Journal of Marine Science: Research & Development
Open Access

Abstract

The structural M-value of antigorite was used as an indicator of the metamorphic grade which ranges from greenschist facies to almandine-amphibolite facies in the Pan-African serpentinites of Egypt. Antigoritization during progressive regional metamorphism yields M-value that is much higher than 8.52 (i.e. tempearture range of 220�300oC). Antigorite was recrystallized in the contact aureoles due to the emplacement of post-orogenic granites in the at T�400-450oC. Near the contact with intrusive granites, M-value of antigorite was much higher than in samples away from the contact (6.48 and 8.52, respectively). Two types of temperature dependant substitutions can be traced in thermal antigorite, on a Tschermak�s (Al3+ and Cr3+ for Si4+ in the tetrahedral sites) and another non-Tschermak�s (2R3++(iv)=3R2++(iv)). Accordingly, antigorite can be used as a qualitative to semi-quantitative geothermometer in regionally metamorphosed ultramafics and those that practice superimposed thermal effects. Ortho- and clinochrysotiles were common with MgO contents of 41-41.13 wt % and 39.38-40.93 wt%, respectively. High-Al and low-Al lizardites can co-exist and chemically distinguishable where total iron (~0.24 cations) as Fe2+ (iv) is constant with variable Mg2+ suggesting that significant R3+ in the octahedral sites which were mostly occupied by Al3+ and Cr3+ and not iron. The negative correlation of Al2O3 with SiO2 was a Tschermak�s substitution of S4+ by R3+ (particularly Al3+) in the tetrahedral sites and such results indicates that Fe3+ is necessary to crystallize lizardite. Some HRTEM images distinguished antigorite from chrysotile and suggested the presence of polyhedral serpentine (spherical with alternating sectors of lizardite). Crack-seal microstructures displayed by chrysotile and brucite veins that were developed by extensional shearing during obduction that superimposed antigoritization viz. of ocean-floor metamorphism. Finally, the current work presents useful discriminating diagrams for serpentine minerals in metamorphic terrains (regional and thermal) that can be applied to any serpentinite worldwide.

Biography

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