Abstract
Angrites are derived from the earliest generation of differentiated planetesimals that accreted sunward of Jupiter’s orbit, and are, thus, key to constraining the timing and source(s) of volatile delivery to planetary bodies in the inner solar system. Here we investigate the nitrogen and hydrogen isotopic signatures of angrite melts by in situ secondary ion mass spectrometry (SIMS) analyses of mineral-hosted melt inclusions and interstitial glass in two of the oldest volcanic angrites: D’Orbigny and Sahara 99555. The most primitive melt trapped in Mg-rich olivines in D'Orbigny is characterized by δ15N values ranging from 0 ± 25 to +56 ± 29‰ and δD values between −348 ± 53 and −118 ± 31‰. This shows that the angrite mantle source sampled by D'Orbigny has a N-H isotopic composition that is similar to that of CM carbonaceous chondrites, whose parent bodies are thought to have accreted in the outer solar system. The low nitrogen and water contents measured in Sahara 99555 possibly indicate that its parental melt underwent a higher degree of degassing compared to D’Orbigny or, alternatively, that the two angrites do not sample the same volatile reservoir within the angrite parent body. Given the very old crystallisation age of D’Orbigny, our findings imply that nitrogen- and water-rich objects, presumably formed beyond the orbit of Jupiter, must have been present in the terrestrial planet-forming region within the first ~4 Ma after the formation of Ca-Al-rich inclusions (CAIs, the oldest materials in the solar system).
Highlights
The origin and timing of the accretion of nitrogen and water on Earth remains a subject of controversy
It is noteworthy that the isotopic composition of N-rich inclusions is remarkably similar to the values of +20.3 ± 3.6‰ and +34. 8 ± 10.7‰ obtained by Abernethy et al (2013, 2018) for D’Orbigny bulk rock chips and glass, respectively, using static mass spectrometry
Outer solar system bodies are thought to have been scattered into the terrestrial planetforming region during the growth and/or gas-driven migration of the giant planets and the resulting destabilization of the outer disk (Raymond and Izidoro, 2017); our results suggest that this scattering must have occurred prior to the extrusion of angrite lavas ~4 Ma after CAI formation, consistent with the timescales required for dissipation of the gaseous protoplanetary disk
Summary
The origin and timing of the accretion of nitrogen and water on Earth remains a subject of controversy. Terrestrial mantle-derived samples record a wide range of hydrogen isotopic compositions (dD), which encompass low, solarlike dD values (Hallis et al, 2015), as well as higher values measured in enstatite and carbonaceous chondrites (Peslier et al, 2017 and references therein; Loewen et al, 2019), suggesting that various sources contributed to Earth’s water inventory.
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