Differentiation of tholeiites - the role of primitive composition and small amounts of H2O

玄武岩是大洋中脊(MORB-大洋中脊玄武岩)、与羽流有关的大洋高原(大型火成岩省、嘴唇)以及许多海洋岛屿(如夏威夷)和海山中的主要玄武岩岩浆类型。大洋岩浆岩的地球化学研究表明，存在多种不同的富镁玻璃成分，代表了各种来自地幔的原始岩浆。由于原始岩浆的多样性，由于不同的地幔矿物学和地幔熔融程度，以及不同的可能结晶条件(P、fO2、挥发活动)和分异机制(结晶、岩浆混合、同化等)，洋壳中聚集了广泛的演化玄武岩。与在极高压力(&gt；1 Gpa)和大气压和干燥条件下的大型实验数据集相比，岩浆分异过程中的地质相关条件(压力从50到700兆帕，流体不饱和系统)以及从其来源到海底的运输仍然缺乏研究。50-700兆帕的压力范围特别重要，因为它涵盖了快速扩张海脊的浅轴向岩浆室和缓慢扩张海脊以下的地幔深层储集层之间的范围，或者是嘴唇的地幔-地壳过渡期的压力范围。因此，现有的预测分异路径的热力学模型是针对有限的条件范围进行校准的，不能考虑原始熔体成分的全部变化。它们中的大多数也不能准确地预测水在分馏过程中的作用。在这个项目中，计划进行结晶实验，以限制六种选定成分的液体下降线，这些成分涵盖原始端元拉斑玄武岩的大成分谱(例如，来自耐火和可育地幔的MORBs、N-MORB、E-MORB、高与低SiO_2)。重点讨论了压力(50、200、500和700 Mpa)和熔体水含量(0.04~1wt%H2O)对不同条件(T、P、H2O)下饱和橄榄石+单斜辉石+斜长石的共晶熔体成分的影响。实验数据库将被用来理解原始拉斑玄武岩的成分差异(以及热力学变量)对伪三元图上投影的Ol+Cpx+Pl共晶位置的作用，这是一种在地压模拟中广泛使用的方法。除了一般意义外，每一种已调查的自然成分都可以利用已公布的现有地球化学数据来解释相应MORB环境(如印度海脊、哥斯达黎加裂谷、东太平洋隆起)的分异条件。

Toleiitic basalt is the predominant type of basaltic magma in mid-ocean ridges (MORB - mid-ocean ridge basalt), in plume-related oceanic plateaus (Large Igneous Provinces, LIPs) and in a number of ocean islands (e.g. Hawaii) and seamounts. The geochemical studies of oceanic magmatic rocks demonstrate the existence of large variety of Mg-rich glass compositions, representative of a variety of mantle-derived primitive magmas. As a result of the diversity of primitive magmas, due to different mantle mineralogy and degree of mantle melting as well as different possible crystallization conditions (P, fO2, volatile activities), and mechanisms of differentiation (crystallization, magma mixing, assimilation etc.), there is a wide compositional range of evolved basalts which have been collected in the oceanic crust. In contrast to the large experimental dataset at very high pressure (> 1 GPa) and at 1 atmosphere and dry conditions, the geologically relevant conditions (pressures from 50 to700 MPa, fluid-undersaturated systems) prevailing during the magma differentiation and the transport from their source to ocean floor have been still poorly investigated. The pressure range of 50-700 MPa is particularly important since it covers the range between shallow axial magma chambers in fast-spreading ridges and deep reservoirs in the mantle below slow-spreading ridges or at the mantle-crust transition for LIPs. As a consequence, the available thermodynamic models predicting differentiation paths are calibrated for a restricted range of conditions and cannot account for the whole variety of primitive melt compositions. Most of them also fail in predicting accurately the role of water on fractionation processes. In this project, crystallization experiments are planned to constrain liquid lines of descent for six selected compositions, which cover the large compositional spectrum of primitive end-member tholeiites (e.g. MORBs derived from refractory and fertile mantle, N-MORB, E-MORB, high vs. low SiO2). The focus will be given to the role of pressure (50, 200, 500 and 700 MPa) and of melt water content (0.04 to 1 wt% H2O) on the composition of cotectic melts saturated with Olivine+Clinopyroxene+Plagioclase at different conditions (T,P,H2O). The experimental database will be used to understand the role of compositional differences in primitive tholeiites (along with thermodynamic variables) on the position of the Ol+Cpx+Pl cotectics projected on pseudoternary diagrams, a method which is widely used in geobarometric simulations. Apart from general implications, each investigated natural composition can be used to interpret the differentiation conditions at the corresponding MORB settings (e.g. Indian Ridge, Costa Rica Rift, East Pacific Rise) using available geochemical data already published.