Abstract Details
(2020) Is Diamond Depth Distribution Systematic?
Nimis P, Preston R, Perritt S & Chinn I
https://doi.org/10.46427/gold2020.1935
The author has not provided any additional details.
03g: Room 1, Thursday 25th June 08:03 - 08:06
Paolo Nimis
View abstracts at 7 conferences in series
Robin Preston
Samantha Perritt
Ingrid Chinn View all 2 abstracts at Goldschmidt2020 View abstracts at 3 conferences in series
Robin Preston
Samantha Perritt
Ingrid Chinn View all 2 abstracts at Goldschmidt2020 View abstracts at 3 conferences in series
Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.
Submitted by Stephanie Greene on Friday 19th June 06:15
Excellent presentation, thank you. I have a few questions: 1) Why did you choose to compare the diamond depths with those of xenocrysts, rather than xenoliths, or xenocrysts and xenoliths? Something I found for the Jericho kimberlite in the Slave craton is that olivine xenocrysts are sourced from shallower depths on average than xenoliths, I suspect because of a change in magma properties during ascent. If xenocryst depths are controlled by magma sampling, comparing diamonds to xenocrysts alone might not be the most useful in determining whether diamond distribution is controlled by kimberlite sampling. 2) How did you correct the cpx thermometry results to be in line with the opx-garnet results? Did you fit the cpx array to the opx-garnet array? Is there any reason to think using cpx-, opx-, garnet-bearing xenoliths with similar compositions as the inclusions to determine how much higher cpx-derived temperatures are would have any impact? 3) The depth distribution of xenocrysts is fascinating. Would you care to speculate on why cpx and garnet xenocrysts are not sourced from the same depths?
Dear Stephanie, thanks for your interesting questions. 1. Our xenocrysts directly derived from the mining processing plant so our xenocrysts actually represent both real kimberlite xenocrysts and crushed xenolith materials. Overall, they provide a better statistical record of what the kimberlites have sampled on route to the surface. In any case, xenoliths from the same sources did not yield much different distributions. 2. Experience derived from thermobarometry of well equilibrated xenoliths showed some systematic discrepancies at high P between, specifically, the Cpx barometer and the Opx-Grt barometer (see Ziberna et al. 2016 AmMin for details). Similar discrepancies were observed also against experiments. So we fitted the results of Cpx barometry against Opx-Grt barometric results for well equilibrated mantle xenoliths and empirically modified the Cpx barometer accordingly. The modified Cpx barometer gives P estimates that are now more consistent with those using the Opx-Grt barometer for well-equilibrated xenoliths, at the expense of somewhat poorer precision at very high P. But this is ok if one has to define general trends for large populations rather than P-T for single samples. 3. Generally speaking, Grt is virtually ubiquitous in cratonic peridotites at P greater than ~2 GPa. Cpx may instead be absent in some refractory lithologies, which may be unevenly distributed with depth. Cpx is also more prone than Grt to resorption during kimberlite transport and to alteration after kimberlite emplacement. Therefore, the depth distribution of Grt xenocrysts should generally provide a better assessment of the kimberlite sampling efficiency at different mantle depths. More specifically, at Kimberley the Cpx distribution is similar to that of the "fertile" Grts. At Cullinan, the xenolith record shows that at intermediate depths there is an abundance or even predominance of nominally Cpx-free harzburgites containing Grts with 'lherzolitic' major and trace element compositions. This suggests that metasomatic processes at those depth may have significantly affected the rock chemistry without causing significant development of modal Cpx. Finally, at Voorspoed, the number of Cpx xenocrysts is too small to allow statistically significant comparisons with the Grt xenocrysts.
Excellent presentation, thank you. I have a few questions: 1) Why did you choose to compare the diamond depths with those of xenocrysts, rather than xenoliths, or xenocrysts and xenoliths? Something I found for the Jericho kimberlite in the Slave craton is that olivine xenocrysts are sourced from shallower depths on average than xenoliths, I suspect because of a change in magma properties during ascent. If xenocryst depths are controlled by magma sampling, comparing diamonds to xenocrysts alone might not be the most useful in determining whether diamond distribution is controlled by kimberlite sampling. 2) How did you correct the cpx thermometry results to be in line with the opx-garnet results? Did you fit the cpx array to the opx-garnet array? Is there any reason to think using cpx-, opx-, garnet-bearing xenoliths with similar compositions as the inclusions to determine how much higher cpx-derived temperatures are would have any impact? 3) The depth distribution of xenocrysts is fascinating. Would you care to speculate on why cpx and garnet xenocrysts are not sourced from the same depths?
