Diverse origin and processes in the formation of diamond and other exotic minerals in ophiolitic chromitites
[eng] The discovery of ultra-high pressure (UHP), super-reduced (SuR), and continental crust-derived minerals in ophiolitic chromitites worldwide challenged the previous hypotheses for chromitite formation and motivated the proposal of complex geodynamic models involving UHP conditions of formation...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2021 |
| País: | España |
| Institución: | Consejo Superior de Investigaciones Científicas (CSIC) |
| Repositorio: | DIGITAL.CSIC. Repositorio Institucional del CSIC |
| OAI Identifier: | oai:digital.csic.es:10261/269418 |
| Acceso en línea: | http://hdl.handle.net/10261/269418 |
| Access Level: | acceso abierto |
| Palabra clave: | Mineralogía Jaciments minerals Diamants Mantell terrestre Mineralogy Mineral deposits Diamonds Mantle of the earth |
| Sumario: | [eng] The discovery of ultra-high pressure (UHP), super-reduced (SuR), and continental crust-derived minerals in ophiolitic chromitites worldwide challenged the previous hypotheses for chromitite formation and motivated the proposal of complex geodynamic models involving UHP conditions of formation for the chromite or recycling of the ophiolitic rocks down to the Mantle Transition Zone (410 – 660 km depth). This thesis investigates in detail the chromitites and the associated ophiolitic rocks from the Moa-Baracoa massif, eastern Cuba, and Bou Azzer, Morocco, with emphasis on identifying the presence of exotic minerals, interpreting their formation, and understanding the related geodynamic processes. The interaction of preexisting chromitites with evolved tholeiitic melts that triggered Fe-Ti-Zr metasomatism in the Potosí chromitites (eastern Cuba) is carefully investigated here. There is evidence of coarse granular textures and strongly variable mineralogy composed of recrystallized/neoformed Fe3+-Ti-rich chromite, Ti-rich amphibole, clinopyroxene, orthopyroxene, olivine, Mg-rich ilmenite, Zr-bearing oxides and silicates, and Fe-Cu-Ni sulfides. Thorough petrographic analysis coupled with the chemical composition of the mineral phases allowed to determine the sequence of mineral formation and to propose a two-stage genetic model: 1) the crystallization of an evolved MORB liquid, via fractional crystallization, produced water-rich residual melts strongly enriched in incompatible elements, and 2) batches of this evolved melt escaped and extensively reacted and metasomatized the surrounding chromitites. U/Pb dating of baddeleyite indicates that the age of the metasomatic event (134.4 ± 14 Ma) matches the age of magmatism related to ophiolite construction in eastern Cuba. The high-Al chromitites of eastern Cuba contain exotic minerals of the three categories: diamond, oriented clinopyroxene and rutile lamellae, graphite-like amorphous carbon, moissanite, native Cu and Si, Fe-Mn alloys, zircon, corundum, and quartz. The studied diamond grains are nanometric in size, located in CH4-bearing fluid inclusions within olivine, and systematically associated with a typical serpentinization mineral assemblage composed of serpentine and magnetite. Textural-mineralogical evidences indicate that nanodiamond formed during low-pressure and low-temperature serpentinization in super-reduced microenvironments (log[fO2;MPa]=−45.3; ΔlogfO2[Iron-Magnetite]=−6.5). The other SuR minerals (moissanite, graphite-like amorphous carbon, native elements, and alloys) also formed in super-reduced CH4-rich microenvironments. On the other hand, nominally UHP phases such as the clinopyroxene lamellae formed as exsolutions after the crystallization of chromite or can represent silicate melt inclusions attached to the growing chromite grains. Oriented rutile lamellae formed after the interaction of chromitites with mafic magma. |
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