Thursday, February 21, 2008
Thursday, February 14, 2008
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 firstname.lastname@example.org) if you have further questions that I might assist with.
Today, Valentine's Day, was a red-letter day for mapping on the Owyhee. NBMG recently obtained some very cool software and hardware for mapping in 3-D. With the help of digital photgrammetry and several other things I only vaguely understand, it is possible to build orthorectified and georectified stereo-models in the digital domain, map on them, and then export the lines (with z-values no less) into ArcGIS. Today, my colleague Nick put me through the preliminary ropes with some Owyhee images, and I am sold. Extremely cool, and not nearly as complicated as a PG-2 plotter to set up.
I plan on fine-tuning the Owyhee map with this device before the river trip. Note, if I have time, I can also extract a very accurate longitudinal profile for the river (or any feature for that matter). Stay tuned. I can even cut cross-sections if necessary...extremely freakin' cool.
Wednesday, February 6, 2008
From Geosphere, February 2008; v. 4; no. 1; p. 183-206
History of Quaternary volcanism and lava dams in western Grand Canyon based on lidar analysis, 40Ar/39Ar dating, and field studies: Implications for flow stratigraphy, timing of volcanic events, and lava dams
Ryan Crow, Karl E. Karlstrom, William McIntosh, Lisa Peters, and Nelia Dunbar
A synthesis of the geochronology on basalt flows from the southern Uinkaret volcanic field indicates that basalts erupted within and flowed into Grand Canyon during four major episodes: 725–475 ka, 400–275 ka, 225–150 ka, and 150–75 ka. To extend the usefulness of these dates for understanding volcanic stratigraphy and lava dams in western Grand Canyon, we analyzed light detection and ranging (lidar) data to establish the elevations of the tops and bottoms of basalt-flow remnants along the river corridor. When projected onto a longitudinal river profile, these data show the original extent of now-dissected intracanyon flows and aid in correlation of flow remnants. Systematic variations in the elevation of flow bottoms across the Uinkaret fault block can be used to infer the geometry of a hanging-wall anticline that formed adjacent to the listric Toroweap fault.
The 725–475 ka volcanism was most voluminous in the area of the Toroweap fault and produced dike-cored cinder cones on both rims and within the canyon itself. Mapping suggests that a composite volcanic edifice was created by numerous flows and cinder-cone fragments that intermittently filled the canyon. Reliable 40Ar/39Ar dates were obtained from flows associated with this period of volcanism, including Lower Prospect, Upper Prospect, D-Dam, Black Ledge, and Toroweap. Large-volume eruptions helped to drive the far-traveled basalt flows (Black Ledge), which flowed down-canyon over 120 km. A second episode of volcanism, from 400 to 275 ka, was most voluminous along the Hurricane fault at river mile 187.5. This episode produced flow stacks that filled Whitmore Canyon and produced the 215-m-high Whitmore Dam, which may have also had a composite history. Basaltic river gravels on top of the Whitmore remnants have been interpreted as “outburst-flood deposit” but may alternatively represent periods when the river established itself atop the flows. Remnants near river level at miles 192 and 195, previously designated as Layered Diabase and Massive Diabase, have been shown by 40Ar/39Ar dating to be correlative with dated Whitmore flow remnants, and they help document the downriver stepped geometry of the Whitmore Dam. The ca. 200 and 100 ka flows (previously mapped as Gray Ledge) were smaller flows that entered the canyon from the north rim between river mile 181 and Whitmore Canyon (river mile 187.5); they are concordant with dates on the Whitmore Cascade as well as other cascades found along this reach.
The combined results suggest a new model for the spatial and temporal distribution of volcanism in Grand Canyon in which composite lava dams and edifices, that were generally leaky in proximal areas, were built from 725 to 475 ka near Toroweap fault and around 320 ka near Whitmore Canyon. New data on these and other episodes present a refined model for complex interactions of volcanism and fluvial processes in this classic locality. Available data suggest that the demise of these volcanic edifices may have involved either large outburst-flood events or normal fluvial deposition at times when the river was established on top of basalt flows.
Check out the interesting graphics the authors provide about lava dams:
Monday, February 4, 2008
|OWY-12||Lower Saddle Butte||452.8 ± 94.1||173.0 ± 144.8||n/a|
|OWY-13||Upper Saddle Butte||8.31 ± 0.62 Ma||7.03 ± 1.00||n/a|
|OWY-22||Upper West Crater||69.86 ± 19.15||37.60 ± 21.01||7.0 ± 8.5|
|OWY-23||Lower West Crater (?)||292 ± 39||182 ± 42||120 ± 130|
|OWY-35||Upper AM-PM||301 ± 24.3||247.6 ± 24.9||179 ± 21|
|OWY-36||Lower AM-PM||194 ± 27||n/a||n/a|
Here is a blurb of related text I received from UNLV:
Nevada Isotope Geochronology Laboratory - Sample Descriptions
Isochrons are the most desirable treatment of 40Ar/39Ar data. This is because the isochron actually defines the isotopic composition of the initial argon in the sample (non-radiogenic argon). Ages calculated for an age spectrum are referred to as "apparent ages" because they are calculated assuming the initial argon is atmospheric in composition - thus, if there is excess argon (40Ar/36Ar > 295.5) the age will be overestimated. Isochrons have their measure of reliability, known as the mean square of weighted deviates (MSWD) which is a statistical goodness of fit parameter. If it is greater than a certain value (which changes depending on the number of points, see Wendt and Carl, 1991, the statistical distribution of the mean squared weighted deviation, Chem. Geol., v. 86, p. 275-285) then there is more scatter than can be explained by analytical errors and it is not a statistically valid isochron. If we provide an isochron it means that the statistical test is valid, if not then no valid isochron was obtained. Also, there are issues of number of data points defining the isochron - the more the better. Four points should be considered a bare minimum for statistical reasons, three points is getting to be a real concern. This can be understood simply by considering two points - a perfectly fit straight line can be put through any two points, so completely accidental data can have a perfect line fit. It follows that with three points there is less of a chance of an accidental line fit, but it is still a very real possibility (especially if analytical errors are fairly large), this possibility gets exponentially smaller as the number of points defining the line (isochron) goes up, thus more points = a more reliable isochron.
If there is no isochron, then a plateau age is next in preference. This is because a sample that gives ages which are analytically indistinguishable from step to step is exhibiting what is known as "ideal" behavior, which suggests it has a simple geologic history, e.g., rapid cooling as a basalt lava, followed by no reheating or alteration, both of which may produce disturbed (discordant) age spectra. A reliable plateau is 3 or more consecutive steps which are indistinguishable in age at the 2 sigma level and comprise >50% of the total 39Ar released. The lack of an isochron or a plateau does not mean the sample provides no useful information, but their presence gives greater confidence in the ages obtained and requires less subjective interpretation.
Of course, you must consider that we run samples such as this "blind" in that we do not know the geologic relations of the samples, either when we analyze them, or when we provide these general interpretations. The geologic constraints must always be considered when interpreting isotopic ages; if any discrepancies arise feel free to discuss them with us, as it can in some cases make a difference in how age data are interpreted. All analytical errors are 1σ.
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