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(2020) The Great Carbonaceous vs. non-CArbonaceous Chondrite Isotope Schism: A Heretical View

MacPherson G & Huss G


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01g: Plenary Hall, View in program

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 Xin Yang on
Dear Dr. MacPherson, thank you for this impressive presentation. In the mixing (or later accretion), the Earth had a larger mass (gravitation) compared to HEDs, so it might have incorporated more CC materials, even it was not closest to the possible CC reservoir in the outer solar system. Is that scenario possible or how could we consider the mass effect? Best regards
I am not sure if I entirely understand your question, but here goes. Larger size (mass) can increase the amount of material accreted, but it cannot control the composition of the material being accreted. If Earth accreted more c.c. material than Vesta, it must be because there was more c.c. material near Earth than near Vesta.

Submitted by Larry Nittler on
How do your reconcile your conclusion with the observation that CCs apparently by and large accreted with far more water ice than ECs or OCs?
I agree this is an issue Larry, and I don't have a good answer. Keep in mind that most CV3s and CO3s are bone dry, and I don't for one minute think that they once were hydrous and then dried out. Anyway, to me the oxygen isotopic patterns (composition vs. heliocentric distance) I showed are just too compelling to ignore. Likewise, some of the apparent isotopic mixing patterns for other systems (some, not all) again are just too obvious to ignore. This is a reasonability argument. The Warren model raises more problems than it solves. One last point: the CAI populations in different chondrite types are different, and I do not see how the Warren model can explain that at all. If all c.c.'s accreted in the outer solar system, why are there such disparities in the CAI populations (e.g. why do only the bone-dry CV3s contain the largest and most numerous CAIs?) But I admit, I don't have all the answers.

Submitted by Emily Worsham on
Could secondary thermal processing account for the opposite expected heliocentric distance on the Cr-Ti isotope plot? Also, if CC material was the early infalling material as in the Nanne et al., 2019 model, and the infall was nearest to the sun before spreading outward, might it have mixed with 16O-rich gas early?
For the first question, I do not see how. The Ti and Cr isotopes are correlated, and the 50Ti bulk anomaly almost certainly resides in the CAIs (which still exist). As for the second question, your idea does not explain the oxygen isotopic bulk compositions vs. heliocentric distance correlation. In the Nanne model it is only the CAIs that preserve the character of the early infalling material. The requiement seems to be that virtually all CAIs were lost from the inner solar system, yet we know that CAIs continued to be thermally processed in the inner solar system for perhaps as long as 1-2 million years. Finally, in general the inner vs. outer solar system model does not explain the differing CAI populations in different chrondrite types.

Submitted by Sheryl Singerling on
If the carrier phases of the isotopic anomalies are presolar grains, wouldn't that be consistent with differences between CCs and NCs, since the former experienced less extensive heating (retained their presolar grains) and the latter experienced more extensive heating ("lost" their presolar grains)? The presolar grains as carriers wouldn't require CCs to have formed closer to the sun, no?
Well, presolar grains certainly contain huge isotope anomalies and thermal processing certainly contributed to the destruction of presolar grains. This may be what is going on with Cr anomalies, which demonstrably resided in Cr-oxide nanograins. But the 16O enrichment certainly does not reside in presolar grains. The bulk 50Ti anomaly likely does reside in CAIs. So presolar grains cannot be the entire answer. And as I already have noted, the inner vs. outer solar system model causes more problems than it solves. Finally, independent of whether you accept our model that CCs actually formed closest to the Sun, you must reconcile the observations that we have raised especially with respect to isotopic compositions of many bodies correlating with heliocentric distance.

Submitted by Yankun Di on
According to your model, do you think both NC and CC formed inside the Jupiter's orbit? Then what might be the physical separation mechanism of the isotopic reservoirs?

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