Early differentiation of planetesimals: Insights from melting experiments of an L6 ordinary chondrite
Abstract
Introduction: Planetary differentiation in small bodies is believed to be ruled by several partial end-states that were dominated by variable degrees of melting and fractionation of the metallic and silicate phases. Studying the melting behaviour of undifferentiated chondritic materials is pivotal for reconstructing differentiation processes occurring during the early evolution of planetesimals and, eventually, leading to the formation of rocky planets and partially differentiated asteroids. In this study, we present results from melting experiments performed at 1 GPa, using an L6 ordinary chondrite (DAV 01001).
Results: At the temperature of 1100 °C, the initial chondritic texture is preserved and melting of silicate minerals is not observed, while the opaque phases (kamacite, taenite and troilite) react forming two immiscible liquid phases, a FeNi metal phase and a S-rich phase. Melting of the silicate domain initiates at 1200 °C and slightly increases with temperature, yielding to a progressive obliteration of the chondrules and textural re-equilibration of the silicate assemblage (re-crystallization at mineral-melt interfaces and within the melt). No substantial textural changes are observed for the FeNi metal and S-rich phase upon further temperature increase, with the FeNi metal phase typically forming spherical or cruciform blebs enveloped by the S-rich phase. The non-modal melting of the silicate mineral assemblage (in the order: plagioclase > high-Ca pyroxene > low-Ca pyroxene > olivine) and subsequent re-crystallisation determines the evolution of the silicate melt from a dominantly trachy-andesitic composition at 1200 °C, to basaltic trachy-andesitic at 1300 °C and andesitic at 1400 °C. The composition of the silicate melt produced in the experiments shows analogies with the trachy-andesitic and andesitic bulk compositions of some anomalous achondrites, whereas the compositional variation of the silicate minerals compares well with that of several achondrite groups, such as pallasites, acapulcoites, lodranites, ureilites, brachinites and IAB inclusions.
Discussion: At the experimental conditions, the FeNi metal and S-rich liquids are always immiscible and the surface tension-dominated regime causes the FeNi metal to be preferentially wet by the S-rich phase, remaining thus insulated from the silicate domain. Under such circumstances, the partitioning of siderophile elements into the metal phase is expected to be limited by the presence of the S-rich phase, which acts as a chemical barrier reducing the exchange of these elements between the silicate and FeNi metal phases. This is consistent with the “excess” of siderophile elements in the Earth’s mantle, relative to the abundance expected from complete core-mantle equilibration. Overall, melting experiments suggest that small degrees of melting and re-crystallisation under magmatic conditions could have been dominant processes at the onset of planetesimal differentiation, explaining the formation of both differentiated (crustal-like) and undifferentiated (mantle-like) lithologies. The absence of evidence for silicate-metal fractionation suggests that, in the lack of differential stress or strain (possibly induced by impact processes or spin rotation) and particularly when silicate melt is present interstitially, the efficiency of metal-sulphide segregation into a core may be severely limited in small planetesimals
Biography
Stefano Iannini Lelarge is a Ph.D. candidate in Earth Sciences at the University of Pisa. He completed his master's with honors at the University of Pisa defending a thesis on a geochemical study of mesosiderites, aimed at investigating their formation and relation to asteroid 4 Vesta. During his Master, Stefano visited and collaborated with people at the Natural History Museum London and the Open University. During his PhD, he started working on the application of experimental petrology in planetary sciences, using the experimental approach to simulate the formation of achondrites in partially differentiated asteroids and constrain the timing and style of aqueous alteration in CM2 chondrites and Ceres. His PhD collaborations include renewed institutions such as the Natural History Museum London (UK), the Natural History Museum Vienna (Austria), The Open University (UK), the Bavarian Research Institute of Experimental Geochemistry and Geophysics (Germany), Elettra Sincrotrone Trieste (Italy), Charles University (Czech Republic) and now at the University of Queensland (Australia). He has also participated in international conferences and the 2021 edition of the EVA Exercise at Meteor Crater of the Center for Lunar Science and Exploration (LPI-JSC) and NASA (SSERVI).
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