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Internal or external magma oceans in the earliest protoplanets – Perspectives from nitrogen and carbon fractionation

Grewal, Damanveer S. and Seales, Johnny D. and Dasgupta, Rajdeep (2022) Internal or external magma oceans in the earliest protoplanets – Perspectives from nitrogen and carbon fractionation. Earth and Planetary Science Letters, 598 . Art. No. 117847. ISSN 0012-821X. doi:10.1016/j.epsl.2022.117847.

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Protoplanets growing within ∼1 Ma of the Solar System's formation underwent large-scale melting due to heat released by the decay of ²⁶Al. When the extent of protoplanetary melting approached magma ocean (MO)-like conditions, alloy melts efficiently segregated from the silicates to form metallic cores. The nature of the MO of a differentiating protoplanet, i.e., internal or external MO (IMO or EMO), not only determines the abundances of life-essential volatiles like nitrogen (N) and carbon (C) in its core and mantle reservoirs but also the timing and mechanism of volatile loss. Whether the earliest formed protoplanets had IMOs or EMOs is, however, poorly understood. Here we model equilibrium N and C partitioning between alloy and silicate melts in the absence (IMO) or presence (EMO) of vapor degassed atmospheres. Bulk N and C inventories of the protoplanets during core formation are constrained for IMOs and EMOs by comparing the predicted N and C abundances in the alloy melts from both scenarios with N and C concentrations in the parent cores of magmatic iron meteorites. Our results show that in comparison to EMOs, protoplanets having IMOs satisfy N and C contents of the parent cores with substantially lower amounts of bulk N and C present in the parent body during core formation. As the required bulk N and C contents for IMOs and EMOs are in the sub-chondritic and chondritic range, respectively, N and C fractionation models alone cannot be used to distinguish the prevalence of these two end-member differentiation regimes. A comparison of N and C abundances in chondrites with their peak metamorphic temperatures suggests that protoplanetary interiors could lose a substantial portion of their N and C inventories with increasing degrees of thermal metamorphism. Provided the thermal metamorphism induced-loss of N and C from the protoplanetary interiors prior to the onset of core formation was efficient, the earliest formed protoplanets, as predicted by previous thermo-chemical models, are more likely to have undergone IMO differentiation resulting in the formation of N- and C-poor cores and mantles overlain by N- and C-rich undifferentiated crusts.

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Grewal, Damanveer S.0000-0002-5653-1543
Additional Information:Amrita P. Vyas created the final versions of the schematic diagrams in Fig. 1, Fig. 8. D.S.G. thanks Linda Elkins-Tanton for providing the seed schematic diagrams for Fig. 1, Fig. 8. Comments by Fabrice Gaillard and Colin Jackson as reviewers of this manuscript are gratefully acknowledged. Paolo Sossi and four anonymous reviewers are also thanked for their comments on earlier versions of this manuscript, which significantly improved our communication. D.S.G. received support from a NASA FINESST grant 80NSSC19K1538, a Lodieska Stockbridge Vaughn Fellowship by Rice University, and a Barr Foundation Postdoctoral Fellowship by California Institute of Technology. R.D. received support from NASA grants 80NSSC18K0828 and 80NSSC18K1314.
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Rice UniversityUNSPECIFIED
Barr FoundationUNSPECIFIED
Record Number:CaltechAUTHORS:20221006-424264000.1
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:117277
Deposited By: Tony Diaz
Deposited On:06 Oct 2022 23:02
Last Modified:06 Oct 2022 23:02

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