¹Shaofan Che,¹Adrian J.Brearley
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.027]
¹Department of Earth and Planetary Sciences, MSC03-2040, 1 University of New Mexico, Albuquerque, NM 87131-0001, USA
Copyright Elsevier

Enstatite is a ubiquitous phase in chondritic meteorites, interplanetary dust particles, and cometary samples. In equilibrium condensation models, enstatite is predicted to condense via a reaction between pre-condensed forsterite and gaseous SiO. However, previous studies have shown that some enstatites in chondrite matrices and AOAs do not have a genetic relationship with forsterite, arguing against formation by the predicted forsterite-gas reaction. Here we report the occurrence of enstatite in a unique, fine-grained, spinel-rich inclusion (FGI) Ef1014-01 in the Efremovka CV3 chondrite. Enstatite in this FGI is present as an outer layer on spinel-anorthite-diopside nodules and separates the FGI from an amoeboid olivine aggregate (AOA) -like material. Enstatite shows elevated CaO and Al2O3 contents (up to a few weight percent). Four FIB sections were extracted from this FGI to investigate the microstructures of enstatite and its relationship with other phases using TEM techniques. The TEM observations show that the enstatite is dominantly low-temperature clinoenstatite (LCLEN), which displays abundant twinning, and is sometimes associated with thin orthoenstatite (OREN) lamellae. Clinoenstatite grains commonly have a crystallographic orientation relationship with adjacent diopside, but do not exhibit any replacement relationship with forsterite in the AOA-like material surrounding the FGI. Investigations of several other fine-grained CAIs from the Efremovka and Leoville CV3 chondrites show that enstatite is more common in these inclusions than previously thought and typically forms discontinuous layers or islands on the diopside layers.

Based on SEM and TEM observations, we suggest that the LCLEN-OREN intergrowths in Ef1014-01 formed by transformation from a protoenstatite (PEN) precursor, which may be a product of direct condensation or reheating in the solar nebula. The crystallographic orientation relationship between enstatite and diopside suggests that epitaxial growth of enstatite occurred, lowering the activation energy for nucleation and facilitating direction condensation of enstatite from the gas phase, rather than by reaction of the gas with forsteritic olivine. The microstructures of enstatite are indicative of an extremely rapid cooling rate (∼104 K/h) that is within the range of chondrule cooling rates. Such a rapid cooling rate may imply that the cooling rates of FGIs are indeed much higher than other types of refractory inclusions. Alternatively, the rapid cooling rate may not reflect the primary cooling of the FGIs, but is the result of rapid cooling after a short-lived secondary reheating event in the solar nebula.

A fractionated gas with a lower Mg/Si ratio than the solar value is required to condense enstatite. Such a gas could be produced by isolation of pre-condensed forsterite or repeated evaporation-recondensation processes. The presence of both enstatite-bearing and enstatite-free CAIs in CV3 chondrites suggests that at least two gaseous reservoirs with different Mg/Si ratios were present in the CAI-forming regions.

^1,2,3A.Stephant,¹M.Wadhwa,¹R.Hervig,¹M.Bose,²X.Zhao,²T.J.Barrett,²M.Anand,²I.A.Franchi
Geochimica et cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.01.004]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA
²School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
³Istituto di Astrofisica e Planetologia Spaziali – INAF, 00111 Roma, Italy
Copyright Elsevier

Recent investigations of meteorites thought to originate from the asteroid 4 Vesta have suggested an early accretion of water on rocky bodies in the inner Solar System from a carbonaceous chondrite-like source. However, these studies have been based on the hydrogen isotope compositions (δD) of late-crystallizing apatite grains in eucrites that likely do not record the primary magmatic composition. We have determined the δD and H2O concentrations in some of the earliest-formed silicates (clinopyroxenes) from several eucrites with the goal of constraining the hydrogen isotope composition of their source reservoir on their parent body. The H2O concentrations in clinopyroxenes from eucrites Juvinas, Stannern and Tirhert range from 5 to 18 μg/g, with a weighted average δD of –263 ± 70 ‰. Their apatites and whitlockites exhibit a higher weighted average δD of –165 ± 73 ‰, possibly as a result of H2 degassing during or after phosphate crystallization. Thermal metamorphism of these eucrites has most probably resulted in the loss of H, and an increase in their original δD values. While the weighted average δD value for the eucrite clinopyroxenes reported here is inferred to reflect an upper limit for the isotopic composition of the silicate mantle reservoir on their parent asteroid 4 Vesta, the average δD value of Stannern clinopyroxenes is considered to be closest to the initial δD of the source mantle (i.e., –373 ±127 ‰), which is lighter than that of Earth’s depleted upper mantle and most carbonaceous chondrites. We suggest that at least some of the water in 4 Vesta (and possibly other rocky bodies in the inner Solar System) was derived from a relatively deuterium-poor reservoir in the protosolar nebula, which was incorporated into planetesimals formed early in Solar System history.