微量元素在斜长石和玄武质熔体间的分配系数及在月球岩浆洋中的应用

A planet-wide magma ocean has been thought to have formed shortly after the birth of the moon since the Apollo era. This canonical magma ocean model has been further applied to Earth, Mars and asteroids. A number of fractional crystallization models have been proposed for the lunar magma ocean so far. These models, however, make completely different predictions due to unconstrained geochemistry of the lunar magma ocean. On the other hand, it is generally accepted that plagioclase, after crystallization, floated in the magma ocean and formed ferroan anorthosite as the primary crust of the moon. Ferroan anorthosite is the only available lithology that is believed to be a primary product of the magma ocean...Aimed to characterize the trace element geochemistry of the lunar magma ocean, this project proposes a two-pronged approach:.(1) Experimental study of trace element partition coefficients between plagioclase (An ≥ 96%) and lunar basaltic melts under conditions of the magma ocean. So far, there do not exist such data for the anorthitic plagioclase. The new results will allow estimation of trace element concentrations of the melt equilibrated with anorthitic plagioclase. Combining the new data with the literature ones, a general model for trace element partition coefficients will be constructed. .(2) Survey of trace elements in plagioclase of ferroan anorthosite in lunar meteorites. Trace elements in ferroan anorthositic plagioclase were partitioned from the lunar magma ocean. Therefore, using the partition coefficients between plagioclase and melt obtained from the experimental study above, constraints on the trace element geochemistry of the lunar magma ocean can be provided...From this multi-disciplinary methodology, this proposal will be able to assess the trace element compositions in the lunar magma ocean. The results from this proposal will essentially place constrains on the geochemistry and crystallization of the magma ocean, a fundamental process that controlled the geological and morphological evolution of the moon.

自从月球岩浆洋模型被提出之后，多项证据证实月球形成后就发生全球性的部分熔融，在月表形成岩浆洋。月球岩浆洋模型还被推广到地球、火星等类地行星以及小行星的研究中。但是，由于严重缺乏岩浆洋的地球化学数据，到目前为止，月球岩浆洋的结晶分异过程还没有一个定论。亚铁斜长岩，作为原始月壳的残余物，是已知月球样品中唯一一个从岩浆洋中直接结晶分异出来的岩石样品。本项目计划通过两项研究对月球岩浆洋的成分及演化进行制约。(1)实验测量月球岩浆洋条件下的微量元素在富钙斜长石(An ≥ 96%)-熔体间的分配系数。目前文献并没有此类数据。结合已有数据，项目还将建立广泛适用的微量元素分配系数模型。(2)测量月球长石质角砾岩陨石中亚铁斜长岩的斜长石主量和微量元素浓度。本项目将利用斜长石成分以及月球岩浆洋条件下的微量元素在斜长石-熔体间的分配系数，计算岩浆洋中微量元素的含量，从而对月球岩浆洋的结晶分异过程进行制约。

自从根据阿波罗11号返回的样品提出月球岩浆洋模型后，多项证据证实了月球形成后就发生全球性的部分熔融，在月表形成了岩浆洋。月球岩浆洋模型还被推广到地球、火星等类地行星以及小行星的研究中。亚铁斜长岩，作为原始月壳的残余物，是已知月球样品中唯一一个从岩浆洋中直接结晶分异出来的岩石样品。本项目通过两项研究对月球岩浆洋成分及斜长岩演化进行制约。.（1）实验测量了月球岩浆洋条件下的镁、铁、微量元素和水在富钙斜长石(An ≥ 96%)-熔体间的分配系数。测量结果显示在熔体水含量较低时，斜长石-熔体间水的分配系数较大。结合已有斜长石-熔体间水分配系数的数据，项目还建立了广泛适用的斜长石-熔体间水分配系数的模型。这项研究的结果为解译月球岩浆洋水含量的变化奠定了基础。(2) 研究了10块月球陨石中的富镁和富铁斜长岩岩屑，这两种类型的斜长岩稀土含量变化都很大，绝对含量相差可达50倍，同时，稀土具有不同的演化趋势线，暗示着这些斜长岩不可能都是从同一个岩浆中结晶形成。根据已有的数据，提出了斜长岩在全球性的岩浆洋中结晶后，组成的原始月壳又受到了KREEP及后期地幔熔体的交代作用。这个模型解释了目前所有关于月球斜长岩的数据，并且解决了斜长岩地球化学数据和岩浆洋模型不一致的危机。