Determination of rare earth elements in black shales by inductively coupled plasma mass spectrometry

We report on the specific occurrence of rare-earth elements (REE) in carbonaceous black shales as determined by inductively coupled plasma mass spectrometry (ICP-MS). Black shales are categorized according to their formation: (i) biogenic (samples SCHS-1 and SLg-1 certified at the Institute of Geochemistry SB RAS); and (ii) bituminous (USGS standard reference samples SDO-1 and SGR-1). Comparison of ICP-MS results obtained with control samples and certified reference materials enabled us to reveal the dependence of error in results on the technique of sample decomposition: (a) open vessel decomposition; (b) autoclave closed system digestion; (c) alkali fusion. It was shown that for SCHS-1 and SLg-1, open vessel and autoclave digestion gave significantly lower results for the heavy REE (from Gd to Lu) with only alkali fusion giving accurate results. For SDO-1 and SGR-1, correct values were obtained for all sample decomposition techniques with the exception of slightly lower results for the heavy REE for SDO-1 using open vessel digestion.