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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Formation of 'Tuffisitic Kimberlites' by phreatomagmatic processes
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Formation of 'Tuffisitic Kimberlites' by phreatomagmatic processes

机译：岩浆过程形成“ Tuffisitic金伯利岩”

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The rock type "Tuffisitic Kimberlite" (TK) occurs in the deeper diatremes of some Southern African, Canadian and Siberian kimberlite pipes and has been considered a unique rock type related to a particular emplacement process. The key features of this rock type are its fragmental nature, its massive, well-mixed appearance and a specific matrix mineralogy characterized by the presence of serpentine and microlitic clinopyroxene and the absence or scarcity of carbonate. Historically, all these features were thought to be related to a highly specific intrusive-extrusive magmatic emplacement process. In this process, the expansion of magmatic volatiles drives the fluidization of a pre-existing, shallow-crustal, vertical magma reservoir (the "embryonic pipe") and its consequent evolution into the final diatreme after breakthrough to the surface. The specific matrix mineralization is explained by the dissociation of carbonate into CO_2 and CaO. While the expanding CO_2 drives the fluidization process, CaO reacts with SiO_2 released from xenolith and olivine alteration to form microlitic clinopyroxene. With the phreatomagmatic process chain we offer an alternative model that can readily explain this specific rock type. Ongoing explosions in a downward penetrating root zone excavate a pipe consisting of a rather regular, cone-shaped diatreme underlain by the irregular root zone. At this stage the pipe in its majority is infilled by warm to hot pyroclastic tephra. During thermohydraulic explosions in the root zone the expansion of water vapor homogenizes the overlying diatreme tephra and mixes it with the adjacent country-rock lithologies. This result in the massive, well-mixed volcaniclastic rock type typical for TKBs. Post-emplacement hydrothermal alteration and mineralization under epi- to mesothermal conditions are considered responsible for the observed specific matrix mineralogy. Carbonate is frequently present in coherent root zone rocks but is rare or absent due to its dissolution in hot hydrothermal fluids ascending from the lower diatreme, which in turn precipitate their dissolved carbonate higher up in the cooler, bedded upper diatreme region. Clinopyroxene also grows under epi- to mesothermal conditions. For the crystallization of serpentine a X(H_2O):X(CO_2) ratio of 20:1 is required, which suggests both a high availability of groundwater and a low abundance of CO_2 after and likely during emplacement. Isotope studies (H, C, O, Sr) of matrix minerals suggest participation of groundwater in the formation of these minerals.
机译：岩石类型“ Tuffisitic Kimberlite”（TK）出现在一些南非，加拿大和西伯利亚金伯利岩烟斗的更深处，被认为是与特定进位过程相关的独特岩石类型。这种岩石类型的主要特征是其碎屑性质，块状，充分混合的外观和特定的基质矿物学，其特征为存在蛇纹石和微晶的斜辉石以及碳酸盐的缺乏或缺乏。从历史上看，所有这些特征都被认为与高度特定的侵入-挤压岩浆沉积过程有关。在这个过程中，岩浆挥发物的膨胀驱使一个早已存在的浅壳垂直岩浆储层（“胚管”）流化，并在其突破地面后演变成最终的异常。特定的基质矿化是由碳酸盐分解为CO_2和CaO所解释的。当膨胀的CO_2推动流化过程时，CaO与从异岩和橄榄石蚀变释放出的SiO_2反应形成微胶体化的次生辉石。借助岩磁过程链，我们提供了可以轻松解释这种特定岩石类型的替代模型。向下穿透的根部区域持续进行的爆炸开挖了一个管道，该管道由不规则的根部区域组成的规则的圆锥形锥形衬砌。在此阶段，管道的大部分被热至热的火山碎屑特菲拉填充。在根部区域发生热液爆炸时，水蒸气的膨胀会使上覆的非特非特菲拉均质化，并将其与邻近的乡村岩石岩性混合。这导致了TKB典型的块状，充分混合的火山碎屑岩类型。上位到中热条件下的位后水热蚀变和矿化被认为是所观察到的特定基质矿物学的原因。碳酸盐常存在于相干的根带岩石中，但由于其溶解在从下部迪阿特级升起的热液中而很少或不存在，这又使它们溶解的碳酸盐在较冷的层状上部迪阿特级区中向上沉淀。 Clinopyroxene也可以在中温条件下生长。对于蛇纹石的结晶，需要20：1的X（H_2O）：X（CO_2）比率，这表明在安置后以及安置期间可能具有较高的地下水利用率和较低的CO_2丰度。基质矿物的同位素研究（H，C，O，Sr）表明，地下水参与了这些矿物的形成。

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