Mineralogical, crystallographic, and isotopic constraints on the precipitation of aragonite and calcite at Shiqiang and other hot springs in Yunnan Province, China

详细信息    查看全文

• 作者：Brian Jones^a ; ^{Brian.Jones@ualberta.ca" class="auth_mail" title="E-mail the corresponding author} ; Xiaotong Peng^b
• 关键词：Hot springs ; Calcite ; Aragonite ; Opal-A ; Isotopes ; Dendrites
• 刊名：Sedimentary Geology
• 出版年：2016
• 出版时间：November 2016
• 年：2016
• 卷：345
• 期：Complete
• 页码：103-125
• 全文大小：9421 K

文摘

Two active spring vent pools at Shiqiang (Yunnan Province, China) are characterized by a complex array of precipitates that coat the wall around the pool and the narrow ledges that surround the vent pool. These precipitates include arrays of aragonite crystals, calcite cone-dendrites, red spar calcite, unattached dodecahedral and rhombohedral calcite crystals, and late stage calcite that commonly coats and disguises the earlier formed precipitates. Some of the microbial mats that grow on the ledges around the pools have been partly mineralized by microspheres that are formed of Si and minor amounts of Fe. The calcite and aragonite that are interspersed with each other at all scales are both primary precipitates. Some laminae, for example, change laterally from aragonite to calcite over distances of only a few millimetres. The precipitates at Shiqiang are similar to precipitates found in and around the vent pools of other springs found in Yunnan Province, including those at Gongxiaoshe, Zhuyuan, Eryuan, and Jifei. In all cases, the δD_water and δ¹⁸O_water indicate that the spring water is of meteoric origin. These are thermogene springs with the carrier CO₂ being derived largely from the mantle and reaction of the waters with bedrock. Variations in the δ¹³C_travertine values indicate that the waters in these springs were mixed, to varying degrees, with cold groundwater and its soil-derived CO₂. Calcite and aragonite precipitation took place once the spring waters had become supersaturated with respect to CaCO₃, probably as a result of rapid CO₂ degassing. These precipitates, which were not in isotopic equilibrium with the spring water, are characterized by their unusual crystal morphologies. The precipitation of calcite and aragonite, seemingly together, can probably be attributed to microscale variations in the saturation levels that are, in turn, attributable to microscale variations in the rate of CO₂ degassing.