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返回地球地幔时，从俯冲物质中释放的流体在地幔楔形体内融化，最终导致新的大陆地壳积聚，火山气体向大气中喷发。由于流体在这样的环境中充当质量和能量传递的主要媒介，了解流体如何以及何时释放对于构建俯冲带正在进行的全球物质循环的连贯图景至关重要。翡翠岩--主要由钠质单斜辉石翡翠岩组成的岩石--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。流体释放的时间(S)可以通过测定某些副矿物如锆石的年代来确定翡翠岩的形成年龄来限制。然而，锆石具有在部分熔融和高级变质作用中幸存下来的能力，这导致了对其在翡翠岩中的主要来源的一些模糊。因此，在这项研究中，建议用二次离子质谱仪(SIMS)对翡翠岩中的其他常见辅料进行原位U-Pb测年，包括钛铁矿、金红石和磷灰石，这些辅料往往与流体的影响直接相关。结合辅助相的SIMS氧同位素分析，所提出的研究可以提供有关翡翠岩形成以及最终俯冲带流体产生的有价值的信息。

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.