The role of chlorine in serpentinization |
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| Authors: | J C Rucklidge G C Patterson |
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| Institution: | (1) Department of Geology, University of Toronto, Toronto, Ontario, Canada |
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| Abstract: | Chlorine present in altered ultramafic rocks resides in a compound which is readily soluble in neutral and acidic aqueous solution. Iron is also dissolved by the same solutions giving Fe Cl = 21. Statistical treatment of multiple microprobe analyses shows a positive correlation between Cl and Fe, and negative correlations between Cl and Mg, and Cl and Si. These, and other data, suggest that Cl is present in the form of submicroscopic grains of a compound for which a likely formula is Fe2(OH)3Cl, and which is dispersed through the regions of the rock suffering active alteration. Elsewhere, where alteration is complete the chloride phase is absent.A mechanism is proposed whereby initially acid chlorine bearing solutions become alkaline in bringing about the conversion of olivine to serpentine and at the same time transfer Fe into Fe2(OH)3Cl which precipitates in the reaction zone. Fresher, acid, solutions, following as the reaction front progresses, dissolve the Fe2(OH)3Cl phase, providing a possibility for long distance transport of Fe and recycling of Cl. pH gradients near the reaction zone may be matched by fO2 gradients which may bring about the formation of magnetite in the centres of veins between the olivine crystals. All evidence of Cl will be removed when the process is complete, and the serpentine which forms will usually have Fe/Mg greater than the original olivine. |
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