Experimental constraints on the onset of planetesimal melting, the formation of primitive achondrites and the distribution of alkali elements in the solar nebula

In this presentation, we will summarize the conclusions of three recent articles [1-3] describing partial melting experiments of ordinary and carbonaceous synthetic chondrites (H, LL, CI, CM and CV). The experiments highlight the role of alkali elements on the melting processes of chondritic planetesimals and provide insights into the distribution of “moderately volatile elements” in the early solar system. They were performed at 2–13 MPa (CO pressure) in a Molybdenum-Hafnium Carbide Pressure Vessel. This approach prevented the loss of alkali elements during experiments, a common limitation of previous studies using gas-mixing furnaces.

Alkali-rich planetesimals, similar in composition to CI, H and LL chondrites, started to melt at low temperature (1040 ºC) and produced silicate melts with high alkali, SiO₂ and Al₂O₃ contents [1], similar in composition to “trachyandesite achondrites” such as GRA 06128 [4] and ALM-A [5]. In addition, the main groups of primitive achondrites (i.e., brachinites, acapulcoites-lodranites and ureilites), which represent mantle residues, all produced similar low-degree melts (<15 wt.%) rich in SiO₂, Al₂O₃ and alkalis that were extracted from the mantle of the different parent bodies [2].

Ureilites (>550 samples), represent the residual mantle of a planetesimal that was catastrophically disrupted and quenched while in the process of melting [3]. Our experiments show that the Ureilite Parent Body (UPB) produced a total of 16-24 wt.% silicate melt as small increments (< 5%). Following the extraction of melts rich in SiO₂, Al₂O₃ and alkalis, the residual mantle produced melts poor in alkalis but rich in CaO. The sampled portion of the UPB reached temperatures as high as 1300 ºC, but the rapid extraction of silicate melts preserved primordial heterogeneities in O, C and Cr isotopes as well as in intrinsic fO₂.

Trachyandesite achondrites, ureilites and other major groups of primitive achondrites were all derived from planetesimals that were initially rich in alkali elements (i.e., not depleted relative to the Sun’s photosphere). They all display nucleosynthetic anomalies characteristic of the inner solar system. Therefore, the depletion of alkalis in other meteorite parent bodies of the inner solar system (e.g., Vesta) likely results from processes that occurred during partial melting. Similarly, the depletion of alkalis in terrestrial planets could result from a secondary loss of alkalis associated with partial melting of the planets building blocks, rather than from the incomplete condensation of the solar nebula.

References:

[1] Collinet and Grove (2020a), GCA, https://doi.org/10.1016/j.gca.2020.03.005, [2] Collinet and Grove (2020b), GCA, https://doi.org/10.1016/j.gca.2020.03.004, [3] Collinet and Grove (2020c), MAPS, https://doi.org/10.1111/maps.13471, [4] Day et al., (2009), Nature, https://doi.org/10.1038/nature07651, [5] Bischoff et al. (2014), PNAS, https://doi.org/10.1073/pnas.1404799111