Limited mobility of argon in a metamorphic terrain
Limited mobility of argon in a metamorphic terrain
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DOI：
10.1016/0016-7037(79)90219-9
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发表时间：
1979-06
影响因子：
5
通讯作者：
K. Foland
中科院分区：
文献类型：
--
作者：
K. Foland
在阿巴拉契亚山前的一些相对无水的炭质岩石中，黑云母中含有过量的ar，表明氩的迁移率有限。麻粒岩相Wilmington杂岩的Arden岩体中的黑云母显然是以辉石和钾长石为代价的逆行产物形成的。所有岩石的黑云母Rb-Sr年龄约为365myr。这些同色岩具有明显的K-Ar年龄，范围在365-590 Myr之间，其中6个明显超过了500 Myr的Sr同位素全岩年龄。它们含有不同数量的过量氩，这些氩是在大约365myr以前的黑云母结晶或再结晶过程中加入的。其他矿物似乎都不含有大量的过量氩。钾-氩表观年龄与全岩钾浓度呈正相关。这些关系产生了黑云母相中过量氩与岩石钾之间的相关性。这表明黑云母中过量的ar是局部衍生的，是由于局部氩活动的印记。如果掺入量大致与当时的氩气分压成正比，则存在实质性差异。在大体积岩石上，氩的化学势不均匀。对间隔较近的样品的分析表明，在小于10米的尺度上，氩气活性不同。这意味着Ar的输运受限，可能是由于无水组合的有效渗透率非常低。
Excess40Ar in biotite from some relatively anhydrous charnockitic rocks in the Appalachian Piedmont indicates limited mobility of argon. Biotite from the Arden pluton of the granulite-facies Wilmington Complex apparently formed as a retrograde product at the expense of pyroxene and K-feldspar. Rb-Sr ages of biotite from all rocks are approximately 365 Myr. Thesamemicas have apparent K-Ar ages which range from about 365–590 Myr, six of which clearly exceed the Sr isotope whole-rock date of 500 Myr. They contain variable amounts ofexcess40Ar incorporated during crystallization or recrystallization of biotite at about 365 Myr ago. None of the other minerals appears to contain significant amounts of excess argon. The K-Ar apparent ages show strong, positive correlation with whole-rock K concentrations. These relations yield a correlation between excess argon in the biotite phase and rock potassium. This suggests that excess40Ar in biotite is of local derivation and is due to an imprint of the local argon activity. If the amount incorporated is roughly proportional to the prevailing argon partial pressure then substantial differences inpArexisted. Argon did not have a uniform chemical potential over large rock volumes. Analysis of closely spaced samples suggests different argon activity over the scale of less than 10m. This implies restricted transport of Ar and is probably due to very low effective permeability of the anhydrous assemblages.