U-Pb Dating of Cave Spar: A New Shallow Crust Landscape Evolution Tool
U-Pb Dating of Cave Spar: A New Shallow Crust Landscape Evolution Tool
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DOI：
10.1002/2017tc004675
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发表时间：
2018-01-01
期刊：
影响因子：
4.2
通讯作者：
Lachniet, M. S.
中科院分区：
文献类型：
--
作者：
Decker, D. D.;Polyak, V. J.;Lachniet, M. S.
在碳酸盐岩地体中，提供磷灰石的岩石类型无法有效地使用磷灰石裂变径迹 (AFT) 或 (U/Th)-He 计时法 (AHe)。在这里，我们建议方解石洞穴晶石可以成为一种有效的计时器，并与碳酸盐地区的 AFT 和 AHe 温度计时器互补，例如我们的研究区域、新墨西哥州东南部的瓜达卢佩山脉和德克萨斯州西部。我们测量的洞穴晶石沉积深度为区域地下水位以下 500 +/- 250 m，形成温度为 40 至 80 摄氏度，这表明这些洞穴及其晶石晶体形成于超临界 CO2-亚临界 CO2 边界附近，我们在此解释了洞穴和晶石的起源。这种深度-温度关系表明地温高于正常水平，可能与区域岩浆活动有关。作为一个案例研究，我们研究了瓜达卢佩山脉的隆升时间，之前归因于压缩拉拉米德造山运动（约 90 至 50 Ma）、后来与盆地和山脉相关的伸展构造（约 36 至 28 Ma）或里奥格兰德裂谷的开放（约 20 Ma 至今）。我们表明，大部分晶石起源与 36 至 28 Ma 之间的火凝灰岩爆发同时发生。我们的结果将瓜达卢佩山脉相对于周围地形的地块隆升起始时间限制在 27 至 16 Ma 之间，并重建了 185 至 28 Ma 块体隆起之前的低洼区域景观的演化，支持将区域表面隆起归因于伸展构造和相关火山作用的模型。 简单语言摘要 一种确定与使用方解石造山相关的隆起时间的新方法 讨论了在深处洞穴中生长的晶体。方解石晶体和含有它们的洞穴是地下深处岩浆释放出高浓度二氧化碳的结果。这些晶体形成的深度由与超临界二氧化碳（既不是固体、液体也不是气体的物质状态）和气态二氧化碳之间的转变相关的温度和压力决定，通常在地下水位以下 250 至 750 m 之间。我们首次对这些类型的晶体使用铀-铅测年技术来确定这些方解石晶体的年龄，以确定晶体形成的时间。我们还利用晶体内的微观流体包裹体来确定它们形成的温度。有了深度、年龄和温度这三项信息，我们就可以计算出晶体何时位于地下水位以下一定深度以及它们生长的地热环境，从而提供了一种新的方法来确定山脉何时可能被抬升到目前的高度。
In carbonate terranes, rocks types that provide apatite are not available to effectively use apatite fission track (AFT) or (U/Th)-He chronometry (AHe). Here we suggest that calcite cave spar can be an effective chronometer and complimentary to AFT and AHe thermochronometers in carbonate regions such as our study area, the Guadalupe Mountains of southeastern New Mexico, and west Texas. Our measured depth of cave spar deposition is 500 +/- 250 m beneath the regional water table, formed at temperatures of 40 degrees to 80 degrees C, indicating that these caves and their spar crystals form near the supercritical CO2-subcritical CO2 boundary where we interpret the origin of both the caves and spar to occur. This depth-temperature relationship suggests a higher than normal geotherm, likely associated with regional magmatic activity. As a case study we examined the timing of uplift of the Guadalupe Mountains previously attributed to the compressional Laramide orogeny (ca. 90 to 50 Ma), later extensional tectonics associated with Basin and Range (ca. 36 to 28 Ma) or the opening of the Rio Grande Rift (ca. 20 Ma to Present). We show that most of the spar origin is coeval with the ignimbrite flare-up between 36 and 28 Ma. Our results constrain the initiation of Guadalupe Mountains block uplift, relative to the surrounding terrain, to between 27 and 16 Ma and reconstruct the evolution of a low-lying regional landscape prior to block uplift from 185 to 28 Ma, in support of models that attribute regional surface uplift to extensional tectonics and associated volcanism.Plain Language Summary A new way of determining the timing of uplift related to mountain building using calcite crystals that grow in deep seated caves is discussed. The calcite crystals and the caves that contain them are the result of high levels of carbon dioxide that is released from magma deep below. These crystals form at a depth determined by the temperature and pressure related to the transition between super-critical carbon dioxide (a state of matter that is neither solid, liquid, nor gas) and gaseous carbon dioxide, usually between 250 and 750 m beneath the water table. For the first time on these types of crystals, we use uranium-lead dating techniques to find the age of these calcite crystals to determine when the crystals formed. We also use microscopic fluid inclusions within the crystals to find the temperatures at which they formed. With these three pieces of information, depth, age, and temperature, we can calculate when the crystals were at a certain depth beneath the water table and the geothermal environment in which they grew, providing a novel way to determine when the mountains may have been uplifted to their present elevation.