Experimental chondrite-water reactions under reducing and low-temperature hydrothermal conditions: Implications for incipient aqueous alteration in planetesimals
Experimental chondrite-water reactions under reducing and low-temperature hydrothermal conditions: Implications for incipient aqueous alteration in planetesimals
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还原和低温水热条件下的实验球粒陨石-水反应：对星子中早期水蚀变的影响
DOI：
10.1016/j.gca.2021.11.006
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发表时间：
2022
影响因子：
5
通讯作者：
Katsuyuki Uematsu
中科院分区：
文献类型：
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作者：
Sakiko Kikuchi;Takazo Shibuya;Mariko Abe;Katsuyuki Uematsu
碳质球粒陨石中含水矿物的存在被认为是母小行星中以前存在液态水的重要证据。然而，含水小行星中水-岩石反应的演化仍然没有得到很好的限制。本文通过低温热液和还原条件下的水岩型实验和化学平衡计算，研究球粒岩早期蚀变阶段的蚀变过程和次生矿物组合。使用合成球粒陨石（橄榄石 (镁橄榄石95)、斜方辉石 (顽辉石95)、硅酸盐玻璃、硫陨石和铁金属的混合物）作为起始材料，我们的实验在 25°C–80°C 的温度下进行，时间为 1 至 460 天，水与岩石的质量比为 10。结合了 X 射线衍射 (XRD) 和透射电子显微镜 (TEM)分析表明，在 80 °C 时，主要次生相由磁黄铁矿、非晶富 SiO2 相和皂石组成，而在 25 °C 时，次生相由非晶富 SiO2 相和皂石组成。在这两个温度下，富SiO2相和皂石密集地覆盖在初生相的表面。皂石相和非晶富 SiO2 相的 Fe/Mg 比在 80 °C 和 25 °C 之间表现出明显的差异。在 80 °C 下形成的皂石比初始硅酸盐相富含 Fe，并且在包裹着硫镁石和金属铁的皂石中获得了最高的 Fe/Mg 比。这些结果表明，从陨铁石和金属铁中分布的铁诱导了富铁皂石的形成。从我们的蚀变实验中观察到的一些次生矿物与化学平衡计算所预期的一致。然而，在长达 460 天的实验中，我们并未观察到蛇纹石（化学平衡计算预期的主要次生矿物）的形成，这可能是因为富含 SiO2 的硅酸盐玻璃在蚀变的最初阶段优先溶解，导致了皂石而不是蛇纹石的形成。通过蚀变实验观察到，次生矿物组合及其形态特征与碳质球粒陨石（如 CM2 和 CO3 球粒陨石）有相似之处。这种变化可能是通过低温下的水-岩反应和短时间的变化来解释的。这些发现更好地限制了含水小行星中水变化的温度和时间尺度，以及水在早期太阳系天体中的作用。
The presence of hydrous minerals in carbonaceous chondrites has been considered important evidence for the former presence of liquid water in parent asteroids. However, the evolution of water–rock reactions in hydrous asteroids remains not well constrained. Here, we conduct water–rock type experiments and chemical equilibria calculations under low-temperature hydrothermal and reducing conditions to investigate the alteration process and secondary mineral assemblages of chondritic rock in the earliest alteration stage. Using synthetic chondrite (mixtures of olivine (forsterite95), orthopyroxene (enstatite95), silicate glass, troilite and Femetal) as a starting material, our experiments were conducted at temperatures of 25 °C–80 °C for time periods between 1 and 460 days at a water-to-rock mass ratio of 10. A combination of X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses revealed that the primary secondary phases consisted of pyrrhotite, an amorphous SiO2-rich phase and saponite at 80 °C, while the secondary phase consisted of an amorphous SiO2-rich phase and saponite at 25 °C. At both temperatures, the SiO2-rich phases and saponite densely covered the surface of the primary phases. The Fe/Mg ratios of saponite and amorphous SiO2-rich phases showed clear difference between 80 °C and 25 °C. Saponite that was formed at 80 °C was richer in Fe than the initial silicate phases, and the highest Fe/Mg ratios were obtained in the saponite encrusting the troilite and Femetal. These results suggest that the Fe distributed from the troilite and Femetalinduced the formation of Fe-rich saponite. Some of the secondary minerals observed from our alteration experiments were consistent with those expected by chemical equilibria calculations. However, the formation of serpentine, the dominant secondary mineral expected from chemical equilibria calculations, was not observed in our experiments up to 460 days, probably because the preferential dissolution of SiO2-rich silicate glass in the earliest stage of alteration induced the formation of saponite rather than serpentine. The secondary mineral assemblage and its morphological characteristics, as observed by our alteration experiments, showed similarities with carbonaceous chondrites such as CM2 and CO3 chondrites. This alteration might be explained by water–rock reactions at low temperatures and by the short time alteration. These findings better constrain the temperatures and timescales of aqueous alterations in hydrous asteroids, as well as the role of water in the early solar system bodies.