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

铬是一种重要的金属，对欧盟的经济和可持续供应具有高度的重要性。铬的矿床类型有四种：层状、足状、砂矿和红土，以铬铁矿矿物的形式存在，但层状和足状铬铁矿是最重要的矿床类型。只有与基性-超基性岩石密切相关的足状铬矿床存在，目前在欧盟开采。铬矿床与地幔岩石的密切联系决定了铬铁矿形成过程中铬的来源必须在地球的上地幔中寻找。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足以产生足状铬铁矿矿床。然而，尽管有大量的研究和各种导致Cr富集的机制，但对于一个可以充分解释足状铬铁矿矿床形成的特定模型缺乏普遍共识。我们的研究旨在了解岩浆系统中从地幔到矿床的Cr富集过程。为了实现这一目标，我们提出了一项结合建模方法的实验研究，重点关注三个阶段的cr富集：1)地幔熔融；2)熔体演化；熔体运动、固态变形和流体动员。在本项目中，我们将重点关注前两个步骤，并进行实验研究，以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：烃源岩的化学性质，地幔熔融程度，铬在不同地幔和岩浆阶段（橄榄石、正辉石、斜辉石和尖晶石）中熔化和结晶过程中的特殊分布。将特别关注水和氧化还原条件对蛇绿岩杂岩承载足状铬矿床的超俯冲构造环境的作用。获得的实验结果将用于校准热力学代码和开发自然系统中铬铁矿富集的一般模型。

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.