In situ peridotitic diamond in Indus ophiolite sourced from hydrocarbon fluids in the mantle transition zone

In recent years ophiolitic diamonds have been reported mostly from podiform chromitites. However, the mechanism of such diamond formation remains unknown. We report in situ diamond, graphite pseudomorphs after diamond crystals, and hydrocarbon (C-H) and hydrogen (H2) fluid inclusions in ultrahigh-pr...

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Bibliographic Details
Published in:Geology (Boulder) 2017-08, Vol.45 (8), p.755-758
Main Authors: Das, S, Basu, A. R, Mukherjee, B. K
Format: Article
Language:English
Subjects:
aliphatic hydrocarbons
alkanes
Asia
Crystals
diamond
Diamonds
Earth mantle
fluid flow
Fluid inclusions
Fluids
Geology
Geophysics
Graphite
Haematite
Hematite
Hydrocarbons
Hydrogen
igneous rocks
inclusions
Indian Peninsula
Indus-Yarlung Zangbo suture zone
Jammu and Kashmir
Ladakh
mantle
Methane
mineral composition
Minerals
native elements
nesosilicates
Nidar Ophiolite
Ocean circulation
Oceans
Olivine
olivine group
Ophiolites
organic compounds
orthosilicates
oxides
Paleoceanography
Peridotite
peridotites
plutonic rocks
pressure
Raman spectra
related to mantle
silicates
spectra
Spreading centres
Transition zone
ultrahigh pressure
ultramafics
Upper mantle
Upwelling
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Summary:In recent years ophiolitic diamonds have been reported mostly from podiform chromitites. However, the mechanism of such diamond formation remains unknown. We report in situ diamond, graphite pseudomorphs after diamond crystals, and hydrocarbon (C-H) and hydrogen (H2) fluid inclusions in ultrahigh-pressure (UHP) peridotitic minerals of the Nidar ophiolite, Indus suture zone. Diamond occurs as octahedral inclusion along with nitrogen (N2) in orthoenstatite. Methane (CH4) also occurs with UHP clinoenstatite (>8 GPa) in orthoenstatite. The graphite pseudomorphs after diamond crystals and primary hydrocarbon (C-H), and hydrogen (H2) fluids are included in olivine. Oriented hematite (α-Fe2O3) exsolutions are also present in the olivines, indicating a precursory β-Mg2SiO4 phase of the host olivines. This assemblage of diamond, graphite, C-H and H2 has not previously been reported from any ophiolitic peridotite. The hydrocarbon fluids in UHP clinoenstatites and retrogressed β-Mg2SiO4 strongly suggest their source from the mantle transition zone or base of the upper mantle. We conclude that the peridotitic diamonds precipitated from C-H fluids during mantle upwelling beneath the Neo-Tethys Ocean spreading center.
ISSN:0091-7613
1943-2682
DOI:10.1130/G39100.1