Experimental Investigation of Condensation Predictions for Dust-Enriched Systems

TL;DR

This study experimentally tests condensation models for dust-enriched systems, comparing predictions with laboratory results to improve understanding of mineral stability and phase boundaries relevant to meteorites.

Contribution

It provides new experimental data validating and challenging existing thermodynamic models of mineral stability in dust-enriched systems.

Findings

Good agreement for forsterite, enstatite, and grossite

Discrepancies in spinel, perovskite, and gehlenite stability

Melilite stability larger than predicted

Abstract

Condensation models describe the equilibrium distribution of elements between coexisting mineral solid solutions, silicate liquid, and vapor in a closed chemical system, vapor phase always present, using equations of state of the phases involved at a fixed total P (< 1 bar) and temperature T. The VAPORS code uses a CaO-MgO-Al$_2$O$_3$-SiO$_2$ liquid model at T above the stability field of olivine and the MELTS algorithm at lower T. Quenched high-T crystal + liquid assemblages are preserved in meteorites as Type B Ca-, Al-rich inclusions (CAIs) and olivine-rich ferromagnesian chondrules. Experimental tests of compositional regions may clarify the nature of the phases present, the phase boundaries, and the partition of trace elements among these phases. Twenty-three Pt-loop equilibrium experiments in seven phase fields on twelve bulk compositions at specific T and dust enrichment factors tested the predicted stability fields of forsteritic olivine (Mg$_2$SiO$_4$), enstatite (MgSiO$_3$), Cr-bearing spinel (MgAl$_2$O$_4$), perovskite (CaTiO$_3$), melilite (Ca$_2$Al$_2$SiO$_7$ - Ca$_2$Mg$_2$Si$_2$O$_7$) and/or grossite (CaAl4O7) crystallizing from liquid. Experimental results for forsterite, enstatite, and grossite are in very good agreement with predictions, both in chemistry and phase abundances. On the other hand, the stability of spinel with olivine, and stability of perovskite and gehlenite are quite different from predictions. Perovskite is absent in all experiments. Even at low oxygen fugacity (IW-3.4), the most TiO$_2$-rich experiments do not crystallize Al-, Ti-bearing calcic pyroxene. The stability of spinel and olivine together is limited to a smaller phase field than is predicted. The melilite stability field is much larger than predicted, indicating a deficiency of current liquid or melilite activity models. In that respect, these experiments contribute to improving the data for calibrating thermodynamic models including MELTS.