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(2020) Diffusivity of Phosphorous in Olivine Revisited: A Joint Experimental and Modeling Approach

Nelson W, Hammer J, Shea T & Chakraborty S


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04h: Room 1, Friday 26th June 22:24 - 22:27

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Submitted by Freya George on Thursday 25th June 21:12
Hi William, thanks for the great talk! As someone unfamiliar with zoning from these sorts of settings, you present preservation of P zoning as a pretty ubiquitous feature. How common is it for there NOT to be heterogeneities/oscillations in the P content? Also, the experimental work sounds very interesting – any preliminary results you could share as a sneak peak? Thanks!
Hi Freya, I certainly didn't mean to present it as ubiquitous, they are just being found in far more samples. I was just searching for some phosphorus heterogeneities in another sample unsuccessfully a few months ago. Rapid crystal growth would appear to be necessary for these skeletal heterogeneities. Experimental work - We have only completed the process on one sample thus far, and had a few problems: We experienced iron loss due to the use of platinum foil to hold the olivine, We overpolished a bit, making comparison hard, and with only 7 days at 1300 degrees C we couldn't see a difference in the sharpness of the diffusive gradient. So, to fix all this, we switched to an olivine crucible and bumped the temperature and time up to the 1400 C for 10 days. Unfortunately, this led to our olivine welding to the crucible! We were testing methods to remove it when we lost access to the lab.

Submitted by Michael Ackerson on Friday 26th June 19:57
Are you assuming the initial Mg zoning was identical to the P zoning? If the olivine grew with diffuse Mg zoning to begin with (e.g., not discrete along the same distribution as the P), then you would require less diffusive re-equilibration to achieve the observed Mg profile, which in turn would bring the calculated P diffusivity back up.
Hi Michael, No. We independently created initial profiles for each, as phosphorus is enriched in skeletal zones, and Mg was distributed in a core-rim enrichment profile. Further, concentrations of P was much, much smaller than magnesium.

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