Primary crystallization and partial remelting of chondrules in the protoplanetary disk: Petrographic, mineralogical and chemical constraints recorded in zoned type-I chondrules

Pape, J.; Rosén, Å. V.; Mezger, K.; Guillong, Marcel (2021). Primary crystallization and partial remelting of chondrules in the protoplanetary disk: Petrographic, mineralogical and chemical constraints recorded in zoned type-I chondrules. Geochimica et cosmochimica acta, 292, pp. 499-517. Elsevier Science 10.1016/j.gca.2020.10.019

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Chondrules from unequilibrated ordinary chondrites are among the oldest Solar system materials and preserve mineralog- ical, chemical and isotopic signatures that link them to their primary formation mechanisms and environments in the early Solar System. Some chondrules record features indicating modifications by high- to low-temperature processes throughout their residence time in the protoplanetary disk. Chondrules that were partially modified after their primary formation record chemical, isotopic and textural information on their initial formation conditions and subsequent reprocessing that are essen- tial to reconstruct their formation environments and interpret the ages recorded by individual chondrules correctly.
The detailed textural and major, minor and trace element analyses of two type-I chondrules from the low petrologic type ordinary chondrites MET 00526 and MET 00452 (L/LL3.05) reveal complex chemical and textural systematics bearing tes- timony of their multi-stage high temperature evolution, including reheating and partial remelting, in the evolving protoplan- etary disk prior to accretion into their parent bodies. During primary crystallization of chondrule MET00526_Ch43, mineral growth, including incipient formation of feldspar in the outer parts of the chondrule, led to the fractionation of melt, even- tually resulting in a chemical gradient in the mesostasis. During a later punctuated reheating that ultimately led to partial remelting of the outer parts of the chondrule, mesostasis and low-Ca pyroxene remelted partially. This partial remelting enhanced the chemical differences within the mesostasis and led to the formation of two chemically distinct mesostases in the inner and the outer zone of the chondrule with almost complementary abundances of Rb, Na, K, Ba, Sr and Eu. The calculated bulk mesostasis composition reveals chondritic relative abundances of these elements in the bulk chondrule with a slight depletion of the most volatile elements. Chemical and textural observations further indicate that this disequilibrium remelting occurred under more reducing conditions than the primary melting event preserved in the chondrule centre, allow- ing for the crystallization of a second generation of low-Ca pyroxene in the outer parts of the chondrule. Very similar pro- cesses are also recorded in chondrule MET00452_Ch22 with the degree of remelting being more extensive.
A previously determined young 26Al-26Mg age of $3 Ma after CAIs determined for chondrule MET00452_Ch22 dates the time of the chondrule remelting rather than its primary formation. This is evidence for a late thermal event in the protoplan- etary disk and generally indicates that multiple, distinct thermal pulses occurred in the chondrule forming region of the protoplanetary disk throughout the time of chondrule formation. The nonconcentric secondary outer zone around a spherical inner zone may indicate a directed heat source as the cause of partial remelting and reprocessing of primary chondrules.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Pape, Jonas; Rosén, Åke Viktor and Mezger, Klaus

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0016-7037

Publisher:

Elsevier Science

Funders:

[4] Swiss National Science Foundation

Language:

English

Submitter:

Jonas Pape

Date Deposited:

14 Jan 2021 10:44

Last Modified:

14 Mar 2021 15:48

Publisher DOI:

10.1016/j.gca.2020.10.019

BORIS DOI:

10.48350/149945

URI:

https://boris.unibe.ch/id/eprint/149945

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