Behaviour of argon during high pressure metamorphic fluid-rock interaction

~(40)Ar/~(39)Ar质谱学是确定地质过程绝对年龄和速率的最有力工具之一。此外，如果已知变质过程中的Ar输运参数，Ar作为一种可流动的痕量元素，可以用来定量流体通量和流体-岩石相互作用。我们的初步工作发现，在西阿尔卑斯山的天然高压岩石中，流体渗透导致了三种不同的矿物组合变质类型：(1)粒内溶解-再沉淀，(2)体积扩散，(3)在反应地点和现有颗粒上过度生长的含K和无K相的新结晶。矿物颗粒的部分叠印可以确定残留物和变质部分斑岩白云母和角闪石中Ar的同位素组成和丰度。此外，还将现场测定Li、Be、B和大离子亲石元素(LILE)等流体敏感元素的浓度。这些数据，连同热力学和微量元素正演模型，将被用来约束流体流入的速率和大小、再结晶机制和晶体内微量元素和同位素再平衡之间的关系。结合众所周知的微量元素信息，新获得的现场Ar数据将使我们能够确定Ar在变质过程中的分配、传输和扩散行为：这些数据非常缺乏。

40Ar/39Ar mass spectrometry is one of the most powerful tools to determine absolute ages and rates of geo-processes. Furthermore, Ar as a fluid-mobile trace element can be used to quantify fluid fluxes and fluid-rock interaction, if Ar transport parameters during metamorphism are known. Our preliminary work has identified, in natural high-pressure rocks from the Western Alps, that fluid infiltration has led to three different modification types of the mineral assemblage: (1) intra-grain solution-reprecipitation, (2) volume diffusion and (3) neocrystallisation of K-bearing and K-free phases that form in reaction sites and in overgrowth on existing grains. Partial overprinting of mineral grains allows to determine the isotopic composition and abundances of Ar in relict as well as modified parts of phengitic white mica and amphiboles. Additionally, concentrations of fluid-sensitive elements, such as Li, Be, B and large ion lithophile elements (LILE) will be determined in situ. These data, together with thermodynamic and trace element forward models will be used to constrain the relationship between rates and magnitudes of fluid influx, recrystallisation mechanisms and intracrystalline trace element and isotopic re-equilibration. Combined with the information from the well-known trace elements, the newly won in situ Ar data will allow us to establish the partitioning, transport and diffusive behaviour of Ar during metamorphism: data that are sorely lacking.