Analytical Suite Metadata

GSC OF 5117 compilation of volcanic rock analyses

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GSC Open File 5117 included compilations of lithogeochemical data as part of a larger compilation of geoscience data for the Cape Smith Belt.  The data were merged together from several different sources, some of which were previously unpublished.  A limited amount of metadata on analytical methods is provided in OF 5117, which indicates that the data were obtained over a period of several years, in several different laboratories, by different methods.  The compilation should be used with great caution, especially for the trace element data.  Information on detection limits was lost in the compilation process.  Values below detection are not identified as such.  The different methods would have had varying detection limits for the same quantities, but it is impossible to determine which values are actually below detection.  For example, for Picard’s data published in ET 87-14, values that were originally reported as below detection are reported in this compilation as simply the detection limit.  For example, “<5 ppm” is reported as “5 ppm”.  The data files contain many values of ‘0”.  It is not clear whether these represent missing data or values below detection.  They are treated here as “missing data”.  OF 5117 states that the measurement units for the trace elements are all in ppm, but for some quantities (Ag, Au, Pd, Pt), the measured values may be in ppb, or a combination of both ppm and ppb, depending on the data source.  In some cases, apparently high values for ultra-trace quantities may be reflecting the removal of the “<” symbol from a “below detection” measurement; for example < 15 ppm Au may become simply 15 ppm Au.  INAA data frequently have variable detection limits for the same element, further confusing the situation.

Fe is expressed in different formats.  Many analytical techniques cannot distinguish between Fe!sup!2+!/sup! and Fe!sup!3+!/sup!.  In XRF analysis, Fe is conventionally reported as Fe!sub!2!/sub!O!sub!3!/sub! (or Fe2O3T, for total).  Fe!sup!2+!/sup! can be measured by titration, and may then be reported as FeO.  This then allows one to calculate a “true” value for Fe!sup!3+!/sup!, or Fe!sub!2!/sub!O!sub!3!/sub! (or Fe2O3V, for vrai (true in French)).  Sometimes, Fe is reported as FeOT rather than Fe2O3T.  Note that the different conventions for reporting Fe will have an effect on the total sum of the major oxides.  Theoretically, this sum should be slightly less than 100.  But reporting Fe entirely as Fe!sub!2!/sub!O!sub!3!/sub! rather than a mix of Fe!sub!2!/sub!O!sub!3!/sub! and FeO may lead to sums of greater than 100.

Fe calculations

Using atomic weights of Fe = 55.845(2) and O = 15.9994(3), we obtain molecular weights for Fe!sub!2!/sub!O!sub!3!/sub! = 159.688 and 2(FeO) = 143.689.  This gives a conversion factor of 159.688 / 143.689 = 1.1113.

We can then use the expressions:

• Fe2O3T – FeO * 1.1113 = Fe2O3V
• Fe2O3T = FeOT * 1.1113