Low pressure differentiation of basalts at the Mid-Atlantic Ridge near Ascension - Constraints from experiments at controlled volatile and oxygen fugacities

最近对Ascension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低压（小于200 MPa）。在如此低的压力下，即使少量的挥发物溶解在熔体中也会导致相对高的挥发物活性，这可能显著影响相的稳定性和分化过程。不同的分馏路径已经确定，但没有实验数据库来解释它们的起源。本研究项目的目的是澄清压力、氧逸度和挥发物逸度（特别是水，但也包括S和C1）对沿着靠近Ascension的MAR玄武岩熔体分异的影响。高压和高温实验将使用以前航行中收集的天然成分进行。代表分化不同阶段的组合物将用于模拟“液体下降线”。将特别注意小的压力变化对分化过程的影响。实验结果将与自然相组合进行比较，以限制分化的条件，并了解来自Ascension Island以东MAR的不同段中不同分馏路径的起源。

Recent investigations on rocks from the Mid-Atlantic Ridge (MAR) near the Ascension Island indicate that major differentiation processes of basaltic melts occurred at low pressure (less than 200 MPa). At such low pressure, even low amounts of volatiles dissolved in the melt result in relarively high volatile activities which may influence significantly the stability of phases and differentiation processes. Different fractionation paths have been identified, but there is no experimental database to interpret their origin. The aim of this research project is to clarify the influence of pressure, oxygen fugacity and fugacities of the volatiles (in particular water, but also S and C1) on the differentiation of basaltic melts along the MAR near Ascension. The high pressure and high temperature experiments will be performed with natural compositions collected during previous cruises. Compositions representing different stages of the differentiation will be used to model the "liquid lines of descent". Particular attention will be given to the effects of small pressure variations on differentiation processes. The experimental results will be compared with the natural phase assemblages to constrain the conditions of differentiation and to understand the origin of different fractionation paths in different segments from the MAR east of the Ascension Island.