Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Test samples for cosmogenic radiocarbon dating  July 2019



Four "inter comparison" samples were submitted for analysis to check the possible interference of  CO rich fluids within fluid inclusions with cosmogenic C dating.

Samples were analysed on the model 215 decrepitometer and were heated to 800 C, the maximum of the decrepitometer. Because low level counts were significant, a lower than normal threshold was used to provide better sensitivity. However the setting used (40) was too aggressive and seems to have resulted in instrumental background interference in some samples. Run numbers from J89 used the normal threshold setting of 60. In addition, some extraneous counts occurred at times, mostly from the "right" furnace. Consequently some of the results for decrepitation between 600 and 800 C are thought to be uncertain. Many additional tests were done to establish the instrumental background using previously decrepitated sample, which should give no counts. Tests were also done using the standard laboratory calibration sample. Two samples (CRN & CO) were analysed twice and one sample (CRA) was also crushed to a finer grainsize of <200 microns, >120 microns and analysed to check for grainsize effects.

The graphs of the data are all smoothed using a weighted rolling mean smoothing over 3 samples weighted 25%, 50%, 25%. Two plots, as stated in the title line, show the raw data results (ie not smoothed) to better show the background variations.

Another approach might be to analyse some quartz samples collected from deep underground ( > 50 metres depth) in which there would be no cosmogenic carbon content. Four samples collected from 2 mines (Dongping and Hougou) in china in 2005 were retrieved and analysed again by decrepitation as potential candidates for further tests. These samples have been stored in the laboratory since 2005.


Cosmogenic sample comparison tests

all comparison samples

Samples CRN and CO were analysed twice. The first analysis of sample CO (run J71) is thought to be incorrect and is omitted here.
Sample CRR (brown) shows the expected absence of fluid inclusion decrepitation above the 573C quartz transition.
Sample CO (J97) (yellow)  also shows the absence of high temperature decrepitation, but there is a small peak at  750C. It should not be, but  may be instrumental interference.
Sample CRA (magenta, J68) shows increasing decrepitation to 800 C, probably due to fluid inclusions. The fine grained re-analysis of this sample (blue, J75) shows a similar decrepitation increase to 800 C, confirming that this is due to fluid inclusions. These inclusions are present despite the sample grainsize being finer (<200 microns, >120 microns).
Sample CRN (green and cyan, J69 & J72) gives conflicting results. J69 shows increasing decrepitation to 800 C, but the reanalysis. J72 does not. There was instrumental interference on J72 from 650 to 720C which has been manually removed in this plot. It is uncertain, but probable that there is no significant decrepitation above 573C on this sample.

sample CRN
The 2 analyses of CRN show unusually poor correlation. The peaks at 450 - 660 C show a low-moderate abundance of fluid inclusions. The rise in decrepitation to 800 C in J69 seemed to be real, but the reanalysis as J72 lacks this decrepitation. Note that there were instrumental problems on run J72 and interference between 650 and 720 was manually removed. Despite this interference, decrepitation was low from 750 to 800 C and does not confirm the high temperature decrepitation on run J69.  Consequently the high temperature decrepitation on sample CRN is uncertain and possibly only at background.

sample co
The initial analysis of sample CO (J71) showed suspiciously high counts at 800 C and was re-analysed as J97. This analysis shows very low decrepitation above 600 C. A small peak at 750 C is most likely real but is close to background levels. The used sample was re-analysed as J98 to show the background level. The small peak at 590 C is due to a few inclusions which survived the first heating run. Some inclusions do manage to survive intact despite being heated beyond the 573 transition. The gradual increase in level from 700 to 800 C on run J98 is most likely the instrumental background rather than fluid inclusion counts.

sample crn and co
Two furnaces are used in alternation in operation of the decrepitometer. During analysis of sample CRN, run J69, in the left furnace, it seemed that there may have been instrumental interference above 600 C and it was re-analysed as run J72 in the right furnace, plotted in the next diagram.  Analysis run J72 shows no significant decrepitation above 600 C and is considered more reliable than the run J69 result for sample CRA.
After adjustments to this furnace this interference was eliminated and analyses of sample CO, runs J97 and J98 gave acceptable results.

