Stable isotope geochemistry and formation mechanisms of quartz veins; extreme paleoaltitudes of the Central Alps in the Neogene
Stable isotope geochemistry and formation mechanisms of quartz veins; extreme paleoaltitudes of the Central Alps in the Neogene
复制标题
稳定同位素地球化学及石英脉形成机制；
DOI：
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
R. Lucchini
中科院分区：
文献类型：
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作者：
Z. Sharp;H. Masson;R. Lucchini
石英脉的大小从小于50厘米长，5厘米宽到大于10米长，5米宽，遍布瑞士中部阿尔卑斯山脉。在某些情况下，矿脉完全充满了乳白色的石英，而在另一些情况下，有时会发现壮观的充满空隙的石英晶体。矿脉充填的类型和大小受寄主岩石成分和变形历史的控制。利用稳定同位素测温法和矿物平衡法估算出的矿脉形成温度范围为450°C至150°C。静脉形成开始于18 ~ 20 Ma，持续时间超过10 Ma。石英脉的氧同位素值在10 ~ 20 permil之间，几乎所有石英脉的氧同位素值都与含矿岩相一致。岩石缓冲力强的矿脉意味着低流体/岩石比和最小流体流动。为了解释块状、近单矿物石英的形成，而没有特别大的流体通量，提出了一种压差和硅扩散结合压力溶液的早期脉状形成机制。晚形成脉体中的流体包裹体和含水矿物δD值极低，与大气水的入渗一致。从岩石缓冲、静态流体到上方渗透的变化，可以用20 ~ 15 Ma大尺度变形样式的变化来解释。先前的研究发现，阿尔卑斯中部的快速冷却，部分原因可能是寒冷的大气水沿着断裂系统渗透到10公里或更深的深度。进入岩石并在40°C的加热下重新出现的平均水通量为0.15 cm3 cm - 2年- 1，足以在500万年中将10公里深处的岩石冷却100°C。晚期流体的极负δD值< -130 permil，远低于今天在该地区大气水中测量的年平均值。低的化石δD值表明中阿尔卑斯在新近纪处于较高的海拔高度。这一结论得到了早先一项研究的支持，该研究基于在远离原产地的低海拔地区发现的大型不稳定巨石，提出了5000米的古海拔。
Quartz veins ranging in size from less than 50 cm length and 5 cm width to greater than 10 m in length and 5 m in width are found throughout the Central Swiss Alps. In some cases, the veins are completely filled with milky quartz, while in others, sometimes spectacular void-filling quartz crystals are found. The style of vein filling and size is controlled by host rock composition and deformation history. Temperatures of vein formation, estimated using stable isotope thermometry and mineral equilibria, cover a range of 450°C down to 150°C. Vein formation started at 18 to 20 Ma and continued for over 10 My. The oxygen isotope values of quartz veins range from 10 to 20 permil, and in almost all cases are equal to those of the hosting Iithology. The strongly rock-buffered veins imply a low fluid/rock ratio and minimal fluid flow. In order to explain massive, nearly monomineralic quartz formation without exceptionally large fluid fluxes, a mechanism of differential pressure and silica diffusion, combined with pressure solution, is proposed for early vein formation. Fluid inclusions and hydrous minerals in late-formed veins have extremely low δD values, consistent with meteoric water infiltration. The change from rock-buffered, static fluid to infiltration from above can be explained in terms of changes in the large-scale deformation style occurring between 20 and 15 Ma. The rapid cooling of the Central Alps identified in previous studies may be explained in part, by infiltration of cold meteoric waters along fracture systems down to depths of 10 km or more. An average water flux of 0.15 cm3 cm - 2 yr - 1 entering the rock and reemerging heated by 40°C is sufficient to cool rock at 10 km depth by 100°C in 5 million years. The very negative δD values of < -130 permil for the late stage fluids are well below the annual average values measured in meteoric water in the region today. The low fossil δD values indicate that the Central Alps were at a higher elevation in the Neogene. Such a conclusion is supported by an earlier work, where a paleoaltitude of 5000 meters was proposed on the basis of large erratic boulders found at low elevations far from their origin.