tag:blogger.com,1999:blog-37371348.post4884730550624596742..comments2023-11-05T03:45:19.337-05:00Comments on Apparent Dip: O.G. (Original Geochronologist) R.J. StruttThermochronichttp://www.blogger.com/profile/08606230819461771700noreply@blogger.comBlogger7125tag:blogger.com,1999:blog-37371348.post-12322474172343446472007-01-19T08:51:00.000-05:002007-01-19T08:51:00.000-05:00That definitely helps - thanks. Hopefully, your e...That definitely helps - thanks. Hopefully, your expansion on the process will help clarify further.Jimhttps://www.blogger.com/profile/08424663298293264533noreply@blogger.comtag:blogger.com,1999:blog-37371348.post-76861560678414832232007-01-19T00:53:00.000-05:002007-01-19T00:53:00.000-05:00I look forward to the Helium post...good timesI look forward to the Helium post...good timesBrianRhttps://www.blogger.com/profile/04930245080333999989noreply@blogger.comtag:blogger.com,1999:blog-37371348.post-13325978205271532302007-01-18T22:59:00.000-05:002007-01-18T22:59:00.000-05:00Excellent question. It turns out that helium does ...Excellent question. It turns out that helium does leak out of geologic materials rather easily, but in some minerals, especially single crystals, helium loss or retention is strictly governed by thermally activated volume diffusion and is in fact very predictable. The temperatures required to diffuse helium out of apatite (common accessory mineral in granitic rocks) you really only need to get to ~70°C or so (on geologic timescales). But, geologists didn't really understand thermally activated volume diffusion in geologic materials until the 1960's, when the potassium-argon geochronometer was being developed. Now it is well understood (an excellent review can be found in a book called "Geochronology and thermochronology by the 40Ar/39Ar method" by McDougall and Harrison.)<br />He is measured nowadays from single crystals, most often of apatite or zircon. They are heated with a laser, and the evolved gas measured on a quadrupole mass spectrometer.<br />I think this weekend I'll work up a post describing the process, with some figures and everything. Hope this makes sense..Thermochronichttps://www.blogger.com/profile/08606230819461771700noreply@blogger.comtag:blogger.com,1999:blog-37371348.post-59439623189271806102007-01-17T14:13:00.000-05:002007-01-17T14:13:00.000-05:00Ok, I'm a little bit confused, so hopefully you ca...Ok, I'm a little bit confused, so hopefully you can teach me something here. How exactly is the helium measured and how does that result help us determine how old the rocks are, especially considering that the helium leaks from the rocks at an unknown rate?Jimhttps://www.blogger.com/profile/08424663298293264533noreply@blogger.comtag:blogger.com,1999:blog-37371348.post-28834568157228358842007-01-17T12:31:00.000-05:002007-01-17T12:31:00.000-05:00Juicy rumor, I am not sure. I have heard that usin...Juicy rumor, I am not sure. I have heard that using a slope Franz to separate zircons can bias the population. I am not sure which way it goes, I think radiation damaged zircons can be pulled off in one of the stronger magnetic fractions, but don't quote me on that. I recently looked at a Frzna magnetic fraction (2.2 Amps, I am not sure the slope) that was entirely apatite, kind of threw me for a loop.Thermochronichttps://www.blogger.com/profile/08606230819461771700noreply@blogger.comtag:blogger.com,1999:blog-37371348.post-21599761724540842932007-01-17T12:28:00.000-05:002007-01-17T12:28:00.000-05:00Another value of refining the technique and making...Another value of refining the technique and making the ages better is that I can just re-analyze samples i've already collected (he he). Sometimes new ages can really fly in the face of long-standing paradigms in tectonics ... gets everyone debating and discussing, which is exciting.<br /><br />I heard that zircon U/Pb SHRIMP ages are systematically too young...is this just a rumor?BrianRhttps://www.blogger.com/profile/04930245080333999989noreply@blogger.comtag:blogger.com,1999:blog-37371348.post-35230864590710284862007-01-17T03:04:00.000-05:002007-01-17T03:04:00.000-05:00And look at those wonderful apparatus figures - my...And look at those wonderful apparatus figures - my lack of artistic ability might have been a serious problem 100 years ago!Chris Rhttps://www.blogger.com/profile/10923865059164569384noreply@blogger.com