^1,4Jangmi Han, ²Kazuhide Nagashima, ¹Changkun Park, ²Alexander N. Krot, ¹Lindsay P. Keller
Geochimica et Cosmochimica acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.09.001]
¹Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
³Division of Glacier and Earth Sciences, Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon 21990, South Korea
⁴ARES, NASA Johnson Space Center, 2101NASAParkway, Houston, TX 77058, USA
Copyright Elsevier

We report the results of coordinated mineralogical, microstructural, and oxygen isotopic analyses of grossite-bearing refractory inclusions from reduced CV (Vigarano type) chondrites to obtain a more complete picture of secondary parent body alteration processes and conditions. Grossite (CaAl₄O₇) occurs in cores of nodules in fine-grained Ca,Al-rich inclusions (CAIs) that likely represent aggregates of nebular condensates. In many occurrences, grossite has been partially replaced by hercynite [(Fe,Mg,Zn)Al₂O₄], which displays complex microstructures and compositions, and magnetite nanoparticles. The alteration of grossite was a crystallographically-controlled, fluid-driven process that occurred via partial dissolution of grossite and subsequent precipitation of hercynite and magnetite during short-lived and low-temperature metasomatic alteration on the CV chondrite parent body. The constituent phases of grossite-bearing CAIs show heterogeneous oxygen isotopic compositions, with grossite and perovskite displaying systematically ¹⁶O-depleted compositions (Δ¹⁷O= − 12 ‰ to − 1 ‰) relative to uniformly ¹⁶O-rich hibonite and spinel (Δ¹⁷O= − 25 ‰ to − 21 ‰). Melilite is variably ¹⁶O-depleted (Δ¹⁷O= − 25 ‰ to − 2 ‰). The observed oxygen isotopic distribution is interpreted as a result of mineralogically controlled oxygen isotopic exchange with an ¹⁶O-poor fluid on the CV chondrite parent body. Collectively, the presence of limited fluids played an important role in preferential alteration of grossite to hercynite and magnetite and various degrees of ¹⁶O depletion in grossite, perovskite, and melilite during thermal metamorphism. We conclude that, among refractory phases in the inclusions, grossite was the most susceptible to metasomatic reactions with Fe-rich fluids and the second most susceptible, after perovskite, to oxygen isotopic exchange with an ¹⁶O-poor fluid during the thermal history of the CV chondrite parent asteroid.

¹Aindrila Pal, ¹Rajdeep Dasgupta
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.08.026]
¹Department of Earth, Environmental and Planetary Sciences, Rice University, Houston TX-77005, USA
Copyright Elsevier

Nitrogen is an essential element for life. Yet the processes of planet formation and early planetary evolution through which rocky planets like Earth obtained their atmospheric and surface nitrogen inventory are poorly understood. In order to understand the effect of early silicate differentiation of the rocky bodies on N inventory, here we study the elemental partitioning of N between the silicate minerals and melts. We conducted laboratory experiments using tholeiitic basalts and Fe + Si alloy mixtures at 1.5–4.0 GPa and 1300 to 1550 ⁰C under graphite saturation at an oxygen fugacity range of IW–1.1 to IW–3.0. The experiments yielded an assemblage of Fe-rich alloy melt (am) + silicate melt (sm) + clinopyroxene (cpx) ± garnet (grt) ± orthopyroxene (opx) ± plagioclase (plag). Using electron microprobe, we determine that under the experimental conditions, N act as an incompatible element with DNcpx/sm (0.11–––0.47) > DNplag/sm (0.40) >DNopx/sm (0.25) >DNgrt/sm(0.06–––0.21). The DNmineral/sm do not show any strong dependence on temperature, pressure, and melt composition. However, through comparison with previous estimates, it appears that with decreasing fO₂, N becomes less incompatible. Under our experimental conditions of alloy melt-mineral equilibria, N behaves as a siderophile element (DNam/mineral ranging from 4.1 to 60.6) with fO₂ playing the strongest control on DNam/mineral. Our data suggest that under reducing conditions, in the early stages of a magma ocean (MO) and/or deeper mantle, silicate minerals would hold a non-negligible fraction of N as N becomes less atmophile and siderophile. Therefore, reduced parent bodies could also retain substantial N in the residual mantle during partial melting. The extraction of N from an internal MO or a solid planetary mantle is thus enhanced only as the system becomes more oxidizing, enriching the surficial reservoirs in N. Thus, Earth’s N₂-rich atmosphere may be intrinsically linked to its mantle oxidation, whereas other rocky planets of the Solar System, such as Mars and Mercury, may have retained a significant portion of their N inventory in nominally N-free mantle silicates through episodes of MO crystallization and mantle melting.