Major element mobility during serpentinization, oxidation and weathering of mantle peridotite at low temperatures.

Abstract

Mantle peridotite in Wadi Fins in eastern Oman exhibits three concentric alteration zones with oxide and sulfide mineralogy recording gradients in fO2 and fS2 (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 fO2 and moderate fS2. 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 fO2 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 fO2. Thermodynamic modelling at 60°C and 50 MPa (estimated alteration conditions) reproduces sulfide assemblages, fO2 changes and Mg and Fe mobility. This article is part of a discussion meeting issue 'Serpentinite in the Earth system'.