Apatite Dating Problem

chenhongjun

I recently made a batch of apatite dating samples. The standard sample is MCclure and the monitoring sample is Durango.However, after data processing, the upper intersection point in the T-W diagram is 0.18-0.2, which is much lower than the normal initial lead value (usually 0.8-1). I don’t know what went wrong.

Bence

Hi @chenhongjun,

Which DRS did you use to process the data?

Which sample's upper intersection are you referring to? McClure or Durango?

chenhongjun

Bence hello，Bence，The parameters of my DRS are as follows：Index channel：U238 ；Reference material:A_McClure; Default fit type:Exponential; Beam seconds method:Gaps between samples; Mask method:Cut off Mask channel:U238 Mask threshold:1000. All analysis values (including samples and standard samples, monitoring samples) on the upper intersection point are low

Bence

Hi chenhongjun,

Sorry, I'm unable to determine the cause of the problem from your drescription. Can you please send the .io4 experiment file to the iolite support email address?

Many thanks,
Bence

chenhongjun

Bence Okay, thank you very much, I will send you the file later

chenhongjun

Bence The .io4 experiment file has been sent to the support email, please check:::

Bence

Hi chenhongjun,

I believe the issue is that you're using the U-Pb Geochronology DRS. This DRS is aimed at zircons (or other minerals with either constant or low common-Pb). For apatites, where the common-Pb component can be variable, it's recommended that you use the VizualAge UComPbine DRS.

Also, to check whether there are any issues with your data reduction, you should also include as many secondary reference materials as you can.

If it helps, we have recorded a webinar on U-Pb data processing, and the recording is available here: https://iolite.xyz/support/webinars/webinar_recordings.php. One of the tips in that recording is that you should meaure at least 13 to 15 analyses of your primary reference material to get a reasonable downhole fractionation model.

Please let me know if you have any questions.

Bence

chenhongjun

Bence I have sent my .io4 experiment file to you, please help me to see if there is any problem with my handling method

chenhongjun

Dear Bence， I recently encountered some problems when dealing with apatite age，The standard sample used in my experiment is Mcclureand MAD，But no matter when I choose either one of the two as the main standard, the other standard cannot pass QA/QC, and is much different from the recommended value. Of course, the age error of my sample is quite large, I don’t know Where is my problem? Can you help me

Bence

Yes, sure. Please send me the experiment files and I can take a look

chenhongjun

Bence

brunovieiraribeiro

I don't know if you are checking yours results within Iolite or outside by plotting with Isoplot or IsoplotR. Given that apatite might have a highly variable content of common-Pb, the final ages presented in Iolite may not make any sense. We need to export it to plot with other tools, potentially calculating the initial 207/206 ratio using Stacey & Kramers for example.

chenhongjun

brunovieiraribeiro Thank you very much for your answers, but I think the problem is not this, because none of my standard samples can pass the QA/AC, and now the latest version of iolite also support normal lead correction , the function is the same as isoplot

Joe

chenhongjun
I had a look at the session you shared with support. There are two things I would suggest changing and @Bence can add his thoughts too.

The settings used for VizualAge UComPbine's beam seconds were not doing their job. The threshold I saw when I loaded your session was U238 at 10000 cps. Some of the McClure U238 is below 10000 so the beam seconds for McClure are not correct. Switching the cutoff to 1000 seems to help.
'Use selection mean' was checked, but it would seem there is variable Pb within individual spots, so you should have that unchecked.

With those 2 changes, I get a very nice linear array for McClure (using MAD as the primary). The (unanchored) lower intercept is ~ 504 +/- 34 Ma (propagated), which is within error of the accepted age, I believe.

All the best,

chenhongjun

Joe Hi Joe，Thank you and Bence for your patience to answer my questions.But some questions keep confusing me：
1.I have never understood what is the function of Beam Seconds? What is Beam Seconds?
2.What is the difference between Cut off threshold and Jump threshold in the beam Seconds method?

In the first suggestion, when you talk about Switching the cutoff to 1000, do you mean to set the sensitivity under Beam Seconds to 1000 or the U isotope cut off under VisualAge UComPbine function to 1000?
In accessory minerals containing common lead, it would be normal there is variable Pb within individual spots,If every point needs uncheck 'Use selection mean' ，So what does this option do?
I am very sorry for asking you a lot of questions, because this kind of common lead content mineral is rarely handled. I have participated in every training session of yours. However, due to language barriers, I really don’t understand some functions.

Bence

Hi chenhongjun,

No worries about having many questions (I know what it's like to have to learn in a different language). I'm sure others will have the same questions as you, so here are some answers:

BeamSeconds is a proxy for ablation pit depth. Fractionation is relative to pit depth, but we never actually measure pit depth, so we plot downhole fractionation vs BeamSeconds instead.
Cutoff threshold is where the beam intensity passes through a value, beamseconds starts counts (i.e. an ablation has started, and the pit depth is increasing). Jump threshold uses the gradient of a channel to determine when ablation starts.
Joe is suggesting you adjust the BeamSeconds setting. The U isotope settings are for when you've measured both U238 and U235 and want to choose under what conditions to use each isotope.
The "Use Selection Mean" option is for determining the 207Pb/206Pb value for your common Pb component when performing a 207Pb correction. This can be helpful if you have noisey data and don't want to use a SK estimate. It's not relevant here though.

Best regards,
Bence

chenhongjun

Bence Thank you very much for your answers. Recently, I have been trying to process apatite age data, but I found that I may still have not learned it. I don’t know how to judge whether the method I chose (such as linear, exponential, etc.) based on Beamseconds Corrected the downhole fractionation. The monitoring samples in the data I processed failed the QA/QC inspection every time. I will send my processed files to your mailbox later. I hope you can help me.::::::

chenhongjun

Was my message ignored? 😃 , or my email was not sent successfully?

Bence

Hi chenhongjun,

We didn't receive your email. Can you please send it again?

chenhongjun

Bence 😄 I just resent it to your email support@iolite-software.com, thank you very much

chenhongjun

chenhongjun 😀 Sorry to disturb you, I don’t know if you successfully received my mail