3 samples
Sample CRA, run J68, shows a significant decrepitation increase near 800 C. This result was confirmed by the re-analysis of the fine-grained fraction of sample CRA plotted below (run J75). 
Sample CRR, J70, shows low counts above 600 C as expected. A repeat analysis of sample CRN was carried out as run J72. (Previous analysis J69, plotted above) There was severe instrument interference in run J72 from 650 to 720 C and this has been removed from this plot. This repeat analysis shows no significant decrepitation above 720 C and is considered to be a more reliable analysis than run J69 of sample CRN.

sample cra finegrained
It was thought that the results might be influenced by the grainsize of the analytical sample. All the original analyses were carried out on samples with grainsize sieved to -500+212 microns. There was enough of sample CRA to permit further crushing and sieving to give a sample with grainsize <200 microns with fines of about less than 120 microns removed. This sample was analysed as run J75. The used sample from this analysis was also re-analysed 3 times to verify the background level and instrument performance. The first re-analysis (used) was run J76 which shows a small peak at 590C due to survival of a few inclusions through the first analysis. Subsequent re-analyses (used) of J77 and J79 show background level and the absence of any peak at 590C. It is clear that a few inclusions do survive the first heating cycle to 800C, but no inclusions survive  the second or third heating of the same material.

The decrepitation increase to 800C on run J75 confirms the result seen on the coarser grainsize fraction of the same sample, run J68 plotted above. Reducing the grainsize of the sample has not made any significant change to the analytical result. Inclusions do survive heating through the 573 C transition temperature on this sample.

Calibration and annealing tests

The calibration standard is a sample of quartz from the Howley gold mine near Darwin which is analysed frequently to check the instrument operation and precision. Run J90 shows a small increase in decrepitation near 800 C. Run J92 is a re-analysis of the used sample from run J90 and shows a small peak at 600 C which is caused by fluid inclusions which survived heating through the quartz alpha-beta transition temperature of 573 C right up to 800 C during run J90. Run J92 shows only background counts above 600 up to 800 C with no detectable further inclusion decrepitation.

annealing quartz
Two additional analyses were carried out on the calibration standard to see if heating to only just through the alpha-beta transition temperature is enough to eliminate inclusions up to 800 C. Run J80 heated the sample only to 620 C and the sample was then cooled to room temperature and the used sample was re-analysed to 800 C as run J81.

Heating to 620 C , above the alpha-beta transition temperature, does not "anneal" the quartz and remove all inclusions. The subsequent analysis to 800 C still shows the small high temperature decrepitation to 800 C as seen on run J90 in the previous plot. The difference in response between 620 and 800 C on these two analyses (run J90 and run J80) may be due to sample aliquot differences but may include some component of instrumental error. It is very difficult to get accurate data at such low count levels at high temperature which may contribute to this small change.

Underground samples from mines in china, extracted 2005

Samples from deep underground should contain no cosmogenic carbon and may be useful to check if fluid inclusion CO2 interferes with cosmogenic carbon measurements.
Four samples were retrieved from the archives and analysed  as potential check samples. The four samples of quartz from 2 underground mines in NE China (Hebei province) were first analysed in 2005 and have H series run numbers. These are plotted using thin lines. These samples were reanalysed on the new decrepitometer with J series run numbers and plotted with thick lines. The duplicate analyses are in agreement and differ in amplitude due to the improved sensitivity of the updated model 215 decrepitometer.  The H series analyses were done on the model 105 decrepitometer.

Samples from the Dongping mine were collected from the "70" ore body at a depth of approximately 300m below surface. The sample from the Hougou mine was collected from a depth of approximately 30m below surface.

china underground

Traces of CO2 are present in samples 1863B, 1865A and 1869C based on the decrepitation counts seen between 200 and 300 C. 
china underground
This plot shows the same data as the above plot with the Y axis scale expanded to more clearly show the low level counts above 600 C.
Dongping samples 1863B and 1865A have minor decrepitation above 600 C to 800C which may be due to fluid inclusions which survived the alpha-beta  quartz transition at 573 C.
Dongping sample 1865B (yellow) and Hougou sample 1869C (red) show only low level decrepitation above 600 C which cannot be distinguished from backgound levels.

dongping 1865a
Sample 1865A does show significant decrepitation from 650 to 800C. The rerun of used sample (J87) shows a small peak at 600 C due to survival of some inclusions. The peak at 720C in run J87 is diabolical as decrepitation of the used sample should be at background level and should not exceed the decrepitation level of the original fresh sample J85! This may be interference due to the excessively sensitive threshold setting used.

dongping 1865b
Sample 1865B shows only trace or no decrepitation above 650 C and the analysis J93 of the used remains of J91 show normal background.

hougou 1869c
Sample 1869C from the Hougou mine shows no significant decrepitation above 600 C with only background levels.

Samples 1863C and 1869B are the cleanest white quartz samples of the 4 samples.  Samples 1865A and 1865B contain perhaps 1% of darker grains and may need heavier pre-cleaning.

Sample details and analytical run number list