Dear Stephanie, thanks for your interesting questions. 1. Our xenocrysts directly derived from the mining processing plant so our xenocrysts actually represent both real kimberlite xenocrysts and crushed xenolith materials. Overall, they provide a better statistical record of what the kimberlites have sampled on route to the surface. In any case, xenoliths from the same sources did not yield much different distributions. 2. Experience derived from thermobarometry of well equilibrated xenoliths showed some systematic discrepancies at high P between, specifically, the Cpx barometer and the Opx-Grt barometer (see Ziberna et al. 2016 AmMin for details). Similar discrepancies were observed also against experiments. So we fitted the results of Cpx barometry against Opx-Grt barometric results for well equilibrated mantle xenoliths and empirically modified the Cpx barometer accordingly. The modified Cpx barometer gives P estimates that are now more consistent with those using the Opx-Grt barometer for well-equilibrated xenoliths, at the expense of somewhat poorer precision at very high P. But this is ok if one has to define general trends for large populations rather than P-T for single samples. 3. Generally speaking, Grt is virtually ubiquitous in cratonic peridotites at P greater than ~2 GPa. Cpx may instead be absent in some refractory lithologies, which may be unevenly distributed with depth. Cpx is also more prone than Grt to resorption during kimberlite transport and to alteration after kimberlite emplacement. Therefore, the depth distribution of Grt xenocrysts should generally provide a better assessment of the kimberlite sampling efficiency at different mantle depths. More specifically, at Kimberley the Cpx distribution is similar to that of the "fertile" Grts. At Cullinan, the xenolith record shows that at intermediate depths there is an abundance or even predominance of nominally Cpx-free harzburgites containing Grts with 'lherzolitic' major and trace element compositions. This suggests that metasomatic processes at those depth may have significantly affected the rock chemistry without causing significant development of modal Cpx. Finally, at Voorspoed, the number of Cpx xenocrysts is too small to allow statistically significant comparisons with the Grt xenocrysts.
Submitted by Paolo Nimis on Monday 22nd June 09:53
Dear Stephanie, thanks for your interesting questions. 1. Our xenocrysts directly derived from the mining processing plant so our xenocrysts actually represent both real kimberlite xenocrysts and crushed xenolith materials. Overall, they provide a better statistical record of what the kimberlites have sampled on route to the surface. In any case, xenoliths from the same sources did not yield much different distributions. 2. Experience derived from thermobarometry of well equilibrated xenoliths showed some systematic discrepancies at high P between, specifically, the Cpx barometer and the Opx-Grt barometer (see Ziberna et al. 2016 AmMin for details). Similar discrepancies were observed also against experiments. So we fitted the results of Cpx barometry against Opx-Grt barometric results for well equilibrated mantle xenoliths and empirically modified the Cpx barometer accordingly. The modified Cpx barometer gives P estimates that are now more consistent with those using the Opx-Grt barometer for well-equilibrated xenoliths, at the expense of somewhat poorer precision at very high P. But this is ok if one has to define general trends for large populations rather than P-T for single samples. 3. Generally speaking, Grt is virtually ubiquitous in cratonic peridotites at P greater than ~2 GPa. Cpx may instead be absent in some refractory lithologies, which may be unevenly distributed with depth. Cpx is also more prone than Grt to resorption during kimberlite transport and to alteration after kimberlite emplacement. Therefore, the depth distribution of Grt xenocrysts should generally provide a better assessment of the kimberlite sampling efficiency at different mantle depths. More specifically, at Kimberley the Cpx distribution is similar to that of the "fertile" Grts. At Cullinan, the xenolith record shows that at intermediate depths there is an abundance or even predominance of nominally Cpx-free harzburgites containing Grts with 'lherzolitic' major and trace element compositions. This suggests that metasomatic processes at those depth may have significantly affected the rock chemistry without causing significant development of modal Cpx. Finally, at Voorspoed, the number of Cpx xenocrysts is too small to allow statistically significant comparisons with the Grt xenocrysts.
Dear Stephanie, thanks for your interesting questions. 1. Our xenocrysts directly derived from the mining processing plant so our xenocrysts actually represent both real kimberlite xenocrysts and crushed xenolith materials. Overall, they provide a better statistical record of what the kimberlites have sampled on route to the surface. In any case, xenoliths from the same sources did not yield much different distributions. 2. Experience derived from thermobarometry of well equilibrated xenoliths showed some systematic discrepancies at high P between, specifically, the Cpx barometer and the Opx-Grt barometer (see Ziberna et al. 2016 AmMin for details). Similar discrepancies were observed also against experiments. So we fitted the results of Cpx barometry against Opx-Grt barometric results for well equilibrated mantle xenoliths and empirically modified the Cpx barometer accordingly. The modified Cpx barometer gives P estimates that are now more consistent with those using the Opx-Grt barometer for well-equilibrated xenoliths, at the expense of somewhat poorer precision at very high P. But this is ok if one has to define general trends for large populations rather than P-T for single samples. 3. Generally speaking, Grt is virtually ubiquitous in cratonic peridotites at P greater than ~2 GPa. Cpx may instead be absent in some refractory lithologies, which may be unevenly distributed with depth. Cpx is also more prone than Grt to resorption during kimberlite transport and to alteration after kimberlite emplacement. Therefore, the depth distribution of Grt xenocrysts should generally provide a better assessment of the kimberlite sampling efficiency at different mantle depths. More specifically, at Kimberley the Cpx distribution is similar to that of the "fertile" Grts. At Cullinan, the xenolith record shows that at intermediate depths there is an abundance or even predominance of nominally Cpx-free harzburgites containing Grts with 'lherzolitic' major and trace element compositions. This suggests that metasomatic processes at those depth may have significantly affected the rock chemistry without causing significant development of modal Cpx. Finally, at Voorspoed, the number of Cpx xenocrysts is too small to allow statistically significant comparisons with the Grt xenocrysts.
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