The effect of oxygen fugacity on the olivine to wadsleyite transformation: Implications for remote sensing of mantle redox state at the 410 km seismic discontinuity
The effect of oxygen fugacity on the olivine to wadsleyite transformation: Implications for remote sensing of mantle redox state at the 410 km seismic discontinuity
复制标题
氧逸度对橄榄石向瓦兹利石转变的影响：410 km地震间断面地幔氧化还原状态遥感的意义
DOI：
10.2138/am.2009.3094
复制
发表时间：
2009
影响因子：
3.1
通讯作者：
C. McCammon
中科院分区：
文献类型：
--
作者：
D. Frost;C. McCammon
摘要通过高温高压多砧实验，研究了不同氧逸度对橄榄石向硅镁石转变的影响。两个胶囊，含有样品的（FeMg）2SiO 4，被放置在每个实验中;第一缓冲的氧逸度与集合的Re和ReO 2。而第二种通过过量Fe金属的存在确保了最低可能的三价铁浓度。测量共存的橄榄石，wadsleyite。和来自Fe金属饱和实验的林伍德石组合物用于使用建立的相关系精确地确定每个实验中的压力。在Re-ReO 2缓冲液的氧化性更强的条件下，发现硅镁石的稳定场相对于橄榄石和林伍德石的稳定场都有所扩大。穆斯堡尔谱测量显示，Re-ReO缓冲的wadsleyite的Fe 3 +/∑Fe比为0.1-0.25。而橄榄石似乎不含Fe 2 O3。采用（Fe 3 +5/3□1/3）Fe 3 + O 4-Fe 2 SiO 4-Mg 2 SiO 4的热动力学模型，研究了地幔体相Fe 3 +/∑Fe比值的变化对410 km地震界面深度和深度间隔的影响。Fe ~（3+）/SFe比值在0.02-0.12范围内，可使410 km不连续面的深度间隔或厚度从-8 km增加到15 km，但对地震可观测绝对深度的影响很小。需要非常大的块状地幔Fe 3 +/∑Fe比值（>0.2）来解释最近的地震观测结果，即某些区域下面410 km的深度间隔可能>20 km。这样的意见是更有可能被解释为适度的本地富集在两个三价铁和H;0在地幔中，最有可能作为一个结果板相互作用。
Abstract High-pressure and -temperature multianvil experiments were performed to test the effect of varying oxygen fugacity on the olivine to wadsleyite transformation. Two capsules, containing samples of (FeMg)2SiO4, were placed in each experiment; the first buffered the oxygen fugacity with an assemblage of Re and ReO2. whereas the second ensured the lowest possible ferric iron concentra- tion through the presence of excess Fe metal. Measurements of coexisting olivine, wadsleyite. and ringwoodite compositions from the Fe metal saturated experiments were used to accurately determine the pressure in each experiment using established phase relations. Under the more oxidizing conditions of the Re-ReO2 buffer, the stability field of wadsleyite was found to expand with respect to both the olivine and ringwoodite stability fields. Mössbauer spectroscopy measurements reveal Fe3+/∑Fe ratios for wadsleyite buffered by Re-ReO: of 0.1-0.25. while olivine appears to be FeJ"-free. A thermodynamic model that employs the wadsleyite end-members (Fe3+5/3□1/3)Fe3+O4-Fe2SiO4-Mg2SiO4 is used to examine the effect of varying bulk mantle Fe3+/∑Fe ratio on the depth and depth interval of the 410 km seismic discontinuity. Fe3+/SFe ratios in the range 0.02-0.12 would cause the depth interval or thickness of the 410 km discontinuity to increase from -8 to 15 km but would haw very little effect on the seismically observable absolute depth. Very large bulk mantle Fe3+/∑Fe ratios (>0.2), unrepresented in recovered mantle samples, would be required to explain recent seismic observations that the depth interval of the 410 km may be >20 km beneath certain regions. Such observations are more likely to be explained by moderate local enrichments in both ferric iron and H;0 in the mantle, most likely as a result of slab interaction.