Lecture 10: Geochronology VI: U-Th decay series dating

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Geol. 655 Isotope GeochemistryLecture 10 Spring 2003much older than the eruption age. Thissuggests the crystals in the magma may havecrystallized as much as 20,000 years or morebefore eruption. The age 226 Ra– 230 Th is lessprecise because the slope is only slightly lessthan 1, and all samples seem to contain excess226 Ra. Nevertheless, the slope of the data isclearly greater than that of a 2 ka isochron,but significantly less than 1, so these data alsosuggest the crystallization significantlypredated eruption. According to Volpe andHammond (1991) the difference in the230 Th– 238 U and 226 Ra– 230 Th ages may reflectchanges in melt composition, perhaps due toassimilation of country rock or addition of aRa-rich gas or fluid phase to the magma.230 Th– 238 U and 226 Ra– 230 Th ages of the morerecent eruptive products of Mt. St. Helens,including the 1980-1986 eruptive phase, werecloser to the eruption ages. For example,Volpe and Hammond (1991) obtained a230 Th– 238 U age of 6±4 ka on the 1982 dacite.One possible interpretation of this is thatwhen the most recent eruptive stage of Mt. St.Helens began about 4000 years ago, there wassignificant volume of older magma stillpresent in the magma chamber, and that thisolder magma has been progressively replacedby fresh magma over the past 4000 years.210 Pb DatingVolpe and Hammond (1991).210 Pb has a much shorter half-life (22.3 yrs) than the nuclides we have discussed thus far, so it isuseful for much shorter time intervals. It has been applied to dating rapidly accumulating lakesediments and glacial ice. Like Th, Pb is only weakly soluble and very particle active and is scavengedby clay and other particles settling through lake water. Lake sediments thus often have significantunsupported 210 Pb in their upper intervals.We can write an equation analogous to the one equation we wrote for unsupported 230 Th (10.14):Again letting t = z/s, we have:210Pb210Pbu =u =210Pb210PbFigure 10.5. 230 Th– 238 U and 226 Ra– 230 Th mineral isochronsfor the Castle Creek andesite of Mt. St.Helens. 14 C and tree ring chronology indicates thislava was erupted 1700-2200 years ago.0u e–l 210t0u e–l 210z/sAfter10.2110.22Thus we would expect the unsupported 210 Pb to decay exponentially with depth.It is generally not useful to ratio 210 Pb to one of the stable isotopes of Pb for several reasons. First, adecaying 238 U will pass through a stage as 222 Rn, a gas, before decaying to 210 Pb. Some of this gas willescape into the atmosphere. However, its half-life is only 4 days and the nuclides between 222 Rn and210 Pb are all very short-lived, so this Rn will quickly decay to Pb. The Pb will be adsorbed on particlesand washed out of the atmosphere by rain. Thus the pathway of some 210 Pb to sediment will bedifferent that that of normal Pb. Second, there has been a very substantial input of anthropogenic(stable) Pb to lakes and streams from leaded gasoline over the 20 th century. Thus for this reason aswell, the geochemistry of stable Pb in modern sediments is different from that of 210 Pb. For both these67 February 12, 2003
.Geol. 655 Isotope GeochemistryLecture 10 Spring 2003reasons, the variation of stable Pb with depth in a corewill likely be quite different than that of 210 Pb.The first step in determining sediment ages using 210 Pbis to estimate the supported 210 Pb so it can be subtractedthat from the total 210 Pb activity to determine unsupportedactivity. One easy solution results from the factthat 210 Pb should reach equilibrium in 5 to 10 half-lives.Thus sediment deposited more than 100-200 years agoshould be in equilibrium, so that the total activity isthe supported activity. This requires the assumptionthat the flux of 210 Pb has been constant over the last 100-200 years. Alternatively, the activity of supported Pbcan be determined by determining the activity of 226 Ra.Once the unsupported activity is calculated, we cansolve equation 10.22 for s, the sedimentation rate.This can be done as follows. By taking the log of equation10.22, we have210ln Pb u = ln 210Pb 0 –lu + 210s z 10.23Equation 10.23 is the equation of a straight line on a plotof ln( 210 Pb) vs depth, where ln( 210 Pb) 0 is the interceptand -l 210 /s is the slope. Applying linear regression tothe data in this form, we can determine both the slopeand the intercept. From the slope, we can easily solvefor s. Figure 10.6 illustrates an example calculated inthis manner.REFERENCES AND SUGGESTIONS FORFURTHER READINGDickin, A. 1995. Radiogenic Isotope Geochemistry.Cambridge: Cambridge University Press.Faure, G., 1986. Principles of Isotope Geology, 2 nd ed.,Wiley & Sons, New York, 589 p.Huh, C.-A. and T.-L. Ku, 1984. Radiochemical observationson manganese nodules from three sedimentary environments in the north Pacific,Geochim. Cosmochim. Acta, 48, 951-964.Volpe, A. M. and P. E. Hammond, 1991. 238 U- 230 Th- 226 Ra disequilibria in young Mount St. Helens rocks:time constraint for magma formation and crystallization, Earth Planet. Sci. Lett., 107, 475-486.Depth, cm024681012141618Cayuga LakeBox CoreJJJJJJ200 0.5 1 1.5 2 2.5 3 3.5 4( 210 Pb), pCiJJJ( 210 Pb) 0 = 5.18 pCis = 0.319 cm/yrFigure 10.6. Unsupported 210 Pb activitymeasured on sediment samples from a boxcore taken from Cayuga Lake. The upper 4-5 cm are probably disturbed, either bybioturbation or in the coring process. Theline shows the calculated decay ofunsupported 210 Pb assuming ( 210 Pb) 0 = 5.19pCi and s= 0.319 cm/yr. This was calculatedby averaging the first 2 points anddiscarding the last 3. Unpublished data ofD. Engstrom.68 February 12, 2003
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Lecture 10: Geochronology VI: U-Th decay series dating

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