不同温度、压强、氧逸度条件下斜方辉石含水性的实验研究

Experimental Study of Water-Bearing Capacity of Orthopyroxene under Different Temperature,Pressure and Oxygen Fugacity

斜方辉石的含水性显著影响着上地幔的物理化学性质。前人对于斜方辉石的水溶解度的制约因素研究并不充分,特别是氧逸度的影响很少被单独考虑。为了厘清斜方辉石的水溶解度的制约因素,我们借助高温高压设备(MAVO LPC250型活塞圆筒压机)在1.0~3.0 GPa,800~1100℃和两种典型的氧逸度条件控制下开展了系统性地实验研究,即Ni+NiO(NNO),Fe+FeO(IW)。通过傅里叶变换红外光谱测量(FTIR),斜方辉石的天然和实验样品显示出7组典型的OH-吸收峰。含水量结果表明,在上地幔浅部条件下,斜方辉石的水溶解度均随温度或压强升高而增大,而随氧逸度值升高而减小,仅氧逸度变化其它条件不变时,还原条件比相对氧化条件高达50%,这与橄榄石等其它名义上无水矿物的氧逸度效应存在不一致。通过估算,橄榄石和斜方辉石间水的分配系数在不同氧逸度条件下可以相差55%。因此,与名义上无水矿物中结构水相关的实验研究,未有效控制氧逸度的结果需要被重新考虑。

The water-bearing capacity of orthopyroxene significantly affects the physical and chemical properties of the upper mantle. Previous studies on limiting factors, especially oxygen fugacity, of the water solubility of orthopyroxene have not been sufficient. This study carries out a systematic experimental study with Mavo LPC250 Piston Cylinder press under 1.0-3.0 GPa, 800-1100℃ and two typical oxygen fugacity conditions(Ni+Ni O(NNO), Fe+FeO(IW). FTIR for orthopyroxene shows 7 typical OH-absorption peaks which water solubility of the orthopyroxene increases with the increase of temperature and pressure and decreases with the increase of oxygen fugacity under the upper mantle condition. The reduction condition is 50% higher than that of the relative oxidation condition when only the oxygen fugacity is changed which is inconsistent with the oxygen fugacity effect of anhydrous minerals such as olivine. Water partition coefficient between olivine and orthopyroxene can vary by 55% under different oxygen fugacity conditions.