In-situ U-Pb age dating and the oxygen isotopic composition of accessory minerals in jadeitites: investigating the timing of fluid release and fluid-rock interactions in subduction zones

当地壳岩石在俯冲带回到地幔时，从俯冲物质中释放出来的流体开始在地幔楔中融化，最终导致新的大陆地壳的形成，火山气体向大气中排放。由于流体在这种情况下是质量和能量传递的主要媒介，因此了解流体是如何以及何时释放的，对于构建俯冲带正在进行的全球物质回收的连贯图像至关重要。硬玉岩——主要由钠斜辉硬石组成的岩石——要么直接见证了这种俯冲带流体，要么代表了这些流体本身的结晶部分。然后，可以通过确定某些辅助矿物（如锆石）的年代来确定翡翠岩的形成年龄，从而限制流体释放的时间。然而，锆石具有部分熔融和高变质作用的能力，这使得其主要成因在翡翠岩中存在一些不确定性。因此，本研究建议利用次级离子质谱法（SIMS）对翡翠岩中的其他常见附件（包括钛矿、金红石和磷灰石）进行原位U-Pb测年，这些附件通常与流体的影响直接相关。结合SIMS副相氧同位素分析，所提出的研究可以提供有关翡翠岩形成的有价值的信息，并最终提供俯冲带流体生产的信息。

When crustal rocks return into Earth’s mantle at subduction zones, fluids released from subducted material initiate melting in the mantle wedge which ultimately leads to the build-up of new continental crust and venting of volcanic gases into the atmosphere. Since fluids act as major agents of mass and energy transfer in such a setting, understanding how and when fluids are released is of paramount importance for constructing a coherent picture of this ongoing global material recycling at subduction zones. Jadeitites – rocks composed mainly of the sodic clinopyroxene jadeite - either directly witnessed such subduction-zone fluids or represent crystallized portions of these fluids themselves. The timing of fluid release(s) could then be constrained by determining the formation ages of jadeitites by dating certain accessory minerals such as zircon. However, zircon has the ability to survive partial melting and high-grade metamorphism which leads to some ambiguity concerning its primary origin in jadeitites. In this study it is therefore proposed to perform in-situ U-Pb dating by secondary ion mass spectrometry (SIMS) of other common accessories in jadeitites including titanite, rutile, and apatite which can often be directly linked to the effect of fluids. In combination with SIMS oxygen isotope analysis of accessory phases, the proposed investigations can provide valuable information about jadeitite formation and, ultimately, fluid production in subduction zones.