Jennifer T. Mitchell
Diogenites are a group of meteorites from asteroid 4 Vesta, composed almost entirely of orthopyroxene with rare examples of olivine-bearing samples. Despite their mineralogical simplicity, how they form and their relationship to other Vestan meteorite groups has been debated for decades. There are two main schools of thought: a) diogenites form from a global magma ocean (40-50< % melt), and crystallising minerals settle out into layers to create an “onion-skin” structure; b) diogenites form later in Vesta’s history as magmatic intrusions into the crust. Whilst NASA’s Dawn mission provided a wealth of information about Vesta, there was no conclusive evidence to help answer this question. It is therefore prudent to determine whether the magma ocean of crustal intrusion model is most appropriate, so that we can better understand the details of planet formation in the early solar system, over 4 billion years ago. This study investigates plausible formation mechanisms of diogenites in order to answer long-standing questions about how they were produced, and the implications this has on our understanding of processes in the early solar system. In particular, this study assesses whether or not the global magma ocean can actually generate diogenite compositions, and how diogenites may relate to other Vestan meteorites. Importantly, all modelled compositions will be compared to natural meteorite compositions and to data from NASA’s Dawn mission.
Early pMELTS models established that small changes in oxygen fugacity have large changes in the Mg-Fe content of the crystallising pyroxenes. However, all the compositions produced at this stage were too high in Ca compared to natural diogenites. In order to deplete the diogenite source of Ca, our models find that removing <20% of a eucrite component best replicates compositions seen in natural diogenites. This suggests that initial eucrite magmatism pre-dates diogenite formation, meaning that a magma ocean scenario is not appropriate for Vesta.
Two THERMOCALC models were produced to determine the conditions needed for magmatism on Vesta – a primitive/undepleted composition and an evolved composition that reflects the removal of a eucrite component. Here, we find that initial eucrite magmatism occurs at approximately 1240°C whilst diogenite magmatism cannot begin until at least 1340°C. This suggests that the first stage of magmatism produced a stagnant lid that trapped the heat generated through radioactive decay inside Vesta, allowing the high temperatures needed for diogenites to be reached. These models also find that only low % of partial melting are required, meaning that a magma ocean scenario is not appropriate to generate these Vestan lithologies.
With this knowledge, thermal models of Vesta’s evolution were developed which investigate the heating effects of short-lived radioisotopes 26Al and 60Fe. These models show that Vesta accreted ~1.5 Myr after CAI formation. If Vesta accreted earlier, the system is too hot. If it accreted later, the system is too cold to produce diogenite compositions. The first stage of magmatism (1240°C) that produced the stagnant lid occurs around 3Myr after CAI formation, and initial diogenite magmatism (1340°C) followed at around 5Myr. This lag in the onset of diogenite magmatism further suggests diogenites do not represent magma ocean cumulate, and instead represent diverse crustal intrusions.
Our models show that Vesta underwent a complex evolution, where multiple stages of magmatism occurred. Our key findings are summarised here; i) Vesta accreted early in the Solar System, allowing the decay of 26Al/60Fe to heat the interior of the asteroid to high enough temperatures to produce both eucrites and diogenites; ii) magmatism on Vesta functions efficiently at low % partial melt; iii) eucrite magmatism began before diogenite magmatism; iv) eucrite and diogenite magmatism are contemporaneous with other achondrite meteorite classes; v) diogenites do not represent Vesta’s mantle nor magma ocean cumulates, and are most likely late-stage crustal intrusions; vi) diogenites require compositional diversity, variable fO2 and high temperatures in a Ca-depleted source.