This study simulates the hydrothermal conditions that existed on carbonaceous chondrite planetesimals in the early solar system. Our experiments are relevant to alteration conditions that existed on the CV parent body and the late stage oxidizing alteration of the CM chondrites. We conducted 11 alteration experiments using chips of the CO3 chondrite Kainsaz. Water was added to each chip and sealed in separate Teflon reaction vessels for 175 days. Samples were altered at different initial water-to-rock ratios (W/R: 0.2–0.8) and temperatures (50 °C and 150 °C). Isotopically doped ¹⁷O-rich heavy water (δ¹⁷O: +64.5‰) was used in five runs. All samples experienced pronounced alteration under a partially open system environment where gases were able to escape the reaction vessels.

The style of alteration (Fe-alkali metasomatism) is similar in all cases. The principal alteration minerals formed are Fe-oxyhydroxides (goethite) and Fe-oxides (magnetite), with smaller quantities of Fe-sulphides. Minor phases formed include fayalite, sulphates (gypsum and Fe-sulphate) and calcite. Nanophase, poorly crystalline phyllosilicates formed in the high-temperature samples but are absent from the low-temperature experiments. In all instances, Mg-rich chondrule silicates remained chemically unaltered although some grains suffered hydrothermal fracture. Chondrule mesostases remained largely unaffected. By contrast, kamacite readily dissolved, acting as a source of Fe and Ni for the fluid phase. A new generation of nanophase Fe-sulphides formed within the matrix, while pre-existing pyrrhotite group sulphides experienced Ni enrichment (<3 at%). In the high temperature samples these sulphides were also partially oxidized, lowering their (Fe + Ni)/S ratio. High-Ni sulphides (pyrrhotite with Ni > 10 at%) were formed in the 150 °C samples, most likely by sulphidation of taenite.

Matrix alteration cemented grains together, reducing porosity. The fine-grained matrix shows highly variable degrees of alteration, with minimally altered matrix in direct contact with regions of heavily altered matrix. Chondrule fine-grained rims (FGRs) were preferentially altered. These textures imply that the unaltered matrix readily reacted with the fluid phase, resulting in an efficient depletion of dissolved ions (Fe²⁺ and S^2-), limiting reactivity until further primary phases were dissolved. At larger length-scales the distribution of heavily altered matrix reveals the presence of large ∼100 µm wide channels that meander through the specimens. Their textures resemble features seen in some CM chondrites and the ungrouped CO-like chondrite MIL 07687. We suggest that alteration fronts developed by sustained rapid reaction of matrix with dissolved cations in solution. Our observations provide a mechanism for the establishment and maintenance of geochemical microenvironments on chondritic asteroids. The effects of open system loss notwithstanding, our experiments demonstrate that more advanced alteration is correlated with higher initial W/R ratios.

The use of ¹⁷O-rich doped water allowed the isotopic effects of aqueous alteration to be observed. Bulk rock compositions evolved towards the initial water composition, reflecting the incorporation of heavy O into hydrated minerals. Additionally, altered samples shifted in δ¹⁸O space, reflecting the competing effects of water–mineral fractionation and mass fractionation due to the preferential escape of isotopically light water.

这项研究模拟了早期太阳系中碳质球粒陨石小行星上存在的热液条件。我们的实验与 CV 母体上存在的改变条件和 CM 球粒陨石的晚期氧化改变有关。我们使用 CO3 球粒陨石 Kainsaz 的碎片进行了 11 次蚀变实验。将水加到每个芯片中并密封在单独的 Teflon 反应容器中 175 天。样品在不同的初始水岩比（W/R：0.2–0.8）和温度（50 °C 和 150 °C）下发生变化。在五次运行中使用了同位素掺杂的富含¹⁷ O 的重水（δ ¹⁷ O：+64.5‰）。在气体能够从反应容器中逸出的部分开放系统环境下，所有样品都经历了明显的变化。

在所有情况下，改变的方式（铁碱交代）是相似的。形成的主要蚀变矿物是铁羟基氧化物（针铁矿）和铁氧化物（磁铁矿），以及少量的硫化铁。形成的次要相包括铁橄榄石、硫酸盐（石膏和硫酸铁）和方解石。在高温样品中形成了纳米相、结晶性差的页硅酸盐，但在低温实验中没有。在所有情况下，尽管一些颗粒遭受热液断裂，但富含镁的球粒硅酸盐在化学上保持不变。球粒内稳态基本不受影响。相比之下，铁纹石容易溶解，作为流体相的 Fe 和 Ni 来源。在基体中形成了新一代纳米相 Fe 硫化物，而预先存在的磁黄铁矿族硫化物经历了 Ni 富集（< 3%）。在高温样品中，这些硫化物也被部分氧化，降低了它们的 (Fe + Ni)/S 比。在 150 °C 的样品中形成了高 Ni 硫化物（Ni > 10 at% 的磁黄铁矿），最有可能是由于条石硫化。

基质改变将颗粒胶结在一起，减少了孔隙度。细粒度矩阵显示出高度可变的变化，最小变化的矩阵与严重变化的矩阵区域直接接触。球粒细纹边缘 (FGR) 被优先改变。这些纹理意味着未改变的基质很容易与流体相反应，从而有效消耗溶解的离子（Fe ²⁺和 S ^2-)，限制反应性，直到进一步的初级相溶解。在较大的长度尺度上，严重改变的基质的分布表明存在蜿蜒穿过标本的约 100 µm 宽的大通道。它们的质地类似于在一些 CM 球粒陨石和未分组的 CO 状球粒陨石 MIL 07687 中看到的特征。我们认为由基质与溶液中溶解的阳离子的持续快速反应形成的蚀变前沿。我们的观测为球粒状小行星地球化学微环境的建立和维持提供了一种机制。尽管存在开放系统损失的影响，但我们的实验表明，更高级的改变与更高的初始 W/R 比相关。

使用富含¹⁷ O 的掺杂水可以观察到水蚀变的同位素效应。大块岩石成分向初始水成分演化，反映了重 O 掺入水合矿物中。此外，改变的样品在 δ ¹⁸ O 空间中移动，反映了由于同位素轻水优先逸出而导致的水-矿物分馏和质量分馏的竞争效应。