| Quote (JonF @ Oct. 04 2007,18:02) | | Quote (C Gieschen @ Oct. 04 2007,07:51) | To Jon F
Here is the quote from the AIG text. Will you accept this or do I have to quote the whole chapter so you can't claim I took it out of context?
"After a mineral containing uranium atoms is formed, the products of uranium decay begin to collect in the mineral. The age of the mineral is found by determining the ratio of the parent material (238U) to the end product (206Pb). Special equipment must be used for determining uranium-to-lead ratios. In using this method, it is assumed that none of the lead escapes from the mineral, that no outside lead is added, and that no lead from a non-radioactive source was present to begin with. If any of these conditions have affected the sample being tested, the results will not be accurate." |
Well, something's crazy, because that's just plain wrong. I still think you're leaving something significant out ... what you posted is wrong.
In the U-Pb dating that's been done since the mid 1950s and currently is used in over 50% of dating studies, both the ratio of 238U/206Pb and the ratio of 235U/207Pb are measured. And this is almost always done on zircons, which are guaranteed by the physics of their solidification to have negligible lead at formation (as is acknowledged by the RATE group in a link I posted earlier). From THE U-TH-PB SYSTEM: ZIRCON DATING:
"Zircon (ZrSiO4) is a mineral with a number of properties that make it extremely useful for geochronologists (Figure 1). First of all, it is very hard (hardness 7-1/2), which means it extremely resistant to mechanical weathering. Second, it is extremely resistant to chemical weathering and metamorphism. For geochronological purposes, these properties mean it is likely to remain a closed system. Third, it concentrates U (and Th to a lesser extent) and excludes Pb, resulting in typically very high 238U/204Pb ratios. It is quite possibly nature's best clock. Finally, it is reasonably common as an accessory phase in a variety of igneous and metamorphic rocks."
Measuring those two ratios is, in essence, dating the sample by two different methods. If no lead or uranium has been added or removed, when the results are plotted on a graph of 206Pb/238U versus 207Pb/235U then the point representing the sample will fall on a curve called "concordia":
If the results plot on the concordia curve, as many do, then it is virtually certain that the system has remained closed. The other possibility is that both addition and removal have happened and have happened in just the correct ratio to keep the plotted point on the concordia curve. It might happen once in a great while, but it doesn't happen every time.
If the points representing the samples do not fall on the concordia curve, all is not lost. It turns out that in several common cases we can still get a good date. The most common case is one relatively brief episode of lead loss (lead is volatile relative to uranium, is unstable even in an udamaged lattice, and is only found in portions of the crystal lattice that have been damaged by radiation from the decay of uranim and its daughter products). In this case and several others, the points representing several co-genetic samples will fall on a line, and the upper intersection of that line with concordia is the age of the samples:
(In some cases the lower intercept is the time since the lead-loss episode, but in other cases it's not, so interpreting the lower intercept is dicier).
Of course concordant dates are preferable to discordant dates, so geochronologists have made great strides in sample selection, treatment, and measurement to obtain concordance. Here's some analyses of Greenland rocks from the 70s and 80s, including what were then the oldest known terrestrial rocks:
(from "The Age of the Earth", G. Brent Dalrymple, Stanford University Press, 1991).
Here's what are currently the oldest known terrestrial rocks from Great Slave lake in Canada, much more concordant (plotted on the Tera-Wasserberg variation of the concordia diagram):
(from Bowring, Samuel A.; Williams, Ian S., "Priscoan (4.00-4.03Ga) orthogneisses from northwestern Canada", Contributions to Mineralogy and Petrology, Volume 134, Issue 1, pp. 3-16 (1999)).
Obviously there's lots more that could be said on the subject; books have been written. I don't know of any really good but not overly technical Web resources. The first two figures in this message came from Radiometric Dating. One of the canonical texts on the subject is on-line at Radiogenic Isotope Geology, and all of chapter 5 is extremely technical coverage of this subject. THE U-TH-PB SYSTEM: ZIRCON DATING is also very good and very technical.
In summary, U-Pb concordia-disocrdia dating does not require any assumptions about the initial amount of lead, it indicates when the system has been opened, and it often yields a reliable date even when the systme has been opened.
And I didn't even get to isochrons and the Ar-Ar method ...
No matter what your book says, no matter who wrote it, the one and only one premise of radiometric dating is the constancy of radioactive decay. |
Chris will undoubtedly trample this under his feet, and turn again and rend you. |
