Iodine-129, 87Sr/86Sr, and trace elemental geochemistry of northern Appalachian Basin brines: Evidence for basinal-scale fluid migration and clay mineral diagenesis
Iodine-129, 87Sr/86Sr, and trace elemental geochemistry of northern Appalachian Basin brines: Evidence for basinal-scale fluid migration and clay mineral diagenesis
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DOI：
10.2475/03.2012.01
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发表时间：
2012-03
影响因子：
2.9
通讯作者：
S. Osborn;J. McIntosh;J. Hanor;D. Biddulph
中科院分区：
文献类型：
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作者：
S. Osborn;J. McIntosh;J. Hanor;D. Biddulph
利用从纽约州西部、宾夕法尼亚州西北部和肯塔基州东部盆地北部边缘中上泥盆统部分收集的地层水的元素和同位素（129I/I、87Sr/86Sr）地球化学，研究了阿巴拉契亚盆地北部盆地规模的盐水迁移和粘土矿物成岩作用的证据。对密西西比伯里亚砂岩和志留纪麦地那砂岩各取一个样品进行分析以进行比较。测得的碘比率范围在 28 至 1,890 × 10−15 之间，与源自泥盆纪有机质的宇宙成因碘相比异常高。水域中的 Iodine-129 主要来自裂变源，即 238U 自发裂变产生 129I，估计 129I/I 值高达 270 × 10−15，这种情况发生在富含有机物的页岩中。三个水样的值分别为 490 × 10−15、860 × 10−15 和 1,890 × 10−15，高于当地裂变 129I 的范围，可能是由地形驱动的盆地规模流体流经区域高裂变源造成的。相对较大的铀矿点位于蓝岭省阿巴拉契亚盆地的构造前缘，并且位于平行于阿勒根造山带主挤压方向的假设流动路径内。这些铀矿点的 129I/I 估计值超过 55,000 × 10−15。盐水的锶同位素组成和 Sr 浓度显示出具有低 Sr (51 mg/L) 的高放射性端元 (0.7210) 和非放射性 (0.7100)、高 Sr (4789 mg/L) 端元之间的混合趋势。相对于蒸发的古生代海水（阿巴拉契亚盆地盐水的假设来源），钾和硼的浓度显着减少。地层水的 K/Rb 值相对于海水值有所减少，但在某些情况下远高于指示水岩反应的值。 Sr 同位素组成、K 和 B 消耗以及中间 K/Rb 比率与蒙皂石成岩作用和可能高于约 150 °C 的古温度一致。考虑到研究区域的埋藏历史，这些温度可能很高，并支持地层水从盆地深处流动。地层水的同位素和元素综合结果为阿巴拉契亚盆地北部盆地规模的流体运移提供了令人信服的证据，并且与之前公布的岩石记录记录的证据一致，包括粘土矿物成岩作用和矿石沉积。
Evidence for basin scale brine migration and clay mineral diagenesis in the northern Appalachian Basin was investigated using elemental and isotope (129I/I, 87Sr/86Sr) geochemistry of formation waters collected from the Middle to Upper Devonian section of the northern basin margin in western New York, northwest Pennsylvania, and eastern Kentucky. One sample from each of the Mississippian Berea sandstone and the Silurian Medina sandstone were analyzed for comparison. Measured iodine ratios range between 28 to 1,890 × 10−15 and are anomalously high compared to cosmogenic iodine sourced from Devonian age organic matter. Iodine-129 in the waters was largely derived from fissiogenic sources, the spontaneous fission of 238U to produce 129I, with estimated 129I/I values up to 270 × 10−15, which occur locally in the organic-rich shales. There are three water samples that have values of 490 × 10−15, 860 × 10−15, and 1,890 × 10−15, which are above the range for local fissiogenic 129I and may be accounted for by topographically driven, basin scale fluid flow through a regionally high fissiogenic source. Relatively large uranium occurrences lie along the structural front of the Appalachian Basin in the Blue Ridge Province and are situated within hypothesized flow paths parallel to the main compressional direction of the Alleghanian orogeny. Estimated 129I/I values for these uranium occurrences are in excess of 55,000 × 10−15. The strontium isotope composition and Sr concentration of brines display a mixing trend between a highly radiogenic end-member (0.7210) with low Sr (51 mg/L) and a non-radiogenic (0.7100), high Sr (4789 mg/L) end-member. Potassium and boron concentrations are notably depleted relative to evaporated Paleozoic seawater, the hypothesized source of Appalachian Basin brines. The K/Rb values of formation waters are depleted relative to seawater values, but in some cases are well above values indicative of water-rock reactions. The Sr isotopic composition, K and B depletion, and intermediate K/Rb ratios are consistent with smectite diagenesis and paleo-temperatures that are likely greater than approximately 150 °C. These temperatures may be high given the burial history of the study area and support the flow of formation waters from deeper within the basin. The combined isotopic and elemental results of formation waters provide compelling evidence for basin scale fluid migration in the northern Appalachian Basin and are consistent with previously published evidence documented from the rock record, including clay mineral diagenesis and ore deposition.