Experimental Evidence for a Reduced Metal-saturated Upper Mantle
Experimental Evidence for a Reduced Metal-saturated Upper Mantle
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DOI：
10.1093/petrology/egq101
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发表时间：
2011-04
影响因子：
3.9
通讯作者：
A. Rohrbach;C. Ballhaus;P. Ulmer;U. Golla‐Schindler;D. Schönbohm
中科院分区：
文献类型：
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作者：
A. Rohrbach;C. Ballhaus;P. Ulmer;U. Golla‐Schindler;D. Schönbohm
捕虏体、橄榄岩包体和原始玄武质熔体取样的最上层地幔似乎相对氧化，氧逸度介于磁铁矿、钨铁矿和铁橄榄石、硅铁矿和磁铁矿平衡之间。氧逸度的这个范围是浅地幔特征还是整个上地幔的代表仍然不清楚，也是一个争论的问题，因为深度超过200 km的地幔区域没有很好的取样。为了限制更深的上地幔的氧化还原状态，我们在1 - 14 GPa和1220 - 16508 C对模型橄榄岩成分进行了实验，包括对流的软流圈地幔到410 km深度的过渡带。实验在铁金属胶囊中进行，以缓冲fO 2接近铁-钨-锑平衡以下约0.5 log单位的氧逸度。三价铁的实验阶段，使用电子能量损失谱分析表明，在高于7 GPa的压力下，亚钙辉石和majoritic石榴石将可观的量的三价铁，即使在实验条件下，他们在氧化还原平衡与金属铁。上地幔中三价铁的主要载体是长角闪岩石榴石，其次是次钙辉石。在大约8 - 1GPa，对应于250 - 30公里的深度在上地幔，足够数量的亚钙辉石和多长岩石榴石稳定，所有的三价铁认为是存在于肥沃的上地幔（即2000 ppm）可以容纳在这些阶段的固溶体，即使它们是在氧化还原平衡与金属铁合成。根据实验结果，可以说，在全球范围内，铁橄榄石-硅铁-磁铁矿平衡附近的氧化上地幔是例外而不是规则。超过75%的地球现今地幔可能被金属铁饱和。
The uppermost mantle as sampled by xenoliths, peridotite massifs and primitive basaltic melts appears to be relatively oxidized, with oxygen fugacities between the magnetite^wu« stite and fayalite^ferrosilite^magnetite equilibria.Whether this range in oxygen fugacity is a shallow mantle signature or representative of the entire upper mantle still is unclear and a matter of debate because mantle regions deeper than 200 km are not well sampled. To constrain the redox state of the deeper upper mantle, we performed experiments from 1 to 14 GPa and 1220 to 16508C on a model peridotite composition, encompassing the convecting asthenospheric mantle down to the Transition Zone at 410 km depth.The experiments were run in iron metal capsules to buffer fO2 close to an oxygen fugacity about 0·5 log units below the iron^wu« stite equilibrium. Analysis of the experimental phases for ferric iron using electron energy loss spectroscopy reveals that at pressures higher than 7 GPa, subcalcic pyroxene and majoritic garnet incorporate appreciable amounts of ferric iron, even though at the experimental conditions they were in redox equilibrium with metallic iron. The major ferric iron carrier in the upper mantle is majoritic garnet, followed by subcalcic pyroxene. At around 8 1GPa, corresponding to 250 30 km depth in the upper mantle, sufficient quantities of subcalcic pyroxene and majoritic garnet are stabilized that all the ferric iron thought to be present in fertile upper mantle (i.e. 2000 ppm) can be accommodated in solid solution in these phases, even though they were synthesized in redox equilibrium with metallic Fe. Based on the results of the experiments, it can be stated that, on a global scale, an oxidized upper mantle near the fayalite^ferrosilite^magnetite equilibrium is the exception rather than the rule. More than 75 vol. % of the Earth’s present-day mantle is likely to be saturated with metallic iron.