Major element mobility during serpentinization, oxidation and weathering of mantle peridotite at low temperatures. (21st February 2020)

• Record Type: Journal Article
• Authors: de Obeso, Juan Carlos; Kelemen, Peter B.
• Is Part Of: Philosophical transactions. Volume 378:Number 2165(2020)
• Publication Date: 2020-02-21
• Subjects: serpentinization -- thermodynamic modelling -- Oman -- ophiolite; Physical sciences -- Periodicals; Engineering -- Periodicals; Mathematics -- Periodicals; 500
• DOI: 10.1098/rsta.2018.0433
• Languages: English
• ISSNs: 1364-503X

Abstract

Abstract : Mantle peridotite in Wadi Fins in eastern Oman exhibits three concentric alteration zones with oxide and sulfide mineralogy recording gradients in f O2 and f S2 (fugacity) of more than 20 orders of magnitude over 15–20 cm. The black cores of samples (approx. 5 cm in diameter) exhibit incomplete, nearly isochemical serpentinization, with relict primary mantle minerals (olivine, orthopyroxene and clinopyroxene) along with sulfide assemblages (pentlandite/heazlewoodite/bornite) recording low f O2 and moderate f S2 . In addition to the black cores, two alteration zones are evident from their colouration in outcrop and hand samples: green and red. These zones exhibit non-isochemical alteration characterized by intergrowths of stevensite/lizardite. All three reaction zones are cut by calcite ± serpentine veins, which are most abundant in the outer, red zones, sometimes are flanked by narrow red and/or green zones where they cut the black zones, and thus may be approximately coeval with all three alteration zones. The alteration zones record progressively higher f O2 recorded by Ni-rich sulfides and iron oxides/hydroxides. These alteration zones lost 20–30% of their initial magnesium content, together with mobilization of iron over short distances from inner green zones into outer red zones, where iron is reprecipitated in goethite intermixed with silicates due to higher f O2 . Thermodynamic modelling at 60°C and 50 MPa (estimated alteration conditions) reproduces sulfide assemblages, f O2 changes and Mg and Fe mobility. This article is part of a discussion meeting issue 'Serpentinite in the Earth system'.