Magma differentiation processes and pre-eruptive conditions of MORBs (7-11°S) near Ascension Island - Constraints from experiments and phase equilibria modeling

最近对阿森松岛附近大西洋中脊（MAR）岩石的研究表明，玄武岩熔体的主要分异过程发生在相对较低的压力下（小于500 MPa）。初步结果表明，在不同结晶深度（和分馏路径）、温度和熔体挥发物含量的条件下，每一段都有不同的结晶历史。然而，目前的实验数据库在解释分馏趋势方面存在一定的困难。特别是，在500兆帕以下，即使少量的挥发物溶解在熔体中，也会导致相对较高的挥发物活性，这可能会显著影响相的稳定性和分化过程。本研究项目旨在阐明压力、氧逸度和挥发物（特别是水）逸度对阿森松附近MAR玄武岩熔体分异的影响。为了实现这些目标，使用了三种方法：(1)在大范围的aH2O和fO2中建立相平衡模型，包括平衡、分数和多压结晶计算；(2)采用天然成分进行高压高温实验，模拟极少量水对主要矿物（橄榄石、斜长石、斜辉石）液相温度的影响；(3)研究了矿物中的玻璃夹杂物和玻璃中的水分含量，以了解挥发物在分化过程和上升过程中的行为和重要性。本研究的最终目的是描述MAR岩浆的分化程度和喷发前条件的演变，并结合其他方法（地球物理，地球化学）的结果，我们的数据将有助于了解海洋地壳的温度分布，并定位MAR可能的岩浆库。

Recent investigations on rocks from the Mid-Atlantic Ridge (MAR) near the Ascension Island indicate that major differentiation processes of basaltic melts occurred at relatively low pressure (less than 500 MPa). Preliminary results indicate that each of the four investigated segments along the MAR have a distinct history with different crystallization depths (and fractionation paths), temperature and volatile contents of melts. However, the current experimental database meets certain difficulties to interpret the fractionation trends. In particular, below 500 MPa, even small amounts of volatiles dissolved in the melt result in relatively high volatile activities which may influence significantly the stability of phases and differentiation processes. The aim of this research project is to clarify the influence of pressure, oxygen fugacity and fugacities of the volatiles (in particular water) on the differentiation of basaltic melts along the MAR near Ascension. To achieve these goals, three approaches are used: (1) phase equilibria are modeled in a wide range of aH2O and fO2 and include equilibrium, fractional, and polybaric crystallization calculations; (2) high pressure and high temperature experiments are performed with natural compositions to model the effect of very small amounts of water on the liquidus temperature of the main minerals (Olivine, plagioclase, clinopyroxene); (3) glass inclusions in minerals and water contents of glasses are investigated to understand the behavior and importance of volatiles in differentiation processes and during ascent. The ultimate goal of this study is to depict the degree of differentiation and the evolution of pre-eruptive conditions of magmas along the MAR. Combined with results from other approaches (geophysics, geochemistry), our data will contribute to the understanding of the temperature distribution in the oceanic crust and to locate possible magma chambers along MAR.