COMPARISON OF SOLUTION ICP-MS AND LA-ICP-MS USING VERY FINE SANDSTONES AND SILTSTONES FROM THE MONTNEY FORMATION

Whole rock geochemical data obtained with solution Inductively Coupled Plasma Mass Spectrometry (ICP-MS) provides an excellent means of quantifying lithological chemical composition. However this method is not ideal because of the time and labour intensive preparation required to produce the sample solutions. In this study, we use laser ablation (LA) - ICP-MS to obtain whole rock geochemical values for siltstone and very fine sandstone from the Lower Triassic Montney Formation of western Canada. By averaging multiple spots measured via LA-ICP-MS, whole rock geochemical values can be obtained, thereby avoiding many of the issues associated with traditional methods. LA-ICPMS requires far less sample preparation and, as the integrity of the sample is maintained, provides less opportunity for sample contamination. Preliminary data analysis has shown this method to yield data in keeping with that of the solution ICP-MS.

Siltstone and very fine grained sandstone from the Montney Formation are composed primarily of quartz, potassium feldspar, plagioclase, micas and dolomite. For this study, 42 drill core fragments from two wells in northern British Columbia, Canada are analyzed. Traditional solution ICP-MS values are obtained in addition to average LA-ICP-MS values using an Elan 6000 Quadrupole Mass Spectrometer equipped with a Perkin-Elmer AS-91 sampling system and a UP213 laser ablation system. Sodium peroxide sintering digestion is used to produce the solutions. During laser ablation, point sizes of 150um and 75um are used with an average of 4-6 points per sample. Linear regression analysis is conducted on the results to yield correlation coefficients for individual elements as well as the data on a whole.

Preliminary findings have shown a strong linear relationship between the two data sets, suggesting that, by averaging multiple spots from LA-ICP-MS, whole rock geochemical data can be obtained. If this proves to be the case, solution ICP-MS may prove to be unnecessary to produce whole rock geochemical data. This would eliminate many problems, such as sample contamination, which makes solution ICP-MS less desirable. Geochemical data, crucial for correlation of fine-grained sedimentary successions such as the Montney Fm., is thus simpler to attain and more cost effective.