Chemical and physical changes during seawater flow through intact dunite cores: An experimental study at 150–200 °C
Chemical and physical changes during seawater flow through intact dunite cores: An experimental study at 150–200 °C
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DOI：
10.1016/j.gca.2017.07.020
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发表时间：
2017-10
影响因子：
5
通讯作者：
A. Luhmann;B. Tutolo;B. Bagley;D. Mildner;Peter P. Scheuermann;J. Feinberg;K. Ignatyev;W. Seyfried
中科院分区：
文献类型：
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作者：
A. Luhmann;B. Tutolo;B. Bagley;D. Mildner;Peter P. Scheuermann;J. Feinberg;K. Ignatyev;W. Seyfried
对完整的泥质岩心进行了两次流动实验，以评价其蛇纹石化作用。渗透率和流体化学表明，在200°C条件下，第二次实验的反应明显多于第一次实验的反应。在150°C和200°C的实验中，渗透率分别下降了2.4和25倍。氢气和甲烷浓度在200°C时分别超过600 μmol/kg和300 μmol/kg，比150°C时分别高出1和2个数量级。蚀变矿物的铁k边x射线吸收近边结构分析表明，在200°C实验期间发生了铁氧化。对实验后岩心的振动样品磁强计测量表明，几乎没有产生磁铁矿，这表明氢主要是由铁氧化产生的，因为橄榄石转化为铁（Fe(III)）蛇纹石和/或皂土。扫描电镜显示，实验后岩心在200°C的实验中形成了二次矿化，形成了蜂窝状结构的二次相以及方解石和硅灰石。扫描电子显微镜图像也显示溶解沿线性带穿过橄榄石晶体内部，可能沿着富含流体包裹体的路径。能量色散x射线光谱鉴定了蛇纹石中Cl的吸收，傅里叶变换红外光谱鉴定了蛇纹石、皂石和滑石的形成。然而，当比较实验前和实验后的x射线计算机断层扫描时，没有观察到变化。利用超小角中子散射数据分析了样品在≈1 nm ~ 10 μm尺度范围内孔隙度、比表面积和分形特征的变化。200°C实验后岩心的结果一般落在两个原始样品的值范围内，而150°C实验后岩心的反应可以忽略不计，这表明任何反应的变化都小于邓恩石的自然变异性。尽管几乎没有蚀变的物理证据，但在200°C时蛇纹石化的初始阶段足以对岩心的流场产生巨大影响。此外，该实验在模拟开放系统动力学时产生了显著的溶解氢浓度。即使开放系统防止氢浓度升高，由于氢的持续损失，我们推测，这一过程负责稳定富铁蛇纹石在自然界中，同时也氧化更多的亚铁（Fe(II)）和累积产生更多的氢比可能在一个封闭系统中。
Two flow-through experiments were conducted to assess serpentinization of intact dunite cores. Permeability and fluid chemistry indicate significantly more reaction during the second experiment at 200 °C than the first experiment at 150 °C. Permeability decreased by a factor of 2.4 and 25 during the experiments at 150 and 200 °C, respectively. Furthermore, hydrogen and methane concentrations exceeded 600 μmol/kg and 300 μmol/kg during the 200 °C experiment, and were one and two orders of magnitude higher, respectively, than the 150 °C experiment. Fe K-edge X-ray absorption near edge structure analyses of alteration minerals demonstrated Fe oxidation that occurred during the 200 °C experiment. Vibrating sample magnetometer measurements on post-experimental cores indicated little to no magnetite production, suggesting that the hydrogen was largely generated by the oxidation of iron as olivine was converted to ferric iron (Fe(III)) serpentine and/or saponite. Scanning electron microscopy images suggested secondary mineralization on the post-experimental core from the 200 °C experiment, portraying the formation of a secondary phase with a honeycomb-like texture as well as calcite and wollastonite. Scanning electron microscopy images also illustrated dissolution along linear bands through the interiors of olivine crystals, possibly along pathways with abundant fluid inclusions. Energy dispersive X-ray spectroscopy identified Cl uptake in serpentine, while Fourier transform-infrared spectroscopy suggested the formation of serpentine, saponite, and talc. However, no change was observed when comparing pre- and post-experimental X-ray computed tomography scans of the cores. Furthermore, (ultra) small angle neutron scattering datasets were collected to assess changes in porosity, surface area, and fractal characteristics of the samples over the ≈ 1 nm- to 10 μm-scale range. The results from the 200 °C post-experimental core generally fell within the range of values for the two pristine samples and the 150 °C post-experimental core that underwent negligible reaction, indicating that any change from reaction was smaller than the natural variability of the dunite. Even though there was little physical evidence of alteration, the initial stage of serpentinization at 200 °C was sufficiently significant to have a dramatic effect on flow fields in the core. Furthermore, this experiment generated significant dissolved hydrogen concentrations while simulating open system dynamics. Even though open systems prevent elevated hydrogen concentrations due to continual loss of hydrogen, we speculate that this process is responsible for stabilizing ferric Fe-rich serpentine in nature while also oxidizing more ferrous iron (Fe(II)) and cumulatively generating more hydrogen than would be possible in a closed system.