Chemical, thermal and infra-red characterization of chrysotile modes from the Wadi Daftah serpentinite (Semail ophiolite), United Arab Emirates

Abstract

Abstract Serpentine minerals, either polymorphs or polytypes (mostly chrysotile, lizardite and antigorite), are hardly distinguished in ophiolitic serpentinites and serpentinized peridotite unless sophisticated mineralogical tools are applied. Among them, chrysotile has special chemical and structural characteristics, which can be used to identify and classify different modes, and accordingly as significant petrographic indicators of serpentinization in ophiolite sequences. Generally, chrysotile characterizes low-grade metamorphism and it occurs in massive serpentinites as well as asbestos veinlets traversing them. The present paper aims to characterize the different modes of chrysotile in serpenitinite and serpentinized peridotite from the Semail ophiolite at the Daftah area in the United Arab Emirates (UAE). The objectives include microscopic and chemical separation of three chrysotile modes; namely slip-fiber (most common), mass fiber (common) and cross-fiber (less common). The cross-fiber mode is considered a “spinning type” in sheared serpentinite with asbestos veinlets whereas the other two belong to “non-spinning type” in massive serpentinite being formed by a contact volume metasomatic process by replacement of olivine and pyroxene. Excess Mg from the transformation of ferromagnesian minerals into chrysotile, in the presence of additional SiO2, results in an association of cryptocrystalline talc with the chrysotile fibers. Differential thermal analysis (DTA) and thermogravimetry (TG) indicate that the process of thermal decomposition of chrysotile leads to the formation of olivine (forsterite). The infra-red (IR) absorption technique for end products of some selected samples appear to indicate that the spectra of the end products are similar and the absorption bands are assigned in the range of (1100–800 cm−1) and (700–200 cm−1) corresponding to Si–O and Mg–O vibration modes. Based on the findings of the present paper, the chrysotile serpentinization process is due to metasomatic replacement of peridotite during the ocean floor early episode and later an episode of ophiolite obduction. The chemical data focalize that the similarity in chemical behaviour of chrysotile veins implies that a close cogenetic relationship among three modes of chrysotile and removal of such major elements occurred during serpentinization; Mg, Fe and Si were removal in solution from the peridotite and precipitated in tension and shear fractures to form fibrous chrysotile and magnetite. Also, it can be concluded that the mode of occurrence and timing of chrysotile vein formation can be fitted according to the type of chrysotile mode. Cross-fiber type develops along tension fractures whereas mass-fiber type is finally developed via CO2 metasomatism. Finally, the slip-fiber type forms by a volume-for volume metasomatic process due to the replacement of olivine and/or pyroxene. In terms of time and tectonism, the different mode witnessed different metamorphic/metasomatic events and structures. Shear deformation is more likely connected to the formation of cross-fibers in NW- and NE-trending asbestos veins and veinlets.

Keywords

• Chrysotile
• Three fiber modes
• Thermal analysis
• IR spectra
• EMPA
• Semail ophiolite