Behind the paper

About ten years ago, we solidified the idea for this paper and had essentially arrived at our answer for how to generate the first magmas on the Moon. But we still needed some more time to fine tune our model and polish up its presentation. This might seem like a simple step to completion, but the journey to publication has been anything but.

I guess you could say it all started with spinel (MgAl₂O₄) c. 2013. I was a graduate student at the time attending Brown University, and there was a lot of buzz circulating about a potential new lunar lithology, pink spinel anorthosite (PSA). PSAs were observed via orbital spacecraft all across the surface of the Moon and commonly associated with fresh and undisturbed impact crater central peaks or basin walls. This geologic context was telling us that the PSA rock type was buried within and all throughout the Moon’s ancient crust.

My colleagues and I each led a series of experimental investigations aimed at constraining the ingredients and process necessary to reproduce this type of spinel on the Moon. What we found was that if you reacted the most primitive magma compositions hypothesized on the Moon with its ancient and primary crust, you could reproduce PSA. This was exciting for two reasons: 1) the primitive melts needed to react with the lunar crust to make this type of spinel are thought to represent the first magmas on the Moon. In turn, these magmas then formed the Mg-suite rock types (which include a troctolite rock bearing the same type of spinel). The Mg-suite rocks are particularly important to the lunar rock record because they provide foundational evidence for the guiding paradigms of planetary differentiation and early lunar mantle convection. The big-picture implication here was that 2) PSA then became a new global proxy for the spatial extent of the earliest episode of lunar magmatism.

So what can the global distribution of Mg-suite tell us about the extent and style of early lunar mantle convection?

Sink or float

There was a postdoc in our department conducting a lot of great 3D dynamic simulations on the earliest form of lunar mantle convection, i.e., the cumulate mantle overturn hypothesis. Mantle overturn sets up from the lunar magma ocean hypothesis, which essentially posits that a global-scale magma ocean (technically, the FIRST first magma on the Moon) crystallized from the bottom upward producing a Mg-rich silicate lower mantle and a more Fe-rich silicate upper mantle. After ~80% crystallization or so the mineral plagioclase begins to precipitate and form floating rock-bergs in the remaining ocean of magma that has yet to crystallize. The coalescence of these floating rock-bergs of plagioclase is thought to make up a majority of the primary lunar crust(!). Sandwiched between those mantle components and this flotation crust is thought to be a layer rich in ilmenite which is super dense compared to the silicate mantle beneath it. Scientists then thought this super dense layer would ultimately sink inward, and many studies since have focused on the fate of these dense ilmenite-bearing cumulates or even if they could form the Moon's core if they sank all the way down(!).

Anyhow, this overturn process is classically linked to Mg-suite production in one way or another, but such origin stories had yet to be fully quantified. And so on one fateful summer afternoon, I sat down with this postdoc and another grad student and we asked,

What happens to the lower lunar mantle when the dense stuff on top of it starts sinking?

Does the dense stuff just sink down in one big glob and expel a single mantle glob upward? Are there multiple drips of the dense stuff and the mantle responds with multiple upwellings of Mg-rich stuff? The basic principle at play in our line of questioning is that when rocky mantles rise upward, moving from a state of high pressure to one of lower pressure as can be the case during mantle convection, they tend to partially melt. Could these overturn-induced partial melts from the Moon's deep interior explain the observations associated with early secondary crust building?

Sketches were drawn. Coffees and tea were brewed. A hypothesis took root.

Things were starting to take shape so as is customary for this type of story, we now meet the curveball….

The detour(s)

As I approached my PhD defense date, I was notified that my postdoc opportunity had fallen through. I reached out to everyone that I knew for help, but there simply was not enough funding to support me at the last minute. I was in effect sprinting toward unemployment. I successfully defended, but shortly thereafter I was waiting tables day and night to make ends meet (shout out to undergrad me that swore he would never wait tables again after getting accepted to grad school 🤦🏼‍♂️).

I worked on publishing papers in between shifts at a nearby coffee shop in hopes to remain relevant in the planetary science community. Serving margaritas always came first though and my publication record quickly developed "gap years." I wasn't ready to move on, so I decided to keep my head down, digging. I saved up enough in tips after a while to pay out of pocket and start attending workshops and meetings again. I listed our apartment as an affiliation on abstracts (in retrospect, I should’ve listed the restaurant).

All this to say that the promising ideas and hypothesis about the first magmas on the Moon were all but a distant memory at this point in my career.

The story unfortunately goes on like this for some time, but not without its momentary highs, and a big thank you to Juliane and Dave for each supporting me at critical moments during this time. Eventually (read: 3 years later) my persistence paid off and I secured postdoc funding for up to 3 years. But within the first year my advisor left for an amazing job opportunity elsewhere. Half a year after that, the pandemic shut everything down. And two months later, our son was born. Thankfully, daddy daycare came somewhat naturally because waiting tables as a second full time job will sharpen your time management skills.

It was during this time in isolation when the otherwise star-crossed project on early lunar mantle convection rematerialized. And when it was my turn to rock the kiddo to sleep, my incredible partner helped me to code data processing and visualization scripts while sitting in a camping chair that was parked in front of our tiny built-in kitchen desk. We were making the most of it.

Many, many moons later

A lot can happen in lunar science over the span of a decade and we had to continuously update our model over the course of the project. For instance, overturn timing remained a critical component to cementing it as a mode of origin for Mg-suite, and near congruent formation ages were being established for both putative primary crust samples and secondary Mg-suite. This is somewhat problematic for defining a timeline of events because the petrologic context requires that the primary lunar crust formed prior to Mg-suite magmas intruding into said crust. Yet the most robust geochronological data was implying that these two events were separated by only tens of millions of years or less (this can be a pretty rapid transition from the rock's perspective).

So one of the major findings of our study is that mantle overturn reconciles this outstanding issue of near congruent formation ages obtained for the ancient primary lunar crust and the very first magmas (secondary crust) on the Moon. Our 3D dynamic simulations demonstrate that the onset of lunar cumulate overturn triggers a rapid and short-lived response of lower mantle melting. We show that these overturn-induced melts from the lower mantle simultaneously explain the key volume, chronological, and spatial characteristics of the Moon's earliest magmas (i.e., Mg-suite). In this way, our work suggests that mantle overturn dominates the geologic history of the Moon in the immediate aftermath of global-scale magma ocean differentiation.

Regardless, my hope is that our study underscores the importance of future sample return missions, detailed surface exploration via orbital spacecraft and robotic landers and rovers, and further radiometric dating toward constraining the dynamical evolution of the terrestrial planets and Moon.

Thank you for reading, sincerely.