Magma ascent in planetesimals: control by grain size

Lichtenberg, T., Keller, T. , Katz, R. F., Golabek, G. J. and Gerya, T. V. (2019) Magma ascent in planetesimals: control by grain size. Earth and Planetary Science Letters, 507, pp. 154-165. (doi: 10.1016/j.epsl.2018.11.034)

Abstract

Rocky planetesimals in the early solar system melted internally and evolved chemically due to radiogenic heating from 26Al. Here we quantify the parametric controls on magma genesis and transport using a coupled petrological and fluid mechanical model of reactive two-phase flow. We find the mean grain size of silicate minerals to be a key control on magma ascent. For grain sizes ≳1 mm, melt segregation produces distinct radial structure and chemical stratification. This stratification is most pronounced for bodies formed at around 1 Myr after formation of Ca, Al-rich inclusions. These findings suggest a link between the time and orbital location of planetesimal formation and their subsequent structural and chemical evolution. According to our models, the evolution of partially molten planetesimal interiors falls into two categories. In the magma ocean scenario, the whole interior of a planetesimal experiences nearly complete melting, which would result in turbulent convection and core–mantle differentiation by the rainfall mechanism. In the magma sill scenario, segregating melts gradually deplete the deep interior of the radiogenic heat source. In this case, magma may form melt-rich layers beneath a cool and stable lid, while core formation would proceed by percolation. Our findings suggest that grain sizes prevalent during the internal heating stage governed magma ascent in planetesimals. Regardless of whether evolution progresses toward a magma ocean or magma sill structure, our models predict that temperature inversions due to rapid 26Al redistribution are limited to bodies formed earlier than ≈1 Myr after CAIs. We find that if grain size was ≲1 mm during peak internal melting, only elevated solid–melt density contrasts (such as found for the reducing conditions in enstatite chondrite compositions) would allow substantial melt segregation to occur.

• Item Type: Articles
• Additional Information: TL was supported by ETH Zürich Research Grant ETH-17 13-1 and acknowledges partial financial support from a MERAC travel grant of the Swiss Society for Astrophysics and Astronomy and from the National Center for Competence in Research PlanetS supported by the Swiss National Science Foundation. TK acknowledges financial support from Jenny Suckale, Stanford University. TK and RFK received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement number 279925.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Keller, Dr Tobias
• Authors: Lichtenberg, T., Keller, T., Katz, R. F., Golabek, G. J., and Gerya, T. V.
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences
• Journal Name: Earth and Planetary Science Letters
• Publisher: Elsevier
• ISSN: 0012-821X
• ISSN (Online): 1385-013X
• Published Online: 14 December 2018