53-1 Chondrule Mesostasis in the Allan Hills 81189 Enstatite Chondrite

Abstract:

Chondrites are among the oldest materials in our solar systemand thought to be the “leftover building blocks” of the terrestrial planets. Enstatite Chondrites (ECs) are the most reduced primitive materials in the solar system, and isotopically closest to the Earth and Moon. Allan Hills (ALH) 81189 is one of the most primitive ECs, so its analysis can offer a clearer glimpse into the early solar system environment in which they formed. Many chondrules– submillimeter, semi-round melt droplets which formed in the protoplanetary disk– contain a glassy quenched silicate melt between olivine and pyroxene grains. This material is called mesostasis and is interpreted to be residual melt which was unable to completely crystallize. The compositions and textures of chondrules and their mesostasis constrain their formation conditions, including their thermal histories and volatile behavior. 

We analyzed chondrules and mesostasis in a thin section of ALH 81189 using optical microscopy, Raman spectroscopy, Scanning Electron Microscopy (SEM), and Electron Probe Microanalysis (EPMA) to examine textures and chemical compositions across 15 chondrules. A Time-Dependent Intensity (TDI) correction was used to compensate for alkali ion migration under the beam, which may contribute to higher sodium values. EPMA analysis reveals that the mesostasis in this EC is especially sodium-rich (up to 12.3 wt%), an interesting result given sodium’s moderately volatile nature and the subsequent expectation of significant depletion during chondrule formation. We further identified the pyroxene mineral pigeonite in two chondrules thus far, which constrains those chondrules’ thermal histories. The pigeonite is Mg-rich, which brings its crystallization temperature up to ~991 ℃, and its preservation indicates a cooling rate of >100℃/hr. This is consistent with the barred olivine (BO) texture seen in one of these chondrules. 

These findings indicate volatile retention despite high crystallization temperatures, which may be due to a high dust to gas ratio and/or high sodium fugacity in the formation environment. The abundance of chondrule mesostasis, and the presence of pigeonite and BO texture indicate very rapid cooling, which may have further contributed to the retention of volatiles, including sodium. The texture of the mesostasis remains glassy throughout, without development of secondary feldspars. This, along with the sharply defined chondrules and their heterogeneous chemical compositions, confirms the very primitive nature of ALH 81189.

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