Enrichment and transport of Cr in magmatic systems in the mantle and crust

铬是一种关键金属，对欧盟经济具有很高的重要性，也是可持续供应的高风险。铬的开采有四种不同的矿床类型：层状、豆荚状、砂矿和红土型铬铁矿矿物，但最重要的类型只有层状和豆荚状铬矿。只有与镁铁质-超镁铁质岩石密切相关的豆荚状铬矿床存在，目前在欧盟开采。铬矿床与地幔岩石的密切共生关系决定了铬铁矿矿床的形成必须在地球上地幔中寻找铬的来源。用铬铁矿饱和岩浆所需的极低浓度的铬表明，在地幔熔融和岩浆结晶过程中，似乎有限的提取过程似乎足以产生豆荚状铬铁矿矿床。然而，尽管有大量的研究和各种机制提出了导致铬的富集化，但对于一个特定的模式来完全解释豆荚状铬铁矿矿床的形成缺乏普遍的共识。在我们的研究中，我们的目的是了解从地幔到矿床的岩浆系统中导致铬富集化的过程。为了实现这一目标，我们提出了一种实验研究和模拟相结合的方法，侧重于铬的富集化的三个阶段：1)地幔熔融；2)熔体演化；3)熔体运动、固态变形和流体活化。在这个项目中，我们将重点放在前两个步骤上，并进行实验研究，揭示和量化不同物理化学参数在地幔和地壳条件下的贡献：源岩的化学、地幔熔融的程度、熔融过程中铬在不同地幔和岩浆相(橄榄石、斜辉石、单斜辉石和尖晶石)中的特殊分布。将特别注意水和氧化还原条件的作用，这些条件与豆荚状铬矿床所在的蛇绿杂岩的超俯冲构造环境有关。所获得的实验结果将被用来校准热力学程序，并建立自然体系中铬矿富集的一般模型。

Chromium is a critical metal with a high importance to the economy of the EU and high rick of sustainable supply. Chromium has been mined from four different deposit types: stratiform, podiform, placer, and laterite deposits in the form of chromite mineral but only stratiform and podiform chromite deposits represent the most significant types. Only podiform Cr deposits, intimately associated with mafic-ultramafic rocks, are present and currently mined in the EU. The close association of Cr deposits with mantle rocks dictates that the source of chromium in the formation of chromite deposits must be sought in the Earth’s upper mantle. The very low concentrations of Cr needed to saturate magmas with chromite indicate that seemingly limited extraction processes both during mantle melting and during magma crystallization appear to be sufficient to generate podiform chromite deposits. However, despite numerous studies and a variety of proposed mechanisms leading to Cr enrichment, there is a lack of general consensus on a particular model which can fully explain the formation of podiform chromite deposits.In our research we aim at understanding the processes leading to the Cr enrichment in magmatic systems from the mantle to the ore deposit. To achieve this goal we propose an experimental investigation combined with a modelling approach focusing on three stages of Cr-enrichment: 1) Mantle melting; 2) Melt evolution; and 3) Melt movement, solid-state deformation and fluid mobilization. In this project we will focus on the two first steps and conduct experimental study unrevealing and quantifying the contributions of different physico-chemical parameters at mantle and crustal conditions: the chemistry of the source rocks, the degree of mantle melting, the particular distribution of chromium among the different mantle and magmatic phases (olivine, orthopyroxene, clinopyroxene and spinel) during melting and consequent crystallization. Particular attention will be paid to the role of water and redox conditions relevant for the supra-subduction tectonic environment of the ophiolitic complexes hosting podiform Cr ore deposits. The obtained experimental results will be used to calibrate thermodynamic codes and to develop a general model of chromite enrichment in natural systems.