(U-Th)/He geochronology and chemical compositions of diagenetic cement, concretions, and fracture-filling oxide minerals in Mesozoic sandstones of the Colorado Plateau
(U-Th)/He geochronology and chemical compositions of diagenetic cement, concretions, and fracture-filling oxide minerals in Mesozoic sandstones of the Colorado Plateau
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科罗拉多高原中生代砂岩中成岩胶结物、结核和裂缝充填氧化物矿物的 (U-Th)/He 年代学和化学成分
DOI：
10.1130/b30983.1
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发表时间：
2014
影响因子：
4.9
通讯作者：
N. S. Evenson
中科院分区：
文献类型：
--
作者：
P. Reiners;M. Chan;N. S. Evenson
成岩胶结物、结核和裂缝填充物中的氧化物矿物反映了古代地下水流的历史，有可能记录构造、地貌和气候随时间的变化。为了更好地了解这些成岩物质的年龄、形成条件和潜在的地质意义，我们测量了科罗拉多高原多个地点中生代砂岩中赤铁矿、针铁矿和氧化锰的 (U-Th)/He 年龄和元素浓度。大多数 (U-Th)/He 年龄为上新世 - 更新世，但一些样品的年龄可达 25 Ma，这是之前在锰氧化物水泥上测定的 40 Ar/ 39 Ar 年龄。在一个区域，质地不同的锰氧化物和铁氧化物产生相对可重复的年龄约。 2-3 Ma，在另一个地区，玻璃态铁氧化物断裂涂层的年龄约为 2-3 Ma。 300 公里。在其他地方，大多数水泥和结核在毫米长度尺度上显示出不同的年龄。以赤铁矿为主的样品显示出年龄和 U-Th 浓度之间广泛的正相关性，而大多数以针铁矿为主的样品则显示出负相关性。球形针铁矿结核壳的外部部分比内部部分具有更高的 U-Th 浓度和更年轻的 (U-Th)/He 年龄。总而言之，这些样品的年龄-成分关系表明，一些样品中的广泛年龄范围在很大程度上反映了相对较新的（上新世-更新世）U-Th 添加、重结晶或后来的氧化物生长，这些影响了可能是氧化物或其他矿物的前体胶结物。在某些情况下，这些前体可能早在 25 Ma 就已形成。至少在两个区域中，(U-Th)/He 日期（或者在样本显示年龄范围较大的情况下，为最小日期）与主要气候或当地切割率变化相关。我们推测，成岩氧化物，以及可能的其他类型的细粒次生氧化物矿物，在最初形成后很长一段时间内，仍然容易被 U-Th 吸收、重结晶或通过与地下水接触而持续生长。当通过浅层地壳挖掘样本时，或者当地壳关键区域和火星的成岩系统中的表面条件随时间变化时，这可能为了解地下水成分和流动状况提供机会。
Oxide minerals in diagenetic cements, concretions, and fracture fill reflect episodes of ancient groundwater flow that have the potential to record tectonic, geomorphic, and climatic changes through time. To better understand the ages, conditions of formation, and potential geologic significance of these diagenetic materials, we have measured (U-Th)/He ages and element concentrations in hematite, goethite, and Mn-oxide in Mesozoic sandstones from several locations in the Colorado Plateau. Most (U-Th)/He ages are Pliocene–Pleistocene, but some samples are as old as 25 Ma, the age of previously determined 40 Ar/ 39 Ar ages on Mn-oxide cement. In one region, texturally diverse Mn- and Fe-oxides yield relatively reproducible ages of ca. 2–3 Ma, and in another region, vitreous Fe-oxide fracture coatings yield ages of ca. 300 ka. Elsewhere, most cements and concretions show a wide range of ages among aliquots taken over millimeter length scales. Hematite-dominated samples show a broad positive correlation between age and U-Th concentrations, whereas most goethite-dominated samples show a negative correlation. Exterior portions of spherical goethite concretion rinds have higher U-Th concentrations and younger (U-Th)/He ages than interior portions. Taken together, the age-composition relationships of these samples suggest that wide ranges of ages in some samples largely reflect relatively recent (Pliocene–Pleistocene) U-Th addition, recrystallization, or later oxide growth that affected precursor cements that may have been oxides or other minerals. In some cases, these precursors may have formed as early as 25 Ma. In at least two areas, the (U-Th)/He dates (or in cases where samples show a wide range in ages, the minimum dates) are associated with major climate or local incision rate changes. We speculate that diagenetic oxides, and possibly other types of fine-grained secondary oxide minerals, remain open to U-Th uptake, recrystallization, or continued growth through contact with groundwater long after initial formation. This may provide opportunities to understand groundwater compositions and flow regimes as samples are exhumed through the shallow crust, or as surface conditions change through time in diagenetic systems in the critical zone of Earth’s crust and on Mars.