Here is a nice summary of the Ar-Ar analyses that surpasses anything that I was putting together. I will follow-up with Terry Spell with a discussion about the strat context of the samples.
Nevada Isotope Geochronology Laboratory - Sample Descriptions – House - NBMG
General Comments: Your samples were run as conventional furnace step heating analyses. This type of sample run produces what is referred to as an apparent age spectrum. The "apparent" derives from the fact that ages on an age spectrum plot are calculated assuming that the non-radiogenic argon (often referred to as trapped, or initial argon) is atmospheric in isotopic composition (40Ar/36Ar = 295.5). If there is excess argon in the sample (40Ar/36Ar > 295.5) then these ages will be older than the actual age of the sample. U-shaped age spectra are commonly associated with excess argon, and this is often verified by isochron analysis, which utilizes the analytical data generated during the step heating run, but makes no assumption regarding the composition of the non-radiogenic argon. Thus, isochrons can verify (or rule out) excess argon, and isochron ages are usually preferred if a statistically valid regression is obtained (as evidenced by an acceptably low MSWD value). If such a sample (U-shaped, or more generally discordant) yields no reliable isochron, the most conservative estimate of the age is that the minimum on the age spectrum is a maximum age for the sample (it could be affected by excess argon, the extent depending on the radiogenic yield). 40Ar/39Ar total gas ages are equivalent to K/Ar ages. Plateau ages are sometimes found, these are simply a segment of the age spectrum which consists of 3 or more steps, comprising >50% of the total gas released, which overlap in age at the ±2σ analytical uncertainty level. Such ages are preferred to total gas or maximum ages if obtained. However, in general an isochron age is the best estimate of the age of a sample, even if a plateau age is obtained.
OWY-36 Basalt Groundmass
The age spectrum for this sample is discordant, with both positive and negative ages which overlap 0 within uncertainties, to ages as high as ~660 ka. The total gas age is 194 ± 27 ka, and is equivalent to a conventional K-Ar age. No plateau age or isochron age was defined by these data. This sample had very low, often negative radiogenic argon (%40Ar*) concentrations (i.e. there was no measurable 40Ar* in two of the steps), likely reflecting both low-K contents and young age. In a case such as this there are two possible interpretations. The first is that the sample contains no excess argon and the total gas age is a reasonable estimate. Unfortunately, with no isochron the presence, or absence, of excess argon cannot be confirmed, making this interpretation somewhat tenuous. The most conservative approach is to assume that the discordance is a result of excess argon, and thus the minimum age on the age spectrum is a maximum age for the sample. In this case, since the minimum ages are actually negative, this interpretation would hold that the sample is effectively 0-age. It should be noted that in such as case as this discordance could simply result from there being very little, to no, measurable 40Ar*, which would result in inaccurate and imprecise age determinations. Which interpretation one should choose depends somewhat on geologic relationships. Does the geology and stratigraphy support an age as old as ~194 ka?
OWY-35 Basalt Groundmass
The age spectrum for this sample is mildly discordant and U-shaped. Ages range from an initial age of ~450 ka, to a plateau segment with ages of ~250 ka, and a higher final step age of ~780 ka. The total gas age is 301 ± 24 ka. Steps 2-10 (94% of the total 39Ar released) define a plateau with a younger age of 248 ± 25 ka. Steps 1-4 (49% of the total 39Ar released) yield an isochron age of 179 ± 21 Ma. The isochron indicates the presence of excess argon (initial 40Ar/36Ar = 305 ± 2) in this sample. Thus, ages calculated for the age spectrum, which assume the initial argon has 40Ar/36Ar = 295.5, should be considered anomalously old. The isochron age is the most reliable for this sample. Note that the radiogenic yields are significantly higher for this sample than for the previous OWY-36 sample, thus the ages should be considered significantly more reliable.
OWY-23 Basalt Groundmass
The age spectrum for this sample is also moderately discordant and U-shaped, with ages which fall from an initial step of ~1.5 Ma to a plateau segment with ages of ~180 ka, and followed by older steps (to ~840 ka) in the final ~15% gas released. The total gas age is 292 ± 39 ka. Steps 2-7 (81% of the total 39Ar released) define a plateau with a younger age of 182 ± 42 ka. Steps 2-7 also yield an imprecise isochron age of 120 ± 130 ka. The isochron does not indicate the presence of excess argon (initial 40Ar/36Ar = 298 ± 6) in this sample. Also, note that all the data points defining the isochron fall near the y-axis in a cluster (similar radiogenic yields, ), thus the y-axis intercept (initial 40Ar/36Ar ratio) is fairly well defined, whereas the x-axis intercept (age) is very poorly defined. Thus, this isochron is not useful for age determination, but does provide important information regarding excess argon, i.e. within uncertainty the sample cannot be said to contain excess argon. Other processes, such as recoil of reactor generated 39Ar during irradiation, can also produce discordant age spectra for fine grained basalt groundmass samples, and this may explain this samples age spectrum in particular. Given these considerations, the plateau age should be considered the most reliable for this sample.
OWY-22 Basalt Groundmass
The age spectrum for this sample is nearly ideally flat and concordant, with the exception of higher ages in the final ~10% gas released. The total gas age is 70 ± 19 ka, and steps 1-8 (88% of the total 39Ar released) define a plateau with a younger age of 38 ± 21 ka. Steps 2-5 define a valid isochron age, however, as for OWY-23 above, the data are tightly clustered at the y-axis due to similar, and low, %40Ar* values, making this isochron useful only for confirming the composition of the initial 40Ar/36Ar ratio, which is indistinguishable from atmospheric argon. Thus, the plateau age can be considered reliable and the best estimate of the eruption age for this sample.
OWY-13 Basalt Groundmass
The age spectrum for this sample is discordant, with ages that fall, rise, and fall again with increasing %39Ar released. The total gas age is 8.3 ± 0.6 Ma. Steps 3-7 (62% of the total 39Ar released) define a plateau with a younger age of 7.0 ± 1.0 Ma. There was no isochron defined by these data. The discordance shown by this samples age spectrum must be considered to be potentially caused by the presence of excess argon, although this cannot be confirmed or denied since no isochron was obtained. Thus, in this case the most conservative interpretation is that the youngest age on the age spectrum (step 10, 3.6 Ma) is a maximum age for the sample.
OWY-12 Basalt Groundmass
This sample is similar to OWY-36 described above, and similar interpretations apply. The total gas age is 453 ± 94 ka. Steps 3-5 (50% of the total 39Ar released) define a plateau with a younger, and imprecise, age of 173 ± 145 ka. There was no isochron defined for this sample. Note that overall the age spectrum is distinctly U-shaped. This may indicate excess argon is present in the sample and thus calculated ages may be anomalously old. This cannot be confirmed as no isochron was obtained. As for OWY-36, since several steps yield negative radiogenic yields and 0-age calculations this sample is best interpreted as being effectively 0-age, i.e. it is so young that we cannot accurately measure the accumulated 40Ar* against the background of initial argon. The plateau age should only be used if stratigraphic constraints suggest it is accurate.
As is typical, these comments are made with little knowledge of geologic relationships and are simply interpretations of the laboratory data. Often knowledge of, e.g., stratigraphic relationships can determine which interpretation is most valid for a particular sample. The first sample above, OWY-36 is a good example of this. Feel free to call or email (best way to contact me terry.spell@unlv.edu) if you have further questions that I might assist with.
