¹Kathleen E. Vander Kaaden,²Francis M. McCubbin
¹Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM, USA
²Institute of Meteoritics, Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM, USA

The range in density and compressibility of mercurian melt compositions was determined to better understand the products of a possible mercurian magma ocean and subsequent volcanism. Our experiments indicate that the only mineral to remain buoyant with respect to melts of the mercurian mantle is graphite; consequently, it is the only candidate mineral to have composed a primary floatation crust during a global magma ocean. This exotic result is further supported by Mercury’s volatile-rich nature and inexplicably darkened surface. Additionally, our experiments illustrate that partial melts of the mercurian mantle that compose the secondary crust were buoyant over the entire mantle depth and could have come from as deep as the core-mantle boundary. Furthermore, Mercury could have erupted higher percentages of its partial melts compared to other terrestrial planets because magmas would not have stalled during ascent due to gravitational forces. These findings stem from the FeO-poor composition and shallow depth of Mercury’s mantle, which has resulted in both low melt density and a very limited range in melt density responsible for Mercury’s primary and secondary crusts. The enigmatically darkened, yet low-FeO surface, which is observed today can be explained by secondary volcanism and impact processes that have since mixed the primary and secondary crustal materials.

Reference
Vander Kaaden KE, McCubbin FM (2015) Exotic Crust Formation on Mercury: Consequences of a Shallow, FeO-poor Mantle. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004733]

Published by arrangement with John Wiley&Sons

¹Evelyn Füri, ^1,2Marc Chaussidon, ¹Bernard Marty
¹Centre de Recherches Pétrographiques et Géochimiques, CNRS-UL, 15 rue Notre Dame des Pauvres, BP20, 54501 Vandoeuvre-lès-Nancy, France
²Now at Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, 75005 Paris, France

Nitrogen and noble gas (Ne-Ar) abundances and isotope ratios, determined by CO2 laser extraction static mass spectrometry analysis, as well as Al-Mg and O isotope data from secondary ion mass spectrometry (SIMS) analyses, are reported for a type B calcium-aluminum-rich inclusion (CAI) from the CV3 chondrite NWA 8616. The high (26Al/27Al)i ratio of (5.06 ± 0.50) × 10-5 dates the last melting event of the CAI at View the MathML source39-99+109 ka after the “time zero”, limiting the period during which high-temperature exchanges between the CAI and the nebular gas could have occurred to a very short time interval. Partial isotopic exchange with a 16O-poor reservoir resulted in Δ17O > -5‰ for melilite and anorthite, whereas spinel and Al-Ti-pyroxene retain the inferred original 16O-rich signature of the solar nebula (Δ17O ⩽ -20 ‰). The low 20Ne/22Ne (⩽0.83) and 36Ar/38Ar (⩽0.75) ratios of the CAI rule out the presence of any trapped planetary or solar noble gases. Cosmogenic 21Ne and 38Ar abundances are consistent with a cosmic ray exposure (CRE) age of ∼14 to 20 Ma, assuming CR fluxes similar to modern ones, without any evidence for pre-irradiation of the CAI before incorporation into the meteorite parent body. Strikingly, the CAI contains 1.4 to 3.4 ppm N with a Δ15N value of +8 to +30 ‰. Even after correcting the measured Δ15N values for cosmogenic 15N produced in situ, the CAI is highly enriched in 15N compared to the protosolar nebula (Δ15NPSN = -383 ± 8 ‰; Marty et al., 2011), implying that the CAI-forming region was contaminated by 15N-rich material within the first 0.15 Ma of Solar System history, or, alternatively, that the CAI was ejected into the outer Solar System where it interacted with a 15N-rich reservoir.

Reference
Füri E, Chaussidon M, Marty B (2015) Evidence for an early nitrogen isotopic evolution in the solar nebula from volatile analyses of a CAI from the CV3 chondrite NWA 8616. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.01.004]

Copyright Elsevier