Evidence for a Metallic Core in the CV Chondrite Parent Planetesimal

Details Evidence for a Metallic Core in the CV Chondrite Parent Planetesimal Evidence for a Metallic Core in the CV Chondrite Parent Planetesimal

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmgp34a..07w&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #GP34A-07

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Logic

[1595] Geomagnetism And Paleomagnetism / Planetary Magnetism: All Frequencies And Wavelengths, [5440] Planetary Sciences: Solid Surface Planets / Magnetic Fields And Magnetism, [6017] Planetary Sciences: Comets And Small Bodies / Erosion And Weathering, [6240] Planetary Sciences: Solar System Objects / Meteorites And Tektites

Scientific paper

The aggregational texture and bulk chemical composition of chondrites demonstrate that they are not the products of planetary melting processes. As a result, carbonaceous chondrites have traditionally been thought to samples bodies that have not undergone large-scale differentiation. However, it has long been known that the meteorite Allende and other CV carbonaceous chondrites contain a natural remanent magnetization. This record has been difficult to interpret because the age and setting of magnetization acquisition were poorly understood. We have conducted a new paleomagnetic study on the Allende CV carbonaceous chondrite. This included high resolution demagnetization analyses, the first detailed shock remanent magnetization and low temperature magnetometry studies, and a variety of new rock magnetic measurements. Our goal was to determine the nature and acquisition time for the magnetization in Allende in order to distinguish between core dynamo and external field sources. Drawing on our new paleomagnetic data, abundant previous paleomagnetic studies, and newly published geochemical, petrologic, and isotopic datasets, we demonstrate that the magnetization in Allende must have been acquired following accretion of the parent body, likely over several million years (Ma) during metasomatism on the CV parent planetesimal ˜8-10 Ma after solar system formation. Therefore, the magnetization in Allende is apparently too young and was acquired over too long a time period to have been produced by early external protoplanetary disk or solar magnetic fields. Thermal modeling of early planetesimals heated by 26Al decay indicate that dynamos were likely generated in convecting metallic cores lasting for ≥11 Ma after solar system formation. Because such bodies melt from the inside out, some may preserve an unmelted, relict chondritic surface which could be magnetized during metasomatism in the presence of a core dynamo. In fact, Allende’s paleointensities are in the range expected for core dynamos in early planetesimals. A simple interpretation of Allende’s paleomagnetic record is therefore that CV chondrites are derived from the outer, unmelted layer of a differentiated body with a convecting, liquid metallic core and core dynamo.

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