BLAISE PASCAL ON THE 40AR/39AR LASER SINGLE CRYSTAL DATING OF CONTAMINATED VOLCANIC ASHES

In cases where two or more grains must be used for an age determination, Pascal's Triangle can be used to predict the possible permutations and combinations of contaminant and primary grains. Older contaminating material is often entrained within newly erupted tephra during explosive volcanic eruptions by the force of the eruption. Incorporation of older materials also occurs secondarily from surficial contaminants picked up during eolian or fluvial redistribution of the tephra. Dating of these “contaminated” tephra has been problematic, although in certain cases, the contaminant materials can be identified through single grain ⁴⁰Ar/³⁹Ar analysis permitting identification of the eruptive population. For the most part, this type of “population approach” requires replicate ⁴⁰Ar/³⁹Ar analyses of single grain, high K minerals of sufficient size and age to yield > 1x10^-14 moles of radiogenic ⁴⁰Ar. Typically, low K phases have not yielded sufficient resolution to identify contaminants unless significant difference in ages exists.

When a single grain is analyzed, one measures either the primary grain A, or the contaminate grain B. These data, plotted on a frequency diagram, yield two nodes (row one of Pascal's Triangle (1 1). Ar volumes less than 1x10^-14 moles generally yield insufficient precision because of low signal to blank ratios and unpredictable isotopic evolution. The low gas volume problem is addressed by using multiple grains for the individual analyses. In these cases, when two grains are drawn and measured, the following combinations, AA, AB, BA, or BB (AB and BA are identical) are obtained. On a frequency diagram, these results yield three nodes, two small nodes on either side of a large node. Row two of Pascal's Triangle (1 2 1). Similarly, when three grains are measured, you are most likely to draw the following combinations: AAA, (AAB, BAA, ABA), (ABB, BAB, BBA), BBB, as predicted by row three of Pascal's Triangle (1 3 3 1). This pattern can be continued indefinitely. As a result, once the number of grains required to obtain a sufficient signal and/or the contaminants increases beyond two, the ability to determine the primary age of the tephra becomes extremely difficult.