Rates of dehydration of olivines from San Carlos and Kilauea Iki
Rates of dehydration of olivines from San Carlos and Kilauea Iki
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DOI：
10.1016/j.gca.2018.08.050
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发表时间：
2018-12
影响因子：
5
通讯作者：
E. Ferriss;T. Plank;M. Newcombe;D. Walker;E. Hauri
中科院分区：
文献类型：
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作者：
E. Ferriss;T. Plank;M. Newcombe;D. Walker;E. Hauri
H+进入、穿过和离开橄榄石的速率对于理解深水循环、评估橄榄石熔融包裹体作为喷发前熔体成分记录器的保真度以及估计火山喷发期间岩浆的减压速率具有重要意义。在这里，我们进行了一系列的实验和观察天然含铁橄榄石，以更好地了解和量化这种H+的流动性，特别关注的速度，H+扩散出橄榄石。我们已经进行了连续的自然，含铁橄榄石单晶体的脱水实验抛光和定向沿着三个晶体取向。这些工作包括四个独立的部分：（1）在800 °C和1000 °C、1 GPa和氧逸度（fO 2）下，在Ni-NiO缓冲（NNO）下，抛光的San卡洛斯橄榄石块体的部分水化;（2）在800 °C、1-atm和fO 2 = NNO-2.6下，San卡洛斯橄榄石块体的连续脱水;（3）在800和1000 °C、1个大气压下，将来自基拉韦厄伊基的橄榄石斑晶的抛光块依次脱水，并且fO 2保持在NNO-2.6直到最后步骤，其在NNO + 1.9下进行;（4）用二次离子质谱法测量基拉韦厄伊基橄榄石斑晶中氢的分带分布，并结合蒙特卡罗模拟确定自然环境中氢的总扩散率。对于所有实验，在每个时间步骤之前和之后，通过傅里叶变换红外光谱法，使用偏振光，在所有三个晶体学方向a、B和c上通过未切割的块测量氢分带分布。||结果表明，总H+的表观扩散系数随时间和橄榄石不同缺陷数的变化而变化，因此H+不会以固有的位点特异性速率扩散。这些变化是由于快速反应，可能会发生在网站之间的功能，不断变化的缺陷浓度。实验揭示了三个新的现象：（1）与Ti-斜齿石缺陷[Ti-2 H]有关的H+能以最快的速率进出橄榄石。(2)特定红外吸收峰的H+损失率以及总的H+损失率可能随着由于Fe 3+的积累和缺陷的再分布（特别是[Ti-2 H]的再水化）而逐渐脱水而改变。(3)以下阿耳忒弥斯定律似乎适用于大多数天然橄榄石在脱水过程中的总H+扩散：Da= 10−5.4exp（−130/RT）;Db= 10−6.9exp（−130/RT）; Dc = 10− 6.6exp（−130/RT），其中R是气体常数8.314 J/mol K; T是温度，扩散系数（D）的单位为m2/s;活化能为130 kJ/mol。这一速率慢于质子极化子氧化还原速率，但快于质子金属空位扩散，并出现在广泛的条件下，从自扩散，熔融包裹体再平衡，地幔捕虏体和火山斑晶环带上升过程中的特征橄榄石的Fo-90组合物。
The rate at which H+moves into, through, and out of olivine is of great interest for understanding the deep water cycle, assessing the fidelity of olivine-hosted melt inclusions as recorders of pre-eruptive melt compositions, and estimating the decompression rate of magma during volcanic eruptions. Here we conduct a series of experiments and observations on natural Fe-bearing olivine to better understand and quantify this H+mobility, with a particular focus on the rate at which H+diffuses out of olivine. We have carried out sequential dehydration experiments on single crystals of natural, Fe-bearing olivine polished and oriented along three crystallographic orientations. These efforts include four separate parts: (1) the partial hydration of polished blocks of xenolithic San Carlos olivine at 800 °C and 1000 °C, 1 GPa, and oxygen fugacity (fO2) at the Ni-NiO buffer (NNO); (2) sequential dehydration of a homogeneous block of San Carlos olivine at 800 °C, 1-atm, and fO2of NNO-2.6; (3) sequential dehydration of a polished block of an olivine phenocryst from Kilauea Iki at 800 and 1000 °C, 1 atm, and fO2held at NNO-2.6 until the final step, which was conducted at NNO + 1.9; and (4) the measurement by secondary ion mass spectrometry of hydrogen zonation profiles in an olivine phenocryst from Kilauea Iki coupled with Monte Carlo modeling to determine total H diffusivities in a natural setting. Hydrogen zonation profiles were measured in all three crystallographic directionsa,b, andcthrough the uncut blocks before and after each time step for all experiments by Fourier transform infrared spectroscopy using light polarized ||aand modeled in 3 dimensions to determine both total and peak-specific dehydration, producing over 50 sets of 3 apparent diffusivities.The results show that the apparent diffusivity of total H+varies both over time and for olivines with different defect populations, and thus H+does not diffuse at inherently site-specific rates. These variations are due to rapid reactions that may occur between sites as a function of changing defect concentrations. Our experiments reveal three new phenomena: (1) H+that is associated with the Ti-clinohumite defect, [Ti-2H], can enter and exit olivine at the fastest rate measured. (2) The rate of H+loss from specific infrared absorption peaks, and in total, may change with progressive dehydration due to the buildup of Fe3+and redistribution of defects, particularly the rehydration of [Ti-2H]. (3) The following Arrhenius laws appear to apply to total H+diffusion out of most natural olivines that have been studied experimentally and naturally during dehydration:Da= 10−5.4exp(−130/RT);Db= 10−6.9exp(−130/RT); andDc= 10−6.6exp(−130/RT), whereRis the gas constant 8.314 J/mol K;Tis the temperature, the units of the diffusivities (D) are in m2/s; and the activation energy is 130 kJ/mol. This rate is slower than proton-polaron redox rate but faster than proton-metal vacancy diffusion and appears to characterize olivine of Fo ∼90 composition in a wide range of conditions, from self-diffusion, to melt inclusion re-equilibration, to mantle xenolith and volcanic phenocryst zonation during ascent.