Diverse origin and processes in the formation of diamond and other exotic minerals in ophiolitic chromitites

Abstract

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. Finally, continental crust-derived minerals, such as zircon, likely represent xenocrystic material derived from the subducting slab that was entrapped by the chromitite parental melts. The investigation of the Neoproterozoic Bou Azzer chromitites (Morocco) allowed the comparison with the younger Cretaceous Cuban chromitites and other Phanerozoic and Precambrian chromitites. It was possible to determine that the Bou Azzer chromitites formed in the fore-arc setting during subduction-initiation with two stages of evolution: 1) formation of intermediate-Cr chromitites from fore-arc basalts (FAB), and 2) formation of high-Cr chromitites after boninitic melts. These chromitites host inclusions of platinum group minerals and exotic minerals. The unusual mineralogy includes oriented clinopyroxene lamellae, moissanite, native Cu, diaspore, and zircon formed after chromite crystallization and post-magmatic serpentinization processes, thus completely ruling out UHP conditions. Altogether, the results of these investigations represent the first descriptions of exotic minerals in the Cuban and Moroccan ophiolites. The corresponding discussions allowed to reject hypotheses of ultra-high pressure formation or recycling for the studied chromitites and the associated ophiolitic sequences and to ultimately provide simpler formation models for the exotic minerals. Additionally, the investigation provided definitive evidence for the formation of metastable nanodiamond, and other SuR phases, during serpentinization of the oceanic lithosphere at low-pressure and low-temperature conditions. Therefore, the presence of diamond in oceanic rocks, in particular chromitites, cannot be taken as a general indication of deep mantle recycling.

En aquesta tesi doctoral s’investiguen en detall les cromitites ofiolítiques de Cuba Oriental i de Bou Azzer, Marroc, per tal d’identificar la presència de minerals exòtics d’ultra-alta pressió, formats en condicions superreduïdes o derivats de l’escorça continental, i interpretar-ne la formació. Els resultats representen la primera descripció de minerals exòtics en aquestes ofiolites i les respectives discussions han permès descartar condicions d’ultra alta pressió per les cromitites i les roques associades i proposar models de formació simples relacionats amb la serpentinització de la litosfera oceànica pels minerals exòtics. S’ha investigat la interacció de líquids toleítics evolucionats amb cromitites preexistents a la zona de Potosí, Cuba, i es proposa un model de formació en dues etapes: primer la cristal·lització d’un magma de tipus MORB genera, mitjançant cristal·lització fraccionada, magmes residuals enriquits en elements incompatibles, i posteriorment alguns d’aquests magmes residuals s’escapen i reaccionen amb les cromitites. A les cromitites ofiolítiques de Cuba Oriental s’ha pogut identificar diamant, lamel·les orientades de clinopiroxè i de rútil, fases superreduïdes (carboni amorf, moissanita, Si i Cu natiu, aliatges de Fe-Mn) i minerals derivats de l’escorça continental. Els grans de diamant són de mida nanomètrica, es troben en inclusions fluides juntament amb metà, serpentina i magnetita. Totes les evidències indiquen que els nanodiamants es van formar durant la serpentinització en ambients superreduïts en condicions de baixa pressió i temperatura, igual que la resta de minerals superreduïts. En canvi, les lamel·les de clinopiroxè es van formar com a exsolucions degut a la cristal·lització de la cromita i les lamel·les de rútil es van formar per la interacció de les cromitites amb gabres. Finalment, el zircó podria representar material de la placa subduint atrapat pels magmes dels quals va cristal·litzar posteriorment la cromita. La investigació de les cromitites neoproterozoiques de Bou Azzer ha permès determinar la seva formació en una zona d’avant-arc durant l’inici de subducció, diferenciant dos estadis de formació relacionats amb basalts d’avant-arc (FAB) i boninites. Aquestes cromitites contenen inclusions de minerals del grup del platí i de minerals exòtics com són lamel·les de clinopiroxè, moissanita, Cu natiu, diàspora i zircó, similar a les cromitites de Cuba.