Direct radiometric dating of bitumen using Sm-Nd isotopes

The Sm-Nd geochronology has great potential to directly constrain the age of hydrocarbon generation, migration, and charging; however, this technique has been rarely applied and poorly understood in the context of timing of petroleum system formation and evolution. In this contribution, we carried out a comprehensive study of the Sm-Nd isotope, mineralogical characteristics, and Raman spec-troscopy of bitumen from the Erdaokan Ag-Pb-Zn hydrothermal deposit located in the arc basin of the northeastern Great Xing'an Range, northeastern China. Bitumen in the orebodies of the Erdaokan Ag-Pb-Zn deposit was formed coevally with the Ag-Pb-Zn mineralization and generally coexisted with galena, sphalerite, pyrargyrite, argentite, and pyrite mainly in the quartz veins. The Sm-Nd dating of the bitumen samples yielded an isochron age of 235.50 +/- 2.18 Ma (uncertainty given as two standard deviations of the mean, n = 8, mean square weighted deviation = 0.095), which is consistent with the ages that were constrained by the pyrite-galena Rb-Sr technique (232.9 +/- 2.3 Ma) and wall-rock zircon U-Pb dating (234.2 +/- 2.8 Ma). Combining the formation temperature (170 degrees C-200 degrees C) and bitumen reflectance (0.72-0.87) estimated by the Raman spectrum parameter, we consider that the bitumen is similar to primary-oil solid bitumen, and the thermal cracking of the oil-prone organic matter from the source rocks during ore mineralization results in the formation of the bitumen. Therefore, the bitumen Sm-Nd age reflects the primary thermal-cracking time of oil-prone organic matter from the source rocks as well as the ore-formation age of the Erdaokan Ag-Pb-Zn deposit. Based on these findings, we strongly recommend the application of Sm-Nd isotope dating on hydrocarbon systems as well as hydrocarbon-bearing hydrothermal metal deposits.