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

在大洋中脊（MORB -大洋中脊玄武岩）、与地幔柱有关的大洋高原（大火成岩省，LIP）和一些海洋岛屿（如夏威夷）和海山中，托利玄武岩是玄武岩浆的主要类型。大洋岩浆岩的地球化学研究表明，存在多种富镁玻璃成分，代表了多种幔源原始岩浆。由于原始岩浆的多样性，由于地幔矿物学和地幔熔融程度的不同，以及可能的结晶条件（P、fO 2、挥发分）和分异机制（结晶、岩浆混合、同化等）的不同，在大洋地壳中收集到的演化玄武岩的成分范围很广。与在非常高的压力（> 1 GPa）和1个大气压和干燥条件下的大型实验数据集相比，在岩浆分异和从其来源到洋底的迁移过程中占主导地位的地质相关条件（压力从50到700 MPa，流体欠饱和系统）仍然没有得到充分的研究。50-700 MPa的压力范围特别重要，因为它涵盖了快速扩张脊中的浅轴向岩浆房和缓慢扩张脊下方地幔中的深部储层之间的范围，或在LIPs的幔壳过渡处。因此，可用的热力学模型预测分化路径校准的条件范围有限，不能占原始熔体组合物的整个品种。他们中的大多数也未能准确地预测水的分馏过程中的作用。在这个项目中，结晶实验计划限制液体下降线为六个选定的组合物，其中包括原始端元拉斑玄武岩的大组成谱（例如，MORB来自耐火和肥沃的地幔，N-MORB，E-MORB，高与低SiO2）。重点将给出的作用压力（50，200，500和700 MPa）和熔体水含量（0.04至1重量% H2O）的组成上的coalites熔体饱和橄榄石+单斜辉石+斜长石在不同的条件下（T，P，H2O）。实验数据库将被用来了解原始拉斑玄武岩（沿着与热力学变量）的位置上的Ol+Cpx+Pl cotectics投影在伪三元图，一种方法，这是广泛使用的地质测压模拟的组成差异的作用。除了一般的影响，每一个调查的自然组成可以用来解释在相应的MORB设置（例如，印度海脊，哥斯达黎加裂谷，东太平洋隆起）使用现有的地球化学数据已经公布的分异条件。

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.