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Past And Present Serpentinisation of Ultramafic Rocks; An Example from the Semail Ophiolite Nappe of Northern Oman

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The Chemistry of Weathering

Part of the book series: Nato ASI Series ((ASIC,volume 149))

Abstract

Spring waters emerging from partly serpentinised ultramafic rocks in the Semail Ophiolite Nappe of Northern Oman are of a calcium hydroxide type. They have pH values of 10 to 12, are unusually saline and possess anomalous stable hydrogen and oxygen isotopic compositions. The springs, which occur mainly at the contact between “mantle” and “crustal” rock divisions, have low discharge rates and originate by chemical evolution from the recharge of magnesium bicarbonate surface waters. Rare, and in some cases, previously unidentified features associated with these springs include; deuterium depleted hydrogen and nitrogen gases, suolunite, magnesian huntite, brucite, portlandite and amorphous calcium hydroxide. Spring water compositions, petrographic features, and variations in the chemical compositions of the ultramafic rock secondary assemblage, provide strong evidence for the occurrence of both low and high temperature serpentisation processes. These low temperature reactions take place under relatively arid hydrological conditions where groundwater flow is restricted, atmospheric CO2 and O2 are completely removed from solution and the normal surface weathering processes are thusinhibited. The unusual isotopic composition of the waters is explained in terms of isotopic exchange between surface waters and hydrous minerals in the groundwater environment related to the low water/rock ratio. The high salt concentrations in the spring waters are derived from the leaching of sea salts introduced during the emplacement of the nappe. Whilst the low temperature serpentinisation processes control the chemical characteristics of the spring waters, it is concluded that the earlier pervasive phase of partial serpentinisation occured at higher temperatures (up to 350°C) whe the ophiolite was obducted from an obducted from an oceanic to an epicontinental environment.

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© 1985 D. Reidel Publishing Company

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Neal, C., Stanger, G. (1985). Past And Present Serpentinisation of Ultramafic Rocks; An Example from the Semail Ophiolite Nappe of Northern Oman. In: Drever, J.I. (eds) The Chemistry of Weathering. Nato ASI Series, vol 149. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-5333-8_15

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  • DOI: https://doi.org/10.1007/978-94-009-5333-8_15

  • Publisher Name: Springer, Dordrecht

  • Print ISBN: 978-94-010-8855-8

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