Alkali feldspar 40Ar/39Ar and apatite UPb geochronological studies have typically invoked two mechanisms to account for apparent loss of radiogenic 40Ar and 206-208Pb. Some studies have suggested that the radiogenic isotopes were lost by volume diffusion and used these dates to constrain temporal variations of rock temperatures; others have argued that the radiogenic isotopes were lost due to interaction with fluids and related these dates to chemical alteration. These two end-member interpretations have fundamentally different implications for tectonic models derived from geochronological data, and therefore it is important to reliably identify the principal mechanism for loss of radiogenic isotopes. Here, we revisit the mechanisms of 40Ar loss in the famous gem-quality alkali feldspar from the Itrongay pegmatite in Madagascar. Previous studies have suggested that volume diffusion is the dominant mechanism of 40Ar loss, providing key evidence to support the use of 40Ar/39Ar dating of alkali feldspar for thermochronology. We attempted to verify these results by obtaining time-temperature paths from petrologically characterised cogenetic feldspar and apatite from the Itrongay pegmatite and comparing them with each other. However, our results suggest that only a minor component in the variability of 40Ar/39Ar dates of Itrongay feldspar is related to the diffusive loss of 40Ar, and that this loss was not compatible with the majority of previously proposed models, which hinders quantitative interpretations. The crystal studied here grew in five episodes related to the influx initially of co-existing dense SiO2-rich solution and CO2-dominated fluid (the first and supposedly the following two episodes) and subsequently of H2O-rich fluid (supposedly the final two episodes). Much greater component in the variability of the acquired 40Ar/39Ar dates is interpreted to reflect the differences in the ages of these growth episodes, which we estimate to span from 477 Ma to 176 Ma (the first four episodes). Apatite inclusions in this crystal are interpreted to be xenocrysts derived from the country rocks of the Itrongay pegmatite. These yield older UPb dates than the estimated age of their host feldspar and have apparently experienced diffusive loss of 206,207Pb prior to entrapment. Our 40Ar/39Ar results indicate that there is a lack of unambiguous evidence for diffusive loss of 40Ar from alkali feldspar that can be readily interpreted for thermochronological purposes. However, in situ 40Ar/39Ar dating of alkali feldspar appears to be a promising tool for tracking fluid-flow events in the Earth's crust whose applicability is not restricted to sedimentary rocks. Our UPb results corroborate previous suggestions that UPb dating of apatite can be used for thermochronology.

中文翻译：

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Itrongay伟晶岩（马达加斯加）中的扩散和流体相互作用：来自宝石级碱长石原位40Ar/39Ar定年和原生磷灰石包裹体U Pb定年的证据

碱长石 40Ar/39Ar 和磷灰石 UPb 地质年代学研究通常使用两种机制来解释放射成因 40Ar 和 206-208Pb 的明显损失。一些研究表明，放射性同位素因体积扩散而丢失，并使用这些日期来限制岩石温度的时间变化；其他人则认为放射性同位素由于与流体的相互作用而丢失，并将这些日期与化学变化联系起来。这两种端元解释对源自地质年代学数据的构造模型有着根本不同的影响，因此可靠地确定放射性同位素丢失的主要机制非常重要。在这里，我们重新审视了马达加斯加伊特龙盖伟晶岩中著名的宝石级碱性长石中 40Ar 的损失机制。先前的研究表明，体积扩散是 40Ar 损失的主要机制，为支持使用碱长石的 40Ar/39Ar 测年进行热年代学提供了关键证据。我们试图通过从具有岩石学特征的共生长石和来自 Itrongay 伟晶岩的磷灰石中获取时间-温度路径并将它们相互比较来验证这些结果。然而，我们的结果表明，Itrongay 长石的 40Ar/39Ar 日期变化中只有一小部分与 40Ar 的扩散损失有关，并且这种损失与之前提出的大多数模型不兼容，这阻碍了定量解释。此处研究的晶体分五次生长，最初与共存的富含 SiO2 的溶液和以 CO2 为主的流体（第一次，推测是接下来的两次）和随后的富含 H2O 的流体（推测是最后两集）的流入有关。 ）。所获得的 40Ar/39Ar 日期的可变性中更大的分量被解释为反映了这些增长事件的年龄差异，我们估计其跨度从 477 Ma 到 176 Ma（前四集）。该晶体中的磷灰石内含物被解释为来自 Itrongay 伟晶岩的围岩的异种晶体。这些产生的 UPb 日期比其寄主长石的估计年龄更旧，并且在捕获之前显然经历了 206,207Pb 的扩散损失。我们的 40Ar/39Ar 结果表明，缺乏明确的证据表明 40Ar 从碱性长石中扩散损失，可以很容易地解释为热年代学目的。然而，碱性长石的原位 40Ar/39Ar 测年似乎是一种很有前途的工具，用于跟踪地壳中的流体流动事件，其适用性不仅限于沉积岩。我们的 UPb 结果证实了先前的建议，即磷灰石的 UPb 测年可用于热年代学。