Chemical and oxygen isotopic properties of ordinary chondrites (H5, L6) from Oman: Signs of isotopic equilibrium during thermal metamorphism

1,2Arshad Ali,1Sobhi J. Nasir,2Iffat Jabeen,3Ahmed Al Rawas,2Neil R. Banerjee,2,4Gordon R. Osinski
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12910]
1Earth Sciences Research Centre, Sultan Qaboos University, Al-Khodh, Sultanate of Oman
2Department of Earth Sciences & Centre for Planetary Science and Exploration, Western University, London, Ontario, Canada
3Department of Physics, College of Science, Sultan Qaboos University, Al-Khodh, Sultanate of Oman
4Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

Mean bulk chemical data of recently found H5 and L6 ordinary chondrites from the deserts of Oman generally reflect isochemical features which are consistent with the progressive thermal metamorphism of a common, unequilibrated starting material. Relative differences in abundances range from 0.5–10% in REE (Eu = 14%), 6–13% in siderophile elements (Co = 48%), and >10% in lithophile elements (exceptions are Ba, Sr, Zr, Hf, U = >30%) between H5 and L6 groups. These differences may have accounted for variable temperature conditions during metamorphism on their parent bodies. The CI/Mg-normalized mean abundances of refractory lithophile elements (Al, Ca, Sm, Yb, Lu, V) show no resolvable differences between H5 and L6 suggesting that both groups have experienced the same fractionation. The REE diagram shows subtle enrichment in LREE with a flat HREE pattern. Furthermore, overall mean REE abundances are ~0.6 × CI with enriched La abundance (~0.9 × CI) in both groups. Precise oxygen isotope compositions demonstrate the attainment of isotopic equilibrium by progressive thermal metamorphism following a mass-dependent isotope fractionation trend. Both groups show a ~slope-1/2 line on a three-isotope plot with subtle negative deviation in ∆17O associated with δ18O enrichment relative to δ17O. These deviations are interpreted as the result of liberation of water from phyllosilicates and evaporation of a fraction of the water during thermal metamorphism. The resultant isotope fractionations caused by the water loss are analogous to those occurring between silicate melt and gas phase during CAI and chondrule formation in chondrites and are controlled by cooling rates and exchange efficiency.

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