A new view on the petrogenesis of the Oman ophiolite chromitites from microanalyses of chromite-hosted inclusions

摘要: To decipher the petrogenesis of chromitites from Moho Transition Zone Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and microprobe investigations ∼500 silicate chromite inclusions their hosts, oxygen isotope measurements seven olivine fractions nodular, disseminated, stratiform ore bodies associated host dunites Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase microcrystal types chromite-hosted to be distinguished. are composed micron-size (1–50 μm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr + Al) up 80] chromite. coronas reduced (oxygen fugacity, fO2, ∼3 log units below quartz–fayalite–magnetite buffer, QFM). chromites, which crystallized between 600 950°C at subsolidus conditions, were overgrown by more oxidized (fO2 ≈ QFM) association (plagioclase An86, clinopyroxene, pargasite) that formed 950 1050°C 200 MPa a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through coronas, inclusions, chromites indicate non-equilibrium fractional crystallization chromitite system fast cooling rates (up ∼0·1°C a−1). Oxygen compositions grains imply involvement mantle protolith (e.g. serpentinite serpentinized peridotite) altered seawater-derived hydrothermal fluids an oceanic setting. Our findings consistent three-stage model formation involving (1) alteration yielding serpentinites harzburgites, probably initial source chromium, (2) owing prograde metamorphism, followed (3) assimilation water-saturated MORB. This study suggests metamorphic occurring level may dramatically affect MORB magma chemistry lead economic chromium deposits.