB（6.8 +/- 1.6）相似，而太古宙岩套的V/Sc值较低（5.2 +/- 0.4，n = 5）。较低的太古代V/Sc被解释为反映更深的熔体提取从最上层的地幔，这变得更加减少与深度，和一个内在的较低的氧化还原状态。太古代样品组的压力校正氧逸度（相对于铁橄榄石-磁铁矿-石英缓冲层表示，Delta FMQ，1 GPa）（Delta FMQ -1.19 +/- 0.33，2s）明显低于后太古代样品组，包括MORB（Delta FMQ -0.26 +/- 0.44）。我们的研究结果表明，减少太古代大气与地球的地幔平衡，并进一步表明，岩浆气体越过阈值，允许建立在大气中的O-2水平约。3000年马，伴随着第一个“呼吸”的氧气沉积物的年龄。
Chemical reduction-oxidation mechanisms within mantle rocks link to the terrestrial carbon cycle by influencing the depth at which magmas can form, their composition, and ultimately the chemistry of gases released into the atmosphere. The oxidation state of the uppermost mantle has been widely accepted to be unchanged over the past 3800 m.y., based on the abundance of redox-sensitive elements in greenstone belt-associated samples of different ages. However, the redox signal in those rocks may have been obscured by their complex origins and emplacement on continental margins. In contrast, the source and processes occurring during decompression melting at spreading ridges are relatively well constrained. We retrieve primary redox conditions from metamorphosed mid-oceanic ridge basalts (MORBs) and picrites of various ages (ca. 3000-550 Ma), using V/Sc as a broad redox proxy. Average V/Sc values for Proterozoic suites (7.0 +/- 1.4, 2 sigma, n = 6) are similar to those of modern MORB (6.8 +/- 1.6), whereas Archean suites have lower V/Sc (5.2 +/- 0.4, n = 5). The lower Archean V/Sc is interpreted to reflect both deeper melt extraction from the uppermost mantle, which becomes more reduced with depth, and an intrinsically lower redox state. The pressure-corrected oxygen fugacity (expressed relative to the fayalite-magnetite-quartz buffer, Delta FMQ, at 1 GPa) of Archean sample suites (Delta FMQ -1.19 +/- 0.33, 2s) is significantly lower than that of post-Archean sample suites, including MORB (Delta FMQ -0.26 +/- 0.44). Our results imply that the reducing Archean atmosphere was in equilibrium with Earth's mantle, and further suggest that magmatic gases crossed the threshold that allowed a build-up in atmospheric O-2 levels ca. 3000 Ma, accompanied by the first "whiffs" of oxygen in sediments of that age.