Rohrbach Arno, Ballhaus Chris, Golla-Schindler Ute, Ulmer Peter, Kamenetsky Vadim S, Kuzmin Dmitry V
Mineralogisches-Petrologisches Institut und Museum, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany.
Nature. 2007 Sep 27;449(7161):456-8. doi: 10.1038/nature06183.
The oxygen fugacity f(O2)of the Earth's mantle is one of the fundamental variables in mantle petrology. Through ferric-ferrous iron and carbon-hydrogen-oxygen equilibria, f(O2) influences the pressure-temperature positions of mantle solidi and compositions of small-degree mantle melts. Among other parameters, f(O2) affects the water storage capacity and rheology of the mantle. The uppermost mantle, as represented by samples and partial melts, is sufficiently oxidized to sustain volatiles, such as H2O and CO2, as well as carbonatitic melts, but it is not known whether the shallow mantle is representative of the entire upper mantle. Using high-pressure experiments, we show here that large parts of the asthenosphere are likely to be metal-saturated. We found that pyroxene and garnet synthesized at >7 GPa in equilibrium with metallic Fe can incorporate sufficient ferric iron that the mantle at >250 km depth is so reduced that an (Fe,Ni)-metal phase may be stable. Our results indicate that the oxidized nature of the upper mantle can no longer be regarded as being representative for the Earth's upper mantle as a whole and instead that oxidation is a shallow phenomenon restricted to an upper veneer only about 250 km in thickness.
地球地幔的氧逸度f(O₂)是地幔岩石学中的基本变量之一。通过铁离子-亚铁离子以及碳-氢-氧的平衡，f(O₂)影响着地幔固相线的压力-温度位置以及小程度地幔熔体的成分。在其他参数中，f(O₂)会影响地幔的储水能力和流变学。以样本和部分熔体为代表的上地幔被充分氧化，足以维持诸如H₂O和CO₂等挥发性物质以及碳酸盐熔体，但尚不清楚浅部地幔是否代表整个上地幔。通过高压实验，我们在此表明软流圈的大部分区域可能处于金属饱和状态。我们发现，在大于7 GPa的压力下与金属铁达到平衡时合成的辉石和石榴石能够容纳足够的铁离子，以至于深度大于250 km的地幔被还原到足以使(Fe,Ni)金属相保持稳定。我们的结果表明，上地幔的氧化性质不能再被视为代表整个地球的上地幔，相反，氧化是一种仅限于厚度约250 km的上部表层的浅层现象。
