The K/Ar radiometric dating technique is based on the radiogenic decay of 40K to 40Ar with the ratio of 40Ar/40K being proportional to the age of a given sample (Dalrymple and Lanphere, 1969). The 40Ar/39Ar dating technique is a variation on this method and involves the irradiation of a sample resulting in the conversion of 39K to 39Ar (Mitchell, 1969). Since the ratio of 39K/40K is constant, the pre-irradiation concentration of 40K in a sample can be determined by measuring the 39Ar which is produced during irradiation.

The K/Ar clock is readily reset by heating through diffusive loss of 40Ar from the mineral. This diffusive loss lowers the 40Ar/40K ratio and results in a lower calculated age for the sample. For most minerals significant argon loss does not occur below 200 degrees C. However, for the potassium feldspar microcline, significant argon loss begins at temperatures as low as 110 degrees C, with argon loss nearly complete by about 180 degrees C (Harrison and MacDougall, 1982). These temperatures are within the range of temperatures corresponding to hydrocarbon maturation and generation.

The rate of argon diffusion from microcline is an Arrenhius function of the form

D = Do exp(-E/RT),

where D is the diffusion coefficient of argon, Do is a constant which is specific to a given sample and includes a time term, E is the activation energy, R is the gas constant, and T is the absolute temperature (Harrison and McDougall, 1982).

The closure temperature for a microcline sample can be calculated from

Tc + E/R/ln (A R Tc2 D0/a2 EdT/dt),

where Tc is the closure temperature, E is the activation energy, Do/a2 is the frequency factor, R is the gas constant, A is the geometry factor (8.7 for a plane sheet model) and dT/dt is the cooling rate. Arrhenius diagrams were plotted with the data points calculated from the %39Ar release and the duration of the heating increment. The activation energy E is proportional to the slope of the line and the y-intercept is equal to the frequency factor (Do/a2).

From these equations it can be seen that the amount of Ar loss from microcline is a function of the maximum temperature and duration of a given thermal event. Thus the maximum temperature and duration of thermal events capable of hydrocarbon generation can be determined through the 40Ar/39Ar age spectrum technique.

For this evaluation, Western Cordillera collected 9 surface samples and obtained cuttings from various intervals in 12 wells drilled within the evaluation area. Both surface and well samples were selected to show a comparison between sections in ranges and in valleys with several thousand feet of Cenozoic basin fill. Microcline mineral separations using heavy liquid and magnetic separation techniques were performed on all 21 samples listed in Appendix VIII.

Only 4 of the surface samples and 9 of the well intervals contained enough microcline for further analysis. The units sampled included Tertiary and Permian sediments, and the Mississippian Chainman, Silurian Elder and Ordovician Valmy Formations. Other formations were examined in the field but did not contain enough microcline on visual inspection to warrant mineral separation.

Analyses were performed by Matt Heizler and Mark Harrison at the State University of New York (SUNY) at Stony Brook. The analytical method employed included ultrasonically cleaning K-feldspar separates in 10 percent HF acid to remove surface contamination. Separates sized between 0.600 and 0.105 mm in diameter and weighing between 100 and 200 mg were irradiated for 80 hours in a fast neutron flux in the core of the Ford reactor, Phoenix Memorial Laboratory, University of Michigan, along with the inter-laboratory standard Fe-mica biotite. This mica has a K-Ar age of 307.3 Ma using the decay constants and isotopic abundances recommended by Steiger and Jager(1977).

The samples were placed in flat-bottomed, evacuated, 100 mm long quartz tubes which were filled to about a 50 mm height with alternating unknowns and monitors. The quartz vials were positioned in the reactor in such a way as to ensure that the middle region of the samples was coincident with maximum flux. This serves to minimize the flux gradient across the 50 mm length of the samples to about 5 percent. Correction factors were used for interfering isotopes produced by nuclear reactions and K and Ca.

Argon extractions were performed in a resistance heated, double-vacuum extraction furnace. Integral temperature control by a solid state controller and thyristor device allows precision temperature monitoring of +/- 0.5 degrees C, more than is sufficient for kinetic studies. The blank imparted by this furnace and the clean up system, consisting of an ion-pump and a SEAS gutter pump, averages about 5 x 10 exp-15 mol 40Ar at temperatures below 1200 degrees C.

Argon was extracted from each sample in a series of incrementally increasing temperature steps, beginning at 400 degrees C and ending at 1450 degrees C. The particular heating steps utilized were tailored for this set of samples by examining the amount of total percentage of argon gas release with given temperature steps. The evolved argon gas extracted during each step was isotopically analyzed using a Nuclide 4.5-60-RSS instrument with automated, digital data acquisition. This analysis provided the radiometric age for each temperature increment which are plotted in the data tables. Uncertainties in the derived 40Ar/39 Ar ages include the precision of the isotope ratio measurements in both the sample and flux monitor, but does not include systematic uncertainty of the age of the flux monitor of about 0.5 percent.