Re-Os Isotope Chemistry and Geochronology of Chromite from Mantle Podiform Chromites from the Zunhua Ophiolitic Mélange Belt, N. China: Correlation with the Dongwanzi Ophiolite

This chapter presents Rhenium–Osmium (Re–Os) data on mantle podiform chromite samples collected from the Zunhua mélange and Dongwanzi ophiolite. The two belts are the same age, and have similar chemical characteristics, supporting their correlation. Rhenium and Osmium are siderophilic elements that concentrate in the Earth's core and mantle during planetary formation. Ophiolitic chromites have a strong resistance to secondary alteration, have high osmium concentrations, and low Re/Os ratios that limit age corrections and uncertainties. The Rhenium–Osmium isotope system was used as a method of dating samples of the Zunhua chromite bodies. The Re–Os concentration in chromites was measured in twelve samples from the Zunhua mélange and Dongwanzi ophiolite. These samples were chosen because of their extremely fresh appearance and their chromite composition. Chromite rich samples were crushed with an alumina and agate crushing apparatus and ground into a powder, and chromite grains under 250 μm were concentrated using a Frantz magnetic separator. The isotopic composition of Re and Os were measured using Mass spectrometry in the DTM lab in Washington, DC. For osmium, the samples were put in Pt filament with 0.5 HBr. Samples were first dried down with a heat lamp and kept loaded in the center of the filament and a nitrogen cold trap was loaded.